Sterrekunde

Hoe weet ons dat pulse twee balke het?

Hoe weet ons dat pulse twee balke het?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Pulsars is neutronsterre wat 'n straal van elektromagnetiese straling uitstraal wat nie in lyn is met hul rotasie-as nie. Ons kan dus slegs 'n neutronster as 'n pols sien as die balk die aarde se pad kruis.

Die algemene voorstelling is dat daar twee balke is, een aan elke kant, maar tensy die magnetiese as en die rotasie-as 90 ° is, kan ons net een van die twee balke sien. Hoe weet ons dat daar twee balke is en nie een, of vier, of enige ander getal nie?


Die magneet- en rotasie-as hoef nie te wees nie presies op 90 grade na mekaar om albei balke te sien. Dit hoef slegs ongeveer 90 grade te wees (afhangend van die 'openingshoek' van die emissiekegel).

Daar is dus baie pulse wat hieraan voldoen en twee pieke in hul profiele het (as ons die emissie daarvan waarneem) wat ongeveer 'n halwe draai-periode uitmekaar is. Elke piek lyk anders vir die ander, so die beste raaiskoot is tans dat ons die balk vanaf die een pool sien en dan die ander.


Dit gaan eintlik oor die aard van fisiese teorieë - ons het waarnemingsbewyse en 'n (vervalsbare) teorie is gestel om te verduidelik wat gebeur. Die getuienis sedert die aanvaarding van die teorie het sy aansprake op korrektheid versterk.

(Maar alle magnete wat ons waargeneem het, is dipole - in die teorie bestaan ​​monopole maar is nog nooit waargeneem nie. Maar daar is geen teorie gestel dat neutronsterre monopole is nie - die energie wat nodig is om so 'n monopool te skep, sou ek seker wees, maar selfs groter as wat in 'n supernova vrygestel is.)


Verskeie pulsars het profiele met dubbele pieke: u kry twee pieke per rotasie, met een piek sterker as die ander. 'N Voorbeeld hiervan is die Crab Pulsar. Dit dui daarop dat ten minste sommige pulse twee balke het. Onthou dat die swaartekrag van lig in die omgewing van die neutronster dit moontlik maak om 'n groter fraksie van die oppervlak te sien, wat die kans verhoog dat beide pole sigbaar is.

Die enkelpiek-ligkurwes verteenwoordig waarskynlik die geval waar die tweede pool nie sigbaar is nie. Natuurlik, as u die advokaat van Devil wil speel, kan u redeneer dat daar enkelstraal-pulsars in die monster skuil ... u sal waarskynlik 'n soort fisiese meganisme moet bedink waardeur so 'n voorwerp kan bestaan, gegewe die fisiese teorieë wat tot dusver gebruik is om pulse te verklaar, voorspel twee balke.


Pulsars

Rooi dwerge, wit dwerge, neutronsterre, swart gate: dit is 'n lys van voorwerpe waarin elkeen kleiner, digter en ekstremer is in sy fisiese toestande as die voorheen. Die verdigting is die gevolg van die bekende swaartekrag, maar die verkorte sterre wat die resultaat is, is buite ons algemene ervaring. 'N Stuk wit dwergmateriaal van vuurhoutjiedosies bevat dieselfde massa as 'n slagskip, terwyl dieselfde massa neutronster-materiaal die ruimte van 'n speldkop inneem. 'N Swart gat is so platgeval dat grootte en digtheid nie meer betekenis het nie.

'N Wit dwerg, wat 'n ster is van ongeveer die grootte van die aarde, maar met 'n massa soortgelyk aan die van die son, word verhinder om verder te krimp deur' ontaarde elektrondruk '- vrye elektrone kan nie nader aan mekaar gepak word nie. In sommige sterre, gewoonlik massiewer as wit dwerge, word hierdie versperring oorkom deur die kombinasie van die elektrone met protone om neutrone te vorm, wat nog nouer saampak en 'n neutronster gee. 'N Neutronster het ongeveer dieselfde massa as die son, maar is ongeveer 30 kilometer breed. So klein het 'n ster 'n klein oppervlakte en kan nie veel van die termiese straling uitstraal wat normale sterre laat skyn nie. Sommige neutronsterre kan op groot afstande waargeneem word deur 'n heel ander soort straling, 'n gereeld polsende radiosignaal. Dit is die pulse.

Pulsars is in 1967 deur Anthony Hewish en Jocelyn Bell by die radiosterrekundige sterrewag (nou die Nuffield Radio Astronomy Observatory) in Cambridge ontdek. Hul kenmerkende radio-emissie is 'n eenvormige reeks pulse, met 'n groot presisie van tyd tot tyd tussen 'n paar millisekondes en 'n paar sekondes. Meer as 300 is bekend, maar net twee, die Crab Pulsar en die Vela Pulsar, gee sigbare pulse uit. Daar is ook bekend dat hierdie twee gammastraalpulse uitstraal, en die een, die Crab, straal ook X-straalpulse uit.

Die reëlmatigheid van die pulse is fenomenaal: waarnemers kan die aankomstye van pulse 'n jaar vorentoe met 'n beter akkuraatheid as 'n millisekonde voorspel.
Hoe kan 'n ster as so 'n akkurate horlosie optree?
Die enigste moontlikheid vir so 'n vinnige en so presiese herhaling is dat die ster vinnig draai en 'n straalstraal uitstraal wat soos 'n vuurtoring deur die lug vee en een keer per rotasie na die waarnemer wys. Die enigste soort ster wat vinnig genoeg kan draai sonder om deur sy eie sentrifugale krag te bars, is 'n neutronster.

Pulsars is baie sterk gemagnetiseerde neutronsterre, met sterktevelde wat 100 miljoen Tesla bereik (1 miljoen miljoen Gauss, vergeleke met minder as 1 Gauss vir die Aarde se magnetiese veld). Die vinnige rotasie maak hulle dus kragtige elektriese kragopwekkers wat in staat is om gelaaide deeltjies te versnel tot energie van duisend miljoen miljoen Volt. Hierdie gelaaide deeltjies is, op die een of ander manier nog onbekend, verantwoordelik vir die straalstraling in radio-, lig-, X-strale- en gammastrale. Hulle energie kom van die rotasie van die ster, wat dus moet vertraag. Hierdie verlangsaming kan opgespoor word as 'n verlenging van die polsperiode. Gewoonlik vertraag 'n pulsar-rotasiesnelheid elke jaar met een deel in 'n miljoen: die Crab Pulsar, wat die jongste en mees energieke is, word elke jaar met een deel in tweeduisend vertraag.


Hoeveel pulse in ons sterrestelsel?

Pulsars word hoofsaaklik in die Melkweg aangetref, binne ongeveer 500 ligjaar vanaf die vlak van die Melkweg. 'N Volledige oorsig van die pulse in die Melkweg is onmoontlik, aangesien swak pulse slegs opgespoor kan word as hulle naby is. Radioopnames het nou bykans die hele lug beslaan, en meer as 300 pulse is opgespoor. Die afstand daarvan kan gemeet word aan 'n vertraging in pols-aankomstye wat by lae radiofrekwensies waargeneem word. Die vertraging hang af van die elektrondigtheid in interstellêre gas en van die afgelegde afstand. Ekstrapolering van hierdie klein monster van waarneembare pulse word beraam dat daar ten minste 200 000 pulse in die hele ons sterrestelsel is. Met die oog op die pulsars waarvan die vuurtoringbalke nie in ons rigting oorwaai nie, moet die totale bevolking een miljoen bereik.

Elke pulsar straal vir ongeveer vier miljoen jaar nadat dit soveel rotasie-energie verloor het dat dit nie waarneembare radiopulse kan lewer nie. As ons die totale bevolking (1.000.000) en die leeftyd (4.000.000 jaar) ken, kan ons aflei dat elke vier jaar 'n nuwe pulsar gebore moet word (as ons aanvaar dat die bevolking bestendig bly).

Onlangs is pulse in bolvormige trosse gevind. Daar word geglo dat hulle daar gevorm is deur materie op wit dwergsterre in binêre stelsels aan te vul. Ander pulse word gebore in supernova-ontploffings. As alle polsare uit supernova-ontploffings gebore is, sou ons kon voorspel dat daar elke vier jaar 'n supernova in ons Melkweg sou wees. Dit is skouspelagtige gebeure, en ons sou verwag om meer daarvan te sien as dit elke vier jaar plaasvind. Die laaste supernova wat direk waargeneem is in ons Melkweg was Kepler se supernova van 1604 nC, maar ons weet wel dat daar ander voorkom wat minder skouspelagtig is of wat deur interstellêre stofwolke vir ons verborge is. Dit is nog nie duidelik of die geboortesyfer van pulse en die snelheid van supernovas volledig met mekaar in ooreenstemming kan kom of hoeveel buite bolvormige trosse in binêre stelsels gevorm kan word nie.

Die krapnevel is die sigbare oorblyfsel van 'n supernova-ontploffing wat in 1054 nC deur Chinese en Japannese sterrekundiges gesien is. Naby die middelpunt van die newel is die Crab Pulsar, wat die mees energieke pulsar is wat bekend is. Dit draai 30 keer per sekonde en word baie sterk gemagnetiseer. Dit dien dus as 'n hemelse kragstasie wat genoeg energie opwek om die hele newel oor feitlik die hele elektromagnetiese spektrum uit te straal.

Die Crab Pulsar straal twee pulse per omwenteling uit: hierdie dubbele polsprofiel is soortgelyk op alle radiofrekwensies vanaf 30 MHz opwaarts, en in die optiese, X-straal- en gammastraalgedeeltes van die spektrum, wat ten minste 49 oktawe in golflengte beslaan.

Die sigbare lig is kragtig genoeg sodat die polsaar op foto's van die newel kan verskyn, waar dit gesien word as 'n ster van ongeveer 16. Normale foto's maak die pulse gladder, maar stroboskopiese tegnieke kan die ster afsonderlik wys as dit 'uit' is en 'op' voorwaardes.


Die binêre pulser en algemene relatiwiteit:

Baie sterre is lede van binêre stelsels, waarin twee sterre om mekaar wentel met periodes van enkele dae of jare. As een van die sterre 'n neutronster is, kan die wentelpaar so naby wees dat die aantrekkingskrag tussen hulle baie groot is, en dat ongewone effekte waargeneem kan word. Verskeie binêre stelsels is bekend waarin die ander ster 'n reus is. In hierdie gevalle kan die neutronster gas uit die buitenste dele van sy metgesel lok en 'n stroom gas val met groot energie op die oppervlak van die neutronster. Hierdie stelsels word as X-straalbronne waargeneem. Sommige van die X-straalbronne toon periodieke variasies as die neutronster draai: dit is die sogenaamde 'X-straal-pulsars'.

Een binêre stelsel, bekend as PSR 1913 + 16, bestaan ​​uit twee neutronsterre, wat so naby aan mekaar is dat hul wenteltyd slegs 775 uur is. Geen gasstroom tussen hierdie sterre nie, wat slegs deur mekaar se swaartekrag aantrek. Die baan van een van hulle kan breedvoerig beskryf word, want dit is 'n pols.
Die periode van hierdie pulsar is 59 millisekondes, en dit lewer 'n baie stabiele reeks pulse met 'n buitengewone lae verlangsamingstempo. Dit is in werklikheid 'n akkurate horlosie wat baie vinnig in 'n sterk gravitasieveld beweeg, wat die klassieke situasie is wat nodig is vir 'n toets van Einstein se Algemene Relatiwiteitsteorie.

Volgens die nie-relativistiese, of Newtonse, dinamiese teorie, moet die wentelbane van albei sterre ellipse met 'n vaste oriëntasie wees, en die wentelperiode moet konstant wees. Metings van die aankomstyd van die pulse het beduidende verskille getoon van die eenvoudige Newtonse wentelbane. Die mees voor die hand liggende is dat die baan met 42 grade per jaar voorlê.
Daar is ook 'n klein, maar baie belangrike, effek op die wenteltydperk, wat nou bekend is dat dit met 89 nanosekondes (minder as een tien miljoenste sekonde) elke baan verminder.

Die verminderde wenteltydperk verteenwoordig 'n verlies aan energie, wat slegs deur swaartekragstraling verreken kan word. Alhoewel gravitasiestraling nooit direk waargeneem is nie, het die waarnemings van PSR 1913 + 16 'n goeie bewys gelewer van die bestaan ​​daarvan. Dit is gepas dat hierdie ontdekking, wat 'n verdere bevestiging is van die voorspellings van die Algemene Relatiwiteitsteorie, in 1979 aangekondig is, wat die eeufees van Einstein se geboorte was.

Geproduseer deur die Departement Inligtingsdienste van die Royal Greenwich Observatory.


Oor

Hierdie kaart toon die posisie van byna elke radiopulsar wat moderne navorsers ken. Die kaart is 'n ewe-hoekige projeksie van die naghemel in galaktiese koördinate, wat die hele lug as sigbaar vanaf die aarde wys met ons Melkwegstelsel wat deur die middel loop. Die grootte van elke sirkel illustreer die relatiewe helderheid (gesien vanaf die aarde) vir elke pulsar, en elke sirkel stuur pulse uit met die regte frekwensie van die pulsar. Met die skuifbalk bo-op die bladsy kan u deur die ruimte beweeg en verskillende afstande van die aarde verken. Kyk na die toonbank regs bo om te sien watter afstandsbereik tans getoon word. Klik op enige pulsar om meer oor die voorwerp te leer.

'N Pulsar is 'n spesifieke tipe sterk gemagnetiseerde neutronster wat ontstaan ​​wanneer 'n massiewe ster 'n kernval-supernova ondergaan. Pulsars is baie klein (vir 'n ster) en draai baie vinnig, wat sterk en smal strale van hul magnetiese pole uitstraal. As ons (op aarde) toevallig binne die pad van hierdie balk val terwyl dit deur die lug vee, tree die pulsar op soos 'n vuurtoring met sy balk wat gereeld oor ons gaan. Net soos 'n vuurtoring, is die waarneembare resultaat vir ons op aarde dat die voorwerp vir 'n kort tydjie baie helder word en vinnig oor en oor en weer wegkwyn, wat die sterk pulse skep waarvoor pulse genoem word. Hierdie kaart bevat pulse wat deur radioteleskope ontdek is.

Navorsers soek en bestudeer pulsars sedert 1967, toe die pulsar B1919 + 21 die eerste keer ontdek is. Aanvanklik was hierdie voorwerp 'n raaisel omdat die polsslag daarvan so konsekwent was dat sommige navorsers (grappenderwys) gewonder het of ons 'n baken van 'n vreemde ras gevind het. Nou weet ons dat pulsars natuurlik voorkom, alhoewel dit ons nog kan help om met vreemdelinge te kommunikeer: NASA het 'n kaart van ons ligging in die sterrestelsel ingesluit met behulp van pulsars as landmerke op die Pioneer- en Voyager-satelliete, wat tans op pad uit ons sonkrag is stelsel. Daar is ook ander (meer praktiese) redes om pulse te bestudeer: hulle het sterrekundiges gehelp om die teorie van algemene relatiwiteit te toets, te leer oor die baie dun gasse tussen die sterre en nog baie meer. Byna al die pulse waarvan ons weet, is in ons eie Melkwegstelsel, alhoewel navorsers seker is dat hulle daar is, is pulse in ander sterrestelsels gewoonlik te ver weg en te flou om ons teleskope te identifiseer. Die enigste uitsonderings is die paar helder pulse wat ons vind in die Melkweg se twee bekendste satellietstelsels, die Magellaanse wolke. Hierdie satellietstelsels is baie naby aan die Melkweg, waarmee navorsers die pulse wat hulle bevat, kan identifiseer.

Tendense

Daar is verskeie interessante tendense wat u in die animasie hierbo kan sien. Ons melkwegstelsel is byvoorbeeld gevorm soos 'n baie groot en dik skyf, en ons (op aarde) is na die radiale rand van die skyf, maar naby die middelvlak daarvan. Dit beteken dat die pulsars (en ander sterre) relatief naby in alle rigtings sigbaar is, maar as u net na voorwerpe kyk wat verder weg is, is dit amper almal in die vlak van die sterrestelsel. Hier is 'n paar ander neigings om na te kyk:

  • U kan 'n wye verskeidenheid in verskillende periodes vind, van baie vinnig tot redelik stadig.
  • Ons sterrestelsel het 'n aantal digte konsentrasies sterre (en pulse) wat bolvormige trosse genoem word, soos minigalaksies. Hierdie bolvormige trosse is geneig om die leërskare te wees vir die verre pulse wat nie in die vlak van die sterrestelsel is nie. Kyk byvoorbeeld hier en hier.
  • Omdat dit die mees afgeleë pulserare is waarvan ons weet, is dit baie maklik om die pulse in ons satellietstelsels te vind, die Groot en die Klein Magellaanse wolke.

Erkennings

Hierdie webwerf is deur Isaac Shivvers gebou terwyl hy sy doktorsgraad in astrofisika aan die UC Berkeley behaal het. Kontak my gerus met enige vrae of kommentaar by [email protected] .

Die agtergrond van die naghemel is met dank aan ESO / S.Brunier, en is vervaardig as deel van die GigaGalaxy Zoom-projek van die European Southern Observatory.

Die webwerf self is gebou met behulp van D3, jQuery, die Twitter Bootstrap en Python. Baie dankie aan al die ontwikkelaars wat aan hierdie wonderlike projekte gewerk het!

Vrae?

Het u vrae oor wat u hier sien, of enige tendense wat u raakgesien het?
Kyk na die About-bladsy vir meer inligting!


'N Oppervlaktekaart van A Pulsar kry 'n NICER-opdatering

Hoe lyk pulsars se magnetiese velde?
Pulsars is vinnig roterende neutronsterre met kort en gereelde rotasietydperke, waargeneem via hul bundels van elektromagnetiese straling. Hierdie waargenome pulse is analoog aan die lig wat op 'n vuurtoring flikker waar waarnemers die ligpulse slegs een keer elke draai kan sien, wanneer die balke langs die siglyn gerig is. Vir rotasie-aangedrewe pulse word hierdie ligpulse aangedryf deur die neutronsterre se vinnige rotasies en uiters sterk magnetiese velde. Die kanoniese (standaard) uitbeelding van die magnetiese velde wat aanleiding gee tot hierdie pulse kan hieronder in Figuur 1 gesien word.

Figuur 1. 'N Handboek-aansig van die magnetiese veldkonfigurasie rondom 'n pulsar, waar die grootskaalse eksterne magnetiese veld 'n dipoolkonfigurasie het wat op die neutronster gesentreer is. Die magnetiese as is nie in lyn met die rotasie-as nie en dus kan elektromagnetiese strale (in hierdie figuur, radiostrale) waargeneem word as pulse met gereelde rotasietydperke. Die ligsilinder, in groen punt gestippel, is die straal waarheen die ko-rotasiesnelheid van die magneetveld die snelheid van die lig benader. Credits: Handbook of Pulsar Astronomy deur Duncan Ross Lorimer en Michael Kramer (https://www.cv.nrao.edu/course/astr534/Pulsars.html)

Hierdie "handboekaansig" van die magnetiese veld van 'n pulsar hang grootliks af van die vereenvoudigende aanname dat die grootskaalse eksterne magnetiese veld 'n dipoolkonfigurasie het wat op die neutronster sentreer. Baie van hierdie dipoolkonfigurasiemodelle gee aanleiding tot verhitte kolle op die neutronster, waar die warm gebiede op teenoorgestelde pole geleë is. X-strale word uitgestraal wanneer die pulsar draai. Hierdie dipoolmodelle is egter nog lank nie omvattend nie, en die begrip van die meganismes van pulsêre emissie is steeds 'n aktiewe navorsingsgebied. Inderdaad, daar is waarnemingsbewyse vir hoër-orde multipoolmomente (bv. Quadrupoles, octupoles, ens.) Naby die oppervlak van neutronsterre. Die kombinasie van beperkte berekeningsvermoë (wat nodig is om hierdie meer komplekse magnetiese velde numeries te simuleer), en geen voor die hand liggende keuse nie watter alternatiewe magneetveldkonfigurasies sou die waargenome pulsêre emissie beter verklaar, dit maak dit moeilik om modelle meer kompleks te bestudeer as die dipoolmodelle.

'N NICER-ontwikkeling
Die Neutron-ster Interior Composition ExploreR (NICER) is 'n NASA-teleskoop wat aan boord van die Internasionale Ruimtestasie geïnstalleer is in 2017. Met sy hoë tydresolusie en ongekende sensitiwiteit in die sagte X-straalband (0,2–12 keV), het NICER van die pulsprofiele van die hoogste gehalte (vorm van waargenome pulse) van bekende röntgenpulsars. Vandag se artikel fokus op NICER-data van een so 'n pulsar, PSR J0030 + 0451. Hierdie pulsar is 'n geïsoleerde pulsar sowel as 'n millisekonde pulsar (aangesien sy rotasietydperk is

4,87 ms). Dit is een van die naaste waargenome millisekondepulsars, met 'n betreklik beperkte afstand van 329 ± 9 parsek (ongeveer 1070 ± 30 ligjare) weg van die aarde.

In hierdie artikel het die skrywers modelle ondersoek wat die waargenome polsprofiel van PSR J0030 + 0451 die beste sou verklaar. Aangesien twee polskomponente wat duidelik voorkom, waargeneem is, het die getoetsde modelle twee verskillende warm streke op die oppervlak van die neutronster ingesluit, met vorms soos sirkelvormige kolle, ringvormige streke en halfmaan. Die standaardkonfigurasie van identiese warm streke op teenoorgestelde pole van die neutronster is ook getoets, asook meer komplekse konfigurasies waar die twee warm streke onafhanklik was en nie teenoorgestelde pole moes wees nie. Die voortplanting van die straling na die waarnemer (rekonstruksie van die emissie van die warm gebiede) en die instrumentele reaksie (die sensitiwiteit van die instrument) is ook in die modelle ingesluit.

Veelvuldige metodes is gebruik om vas te stel watter fisieke opset optimaal was, insluitend (maar nie beperk nie tot) die evaluering van die modelbewyse (ook bekend as die marginale waarskynlikheid), dit wil sê hoe goed die model die vraag beantwoord & # 8220 gegewe 'n model, hoe waarskynlik is dit dat die waargenome data van hierdie model af kon kom? & # 8221 Na die ondersoek van verskillende modelle, het die outeurs gevind dat die fisiese konfigurasie wat die waargenome data die beste blyk te verklaar, warm streke gehad het as 'n klein sirkelvormige kol en 'n uitgebreide dun sekel, albei geleë in die dieselfde halfrond van PSR J0030 + 0451 (Figuur 2).

Figuur 2. 'N Uitsig van die afgeleide warm streke op die oppervlak van PSR J0030 + 0451, in lyn met die ewenaar en op drie verskillende posisies in die rotasie gesien. In hierdie model is die afgeleide warm streke die klein sirkelvormige kol en die uitgebreide dun sekel, en hulle het ongeveer dieselfde effektiewe temperatuur. (Figuur 17 in vandag se vraestel.)

Figuur 3. Nog 'n visualisering van die warm streke vanaf PSR J0030 + 0451. Die kleur blou beeld warm streke op teenoorgestelde pole van die neutronster af wat in 'n vorige studie afgelei is (Johnson et al. 2014), terwyl die rooi kleur warm streke in dieselfde halfrond uit die huidige papier voorstel. (Figuur 2 in Bilous et al. 2019, 'n bygaande referaat tot vandag se vraestel.)

Die studie van vandag het bevind dat die kanonieke "handboekaansig" van pulserende magnetiese veldkonfigurasies, waar die gevolglike warm streke op teenoorgestelde pole van die neutronster geleë is, 'n sterk ongunstige model vir PSR J0030 + 0451 was. 'N Vergelyking tussen warm streke vanaf 'n kanonieke magnetiese veldkonfigurasie (met behulp van gammastraal- en radiopulse) en met die optimale konfigurasie wat in die hedendaagse koerant gevind word, kan gesien word in Figuur 3. Hierdie buitengewone bevinding - waar die konfigurasie van die warm streke van die pulser is is baie ingewikkelder as wat standaardmodelle voorspel - stem saam met 'n ander onafhanklike studie van NICER-data van PSR J0030 + 0451, wat 'n opvallende soortgelyke resultaat gevind het. Vir 'n geanimeerde visualisering van die afgeleide warm streke uit albei studies, kyk na die ingeslote video wat deur NASA verskaf word!

Astrofisiese implikasies
Die vreemde opset van warm streke op 'n pulsar sou moes geskep word deur 'n vreemde magneetveldkonfigurasie. So 'n ingewikkelde magneetveldkonfigurasie sou implikasies hê vir die bestudering van meganismes vir die pulserende emissie, soos om die interpretasie van multi-golflengte-emissie vanaf pulse moontlik te verander. Met die optimale model uit die hedendaagse artikel kan die outeurs ook 'n skatting gee van die massa en radius van die neutronster van PSR J0030 + 0451 (ongeveer 1,34 + 0,15 / -0,16 sonmassas en 12,71 + 1,14 / -1,19 km , onderskeidelik). Beperkings op die massa en radius van 'n neutronster is van onskatbare waarde vir die bestudering van die interne samestelling daarvan en die digte materie-vergelyking, wat 'n langdurige astrofisiese raaisel is. Met die NICER-missie is dit moontlik dat die digte materie-vergelyking van die staat uiteindelik verstaan ​​kan word!


23.4 Pulsars en die ontdekking van neutronster

Nadat 'n tipe II-supernova-ontploffing verdwyn, is net 'n neutronster of iets selfs vreemds, 'n swart gat. Ons sal die eienskappe van swart gate in swart gate en geboë ruimtetyd beskryf, maar vir eers wil ons ondersoek instel na hoe die neutronsterre wat ons vroeër bespreek het, waarneembaar kan word.

Neutronsterre is die digste voorwerpe in die heelal. Die swaartekrag op hul oppervlak is 10 11 keer groter as wat ons op die aarde se oppervlak ervaar. Die binnekant van 'n neutronster bestaan ​​uit ongeveer 95% neutrone, met 'n klein aantal protone en elektrone ingemeng. In werklikheid is 'n neutronster 'n reuse atoomkern met 'n massa van ongeveer 10 57 keer die massa van 'n proton. Die deursnee daarvan is meer soos die grootte van 'n klein dorpie of 'n asteroïde as 'n ster. (Tabel vergelyk die eienskappe van neutronsterre en wit dwerge.) Omdat dit so klein is, tref 'n neutronster jou waarskynlik as die voorwerp wat die minste waarskynlike waargeneem sal word vanaf duisende ligjare. Tog slaag neutronsterre daarin om hul teenwoordigheid oor groot klowe ruimte aan te dui.

Eienskappe van 'n tipiese wit dwerg en 'n neutronster
Eiendom Wit dwerg Neutronster
Massa (Son = 1) 0.6 (altyd & lt1.4) Altyd & gt1.4 en & lt3
Radius 7000 km 10 km
Digtheid 8 × 10 5 g / cm 3 10 14 g / cm 3

Die ontdekking van neutronsterre

In 1967 studeer Jocelyn Bell, 'n navorsingsstudent aan die Universiteit van Cambridge, radiobronne in die verte met 'n spesiale detektor wat deur haar adviseur Antony Hewish ontwerp en gebou is om vinnige variasies in radiosignale te vind. Die rekenaars van die projek het papiertjies uitgespoeg wat getoon het waar die teleskoop die lug opgemerk het, en dit was die taak van Hewish se gegradueerde studente om alles deur te gaan en na interessante verskynsels te soek. In September 1967 ontdek Bell wat sy ''n bietjie skroef' 'noem - 'n vreemde radiosein wat niks anders gesien het nie.

Wat Bell in die konstellasie Vulpecula gevind het, was 'n bron van vinnige, skerp, intense en uiters gereelde pulse van radiostraling. Soos die gereelde tik van 'n horlosie, het die pulse presies elke 1.33728 sekondes opgedaag. Sulke akkuraatheid het die wetenskaplikes eers laat bespiegel dat hulle miskien seine van 'n intelligente beskawing gevind het. Radiosterrekundiges noem selfs die bron 'LGM' vir 'klein groen mannetjies'. Binnekort is daar egter drie soortgelyke bronne in 'n wye rigting in die lug ontdek.

Toe dit blyk dat hierdie soort radiobron redelik algemeen is, het sterrekundiges tot die gevolgtrekking gekom dat dit waarskynlik nie seine van ander beskawings sou wees nie. Vandag is meer as 2500 sulke bronne ontdek. Hulle word nou pulsars genoem, afkorting vir 'pulserende radiobronne'.

Die polsperiodes van verskillende pulse wissel van iets langer as 1/1000 van 'n sekonde tot byna 10 sekondes. Aanvanklik het die pulsars besonder geheimsinnig gelyk omdat niks op hul foto's op sigbare ligfoto's gesien kon word nie. Maar toe word 'n pulsar reg in die middel van die krapnevel ontdek, 'n wolk gas wat deur SN 1054 geproduseer word, 'n supernova wat in 1054 deur die Chinese opgeneem is (Figuur 1). Die energie van die Crab Nebula pulsar kom in skerp sarsies wat 30 keer per sekonde voorkom - met 'n reëlmaat wat die afguns van 'n Switserse horlosiemaker sou wees. Behalwe pulse van radio-energie, kan ons ook pulse van sigbare lig en X-strale vanaf die krapnevel waarneem. Die feit dat die pulsar net in die supernova-oorblyfsel was, waar ons verwag dat die oorblywende neutronster onmiddellik sterrekundiges daarop gewys sal word dat pulsars moontlik verbind kan word met hierdie ontwykende "lyke" massiewe sterre.

Krapnevel.

Figuur 1. Hierdie beeld toon X-straal-emissie uit die krapnevel, wat ongeveer 6500 ligjaar weg is. Die pulser is die ligpunt in die middel van die konsentriese ringe. Gegewens wat ongeveer 'n jaar geneem is, toon dat deeltjies ongeveer die helfte van die ligspoed van die binneste ring af stroom. Die straal wat loodreg op hierdie ring is, is 'n stroom materie en antimaterie-elektrone wat ook teen die helfte van die ligspoed beweeg. (krediet: wysiging van werk deur NASA / CXC / SAO)

Die krapnevel is 'n boeiende voorwerp. Die hele newel gloei van straling op baie golflengtes, en die totale energie-uitset daarvan is meer as 100 000 keer dié van die son - nie 'n slegte truuk vir die oorblyfsel van 'n supernova wat byna duisend jaar gelede ontplof het nie. Sterrekundiges het gou begin soek na 'n verband tussen die pulsar en die groot energie-uitset van die omliggende newel.

'N Spinning Lighthouse Model

Deur 'n kombinasie van teorie en waarneming toe te pas, het sterrekundiges uiteindelik tot die gevolgtrekking gekom dat pulsars moet wees draai neutronsterre. Volgens hierdie model is 'n neutronster iets soos 'n vuurtoring aan 'n rotsagtige kus (Figuur 2). Om skepe in alle rigtings te waarsku en tog nie te veel kos om te bedryf nie, draai die lig in 'n moderne vuurtoring en vee sy balk oor die donker see. Vanaf die uitkykpunt van 'n skip sien u elke keer 'n polsslag wanneer die straal in u rigting wys. Op dieselfde manier vee bestraling van 'n klein gebied op 'n neutronster oor die oseane van die ruimte en gee ons 'n polsslag elke keer as die straal na die aarde wys.

Vuurtoring.

Figuur 2. 'N Vuurtoring in Kalifornië waarsku skepe op die see om nie te naby aan die gevaarlike oewer te kom nie. Die verligte gedeelte bo draai om sodat die balk alle rigtings kan bedek. (krediet: Anita Ritenour)

Neutronsterre is ideale kandidate vir so 'n werk, omdat die ineenstorting hulle so klein gemaak het dat hulle baie vinnig kan draai. Onthou die beginsel van die behoud van hoekmomentum uit die Great Synthesis van Newton: as 'n voorwerp kleiner word, kan dit vinniger draai. Selfs as die ouerster baie stadig gedraai het toe dit op die hoofreeks was, moes die rotasie daarvan versnel terwyl dit ineengestort het om 'n neutronster te vorm. Met 'n deursnee van slegs 10 tot 20 kilometer kan 'n neutronster een volle draai in slegs 'n breukdeel van 'n sekonde voltooi. Dit is net die soort tydperk wat ons tussen pulserende pulse waarneem.

Enige magnetiese veld wat in die oorspronklike ster bestaan, sal sterk saamgepers word as die kern in 'n neutronster ineenstort. Op die oppervlak van die neutronster, in die buitenste laag wat bestaan ​​uit gewone materie (en nie net suiwer neutrone nie), word protone en elektrone in hierdie draai-veld vasgevang en byna tot die snelheid van die lig versnel. Op slegs twee plekke - die noord- en suidmagnetiese pole - kan die vasgekeerde deeltjies uit die sterk greep van die magneetveld ontsnap (Figuur 3). Dieselfde effek kan gesien word (omgekeerd) op die Aarde, waar gelaaide deeltjies uit die ruimte is uitgehou deur ons planeet se magnetiese veld oral behalwe naby die pole. As gevolg hiervan word die Aura's van die Aarde (veroorsaak wanneer gelaaide deeltjies teen 'n hoë spoed die atmosfeer tref) hoofsaaklik naby die pole gesien.

Model van 'n Pulsar.

Figuur 3. 'N Diagram wat toon hoe stralingsstrale by die magnetiese pole van 'n neutronster aanleiding kan gee tot emissiepulse as die ster draai. Terwyl elke balk oor die aarde vee, soos 'n vuurtoring wat oor 'n ver skip vee, sien ons 'n kort polsslag. Hierdie model vereis dat die magnetiese pole op verskillende plekke as die rotasiepale geleë is. (krediet "sterre": wysiging van werk deur Tony Hisgett)

Let daarop dat die magnetiese noord- en suidpool in 'n neutronster nêrens naby die noord- en suidpool hoef te wees wat deur die rotasie van die ster gedefinieer word nie. Op dieselfde manier het ons in die hoofstuk oor The Giant Planets bespreek dat die magnetiese pole op die planete Uranus en Neptunus nie in lyn is met die pole van die planeet se draai nie. Figuur 3 toon die pole van die magneetveld loodreg op die draaipale, maar die twee soorte pole kan enige hoek maak.

In werklikheid speel die verkeerde uitlê van die rotasie-as met die magnetiese as 'n deurslaggewende rol in die opwekking van die waargenome pulse in hierdie model. Aan die twee magnetiese pole word die deeltjies van die neutronster in 'n smal straal gefokus en kom dit uit die draaiende magnetiese gebied teen enorme snelhede. Hulle straal energie uit oor 'n wye verskeidenheid van die elektromagnetiese spektrum. Die bestraling self is ook beperk tot 'n smal straal, wat verklaar waarom die pulsar soos 'n vuurtoring optree. Aangesien die rotasie eers een en dan die ander magnetiese pool van die ster in ons sien, sien ons elke keer 'n polsslag.

Toetse van die model

This explanation of pulsars in terms of beams of radiation from highly magnetic and rapidly spinning neutron stars is a very clever idea. But what evidence do we have that it is the correct model? First, we can measure the masses of some pulsars, and they do turn out be in the range of 1.4 to 1.8 times that of the Sun—just what theorists predict for neutron stars. The masses are found using Kepler’s law for those few pulsars that are members of binary star systems.

But there is an even-better confirming argument, which brings us back to the Crab Nebula and its vast energy output. When the high-energy charged particles from the neutron star pulsar hit the slower-moving material from the supernova, they energize this material and cause it to “glow” at many different wavelengths—just what we observe from the Crab Nebula. The pulsar beams are a power source that “light up” the nebula long after the initial explosion of the star that made it.

Who “pays the bills” for all the energy we see coming out of a remnant like the Crab Nebula? After all, when energy emerges from one place, it must be depleted in another. The ultimate energy source in our model is the rotation of the neutron star, which propels charged particles outward and spins its magnetic field at enormous speeds. As its rotational energy is used to excite the Crab Nebula year after year, the pulsar inside the nebula slows down. As it slows, the pulses come a little less often more time elapses before the slower neutron star brings its beam back around.

Several decades of careful observations have now shown that the Crab Nebula pulsar is not a perfectly regular clock as we originally thought: instead, it is gradually slowing down. Having measured how much the pulsar is slowing down, we can calculate how much rotation energy the neutron star is losing. Remember that it is very densely packed and spins amazingly quickly. Even a tiny slowing down can mean an immense loss of energy.

To the satisfaction of astronomers, the rotational energy lost by the pulsar turns out to be the same as the amount of energy emerging from the nebula surrounding it. In other words, the slowing down of a rotating neutron star can explain precisely why the Crab Nebula is glowing with the amount of energy we observe.

The Evolution of Pulsars

From observations of the pulsar s discovered so far, astronomers have concluded that one new pulsar is born somewhere in the Galaxy every 25 to 100 years, the same rate at which supernovae are estimated to occur. Calculations suggest that the typical lifetime of a pulsar is about 10 million years after that, the neutron star no longer rotates fast enough to produce significant beams of particles and energy, and is no longer observable. We estimate that there are about 100 million neutron stars in our Galaxy, most of them rotating too slowly to come to our notice.

The Crab pulsar is rather young (only about 960 years old) and has a short period, whereas other, older pulsars have already slowed to longer periods. Pulsars thousands of years old have lost too much energy to emit appreciably in the visible and X-ray wavelengths, and they are observed only as radio pulsars their periods are a second or longer.

There is one other reason we can see only a fraction of the pulsars in the Galaxy. Consider our lighthouse model again. On Earth, all ships approach on the same plane—the surface of the ocean—so the lighthouse can be built to sweep its beam over that surface. But in space, objects can be anywhere in three dimensions. As a given pulsar’s beam sweeps over a circle in space, there is absolutely no guarantee that this circle will include the direction of Earth. In fact, if you think about it, many more circles in space will nie include Earth than will include it. Thus, we estimate that we are unable to observe a large number of neutron stars because their pulsar beams miss us entirely.

At the same time, it turns out that only a few of the pulsars discovered so far are embedded in the visible clouds of gas that mark the remnant of a supernova. This might at first seem mysterious, since we know that supernovae give rise to neutron stars and we should expect each pulsar to have begun its life in a supernova explosion. But the lifetime of a pulsar turns out to be about 100 times longer than the length of time required for the expanding gas of a supernova remnant to disperse into interstellar space. Thus, most pulsars are found with no other trace left of the explosion that produced them.

In addition, some pulsars are ejected by a supernova explosion that is not the same in all directions. If the supernova explosion is stronger on one side, it can kick the pulsar entirely out of the supernova remnant (some astronomers call this “getting a birth kick”). We know such kicks happen because we see a number of young supernova remnants in nearby galaxies where the pulsar is to one side of the remnant and racing away at several hundred miles per second ( Figure 4 ).

Speeding Pulsar.

Figure 4. This intriguing image (which combines X-ray, visible, and radio observations) shows the jet trailing behind a pulsar (at bottom right, lined up between the two bright stars). With a length of 37 light-years, the jet trail (seen in purple) is the longest ever observed from an object in the Milky Way. (There is also a mysterious shorter, comet-like tail that is almost perpendicular to the purple jet.) Moving at a speed between 2.5 and 5 million miles per hour, the pulsar is traveling away from the core of the supernova remnant where it originated. (credit: X-ray: NASA/CXC/ISDC/L.Pavan et al, Radio: CSIRO/ATNF/ATCA Optical: 2MASS/UMass/IPAC-Caltech/NASA/NSF)

TOUCHED BY A NEUTRON STAR

On December 27, 2004, Earth was bathed with a stream of X-ray and gamma-ray radiation from a neutron star known as SGR 1806-20. What made this event so remarkable was that, despite the distance of the source, its tidal wave of radiation had measurable effects on Earth’s atmosphere. The apparent brightness of this gamma-ray flare was greater than any historical star explosion.

The primary effect of the radiation was on a layer high in Earth’s atmosphere called the ionosphere. At night, the ionosphere is normally at a height of about 85 kilometers, but during the day, energy from the Sun ionizes more molecules and lowers the boundary of the ionosphere to a height of about 60 kilometers. The pulse of X-ray and gamma-ray radiation produced about the same level of ionization as the daytime Sun. It also caused some sensitive satellites above the atmosphere to shut down their electronics.

Measurements by telescopes in space indicate that SGR 1806-20 was a special type of fast-spinning neutron star called a magnetar. Astronomers Robert Duncan and Christopher Thomson gave them this name because their magnetic fields are stronger than that of any other type of astronomical source—in this case, about 800 trillion times stronger than the magnetic field of Earth.

A magnetar is thought to consist of a superdense core of neutrons surrounded by a rigid crust of atoms about a mile deep with a surface made of iron. The magnetar’s field is so strong that it creates huge stresses inside that can sometimes crack open the hard crust, causing a starquarke. The vibrating crust produces an enormous blast of radiation. An astronaut 0.1 light-year from this particular magnetar would have received a fatal does from the blast in less than a second.

Fortunately, we were far enough away from magnetar SGR 1806-20 to be safe. Could a magnetar ever present a real danger to Earth? To produce enough energy to disrupt the ozone layer, a magnetar would have to be located within the cloud of comets that surround the solar system, and we know no magnetars are that close. Nevertheless, it is a fascinating discovery that events on distant star corpses can have measurable effects on Earth.

Key Concepts and Summary

At least some supernovae leave behind a highly magnetic, rapidly rotating neutron star, which can be observed as a pulsar if its beam of escaping particles and focused radiation is pointing toward us. Pulsars emit rapid pulses of radiation at regular intervals their periods are in the range of 0.001 to 10 seconds. The rotating neutron star acts like a lighthouse, sweeping its beam in a circle and giving us a pulse of radiation when the beam sweeps over Earth. As pulsars age, they lose energy, their rotations slow, and their periods increase.


A second discovery

Bell Burnell would later report that Hewitt called a meeting without her, in which he discussed with other members of the department how they should handle presenting their results to the world. While their fellow scientists might practice restraint and skepticism, it was likely that the possible detection of an intelligent alien civilization could create chaos among the public, the scientists said. The press would very likely blow the story out of proportion and descend on the Cambridge researchers. According to Hewitt, one person even suggested (perhaps only partly joking) that they burn their data and forget the whole thing.

Years later, Burnell wrote that she was rather annoyed at the appearance of the strange signal for another reason. As a graduate student, she was trying to get her thesis work done before her funding ran out, but work on the pulsar was taking away from her primary pursuit.

"Here I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us," she wrote in the article for Cosmic Search Magazine.

But then, Bell Burnell resolved the problem. She went back through some of the data from the radio array and found what looked like a similar, regularly repeating signal, this one coming from an entirely different part of the galaxy. That second signal indicated that this was a family of objects, rather than a single civilization trying to make contact.

"It finally scotched the little green men hypothesis," Bell Burnell said in the a BBC documentary filmed in 2010. "Because it's highly unlikely there's two lots of little green men, on opposite sides of the universe, both deciding to signal to a rather inconspicuous planet, Earth, at the same time, using a daft technique and a rather commonplace frequency."

"It had to be some new kind of star, not seen before," she said. "And that then cleared the way for us publishing, going public."

In 1974, the Nobel Prize in Physics was awarded to Hewish, along with radio astronomer Martin Ryle, "for their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular of the aperture-synthesis technique, and Hewish for his decisive role in the discovery of pulsars." The omission of Bell Burnell's name as a contributor to the pulsar discovery has stirred controversy among scientists and members of the public, though Bell Burnell has not publicly contested the Nobel committee's decision.


Astronomers Use Pulsars to Listen for Gravitational Waves

One of the most spectacular achievements in physics so far this century has been the observation of gravitational waves, ripples in space-time that result from masses accelerating in space. So far, there have been five detections of gravitational waves, thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) and, more recently, the European Virgo gravitational-wave detector. Using these facilities, scientists have been able to pin down the extremely subtle signals from relatively small black holes and, as of October, neutron stars.

But there are merging objects far larger whose gravitational wave signals have not yet been detected: supermassive black holes, more than 100 million times more massive than our Sun. Most large galaxies have a central supermassive black hole. When galaxies collide, their central black holes tend to spiral toward each other, releasing gravitational waves in their cosmic dance. Much as a large animal like a lion produces a deeper roar than a tiny mouse’s squeak, merging supermassive black holes create lower-frequency gravitational waves than the relatively small black holes LIGO and similar ground-based experiments can detect.

“Observing low-frequency gravitational waves would be akin to being able to hear bass singers, not just sopranos,” said Joseph Lazio, chief scientist for NASA’s Deep Space Network, based at NASA’s Jet Propulsion Laboratory, Pasadena, California, and co-author of a new study in Nature Astronomy.

To explore this uncharted area of gravitational wave science, researchers look not to human-made machines, but to a natural experiment in the sky called a pulsar timing array. Pulsars are dense remnants of dead stars that regularly emit beams of radio waves, which is why some call them “cosmic lighthouses.” Because their rapid pulse of radio emission is so predictable, a large array of well-understood pulsars can be used to measure extremely subtle abnormalities, such as gravitational waves. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a Physics Frontier Center of the National Science Foundation, is one of the leading groups of researchers using pulsars to search for gravitational waves.

The new Nature Astronomy study concerns supermassive black hole binaries — systems of two of these cosmic monsters. For the first time, researchers surveyed the local universe for galaxies likely to host these binaries, then predicted which black hole pairs are the likeliest to merge and be detected while doing so. The study also estimates how long it will take to detect one of these mergers.

“By expanding our pulsar timing array over the next 10 years or so, there is a high likelihood of detecting gravitational waves from at least one supermassive black hole binary,” said Chiara Mingarelli, lead study author, who worked on this research as a Marie Curie postdoctoral fellow at Caltech and JPL, and is now at the Flatiron Institute in New York.

Mingarelli and colleagues used data from the 2 Micron All-Sky Survey (2MASS), which surveyed the sky from 1997 to 2001, and galaxy merger rates from the Illustris simulation project, an endeavor to make large-scale cosmological simulations. In their sample of about 5,000 galaxies, scientists found that about 90 would have supermassive black holes most likely to merge with another black hole.

While LIGO and similar experiments detect objects in the final seconds before they merge, pulsar timing arrays are sensitive to gravitational wave signals from supermassive black holes that are spiraling toward each other and will not combine for millions of years. That’s because galaxies merge hundreds of millions of years before the central black holes they host combine to make one giant supermassive black hole.

Researchers also found that while bigger galaxies have bigger black holes and produce stronger gravitational waves when they combine, these mergers also happen fast, shortening the time period for detection. For example, black holes merging in the large galaxy M87 would have a 4-million-year window of detection. By contrast, in the smaller Sombrero Galaxy, black holes mergers typically take about 160 million years, offering more opportunities for pulsar timing arrays to detect gravitational waves from them.

Black hole mergers generate gravitational waves because, as they orbit each other, their gravity distorts the fabric of space-time, sending ripples outward in all directions at the speed of light. These distortions actually shift the position of Earth and the pulsars ever so slightly, resulting in a characteristic and detectable signal from the array of celestial lighthouses.

“A difference between when the pulsar signals should arrive, and when they do arrive, can signal a gravitational wave,” Mingarelli said. “And since the pulsars we study are about 3,000 light-years away, they act as a galactic-scale gravitational-wave detector.”

Because all supermassive black holes are so distant, gravitational waves, which travel at the speed of light, take a long time to arrive at Earth. This study looked at supermassive black holes within about 700 million light-years, meaning waves from a merger between any two of them would take up to that long to be detected here by scientists. By comparison, about 650 million years ago, algae flourished and spread rapidly in Earth’s oceans — an event important to the evolution of more complex life.

Many open questions remain about how galaxies merge and what will happen when the Milky Way approaches Andromeda, the nearby galaxy that will collide with ours in about 4 billion years.

“Detecting gravitational waves from billion-solar-mass black hole mergers will help unlock some of the most persistent puzzles in galaxy formation,” said Leonidas Moustakas, a JPL research scientist who wrote an accompanying “News and Views” article in the journal.

2MASS was funded by NASA’s Office of Space Science, the National Science Foundation, the U.S. Naval Observatory and the University of Massachusetts. JPL managed the program for NASA’s Office of Space Science, Washington. Data was processed at IPAC at Caltech in Pasadena, California.

Publication: Chiara M. F. Mingarelli, et al., “The local nanohertz gravitational-wave landscape from supermassive black hole binaries,” Nature Astronomy (2017) doi:10.1038/s41550-017-0299-6


Part-time Pulsar Yields New Insight Into Inner Workings Of Cosmic Clocks

Astronomers using the 76-m Lovell radio telescope at the University of Manchester's Jodrell Bank Observatory have discovered a very strange pulsar that helps explain how pulsars act as 'cosmic clocks' and confirms theories put forward 37 years ago to explain the way in which pulsars emit their regular beams of radio waves - considered to be one of the hardest problems in astrophysics. Their research, now published in Science Express, reveals a pulsar that is only 'on' for part of the time. The strange pulsar is spinning about its own axis and slows down 50% faster when it is 'on' compared to when it is 'off'.

Pulsars are dense, highly magnetized neutron stars that are born in a violent explosion marking the death of massive stars. They act like cosmic lighthouses as they project a rotating beam of radio waves across the galaxy. Dr. Michael Kramer explains, "Pulsars are a physicist's dream come true. They are made of the most extreme matter that we know of in the Universe, and their highly stable rotation makes them super-precise cosmic clocks. But, embarrassingly, we do not know how these clocks work. This discovery goes a long way towards solving this problem."

The research team, led by Dr. Michael Kramer, found a pulsar that is only periodically active. It appears as a normal pulsar for about a week and then "switches off" for about one month before emitting pulses again. The pulsar, called PSR B1931+24, is unique in this behaviour and affords astronomers an opportunity to compare its quiet and active phases. As it is quiet the majority of the time, it is difficult to detect, suggesting that there may be many other similar objects that have, so far, escaped detection.

Prof. Andrew Lyne points out that, "After the discovery of pulsars, theoreticians proposed that strong electric fields rip particles out of the neutron star surface into a surrounding magnetised cloud of plasma called the magnetosphere. But, for nearly 40 years, there had been no way to test whether our basic understanding was correct."

The University of Manchester astronomers were delighted when they found that this pulsar slows down more rapidly when the pulsar is on than when it is off. Dr. Christine Jordan points out the importance of this discovery, "We can clearly see that something hits the brakes when the pulsar is on."

This breaking mechanism must be related to the radio emission and the processes creating it and the additional slow-down can be explained by a wind of particles leaving the pulsar's magnetosphere and carrying away rotational energy. "Such a braking effect of the pulsar wind was expected but now, finally, we have observational evidence for it" adds Dr Duncan Lorimer.

The amount of braking can be related to the number of charges leaving the pulsar magnetosphere. Dr. Michael Kramer explains their surprise when it was found that the resulting number was within 2% of the theoretical predictions. "We were really shocked when we saw these numbers on our screens. Given the pulsar's complexity, we never really expected the magnetospheric theory to work so well."

Prof. Andrew Lyne summarized the result: "It is amazing that, after almost 40 years, we have not only found a new, unusual, pulsar phenomenon but also a very unexpected way to confirm some fundamental theories about the nature of pulsars."


How do we know pulsars have two beams? - Sterrekunde

Pulsars are, at least in my opinion 1 , some of the most interesting objects in the universe. They are extremely dense stars, supported by neutron degeneracy pressure, shooting out beams of radiation along their magnetic axes. These extreme behaviors can both serve as tools, helping astronomers get a better sense of their surroundings, as well as laboratories, allowing astronomers to study extreme situations that may not be as easily observable in other locations in the universe. I recently wrote a post detailing briefly some of the important and interesting characteristics of pulsars. This post goes beyond that relatively simple look at pulsars. Instead, it focuses on a few of the neatest phenomena that take place in the lives of some very special pulsars that have orbiting companion stars, helping deviate their behaviors from those of ordinary pulsars.

A Shocking Discovery

The first millisecond pulsar discovery (known as PSR B1937+21) took place in 1982 by Don Backer and Shrinivas Kulkarni from the Radio Astronomy Lab at UC Berkeley. Previous observations had suggested two different objects in this position of the sky, with the compact object out of these two possibly being a pulsar with a period shorter than 10 ms. To hunt out the pulsar, the team of radio astronomers implemented subsequently higher and higher sampling rates to increase the frequency range of their search. Ultimately, a pulsar was discovered, with a period of net 1.558 ms.

To truly appreciate how shocking this result appeared at the time (and still is), recall that pulsars are neutron stars with masses on the order of that of the Sun, and have radii on the order of 10 km. The fastest pulsars known at the time had periods of a little less than a second, spinning just a few times every second. To create pulses less than every 10 ms, one of these stars has to be spinning more than hundred times a second. Even more mind-blowing is the fact that on the surface of the neutron star, matter is traveling at velocities of 0.13c (that&rsquos 0.13 times the speed of light!) Furthermore, the short period puts it very close to the spin rate limit of 2000 Hz (the pulsar&rsquos rate is 642 Hz) where centrifugal forces would begin to exceed the gravitational force on the star&rsquos surface and rip it apart.

The biggest questions resulting from the discovery centered around how the pulsar was formed and what gave it its millisecond period. Pulsars&rsquo spin rates slow down over time as they lose energy (known as their spin-down rates), and this can be determined by measuring changes in the period of the pulses coming from pulsars. Just using this pulsar&rsquos spin-down rate and assuming a maximum possible spin rate of 2000 Hz, the pulsar would only have been about 750 years old! Yet, no supernova remnants could be found in the area, as would have been expected for a pulsar this young. All of this suggests that the pulsar, and its millisecond period, was not the direct result of a supernova. Something else must have resulted in giving it its short period.

The Current Picture

The model for the formation of millisecond pulsars most widely supported today centers around binary objects. In a binary star system, the more massive star first undergoes a supernova. In the process, the star system could disrupt, meaning that the less massive star could be ejected due to the supernova, if the collapse is not symmetrical or due to its position in orbit. This case results in an ordinary, isolated pulsar, the same result as if there were just a solitary heavy star rather than a binary star system.

More interesting is if the binary system is not disrupted, and instead resulting in a young pulsar and its ordinary (main sequence) companion star. As time progresses, the pulsar evolves like all pulsars and its rotation period gets longer as it loses energy. However, the companion star is also aging alongside the pulsar, and eventually it reaches the red giant phase, with a dense, white dwarf core and an atmosphere that extends out to a further distance as it becomes older. The pulsar&rsquos gravitational attraction begins to pick off matter off of the companion star&rsquos atmosphere, and this material starts accreting onto the neutron star. The gravitational energy of the accreting matter is released through thermal emission in X-ray, while the angular momentum of the orbiting companion star is converted into the angular momentum of the pulsar&rsquos rotation. This causes the pulsar&rsquos rotation to speed up, rotating hundreds of times every second, and making it a millisecond pulsar.

The process gives millisecond pulsars their other name: recycled pulsars. If not for a orbiting companion, a pulsar would rotate more slowly and eventually lose brightness as it ages, eventually becoming non-observable. However, the presence of a companion makes the pulsar spin up again, this time much faster, and results in a greater brightness.

Going back to our binary star system again, something interesting can happen at this stage as well as the system encounters another fork in the road. The companion to the pulsar itself could be massive enough to undergo a supernova explosion, and become a second pulsar in the system. Again, if the explosion is not symmetrical enough, the two pulsars can get separated. However, if the system isn&rsquot disrupted, we can expect to find two pulsars orbiting one another: one of them being an ordinary young pulsar (with a relatively long period) and the other being a recycled (millisecond) pulsar. The exciting part of this result is that if the beams of both pulsars happened to hit the Earth, we could see two pulsars in the same spot of the sky, each with different periods. It turns out that we do have observational evidence for the existence of a system like this. The double pulsar system PSR J0737-3039 has an ordinary pulsar and a millisecond pulsar with beams that happen to both point towards the Earth. Typically, though, the younger pulsar fades away quickly, so we aren&rsquot always lucky enough to be able to view both. In fact, the younger component of the PSR J0737-3039 faded away in 2010 and is no longer observable.

A Bit of Stamp Collecting 2

One useful way to organize discovered pulsars is to plot them by two of their most easily observed characteristics: their periods and period derivatives (the rate at which the period is changing). This diagram (shown above), a (P-dot

) diagram, is especially useful to gain a sort of visualization of a pulsar&rsquos life cycle. The diagram plots discovered pulsars based on the two characteristics. The period is plotted along the horizontal axis, while the period derivative is on the vertical axis. It turns out that these characteristics are enough to determine a pulsar&rsquos age and their energy output, both of which are plotted in the diagram as diagonal dotted lines.

Pulsars typically begin life in the big clump in the upper portion of the diagram, starting with high energy outputs (left side of the big clump) and often with observable supernova remnants associated with them. As they get older, they migrate towards the right side of the clump, slowly diminishing their energy output and entering the portion of the diagram affectionately labelled the &lsquoGraveyard&rsquo. However, if the pulsars are in binary systems, they can cheat death and avoid the Graveyard by the accretion of matter. This speeds up their spins, and the resulting pulsars migrate towards the bottom left of the diagram, where the recycled millisecond pulsars reside. Notice that most of these are found in binary systems, helping support the binary system model for their formation.

Black-Widows

There&rsquos one subset of the millisecond pulsar population that is extremely interesting: the black-widow pulsars. These pulsars have very low mass companion stars, masses on the order of 1 percent of the Sun&rsquos mass. A companion&rsquos atmosphere spreads out to large distances, and leads to large portions of the companion star&rsquos mass to be absorbed by the pulsar. Since the pulsar is &ldquoeating&rdquo its companion, they have been named black-widow pulsars 3 . This can be seen in optical wavelengths since the side of a companion star facing the pulsar is much brighter and leads to measurable changes in brightness as the companion and pulsar orbit around each other. Most of these black-widows are found in globular clusters, which have high densities of stars, suggesting that their system formations are due to the original companion stars being swapped for much less massive companions.

What&rsquos more interesting about these stars is that they can serve as useful scientific laboratories. The black-widow pulsars for which masses can be measured tend to be very massive 4 . PSR B1957+20 is believed to have a mass of 2.40 solar masses, while the newly discovered PSR J1311-3430 is thought to come in at around 2.7 solar masses. These values are about double of most other pulsars. The massive, and dense stars, can help provide some clues about how matter behaves at very extreme densities, one of the areas of physics which is still not very well understood. Conditions with densities a little higher than those found in atomic nuclei and very high temperatures can be created with particle colliders, but even higher densities at low temperatures can only be found inside neutron stars so far. Studying the behavior of these black-widows could provide valuable constraints on the Equation of State of dense matter, and either support or disprove some of the theories describing matter in that realm.

It isn&rsquot often that astronomy can provide constraints on physics. Often, to test new physics theories, experiments are constructed on the ground. Ground experiments can be better and more easily controlled while measurements are often easier to obtain. Pulsars themselves are often just used as scientific tools. They are often implemented as probes to determine the density of the interstellar medium, or used as very precise clocks to help conduct tests on relativity or detect gravitational waves. While those are exciting, the high densities inside neutron stars can make them one of the few areas of astronomy that can help give back to physics, making them laboratories as well as tools. This is an opportunity where instead of using physical laws to understand more about the universe, astronomers could help define those very laws.

Sources and Further Exploration

  • Backer, D. C., Kulkarni, Shrinivas R., and Heiles, Carl. &ldquoA millisecond pulsar&rdquo, 1982, Nature 300:615-618.
  • Lyne, Andrew and Francis Graham-Smith. &ldquoPulsar Astronomy&rdquo, 2012 (4th Ed.), Cambridge University Press.
  • van Kerkwijk, M.H., Breton, R.P., Kulkarni, S.R. &ldquoEvidence for a massive neutron star from a radial-velocity study of the companion to the black-widow pulsar PSR B1957+20&rdquo, 2011, ApJ 728:95-102.
  • Kramer, M. and Stairs, I.H.. &ldquoThe Double Pulsar&rdquo, 2004, Annu. Rev. Astron. Astrophys. 46:541-72.
  • Phinney, E. S. and Kulkarni, S.R. &ldquoBinary and Millisecond Pulsars&rdquo, 1994, Annu. Rev. Astron. Astrophys. 32:591-639.
  • Pletsch, H.J., Guillemot, L., Fehrmann, H., et al. &ldquoBinary millisecond pulsar discovery via gamma-ray pulsations&rdquo, 2012, Science 338:1314-1317.
  • Radhakrishnan, V. and Srinivasan, G. &ldquoOn the origin of the recently discovered ultra-rapid pulsar&rdquo, 1982, Current Science 51:1096-1099.
  • Romani, Roger W., Filippenko, Alexei V., Silverman, Jeffrey M., et al. &ldquoPSR J1311-3430: A heavyweight neutron star with a flyweight helium companion&rdquo, 2012, ApJ 760:L36-41.
  • Stairs, Ingrid H. &ldquoPulsars in Binary Systems: Probing Binary Stellar Evolution and General Relativity&rdquo, 2004, Science 304:547-552.

Image Credits

I may be a little biased by my current research project at Berkeley. I&rsquom working on an automatic pipeline to search and study pulsars in the same field of view as the Kepler spacecraft and around the galactic center. ↩︎

Although astronomy is mostly physics, stamp collecting tends to creep in a little bit. ↩︎

I have to abandon one of my original premises when starting the blog: astronomers do not give very imaginative names. Writing on this blog has given me plenty of evidence to the contrary (another notable example: the Musket Ball Cluster). ↩︎

There isn&rsquot a very well supported explanation yet why black-widow pulsars tend to be more massive. ↩︎


What's a double pulsar?

You would not be wrong if you thought that a "pulsar" sounds like a great addition to your weekend rave. (You live in 1995.) A pulsar does kind of resemble a big, galactic strobe light and — with its steady rhythm — it could even allow you to keep time as you trip the light fantastic. But you probably wouldn't want one at your weekend party — let alone two.

Before we trip even harder imagining double pulsars, let's talk about how a pulsar works in general. When a massive star collapses, it goes out in a giant explosion called a supernova. Now if the star is big enough, it'll collapse into itself to form a black hole — end of the story, as we know it. But if it's just a little smaller (and we're still talking massive stars here, several times bigger than our sun), a pretty cool phenomenon will occur.

Instead of collapsing upon itself into a super-dense point source (the black-hole scenario), the protons and electrons at the sun's core will crush into each other until they actually combine to form neutrons. What you get is a neutron star that might be just a few miles across but has as much mass as our sun [source: JPL]. That means that the tough little star is so dense that a teaspoon full of its neutrons would weigh 100 million tons (90,719,000 metric tons) here on Earth [source: Goodier].

But let's not forget the "pulsing" part of pulsars. A pulsar might also emit beams of visible light, radio waves — even gamma and X-rays. If they are oriented just right, the beams can sweep toward Earth like a lighthouse signal, in an extremely regular pulse — perhaps more accurate than even an atomic clock. Pulsars also spin very quickly — as often as hundreds of times per second [source: Moskowitz]. But let's get to the good stuff — what's a double pulsar?

As a close and astute reader, you've probably already figured out that a double pulsar is two pulsars. And while it's not unusual to find a binary pulsar — where a pulsar is orbiting around another object, like a star or white dwarf — it's a lot more unusual to find two pulsars orbiting each other. In fact, we only know of one of these systems, discovered in 2003 [source: University of Manchester].

One of the coolest things about double pulsars is that they can help us understand or even confirm some huge, theoretical physics principles. Because they are such reliable astrophysical clocks, scientists immediately set to work testing parts of Einstein's Theory of General Relativity.

One section of that theory suggests that huge events, like merging two enormous black holes, could create ripples in space-time (called gravitational waves) that spread throughout the universe.

Thanks to pulsars, scientists have discovered that stars wobble like tops in the curved space-time of their orbit, as predicted by Einstein. They have also observed that the orbits are becoming smaller as energy is lost because of gravitational waves carting it away – another Einstein prediction proved correct [sources: University of Manchester, Weisberg].