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AstroTalk:BehindthenewsheadlinesofJanuary2017 RicharddeGrijs(何锐思) (KavliInstituteforAstronomyandAstrophysics,PekingUniversity) Pulsars—enigmaticobjectsthatremainasmysteriousaswhentheywere firstdiscovered… Whenastarwithabirthmassofatleast20timesthatofourSuneventually explodesasasupernova,itsremainingcoreofabout500,000Earthmassesis compressedtoasizeofonlyabout30kilometreswhileitcollapsesintoasocalled‘neutronstar.’ Neutronstarsrotaterapidly,whichissimplyaconsequenceofoneofthe fundamentallawsofNature,thatoftheconservationofangularmomentum. Becauseofthecollapseofthemassiveprogenitorstarintoaverysmallvolume, therotationrateofthecoreneedstoincreasesignificantlytomaintainthetotal angularmomentuminthesystem.Afterall,thesystem’stotalenergymustgo somewhere,soitisredistributedintheformofrotationalenergy. Thisrapidrotationcauseschargedparticlestostreamoutfromthemagnetic poles–mostlyintheformofradioorX-rayemission,althoughopticaland/or gamma-rayemissionhasalsobeenreported–alongthespinningneutronstar’s magneticaxis.Thelatterisnotnecessarilyalignedwiththeobject’srotationaxis, whichthereforecausestheappearanceofapulsedbeam,hencethename ‘pulsatingstar’orpulsar. LikecosmiclighthousessweepingtheUniversewithburstsofenergy,pulsars havefascinatedandbaffledastronomerssincetheywerefirstdiscovered50 yearsago.Despitethediscoveryofthefirstpulsaraslongagoas1967,they remainmysteriousobjectsuntiltoday,stillrevealingsurprisingnewdiscoveries fromtimetotime. Infact,anewdiscoverypublishedthismonthhasupendedthewidelyheldview thatallpulsarsareamongthemostorderlytickingclocksoftheUniverse.A surveydoneattheAreciboObservatoryinPuertoRicohasfortuitously discoveredtwoextremelystrangepulsarsthatundergoa‘cosmicvanishingact.’ Sometimestheyarethere,andthenforverylongperiodsoftime,theyarenot. Recognizingtheexistenceofthisstrangebehaviourwasfortuitousinitself.It tookgreatpatienceonthepartofateamofradioastronomersatJodrellBank ObservatoryintheUK,ledbyProfessorAndrewLyneoftheUniversityof Manchester,toconfirmtheexistenceofthesemostlyinvisiblepulsars. Intermittentpulsarsareararelyobservedpopulationofpulsars,whichhavetwo states—onewhentheypulselikenormalpulsars(theONstate),andanother whentheymysteriouslyfailtowork,producingnoradiowavesatall(theOFF state). “Theyswitchinstantaneouslybetweenthestates,”notesLyne.“They’reON andthenthey’regone,disappearingwithoutanyapparentwarning.” A34-memberpulsarstudyteamusedtheseven-beamreceiveroftheArecibo ObservatorytoconductroutinepulsarsearchesinwhattheycallthePulsar AreciboL-BandFeedArray(orPALFA)Survey.Thetworecentlydiscovered intermittentpulsarsspendmostoftheirtimeintheOFFstate.Threeother similarpulsarsarealsoknown,buttheyaremostlyON. InSeptember2012,oneofthesenewobjectswasdiscoveredtoemitverybright pulses,anditwaslabeledPSRJ1929+1357.Of169newpulsars,follow-up observationsofhalfofthosehadbeeninitiatedatJodrellBankObservatory,and thiscandidatewasconfirmedasapulsarinFebruary2013usingthe Observatory’s250-footLovellTelescopeatthesecondattempt. “Duringthenext9monthsitwasobservednofewerthan650times—for 100hoursintotal,”saidProfessorBenjaminStappersoftheJodrellBank CentreforAstrophysicsandaco-authoronthenewpublication.“Itwas ONononlyfiveoccasions—just0.8%ofthetime.” Themostimportantimplicationofthisdiscoveryisthattheremustexistan extremelylargenumberofthese‘vanishingact’pulsars.ThePALFAsurvey, whichisaimedatasectionoftheMilkyWayvisibletoArecibo’sradiodish,only coverseachpositioninthesurveyonce.Itprobablypassedover130similar pulsars,butthiswastheonlyonethatwasONatthetimeoftheobservation. Inaddition,ifitwerenotfortheearlysignalsdetectedatJodrellBank Observatory,thispulsarcouldeasilyhavebeendiscardedasafalsedetection, likelyarisingfromso-called‘radio-frequencyinterference.’ThePALFAteam estimatesthatthereareabout3,000suchintermittentpulsarsinthesurveyarea, farmorethanthepopulationofnormalpulsars. “Thesedisappearingpulsarsmayfaroutnumbernormalpulsars,”said VictoriaKaspiofMcGillUniversityinCanadaandtheprincipal investigatorofthePALFAproject.“Infact,theymayredefinewhatwe thinkofasnormal.” Whythisoddbehaviour?Afterall,sincetheoriginalpulsardiscovery,theyhave beenreferredtoasmarvelouslyaccuratecosmicclocksthatticksteadilyfor millionsyearswitharegularitythatsurpassesthetickingofourbestlaboratory clocks.Buttheselong-termintermittentpulsarsaremostlyinvisible,whichis aboutasusefulashavingtheclockonthewallthatishiddenbehindacurtainfor mostofthetime. “TheexplanationoftheON–OFFbehaviourremainsapuzzle,”says Seymour.“Itindicatesthatthepulsarenvironmentischanging,butjust whatthosechangesentailisopentodebate.” Apropertyof‘normal’pulsarsisthattheirpulserateslowsverygraduallyover time.However,recentobservationsoftheseoddpulsarssuggestthattheir rotationalslow-downratewhenOFFisonly80%oftheratewhenON.The PALFAteamsuspectsthatthestreamofchargedparticles,whichdrivetheradio beamsemanatingfromthepulsar,alsocausesthepulsartospindownmore rapidly. WhenOFF,thisparticlestreamfailsforsomereasonandthespin-downrateis reduced.But,asSeymournotes,thereisasyetnoagreementinthepulsar communityastotheON–OFFmechanism.Thechangingspinrateisinferredby calculatinghowmanybeatsweremissedduringthepulsar’sinvisiblephases. PALFAsurveysareongoing,andnoonecanpredictifandwhenmoreexamples ofthisfascinatingnewphenomenonwillbefound.Catchinganotherintermittent pulsarinitsONmodeisuptochance.Isthereanothercandidateoutthereready torevealitssecrets,orwillitforeverlurkhiddeninthedarkunknownsofspace? Lynehopesthatfollow-upmeasurementsofPSRJ1929+1357willprovidearare insightintothephysicsofthepulsaremissionmechanismandthechangingspindownphenomenon. Twootherstudiespublishedthismonthalsofocusontheintriguingnatureof radioandX-raypulsars.Internationalteamsofastronomerssuggestthatrecent imagesfromNASA’sChandraX-rayObservatoryoftwopulsars—Gemingaand B0355+54—mayhelpshinealightonthedistinctiveemissionsignaturesof pulsars,aswellastheiroftenperplexinggeometry. Interestingly,thepulsarbeamsrarelymatchupacrossthewavelengthrange, saidBettinaPosseltfromPennStateUniversity(USA).Theshapesofobserved radioandgamma-raypulsesareoftenquitedifferentandsomeoftheobjects showonlyonetypeofpulseortheother.Thesedifferenceshavegenerated debateaboutthepulsarmodel. “It’snotfullyunderstoodwhytherearevariationsbetweendifferentpulsars,”said Posselt.“Oneofthemainideashereisthatpulsedifferenceshavealottodowith geometry—anditalsodependsonhowthepulsar’sspinandmagneticaxesare orientedwithrespecttolineofsightwhetheryouseecertainpulsarsornot,aswell ashowyouseethem.” Chandra’simagesaregivingtheastronomersacloser-than-everlookatthe distinctivegeometryofthechargedparticlewindsradiatinginX-raysandother wavelengthsfromtheobjects,accordingtoPosselt.Pulsarwindnebulae(PWN) areproducedwhentheenergeticparticlesstreamingfrompulsarsshootalong thestars’magneticfields,formtori—doughnut-shapedrings—aroundthe pulsar’sequatorialplane,andjetalongthespinaxis,oftensweepingbackinto longtailsasthepulsarstravelatspeedsofuptohundredsofkilometresper secondthroughtheinterstellarmedium. “Thisisoneofthenicestresultsofourlargerstudyofpulsarwindnebulae,” saidRogerRomani,professorofphysicsatStanfordUniversity(USA)and principalinvestigatoroftheChandraPWNproject.“Bymakingthe3D structureofthesewindsvisible,wehaveshownhowonecantracebackto theplasmainjectedbythepulsaratthecentre.Chandra’sfantasticX-ray acuitywasessentialforthisstudy,sowearehappythatitwaspossibleto getthedeepexposuresthatmadethesefaintstructuresvisible.” AspectacularPWNisseenaroundtheGemingapulsar.Geminga—oneofthe closestpulsarsatonly800light-yearsdistancefromEarth—hasthreeunusual tails,saidPosselt.ThestreamsofparticlesspewingoutofthepolesofGeminga stretchoutformorethanhalfalight-year,morethan1,000timesthedistance betweentheSunandPluto.Another,shortertailalsoemanatesfromthepulsar. TheastronomerssaidthataverydifferentPWNpictureisseenintheX-ray imageofanotherpulsar,B0355+54,whichislocatedatabout3,300lightyears fromEarth.Thetailofthispulsarhasacapofemission,followedbyanarrow doubletailthatextendsalmostfivelight-yearsawayfromthestar. WhileGemingashowspulsesatgammarays,butisradio-quiet,B0355+54isone ofthebrightestradiopulsars,butfailstoshowgammarays. “Thetailsseemtotelluswhythatis,”saidPosselt,addingthatthepulsars’spin andmagneticaxisorientationsinfluencewhatkindofemissionisseenonEarth. Gemingamayhavemagneticpolesquiteclosetothetopandbottomofthe object,andnearlyalignedspinpoles,muchlikeEarth.Oneofthemagneticpoles ofB0355+54coulddirectlyfacetheEarth.Becausetheradioemissionoccurs nearthemagneticpoles,theradiowavesmaypointalongthedirectionofthe jets.Gamma-rayemission,ontheotherhand,isproducedathigheraltitudesina largerregion,allowingthosepulsestosweeplargerareasofthesky. “ForGeminga,weviewthebrightgamma-raypulsesandtheedgeofthe pulsarwindnebulatorus,buttheradiobeamsnearthejetspointofftothe sidesandremainunseen,”Posseltsaid. Thestronglybenttailsoffertheastronomerscluestothegeometryofthepulsar, whichcouldbecomparedtoeitherjetcontrailssoaringintospaceortoabow shocksimilartotheshockwavecreatedbyabulletasitisshotthroughtheair. OlegKargaltsevfromGeorgeWashingtonUniversity(USA),whoworkedonthe B0355+54study,saidthattheorientationoftheobjectplaysaroleinhow astronomersseethepulsaraswell. “ForB0355+54,ajetpointsnearlyatussowedetectthebrightradiopulses whilemostofthegamma-rayemissionisdirectedintheplaneoftheskyand missestheEarth,”hesaid.“Thisimpliesthatthepulsar’sspinaxisdirection isclosetoourline-of-sightdirectionandthatthepulsarismovingnearly perpendicularlytoitsspinaxis.” NoelKlingler,agraduatestudentatGeorgeWashingtonUniversityandlead authoroftheB0355+54paper,addedthattheanglesbetweenthethree directions—thespinaxis,theline-of-sight,andthevelocity—aredifferentfor differentpulsars,thusaffectingtheappearancesoftheirnebulae. “Inparticular,itmaybetrickytodetectaPWNfromapulsarmovingclose totheline-of-sightandhavingasmallanglebetweenthespinaxisandour line-of-sight,”saidKlingler. Inthebow-shockinterpretationoftheGemingaX-raydata,thepulsar’stwolong tailsandtheirunusualspectrummaysuggestthattheparticlesareacceleratedto nearlythespeedoflightthroughaprocesscalledFermiacceleration.Fermi accelerationtakesplaceattheintersectionofapulsarwindandtheinterstellar material,accordingtotheresearchers. AlthoughdifferentinterpretationsremainpossibleforGeminga’sgeometry, PosseltsaidthatChandra’simagesofthepulsararehelpingastrophysicistsuse theseobjectsasparticlephysicslaboratories.Studyingtheobjectsgives astrophysicistsachancetoinvestigateparticlephysicsinconditionsthatwould beimpossibletoreplicateinaparticleacceleratoronEarth. “Inbothscenarios,Gemingaprovidesexcitingnewconstraintsonthe accelerationphysicsinpulsarwindnebulaeandtheirinteractionwiththe surroundinginterstellarmatter,”shesaid. Figure1:Thepulsarpicturedhere,whichresidesinthegalaxyMessier82,12millionlight-years away,sendsoutX-raybeamsthatpassEarthevery1.37seconds.Scientistsstudyingthisobject withNuSTARoriginallythoughtitwasamassiveblackhole,butitsX-raypulserevealeditstrue pulsaridentity.(Credit:NASA/JPL-Caltech) Figure2:Artist’srepresentationofwhatthethreeunusualtailsoftheGemingapulsarmaylook likeclose-up.(Credit:NahksTr’Ehnl) Figure3:TheCrabNebulaseenintheopticalbytheHubbleSpaceTelescope.TheCrabisan exampleofapulsarwindnebula.(Credit:NASA/ESA-HubbleSpaceTelescope) Figure4:SignificancemapsofthepulsarwindnebulaHESSJ1825-137regioninthreedifferent energybands.Thesizeofthesourcesisclearlymuchreducedathighenergies.Othersources withinthefieldofviewincludethebinarysystemLS5039andthehardspectrumsourceHESS J1826–130.(Credit:Mitchelletal.,2016) Figure5:Thisimagecontainsinfrared,X-ray,andradiodataofthepulsarknownasPSRB150958.Itshowstheenvironmentintowhichthepulsar’snebulaisexpanding.(Credit:X-ray: NASA/CXC/SAO/P.Slaneetal;Infrared:2MASS/UMass/IPAC-Caltech;Radio:MolongloObservatory SynthesisTelescope) Figure6:AtthecentreofthisChandraimage,apulsar–only20kmindiameter–isresponsible forthisX-raynebulathatspans150light-years.Thispulsarisspinningaroundalmostseven timesasecondandhasamagneticfieldatitssurfaceestimatedtobe15trilliontimesstronger thantheEarth’smagneticfield.Thiscombinationofrapidrotationandultra-strongmagnetic fielddrivesanenergeticwindofelectronsandions,ultimatelycreatingtheelaboratenebulaseen byChandra.(Credit:NASA/CXC/SAO/P.Slaneetal.) Figure7:InthisMarch26,2003,filephoto,theworld’ssecond-largestradiotelescopeisseen fromtheair,attheAreciboObservatory,inPuertoRico.(Credit:APPhoto/TomasvanHoutryve)
