MeerKAT’s first paper explained ... - intranet.sarao.ac.za · construction, with only 16 of its...

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MeerKAT’sfirstpaperexplained:basicsofthelifeofstars,supernovae,andmagnetars;radioandX-rayobservationsThefirstscientificarticlebasedonMeerKATdata,entitledRevivalofthemagnetarPSRJ1622−4950:observationswithMeerKAT,Parkes,XMM-Newton,Swift,Chandra,andNuSTAR,hasjustbeenpublishedinTheAstrophysicalJournal(5April2018).What’samagnetar?What’sParkes,orChandra?Whatdoesitallmean?Here’sabrieflookintosomeofthesematters,andmore,forthegeneralinterestedreader.1.ThelifeofstarsliketheSunTypicalstarsliketheSunshinebyconvertingtheirhydrogenfuelintoheavierelementssuchashelium,throughachainofnuclearreactions.Whenlighterelementsarefusedintoheavierelementsinthisway,abitofenergyisreleasedthatweseeaslight.TheSunismid-waythroughits10billion-yearlifespan(theEarthisslightlyyounger,about4.5billionyearsold).Whenitrunsoutoffuel,itscorewillcontractintoaballthesizeoftheEarth,withapproximatelyhalfofthecurrentSun’smassandmadeupmostlyofhelium,carbonandoxygen.Itwillnolongergenerateenergythroughnuclearreactions,butitwillbeveryhot.Graduallyitwillloseitsstoredenergy,coolingoverbillionsofyears.Theseendpointsofstellarevolutionarecalledwhitedwarfstars.2.Thedramaticlifeofverymassivestars–themostviolentcosmicexplosionsStarsthatstarttheirliveswithbetween8and30timesasmuchmassastheSunfollowadifferentpathandhaveadifferentending.Theseareveryrare:themoremassivethestar,thefewerlikethemexistinourMilkyWaygalaxy.Massivestarsstart,liketheSun,byconvertinghydrogenintohelium.Thishappensintheirdenseandhotcores(theSun’ssurfacethatweseehasatemperatureof5000degrees,butthecoreisinexcessof10milliondegrees).Thehightemperatureforcesprotons(thepositivelychargednucleiofhydrogenatomsthatwouldotherwiserepel)tofuseintoheavierelements.Duetotheirincreasedgravity,thecoresofverymassivestarsaredenserandhotterthantheSun’s.Thiscausesthemtoconsumetheirnuclearfuelmorerapidly,inonlyafewmillionyears.Whentheyrunoutofhydrogen,theircoresfirstcontractduetogravity,whichcausesthemtobecomeevendenserandhotter,nowfusingheliumnucleiintograduallymoremassiveelements.Thisprocesscontinues,evermorerapidly,untilthestarburnssiliconattemperaturesofbillionsofdegreesinonlyafewdays.ThisresultsinaballofironwiththemassoftheSunandthesizeoftheEarthsurrounded,onion-like,bylayersofgraduallylighterelements.Atthispoint,veryrapidly,somethingdramatichappens.Ironcannotbefusedintoheavierelementswhilereleasingenergy.However,gravitycontinuesitswork,pullingatomseverclosertothecenter,everdenser.Suddenly,eachprotonpluselectroninthecoretransformsintoaneutron,andwithinafractionofasecondthecoreshrinkstoaballmadeupmostlyofneutrons,20kilometresindiameter–thesizeofacitybut

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slightlymoremassivethantheSun.AteaspoonofthismaterialhasmoremassthanTableMountain!Theouterlayersofthestar,fallingduetogravity,crashintothishardyneutronstar,whichreboundslikearubberball,launchingelementsintospaceattensofthousandsofkilometrespersecond!Astheatomscollidewitheachother,newelementsarecreated.Thesearesupernovaexplosions–themostviolenteventsknownintheuniverse(seeFigure1).3.Theoxygenthatyoubreatheandthesaltthatyoueat:productsofexplodedstars!Onaverage,asupernovahappensinagalaxyliketheMilkyWayoncepercentury.Theelementscreatedanddispersedbyasupernovainthiswaymakeuptherawmaterialsthatformsubsequentgenerationsofstarsandplanetsinthegalaxy.Theatomsintheoxygenthatwebreathe,theatomsinthesaltthatweeat,wereoncemadeinsidemassivestarsandlaunchedintospacebysupernovas.4.Fromthediscoveryofthesub-atomicneutrontothediscoveryofneutronstarsandpulsarsNeutronstarswerepredictedtoexistinthe1930s,soonafterthediscoveryoftheneutronparticleitselfinlaboratoryexperiments.However,theywereexpectedtobesosmallthatitwashardtoimaginethattheycouldeverbeseen–thestarsthatweseeinthenightskyarevisibleeventhoughtheyareveryfarawaybecausetheyarehotandbecausetheyarehuge(theSunisabout1millionkilometresindiameter).Nevertheless,neutronstarswerediscoveredin1967,completelyunexpectedly,usingradiotelescopes.Wenowknowthattheneutronstarremnantleftoverfromasupernovaexplosiontypicallyspinsveryrapidly,tensoftimespersecond(Figure1).Italsohasanextremelystrongmagneticfield,onetrilliontimesstrongerthantheEarth’s.Thecombinationofthesefactorsleadstobeamsofradiowavesbeingproducedthatarefocusedalongthemagneticfields.Asthestarspins,givenafortuitousalignment,telescopesonEarthcandetectburstsofradiowaveswitheveryturnofthestar,inlighthouse-likefashion.Theseneutronstarsarethereforesometimesalsoknownaspulsars,sincetheyappeartopulsate,althoughinfacttheyarerotating.Fiftyyearsafterthediscoveryofthefirstone,about3000pulsarsareknownintheMilkyWay,afewpercentofthetotalpopulationthoughttoexist.Bycomparison,ourgalaxycontainsmorethan100billionordinarystars.5.Magnetars:themostmagneticobjectsintheuniverseMagnetarsareaveryraresubsetofneutronstars/pulsars.Onlytwodozenareknowninthegalaxy.Theirmagneticfieldsareupto1000timesstrongerthanthoseofordinarypulsars.Theenergyassociatedwithsuchfieldsissolargethatitalmostbreaksthestarapartinmassivestarquakes.Magnetarsthereforetendtobeunstable,displayinggreatvariabilityintheirphysicalpropertiesandelectromagneticemission.Sometimestheydisplayenormousoutburstsofenergy–andsometimesthey“turnoff”:wecannolongerseethem(atleastforawhile),evenwiththebesttelescopeintheworld.

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6.ObservingmagnetarswithradioandX-raytelescopesWhiletypicalpulsarsaremosteasilyobservedthroughtheirradioemission,until2006nomagnetarhadbeenobservedtoemitradiowaves–somuchsothattheorieshadbeenadvancedtoexplainwhytheycouldn’temitradiowaves.Thosetheoriesareincorrect,sincewenowknowoffourmagnetarsthatemitradiowaves.Nevertheless,wedon’tunderstandverywellwhatcausesthisemission,whichhassomeunusualproperties.Radioemissionfrommagnetarsisrareandpoorlyunderstood,butallmagnetarsareknowntoemitX-rays.Thesearesimilartothoseusedatthedoctor’sofficeorinairportX-rayscanners,butareproducedatthesurfaceofamagnetarheatedtomillionsofdegreesbythedecayofitsultra-strongmagneticfields.BythetimetheyarriveattheEarth,aftertravelingthousandsoflight-years(alight-yearisthedistancecoveredbylightoranyelectromagneticradiationinoneyear,movingataspeedof300thousandkilometrespersecond),theseX-raysarefaint,requiringverysensitiveinstrumentsinordertobedetected.Also,X-raysfromastronomicalsourcessuchasstarsandgalaxiesmustbedetectedabovetheEarth’satmosphere(whichabsorbsX-rays,protectinglifeonEarthfromdangerousradiation).X-raytelescopesarethereforeplacedinorbitingsatellites.7.Whystudymagnetars?Astronomers,withoutleavingtheEarth,usetelescopestoexploretheuniverse.Thestudyofmagnetarsallowsustolearnaboutthebehaviorofmatterinthemostextremeconditionspresentintheuniverse,quiteunlikeanythatcanbeexperiencedonEarth.ThestudyofthefourmagnetarsknowntoemitbothX-raysandradiowavesopensanewwindowforunderstandingtheserareandexoticobjects.8.RadiomagnetarsturningonandoffRadiowaveswerefirstdiscoveredfromamagnetarafteranX-rayoutburstcausedbyasuddenrearrangementofitsmagneticfield.Afteralltheenergywasreleasedandthemagneticfieldrelaxedtoamorestablestate,radioemissionceasedin2008,andthestarhasbeenquietsince.Itshouldturnonagainoneday–whethertomorrowor100yearsfromnow,nobodyknows.9.ThesubjectofthefirstMeerKATpaper:discovery,radioturnoff,andrevivalThemagnetarPSRJ1622-4950isthesubjectofthefirstscientificpublicationbasedonMeerKATdata(thenameisderivedfromitsposition,intheNormaconstellationalongtheMilkyWay,closetothetailofScorpius).Itwasdiscoveredasapulsarwitharotationperiodof4secondsin2009,usingtheParkesradiotelescopeinAustralia(seeFigure2).AlthoughthemagnetarwasalsodetectedinX-rayimages,itwasalreadybecomingfaintandnoX-raypulsationscouldbedetected.Byearly2015,itsradioemissionturnedoff,butregularmonitoringobservationscontinued.

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On26April2017,anAustraliancolleagueusingtheParkestelescopenoticedthatPSRJ1622-4950wasonceagainemittingbrightradiopulsesevery4seconds!AfewdayslaterParkesunderwentaplannedmonth-longmaintenanceshutdown.AlthoughMeerKATwasstillunderconstruction,withonly16ofitseventual64dishesavailable,theSARAOcommissioningandoperationsteamstartedregularmonitoringofthisunusualstar30,000lightyearsfromEarth(Figure4).10.ParkesandMeerKAT:twodifferenttypesofradiotelescopes,bothgreatforpulsarsParkesisasingle-dishradiotelescope64metresindiameter,oneofthemostsuccessfulintheworldforpulsarstudies.MeerKATisaninterferometer,whichcanmakesharpradioimagesofawideareaofskybycombiningsignalsfromitsmanysmallerantennasseparatedbyupto8kilometres.Inthiscase,thedistinctiondoesn’tmatter,becausepulsarsaresosmallandsofarawaythattheyalwayslooklikepointsof(radio)light.MeerKATwasthereforeusedina(beamforming)modewherebythesignalsfromthedifferentantennasarecombinedtolookatjustonepointonthesky,aswithasingle-dishtelescope.Withonly16dishesoperatingatL-band(overafrequencyrangeof900to1670MHz),MeerKATalreadyhadcomparablesensitivitytothemuchlargersingleParkesdish.11.UsingX-raytelescopestostudythemagnetar–andneedingMeerKATdataAssoonaswerealizedthatthemagnetarhadrevivedatradiowavelengths(itwasnowatleast100timesbrighterthananytimesince2015),ourteamwantedtodeterminewhetheritwasalsobrighterinX-rays–andtotrytodetectX-raypulsations(thiswastheonlymagnetarknowninthegalaxyforwhichX-raypulsationshadneverbeendetected).TogetherwithcolleaguesinCanadaweweresuccessfulinmakingthecaseforusingNASAX-raytelescopesforthispurpose.IntheendweusedtheChandra(seeFigure3),NuSTAR,andSwifttelescopes,inadditiontoearlierarchivalX-raydatafromtheXMM-Newtontelescope.TheabilitytoobservethestarseveraltimesaweekwithMeerKATprovedcritical.That’sbecauseweexpectedveryfewX-rayphotonsfromthestar(manyfewerthanoneeverysecond),andthestar’srotationbehaveserraticallyduetothechangingmagneticfieldstillrecoveringfromitsrecentviolenteruption.ThiswouldmakeitimpossibletosearchforfaintX-raypulsations–unlessweknewwhatthepreciserotationalbehaviorofthestarwasatthesametimethatwewerecollectingX-raydata.Whichwedid–bymeasuringitwithMeerKAT!InthiswaywediscoveredX-raypulsesfromthestar,every4seconds(Figure5).Fromourinvestigationwealsolearnedthatthemagnetarawokefromits2-yearslumberinMarchorApril2017.InearlyMayitwasatleast800timesbrighterinX-raysthanwhenitwasdormant.Butitwasalreadyfadingfast:hadwewaitedafewmonths,wewouldn’thavedetectedanyX-raypulsations.UsingdatafromParkesafteritreturnedtoactionfromitsshutdown,wealsodeterminedthatradioemissionfromthemagnetarnowarisesfromadifferentlocationonitsso-calledmagnetosphere.Wedon’tknowwhythisshouldbeso–nothinglikeithasbeenobservedbefore.Moremysteriestobetackledbyfuturestudies…

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12.MeerKAT:apowerfulnewtelescopebuiltbyaremarkableteamThediscoveriesreportedinthearticlepublishedtodaycouldnothavebeenmadewithouttheMeerKATtelescope.Whilenotyetcomplete,MeerKATisnowclearlyanexcitingnewscientificinstrument.Itwillbecomemoresowithtimeasitscapabilitiescontinuetobedeveloped.It’stakenmorethanadecadeofhardworkbyteamsofhundreds,SouthAfricansforthemostpart,workingatthecuttingedgeoftechnology,tobuildthisbeautifulinstrument.Manyofthosewho’vebuiltit–membersoftheso-calledMeerKATBuildersList–areamongthe208co-authorsofthisarticle,thefirstofmanytofollowintheyearstocome. F.Camilo,L.Magnus,M.Venter,M.Geyer

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FIGURESFigure1:TheCrabNebula,viewedwiththeHubbleSpaceTelescope.(CreditandCopyright:NASA,ESA.)

Everythinginthisimage(otherthanthepointsoflightwhichareunrelatedstarsinourgalaxy)ispoweredbyahighlymagnetizedneutronstarthesizeofacityspinning30timespersecond–theCrabpulsar.Thisistheremnantofasupernovaexplosionobservedon4July1054AD.Locatedapproximately6000light-yearsfromEarth,theCrabNebulaspans10light-years.

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Figure2:TheParkes64-metreradiotelescopeinAustralia.(CreditandCopyright:JohnSarkissian,CSIROParkesObservatory.)

Inthisphototakenon21July2011,theSpaceShuttleAtlantis(streakbehindthefocuscabinofthetelescope)hasjustundockedfromtheInternationalSpaceStation(streaktothelowerright)forthefinaltime.ThemagnetarPSRJ1622-4950wasdiscoveredwiththeParkestelescopein2009,whereithasbeenstudiedatradiowavelengthssince–untilMeerKATjoinedin2017.

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Figure3:TheChandraX-rayObservatory,withtheNamibiandesertinthebackground,justbeforedeploymentfromtheSpaceShuttleColumbia’spayloadbayon23July1999.(CreditandCopyright:NASA.)

TheX-raytelescopehasaverylongfocallength(10metres)becauseX-rayscannotbefocusedwithmirrorsinthesamewayasvisiblelight:inordernottobeabsorbed,theyare‘skipped’atgrazingincidencereflection,analogouslytoastonebeingskippedalongapondsurface,withanglesof2-3degreestothesurfaceoftheparaboloidandhyperboloidChandramirrors,beforebeingfocusedtothescienceinstrumentsthatdetecttheX-rays.Chandra,withitsAdvancedCCDImagingSpectrometer(ACIS),wasusedtoobservethemagnetarPSRJ1622-4950inMayandSeptember2017incoordinationwithMeerKAT.

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Figure4:ThefirstdetectionofthemagnetarPSRJ1622-4950usingMeerKAT,on27April2017.Thepulseprofile(phaseof0–1correspondstotherotationperiodof4.3seconds)isshowntwice,asafunctionoftimeduringthe12-minuteobservationwiththeL-bandreceiver,andsummedatthetop.(AdaptedfromCamiloetal.2018,ApJ,856,180.)

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Figure5:X-raypulsesfromthemagnetarPSRJ1622-4950,detectedforthefirsttimein2017.ThesparseX-rayphotonscollectedwithNASA’sChandraandNuSTARtelescopeswerefoldedinrotationalphase(longitude)usingthepreciserotationalperiodpredictedfortheneutronstar,obtainedfromcontemporaneousMeerKATradioobservations.Thefullrotationalphaseof4.3secondsisshowntwiceforclarity,forthreecombinationsofX-rayinstruments.(FigurefromCamiloetal.2018.)