Gamma-rayConstraintsonElectromagneticCounterpartsofFastRadioBursts

Collaborators:TomonoriTotani,NoritaKawanaka

@Resuceu-RIKENJointSeminar201625July2016

ShotaroYamasaki(U-Tokyo)

“A blind search for prompt gamma-ray counterparts of fast radio bursts with Fermi-LAT data”

Yamasakietal.2016(arXiv:1604.03077,MNRASinpress)

Outline

1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork

TheDawnofFRBAstronomy(2007-)

• Fastradioburst(FRB)wasfirstdiscoveredbyLorimer(2007)throughpulsarsearch.FRB010724

• FRBsarefurtherconfirmedwithaconsiderablesample(17FRBsatthemoment,i.e.,Thorntonetal.2013;Keaneetal.2011&2016;Burke-Spolaor&Bannister2014;Spitleretal.2014&2016;;Championetal.2015;Ravietal.2015).

freq

uency

Time

Afewmsecduration

FastRadioBursts• Duration𝛿𝑡 ∼ afewmilliseconds

Indicatescompactobjects:

• Fluxdensity~ afewJyat~ GHzCoherentemissions:

• AnomalouslyhighDispersionMeasure(DM)(375-1500pc/cm3)PropagatingthroughtheionizedISM:

7FRB ~ ~ 3 1 cm0 ( / )r c t t msδ δ×

Evenat~kpc,TB isstillhigh(1012 K:upperlimitofastrophysicalsources)

-2

-4

Timedelay DMPulse width

ν

ν

∝ ×

∝

Comological origin?(DM-bplot)Thorntonetal.2013

Pulsars

FRBs

Latitude

DM

HighDMsatrelativelyhighlatitudes

FastRadioBursts• CosmologicalDistances

HighDMvaluessuggest→ indicatingcosmoloicalorigin(ifdilationoccursbypropagationthroughISM)

• EventrateØ EventrateofGRBs:Ø Eventrateofcore-collapseSNe:

• Nocounterpartssofarinotherwavelength(exceptforFRB150428)

• ConstraintsonprompthighenergyemissionØ 10-100keV:(Tendulkar+16,Murase+16)Ø 1-100GeV:(Yamasaki+16,thiswork)

3 1 14 ~1010 / sky / day or gal yr− − −:6 1 1gal r~10 y− − −

2 1 1gal r~10 y− − −

Recentobservations(Totani-san’stalk)

• Keane+16Ø ATCAdetected~6dayradioafterglowassociatedwithFRB150428Ø AhostgalaxywasfoundinSubarui’bandimageforthefirsttimeØ ThehostgalaxyofFRB150428iselliptical(hencenotstar-forming)

→progenitor=oldstellarpopulation(i.e.NS-NSmerger,etc..)

• Spitler+16&Scholtz+16Ø AreciboFRB(FRB121102)repeats17timestotalØ AllDMsconsistentwithasinglevalue(~559pc/cm3)→prgenitor=super-giantpulsesfromyoungpulsarsormagnetars?

Ø NopromptX-rayemissiondetected

→ AllFRBsrepeat?Atleasttwopopulations?

…butcurrently,itsassociationunderdebate(AGNactivity?)

LeadingProgenitorModels

• Pulsaremissions• SGR(Magnetar)flares• AsteroidscollisionwithNS

• NS-NS(BH)merger• HMNScollapse• WD-WDmerger• BH-BHmerger

(ImageCredit:NASA/CXC/SAO) (Rezzollaetal.2011)

Catastrophic(One-off)Repeatable

Possiblesiteofgamma-rayemissions

FRB121102(Arecibo/GBT) FRB150428(Parkes)

Outline

1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork

Motivations

• Whatevertheprogentoris,itisreasonabletoexpectcorrespondingms-durationhigh-energyradiationsattheonsetofFRBs

• NoassociationwithX-ray(Swift-BAT)andgamma-ray(Fermi-GBM)emissiondetectedyet

→UseFermi-LATPass7Rep (1-100GeV,2008-2015)all-skydata

• RadiotelescopesfindingFRBsatcosmologicaldistanceshavenarrowerfield-of-views(∼ 15arcminbeamsizeforParkes)thanFermi-LAT

→RatherthanexaminingtheFermi-LATdataatknownFRBlocations(z=0.5-1.0),wesearchforms-durationgamma-rayflashesfromnearbypotentialFRBs(z<0.02)

Fermi-LAT(LargeAreaTelescope)

• Effectivearea:8000cm2

• EnergyRange:20MeV-300GeV• Angularresolution:0.8deg @1GeV• Pair-conversiontelescope(γ→e+ +e- )

Resolvedgamma-raysky(0.1-300GeV) Widefieldofview(~2.4sr)

(ImageCredit:NASA)

SchematicPicture

Gamma-raytelescopeFOV(wide&shallow)

RadiodetectedFRB(z=0.5- 1.0)

NearbypotentialFRBs

Parkes(Radio)FOV(narrow&deep)

Previousstudy(Tendulkar+16) Thiswork(Yamasaki+16)

observer observer

→ Weimprovethedetectionsensitivitybyafactorof≳100

Sensitivity line

Data&SearchAlgorithm

• Timespan:6.8yearsfromJuly31,2008(UTC)toJune18,2015(UTC),thetotalamountofgoodtimeintervalsis79%ofthis.

• Photondata:“P7SOURCE”(smallercontamination)

• EnergyRange:1-100GeV(Initiallyweused30MeV-300GeVwhichturnedouttoincludelargebackgroudcontamination)

• Searchtimewindow:∆t=1,2,5,and10msaretried.

• Weconsiderareferenceγ-rayevent,andotherγ-rayeventsaresearchedwithin2◦ radiusfromthereferenceinthetimeintervalof∆tstartingfromthereferenceevent.

Outline

1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork

Results

• ~100flashcandidatesaredetected

• Aretheygenuine?orfalseevent?

→Weexaminedthesignificanceoftheseevents

TimeWindows

FalseEventsfrom brightpointsources

> 4 x 10-9 phs/cm2/s 𝜃 < 2.0 deg

#1.Thephotonfluxdistributionofthenearestpointsourcetoeachevent

#2.Angularseparationdistributionfromeacheventtothenearestpointsource

CUT CUTEventsoriginatingfromneabypoint sourcesEventsoriginating from

brightpointsources

FalseEventsfromdiffusebackground

• Afterremovingeventsfrombrightsteadypointsources,wefoundnoflasheventsat|b|>20°

• Theexpectednumberofeventsfromthediffusebackground(black)canaccountfortheremainingflasheventsat|b|<20° (grey)

• Weusedthisfinalresult(noganmma-rayflasheventsat|b|>20°,remainingskyfraction~0.68) tosetanupper-limitonthefluxofms-scalegamma-rayflashes #Galacticlatitudedistributionofremainingevents

(Nphoton =2,Δ𝑡 =10ms)

黒:Diffuse BG からの理論値

非検出 非検出Non-detection Non-detection

Outline

1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork

Upper-limitongamma-ray/radiofluxratio• TheStandardcandleapproximation:wedefinegamma-raytoradioluminosity

ratiointherest-frameofanFRBas

• WeassumethatξisthesameforallFRBs,andthatFRBshaveapower-lawgamma-rayspectrumwithaphotonspectralindexΓ(herewetakeΓ=2)

• Themaximumredshiftofdetection()isdeterminedby:

• VolumetricFRBrateevolution isincludedas

• Theupper-limit onξ(2-sigma)isderivedforβ=0-4bysolving

w/

Duration(3ms)Numberofobserved

γ-rayflashes (< 2)withinΔt Differentialγ-rayfluxofanFRBEffectiveareaofLAT

(ξ:thekeyparameterinouranalysis)

NumberofFRBsinz<zmax

Exposuretime(fori-thtimebin)

Direction(forj-thskygrid)

Numberofdetectableγ-rayflashesfromFRBs(fori-thtimebinandj-thsky direction)

Result

ξ≲ 4.2 − 12 ×103 (@GeVband,thiswork)Ourlimitonξisstrongerthanthatobtainedfromnon-detectionofradio-detectedFRBsbyFermi-LAT(typicallyξ≲ 4×1045)

Themulti-wavelengthspectraoftypicalgamma-raypulsars

Thompson, D.J.2004a,inIAUSymp.218

• Typicallyξ =106 – 108

• Someyoungpulsarsexhibitξ <104

• ξofpulsarsaredistributedinawiderange(ξPSR =104 –108),andourresult(ξ≲ 106)doesnotexcludeapossibilitythatFRBshaveasimilarGeVgamma-raytoradiofluxratiotopulsars

SGR1806-20(magnetar)flaresinDec2004Initialflare

(IF)

Pulsating tails(PT)

TheSEDofInitialflare

~10– 100keVobservationonly

• Thebrightestgamma-rayeventbutnon-detection intheradioband:

(𝜈𝐹𝜈);<=>?< 1.1MJy(Tendulkar+16)

• Gamma-ray:~keVobservationonly→WeestimatetheGeV fluxofIF

usingPT’sSED:

(𝜈𝐹𝜈)A<;=BC~0.51 ergcm-2 s-1

• Thelower-limitonξforSGRisderivedas:ξSGR≳ 103.E

• Thisismarginallyconsistentwithourlimit: ξ≲ 4.2− 12 ×103

Outline

1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork

Summary• Wesearchedms-scalegamma-rayflashesinthe7-yrFermi-LATdata,

motivatedbythepossiblegamma-raycounterparttoFRBs– About100flashcandidatesaredetected

– Aftercarefuleventcuts,theremainingeventsatlowlat.regions(|b|< 20°)areconsistentwiththestatisticalflukesoriginating fromdiffusegamma-raybackground

– Noms-scale(∆t<10ms)gamma-rayflashesfoundathigh lat.regions(|b|>20°, skyfraction~0.68)

• Consideringthez-evolutionofFRBratedensity,wesetupperboundsonGeVgamma-raytoradiofluxratio(≡ ξ)ofFRBs:ξ≲ 4.2 − 12 ×103

– ξofpulsars:ξPSR =104 –108→ wecannotexcludethatFRBshavesimilarvaluesofξtopulsarsingeneral

– ξofamagnetar(SGR1806-20):ξSGR≳ 103.E→ theupperboundontheradiofluxforSGR1806-20ismarginallyconsistentwithourlimitofξ

Outline

1. GeneralfeaturesofFRBs2. Motivation,Data&Searchalgorithm3. Results&Discussion4. ImplicationsforFRBs5. Summary6. Currentwork

TestingbinaryNSmergermodelforFRBs• BinaryNSmergersarepossibleoriginofFRBs(e.g.Totani2013,

Zhang+2016,Wang+2016)

• Theemissionmechanismproducingcoherentradioemissionisunclear(Magneticdipolaremission,curvatureradiation,synchrotron,InverseCompton…)

• Theoreticalstudiespredictpossibleelectromagneticemissionsduringinsipralphase(priortomerger)(e.g.Hansen&Lyutikov2000,Lai2012,Wang+2016)

• UsingbinaryNSmergersimulationdata(K.Kiuchi,M.Shibata),wetestproposedscenariosandconsiderarealisticmodelforFRBsifpossible

t=tmrg – 1.4ms(i=22)

t=tmrg – 0.7ms(i=23)

H4EOS,1.35MSUN – 1.35MSUN

t=tmrg(i=24)

t=tmrg +0.7ms(i=25)

H4EOS,1.35MSUN – 1.35MSUN

Angularvelocityevolution(beforemergertmrg- 4.2ms<t<tmrg)

V =R×Ω

Theangularvelocityenhancedbyafactorof~3-10inthelast0.7msbeforemerger

• HowenergyisextractedfromthespinandorbitalmotionofinspiralNS?

• Iscoherentemissionpossible?

• Themagnetosphericenvironment(density,opacity…）

Openquestions:

(Totani2013)