LOFT as a discovery machine for jetted Tidal Disruption Events

1.4 MB·English

by  E. M. Rossi

LOFT as a discovery machine for jetted
Tidal Disruption Events
5
1
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White Paper in Support of the Mission Concept of the
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Large Observatory for X-ray Timing
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h Authors
p
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o E.M.Rossi1,I.Donnarumma2,R.Fender3,P.Jonker4,9,
tr S.Komossa5,Z.Paragi6,I.Prandoni7,L.Zampieri8
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1 1 LeidenObservatory,LeidenUniversity,P.O.Box Hügel69,53121Bonn,Germany
v 9513,2300RA,Leiden,TheNetherlands 6 JointInstituteforVLBIinEurope,Postbus2,NL-
4 2 INAF-IAPS,ViaFossodelCavaliere100,00133, 7990AADwingeloo,theNetherlands
7
Rome,Italy 7 INAF-IRABologna,ViaP.Gobetti101,40129
7
2 3 DepartmentofPhysics,OxfordUniversity,Keble Bologna,Italy
0 Road,OX13RH,Oxford,UK 8 INAF-OsservaorioAstronomicodiPadova,Vicolo
. 4 NetherlandsInstituteofSpaceResearch,,Nether- dell’Osservatorio5,I-35122Padova,Italy
1
0 landsInstituteofSpaceResearch,Utrecht,theNether- 9 DepartmentofAstrophysics/IMAPP,RadboudUni-
5 lands versityNijmegen,P.O.Box9010,6500GLNijmegen,
1 5 Max-Planck-InstitutfürRadioastronomie,Aufdem TheNetherlands
:
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LOFT asadiscoverymachineforjettedTidalDisruptionEvents
Preamble
TheLargeObservatoryforX-rayTiming,LOFT,isdesignedtoperformfastX-raytimingandspectroscopy
with uniquely large throughput (Feroci et al., 2014). LOFT focuses on two fundamental questions of
ESA’s Cosmic Vision Theme “Matter under extreme conditions”: what is the equation of state of ultra-
dense matter in neutron stars? Does matter orbiting close to the event horizon follow the predictions of
generalrelativity? ThesegoalsareelaboratedinthemissionYellowBook(http://sci.esa.int/loft/
53447-loft-yellow-book/)describingtheLOFT missionasproposedinM3,whichcloselyresembles
theLOFT missionnowbeingproposedforM4.
TheextensiveassessmentstudyofLOFT asESA’sM3missioncandidatedemonstratesthehighlevel
ofmaturityandthetechnicalfeasibilityofthemission,aswellasthescientificimportanceofitsunique
core science goals. For this reason, the LOFT development has been continued, aiming at the new M4
launch opportunity, for which the M3 science goals have been confirmed. The unprecedentedly large
effectivearea,largegrasp,andspectroscopiccapabilitiesofLOFT’sinstrumentsmakethemissioncapable
of state-of-the-art science not only for its core science case, but also for many other open questions in
astrophysics.
LOFT’s primary instrument is the Large Area Detector (LAD), a 8.5m2 instrument operating in the
2–30keVenergyrange,whichwillrevolutionisestudiesofGalacticandextragalacticX-raysourcesdown
totheirfundamentaltimescales. ThemissionalsofeaturesaWideFieldMonitor(WFM),whichinthe
2–50keVrangesimultaneouslyobservesmorethanathirdoftheskyatanytime,detectingobjectsdownto
mCrabfluxesandprovidingdatawithexcellenttimingandspectralresolution. Additionally,themissionis
equippedwithanon-boardalertsystemforthedetectionandrapidbroadcastingtothegroundofcelestial
brightandfastoutburstsofX-rays(particularly,Gamma-rayBursts).
This paper is one of twelve White Papers that illustrate the unique potential of LOFT as an X-ray
observatoryinavarietyofastrophysicalfieldsinadditiontothecorescience.
Page2of10
LOFT asadiscoverymachineforjettedTidalDisruptionEvents
1 Introduction
Mountingobservationalevidenceissupportingascenariowheremostgalacticnucleihostsupermassiveblack
holes(SMBHs). Gasinflowfromlargerscalescausesasmall( 1%)fractionofSMBHstoaccretecontinuously
∼
formillionsofyearsandshineasquasars. Mostofthemareinstead“quiescent”,accreting–ifatall–atahighly
sub-Eddingtonrate. Observationally,itisthereforehardtoassessthepresenceandmeasurethemassofmostof
SMBHs,beyondthelocaluniverse. Occasionally,however,asuddenincreaseoftheaccretionratemayoccur,
ifalargemassofgas,e.g. astar,fallsintothetidalsphereofinfluenceoftheblackholeandfindsitselftorn
apartandaccreted. Wecalltheseevents“tidaldisruptionevents”(TDEs). TDEscanresultinasuddenriseof
electromagneticemission. Theycanreachtheluminosityofaquasarbuttheyarerare( 10 5yr 1 pergalaxy)
− −
∼
andlastseveralmonths,oryearsinsoftX-rayKomossa(2002).
Thedetectionandstudyoftheseflarescandeliverotherimportantastrophysicalinformation,beyondprobing
the presence of a SMBH. After stellar disruption, part of the stellar material is accreted onto the black hole,
causingaluminousflareofradiation. Ifthestariscompletelydisrupted,itsdebrisisaccretedatadecreasing
rateof M˙ t 5/3 (Rees,1988;Phinney,1989). Therefore,TDEsallowustostudytheformationofatransient
−
∝
accretiondiscanditscontinuoustransitionthroughdifferentaccretionstates. Thesuper-Eddingtonphase—
whichoccursonlyforSMBHmasses M < 107 M —istheoreticallyuncertain,butitmaybeassociatedwitha
powerfulradiativelydrivenwind(Rossi&∼Begelm(cid:12)an,2009),thatthermallyemits 1041 1043 ergs 1,mainly
−
∼ −
atopticalfrequencies(Strubbe&Quataert,2009;Lodato&Rossi,2011). Thediscluminosity( 1044 1046
∼ −
erg s 1), instead, peaks in the far-UV/soft X-rays (Lodato & Rossi, 2011). Of great theoretical importance
−
wouldalsobethepossibilitytoobservetheformationandevolutionofanassociatedjet,poweredbythissudden
accretionepisode. Furthermore,TDEsaresignpostsofsupermassivebinaryblackholes,astheirlightcurveslook
characteristicallydifferentinthepresenceofasecondBH,whichactsasaperturberonthestellarstream(Liu
etal.,2009),asrecentlyobservedinafirstcandidateevent(Liuetal.,2014). Finally,wenotethat,inX-rays,
TDEsrepresentanewprobeofstronggravity,forinstancetracingprecessioneffectsintheKerrmetric.
ThefirstTDEswerediscoveredinROSATsurveysoftheX-raysky(e.g.,Komossa&Bade,1999;Grupeetal.,
1999)—seeKomossa(2002)forareview. Later,GALEXallowedfortheselectionofTDEsatUVfrequencies
(Gezari et al.,2009;Gezarietal.,2012);manyofthemostrecentTDEcandidatesarenowfoundinoptical
transientsurveys(VanVelzenetal.,2011a;Cenkoetal.,2012;Chornocketal.,2014). Analternativemethodto
selectTDEcandidatesistolookforopticalspectrawithextremehigh-ionizationandBalmerlines(Komossa
etal.,2008;Wangetal.,2012).This“thermal”SpectralEnergyDistribution(SED)ofTDEsisbelievedtobe
associatedwithemissionfromthediscortheradiativelydrivenwind.
RecentlytheSwiftBurstAlertTelescope(BAT)instrument(Bloometal.,2011;Burrowsetal.,2011;Cenko
etal.,2012),triggeredtwoTDEcandidatesinthehardX-rayband. Amulti-frequencyfollow-upfromradio
toγ-raysrevealedanewclassofnon-thermalTDEs. Itiswidelybelievedthatemissionfromarelativisticjet
(Γ 2,Zaudereretal.,2013;Bergeretal.,2012)isresponsibleforthehardX-rayspectrum(withpower-law
≈
photonindexα 1.6 1.8)andtheincreasingradioactivity(Levanetal.,2011),detectedafewdaysafterthe
∼ −
trigger. ThebeststudiedofthetwoeventsisSwiftJ1644+57(SwJ1644inshort). Thetwomainfeaturesthat
supporttheclaimthatSwJ1644isaTDEarei)theX-raylightcurvebehaviour,thatfollowsL M˙ (t τ) 5/3
x −
∝ ∝ −
afterafewdays(τ 3day)fromthetrigger,andii)theradiolocalizationoftheeventwithin150pcfromthe
≈
centreofaknownquiescentgalacticnucleus(Zaudereretal.,2011).
In this White Paper, we use the Sw J1644 X-ray lightcurve (see next section) to estimate detection rate
prospects for the proposed M4 mission LOFT (Feroci et al., 2012). We chose to focus on predictions for
non-thermalTDEs,bestsuitedtobeaLOFT target(LOFT couldalsoseethelatecoronahardX-rayemission
in disk-dominated TDEs but only in the local Universe; Strubbe & Quataert, 2009). On board there are two
maininstruments: theLargeAreaDetector(LAD,2-50keV)optimallysuitedforX-raytimingstudiedthanksto
the8.5m2 areapeakingat 8keVandtheWideFieldMonitor(WFM)witha 4srFoVaimedattriggering
∼ ∼
Page3of10
LOFT asadiscoverymachineforjettedTidalDisruptionEvents
follow-upobservationswiththeLADforavarietyofsources(bothgalacticandextra-galactic). Weconsider
bothandgivepossiblediscoveryandidentificationstrategies. Ourconclusionscanbesummarizedasfollows:
theWideFieldMonitorcanallowtoserendipitouslydetectnon-thermalTDEs,atahigherrates(between
•
oneandafewtensperyear)thananycurrentlyplannedsurveymissions.
theLargeAreaDetectorcaneffectivelyfollowupandidentifyradiotriggeredcandidates. Insynergy
•
withSKAinsurveymode,LADcaninprinciplesuccessfullyrepointeachradiocandidate,andmeasure
itslightcurvedecayindexandspectralproperties. LAD,togetherwithAthena, willbethebestX-ray
followupinstrumentintheSKAera. Theexpectedrateisbetweenafewtoafewhundredsperyr.
LOFT isauniquemissionforjettedTDEs,becauseitcancombinethebenefitsofaWFMandafollow-up
•
instrumentwithawideenergyrange(upto50keV).Theattractiveprospectisthatofbuildingforthe
firsttimeasampleofwellstudiedTDEsassociatedwithnon-thermalemissionfromjets. Thiswillallow
ustostudydisc-jetformationandtheirconnection,inacomplementarywaywithrespecttoothermore
persistentsourcessuchasactivegalacticnucleiandX-raybinaries. Inaddition,jettedTDEscanuniquely
probethepresenceofsupermassiveblackholeinquiescentgalaxieswellbeyondthelocaluniverse.
Throughout this paper we use the following cosmological parameters: Ω = 0.25, Ω = 0.75 and H =
M λ 0
70km/s/Mpc.
2 X ray emission of jetted TDEs
−
In2011, BATdiscoveredwhatwasfirstthoughttobeapeculiargamma-rayburst, nowknownasSwJ1644.
Whatfirstcaughttheattentionwasitsflaringactivity,whichcausedmultipleBATtriggers,followedbyanX-ray
lightcurvethatextendedtomuchlongertimescalesthananyknownGRB(Levanetal.,2011). Thesourcewas
confirmedtobeofextra-galacticorigin,withapeakluminosity 100timeshigherthanthatofbrightactive
∼
galacticnuclei(Burrowsetal.,2011;Levanetal.,2011). Theobserved0.3-10keVlightcurveofSwJ1644is
showninFig.1(bluesquares). Activitywasalreadydetected 3daysbeforetheBAT“official”triggerandthe
≈
startofXRTobservations(Burrowsetal.,2011). Consideringthisdelay,theX-raylightcurveasafunctionof
time∆tsincethetrigger(∆t = 0)maybedescribedby
τ+∆t −5/3
Lx,iso(∆t) 1.5 1048 ergs−1 , (1)
≈ × τ
!
(Fig.1, solid line). L is the isotropic equivalent luminosity, computed from the X-ray flux. The X-ray
x,iso
luminosity can be related to the jet luminosity assuming a constant radiation efficiency in X-rays (cid:15) , L =
x j
L (1 cosθ)/((cid:15) δ2),whereθ isthejetopeningangleandδtheDopplerfactor. Theradiationefficiencyof
x,iso j x j
−
J1644in1-10keVband(i.e. thefractionofthetotalluminosityemittedinthatband)isinferredfromBAT,XRT
andFermispectralinformationtobe(cid:15) 0.2(Burrowsetal.,2011). WenotethattheX-rayspectrumofSw
x
≈
1644remainedhard(α 1.4 1.7)overthedurationoftheeventwiththeexceptionoftheshortdipsinthe
∼ −
initialvariablephase(Saxtonetal.,2012).
Inthefollowing,weassumethatthelightcurveofJ1644istheTDEprototypicalX-raylightcurveinthe1-10
keVband,foreventswhereweobservenon-thermalandrelativisticallyboostedemission,associatedwithjets.
Then,ifthejetluminositytracesthemassaccretionrate,L M˙,itispossibletorescaleeq.1foreventswith
j
∝
differentdelayoftriggertime,redshift,supermassiveblackhole(SMBH)andstellarmass(seeDonnarumma&
Rossi,2014,fordetails).
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LOFT asadiscoverymachineforjettedTidalDisruptionEvents
Figure1: TheX-ray(0.3-10keV)lightcurveofJ1644asafunctionoftimefromtheX-raytrigger: data(absorbedflux,
bluesquarestakenfrompubliclyavailableXRTlightcurvesathttp://www.swift.ac.uk/xrt_curves/)versusourmodeling
(dot-dashedline). Thedecaycanbefittedby(t τ) 5/3,withτ=3day.
−
−
3 Monte Carlo calculations for rate predictions
WeconstructapopulationofTDEs,witheacheventcharacterizedbyatriggertimeτ,aredshift,aSMBHmass
M and a mass of the star m that is disrupted. The trigger time is chosen randomly from a one year interval.
∗
ThemassdistributionofSMBHsasafunctionofredshiftistakenfromShankaretal.(2013). Inparticular,we
considerthetwoaccretionmodelsthatyieldthelargestandthelowestmassfunctionpredictions,toquantify
theuncertaintyonourexpectedTDErates,duetotheimperfectknowledgeoftheblackholemassdistribution.
Finally,weassumeapopulationofmainsequencestarsingalacticnucleithatfollowsaKroupaInitialMass
Function,(Kroupa,2001).
Thisintrinsicdistributionisthenrescaledby2πΓ 2/(4π) = 1/(2Γ2) 0.125,i.e. thefractionofsolidangle
−
≈
subtendedbytheemission,fromatwosidedjet. Wearenowinthepositiontomakepredictionsforspecific
instruments,onceanobservingstrategyisspecified. Inthefollowingtwosections,wefocusontheinstruments
onboardofLOFT.Wethencomparetheirperformanceswiththoseofotherplannedmissionsinsec. 6.
4 Prospect for direct detection with LOFT Wide Field Monitor
TheWideFieldMonitor(WFM)ofLOFT (Ferocietal.,2012)willsurvey1/3rd oftheskyineachpointing,
reachinga5-σsensitivityof 8 9 10 11 ergcm 2s 1perdayintheenergyrange2-50keV.Thecombination
− − −
∼ − ×
ofitsskycoverageandsensitivitymakestheWFMagoodhunterforserendipitouslydetectingTDEs. Tobe
abletodetectandidentifyaTDE,weadoptadailyfluxthresholdthatcorrespondsroughlytoa25-σdetection,
allowingustomeasureitstemporaldecayingindex. Wethereforestartfromthe1day5-σfluxlimitin2 50keV
−
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LOFT asadiscoverymachineforjettedTidalDisruptionEvents
Figure2: Leftpanel: percentageof
eventratesasafunctionofBHmass.
Rightpanel:eventratesfortheWFM
onboardofLOFT asafunctionof
redshift. In both panels, two dif-
ferent black hole distribution func-
tions are shown (blue solid line: G
model; blue dot-dashed line: Gz
fromShankaretal.(2013)).
bandandthenweextrapolateitbackwardusingtheexpectedt 5/3 decay. Wethentakethefluxcorrespondingto
−
4pointsoverthisperiodandcomputetheiraverage. Thisaveragefluxdefinesouridentificationfluxthreshold.
Toclaimidentification,eacheventinoursimulationsneedstohaveafluxaveragedoveronedayabovethisflux
limit.
The results are shown in Fig.2 and Table 1. As showed in Fig. 2 (left panel), the TDE rate distribution
steadilyincreasesgoingtolowerBHmassesfromafewpercentatM 108M uptoamaximumof 20%
BH
atM 106 M . TheratedistributiondroptoafewpercentsatM ∼108M(cid:12) isanaturalconseque∼nceof
BH BH
∼ (cid:12) ∼ (cid:12)
the BH mass dependence in Eq. 1. In total, we estimate tens of events per year up to z 0.6, where we
max
≈
definez astheredshiftwheretheratedistributionreachesavalueof0.5yr 1. Thepeakrateis 6yr 1 at
max − −
∼
z = 0.2 0.3(seeFigure2,rightpanel). WenotethatthereportedvaluesareforabeamingfactorofΓ = 2,as
−
measuredfromradioobservations. WhencomparingourresultswithBAT/Swiftobservations,however,one
realizesthatasuppressionfactorfortheseratesmaybeneeded(see Donnarumma&Rossi,2014,fordetails).
ThequantificationofthisfactorischallengingbecauseBATdoesnotoperateinsurveymodeandthereareover
500 trigger criteria, that make our use of a flux limit survey a rather simplified approach. A somewhat hard
lowerlimitestimatesuggestsasuppressionfactoroftwoordersofmagnitude. Thiscanbeduetoacombination
ofalargerbeamingfactorandajetproductionefficiency< 1. Forexample,abeamingfactorofΓ = 20inthe
X-rayemissionregionwouldaccountfortheBATrateandbeconsistentwithhighenergyobservation(Burrows
etal.,2011). ThiswouldimplyphysicallydifferentradioandX-rayemittingregions. Thereforeinthemost
pessimisticcase,theWFMwouldobserveTDEsatarateof 0.3 0.8yr 1,similartothatofBAT.Foralower
−
≈ −
limitontheratesforallX-rayinstrumentsmentionedinthefollowing,thesamefactorshouldbeapplied. Of
course,thisdoesnotechangethediscussionontherelativeperformanceofthemissions.
5 LAD as excellent follow-up instrument of SKA triggers
The LAD on board of LOFT (2-50 keV) is a collimated instrument with 1 degree field of view, and a wide
energyrange(2-50keV).Itsbackgroundlimited10 σsensitivityis 3 10 12 ergcm 2 s 1 inthe2-10keV
− − −
− ≥ ×
band,fora1000sexposure(seeFig.3).
GiventhesmallerfieldofviewandthehighersensitivitywithrespecttotheWFM,LADismoreeffective
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LOFT asadiscoverymachineforjettedTidalDisruptionEvents
Figure3: LADsensitivitylimitasafunction
ofexposuretimefordifferentS/Nratioscor-
responding to a systematics of 0.25% on the
backgroundknowledge. Frombottomtotop:
S/N 3,5,10,150,250.
∼
asafollow-upinstrument. JettedTDEshaveastrongradiocounterpartandtheSquareKilometerArray(SKA
Dewdneyetal.,2013)insurveymodehasthepotentialtodetectafewhundredeventsperyearVanVelzenetal.
(2011);Donnarumma&Rossi(2014). Wethereforeinvestigateapossiblestrategytoefficientlyfollowing-up
SKAtriggeredcandidateeventsandthenassesstheactualrateoftheirX-raycounterparts. SincetheradioTDE
emissionisquitefeaturelessandtheTDEsignature(i.e. thet 5/3 decay)seemstobeaprerogativeofitsX-ray
−
emission,theX-rayfollow-upofradiocandidatesisessentialfortheiridentification,togetherwithaprecise
radiolocalisation. TheroleofLOFT inthisrespectbecomesfundamentalbecauseSwift,ChandraandXMM
areunlikelytobeoperatingbythetimeSKAinsurveymodewillbedetectingcandidates.
Weconsidera1-daydelayintheX-rayrepointing. SimilarlytothestrategyadoptedfortheWFM,werequire
atleast4pointingsspreadover4days,eachwithaS/Nratioofatleast10. Eachofthemrequiresanexposure
timelessthan1000s. ThishighS/Nisrequiredinordertocharacterizeboththetemporalandspectralbehavior
ofthesource. Followingthesameprocedureexplainedintheprevioussection,wedefineanidentificationflux
thresholdof 10 11 ergcm 2 s 1 inthe2-10keVband,thatisthentranslatedinthecorrespondingun-absorbed
− − −
≥
valueinthe1-10keVband,theenergyrangeadoptedinourmodelling.
ForeacheventintheMonteCarlosimulations,wecalculatetheaverageX-rayfluxoverthe4pointingsand
compareitwiththeidentificationfluxthreshold. Finally,ourdetectionratesarerescaledaccordingtothefact
thattheLOFT repointingvisibilityallowsaskyaccessibilityof 75%(Ferocietal.,2014).
∼
Figure4showstheexpectedrateofjettedTDEsasafunctionofBHmass(leftpanel)andofredshift(right
panel). AsregardstheirBHmassdistribution, 25%ofalleventshaveBHswithmasses 106 M ,andevents
withBHmasses< 107 M dominatetheredsh∼iftdistributionatallepochs. ≈ (cid:12)
(cid:12)
Thistime,theredshiftdistributionextendsabovez = 1(z 1.2)(seeTable1,bottomsection),withmost
max
≈
of the TDEs expected around z w 0.4 (right panels). The peak rates are roughly between 4 and 20 events
≈
peryear. Intotal,LADshouldbeabletodetectroughlyhalfoftheSKAtriggeredTDEs,i.e. between 130
≈
and 350 yr 1. For these events, the instrument broad energy band (2 50 keV) should enable us to better
−
’ −
characterizetheenergybudgetoftheX-raycomponent.
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LOFT asadiscoverymachineforjettedTidalDisruptionEvents
Figure4: The same as
Fig.2,butfortheLAD
instrument. These
events have been first
detected by SKA in
wide radio surveys
at 1.4 GHz. For
details on the radio
lightcurve modelling
see Donnarumma &
Rossi (2014), and in
particular their model
"MLD".
6 Discussion and conclusions
Although the LAD instrument would be roughly 5 times more effective in identifying TDEs, the WFM has
theabilitytoserendipitouslycatchevents,closertotheironset. ThisisshowninFig.5,wherethecumulative
distributionofthetriggerdelayisshownasafunctionofredshift(fromz=0.1upto0.6,thatisthez ofWFM
max
TDEs). WhiletheWFMwillbeabletodetectTDEsatanyepoch,theLADwillpreferentiallycatchthemat
latertimes,after 10daysfromtheirbeginning. ThislatterdelayisnotduetoanintrinsicproblemoftheLAD
≈
instrument,butitisanaturalconsequenceofthedelayintroducedbytheSKAtrigger: theradiofluxincreases
overtime,andbecomesbrightenoughforaSKAdetectiononlyafteratleast 10daysfromthebeginningofthe
≈
event(Donnarumma&Rossi,2014). Anotherdifferenceintheperformanceofthetwoinstrumentsistheability
ofLADtogotwiceasfarinredshift(z 1.2)thanWFM(z 0.6). Synergybetweenthetwoinstrumentscan
∼ ∼
alsobeenenvisaged. TheLADinstrumentcanrepointwithin< 1daycandidatesdetectedwithWFM,allowing
thestudyoftheeventlightcurve,alsoatearlytimes(unlikeSKAtriggeredTDEs).
ThepotentialoftheWFMforjettedTDEdiscoveriescanbeappreciatedbettercomparingitsperformance
witheRosita. Currently,thissurveyrapresentsthebestchancetoserendipitouslydiscoverTDEsinX-ray,in
thenearfuture(Merlonietal.,2012). Indeed,thecombinationofagreatsensitivityin0.2-12keVbandand
thepointingstrategy(4-yearallskysurvey)makesittheoptimalhunterofthermalTDEs,with 1000objects
∼
perscanatasensitivitylevelseveraltimesbetterthanRASS.However,itwillbelessperformingatdetecting
non-thermalTDEs. Followingtheidentificationstrategydetailedabove,eRositawillbeabletodetectafactor3
to4lessjettedTDEsthantheWFMinthehardX-rayband(seealsoKhabibullinetal.,2014). Thisisbecause
eRositawillnotcoveraneventrepeatedly,butonlyevery6-month.
LADcaninsteadbecomparedwiththeWideFieldImager(WFI)onboardofAthena,theESAL2mission,
planned to operate in 2028. We stress that radio counterparts need an X-ray follow-up to be confirmed and
LOFT,togetherwithAthena,islikelytobetheonlyeffectiveinstrumentoperatingsimultaneouslywithSKA.If
weusetheverysamefollow-upstrategyandweconsideranoptimisticrepointingefficiencyof50%,thenAthena
maybeabletoidentifyslightlymoreTDEs(afactor1.1-1.2). Thereasonofthissmalldifferenceinidentification
Page8of10
LOFT asadiscoverymachineforjettedTidalDisruptionEvents
Figure5: CumulativedistributionsofdelaysindetectingtheTDEfromtheexplosiontime,fordifferentredshifts. Weshow
resultsfortheWFM(blacklines)andLADfollow-upsofradiotriggeredTDEs(bluelines). Thedifferentlinestylesarefor
z=0.1,0.2,0.37,0.6fromrighttoleftforWFMandviceversaforLAD.NotethattheLADinstrumentcanrepointevents
onlyafter10days,becausetheradiolightcurvebecomessufficientlybrighttobedetectedbySKAonlyafterthattime.
ratesisthatonlyamoderateX-raysensitivity( > 5 10 12 erg cm 2 s 1)isrequiredtoobtainacompleteset
− − −
∼ ×
ofX-raycounterpartsoftheradiotriggeredTDEs. However,theLADwiderenergyrangehastheadvantage
ofbetterconstrainingthetotalemittedluminosityandthusthejetenergycontent. Mostimportantly,Athena
willnothaveawidefieldmonitorinstrumentonboardandwithoutaradiotriggerwillbeblindtoTDEs,while
LOFT ontheotherhandcandoTDEscienceautonomously. Thus,inthisrespect,LOFT isamorecomplete
mission,combiningthepotentialforserendipitousdiscoverieswiththeWFMandrepointingabilitywithLAD.
Insummary,LOFT hasthepotentialtofinallyopenaneweraofstatisticalstudiesofTDEsandexploitthem
toaddressopenquestionsingalaxyformationandhighenergyastrophysics. Withafewtenstoafewhundreds
ofeventsandtheirradiocounterparts,onemayconstraintheSMBHmassfunctionbelowredshift1.5andthe
physicsofjetformationduringtransientaccretionepisodes: e.g. jetefficiency,Lorentzfactor,andwhatisthe
connectionwithasuper-Eddingtonaccretionphase.
References
Burrows,D.N.etal.2011,Nature,476,421 333,203
Donnarumma,I.,&Rossi,E.M.2014,Ap.J.,accepted Bower,BowerG.C2011ApJ,732,L12
Kroupa,P.2001,Mon.Not.Roy.Astro.Soc.,322,231 Campana,S.2011,Nature,480,69
FerociM.,etal.2014,SPIE,9144,91442 CannizzoJ.K.,TrojaE.,&LodatoG.,2011,Ap.J.,742,32
Berger,E.,Zauderer,A.,Pooley,G.G.,Soderberg,A.M.,Sari,R., CarilliC.L.,RawlingsS.,2004,NewAstron.,48,979
Brunthaler,A.&Bietenholz,M.F.,2012Ap.J.,748,36 CenkoS.B.,etal.,2012,Ap.J.,753,77
Bloom,J.S.,Giannios,D.,Metzger,B.D.,etal.2011,Science, ChornockR.,etal.,2014,Ap.J.,780,44
Page9of10
LOFT asadiscoverymachineforjettedTidalDisruptionEvents
R1 R2 z R1 R2 z
peak peak peak max
yr 1 yr 1 yr 1 yr 1
− − − −
X-raysurveys
LOFT WFM 24.5 67 0.2-0.3 2.3 6 0.6
eRosita 8 15 0.4 0.15 0.5 0.4
Radioselectedsamples
LOFTLAD 135 352 0.4 8 22 1.2
Athena 163 385 0.4 7 20 1.4
Table1: FutureX-raysurveyspredictions: 1stand2ndcolumnsaretotalyearlyrate(thesubscripts1and2arefortheGz
andGMFs,respectively),3rdcolumnredshiftatthepeakrate,4thand5thcolumnsmaximumpeakrateand6thcolumn
maximumredshift,definedasthezwheretherateis0.5. X-rayexpectedratesarederivedforΓ=2.
Donley,J.L.,Brandt,W.N.,Eracleous,M.&Boller, 359
DewdneyP.E.,etal.,2013,SKA1SystemBaselineDesign.Tech. MerloniA.etal.,2012,ArXive-prints,arXiv1209.3114M
rep.,SKAProgramDevelopment,DocumentNumber: SKA- Metzger,B.D.,Giannios,D.&Mimica,P.,2012,Mon.Not.Roy.
TEL-SKO-DD-001 Astro.Soc..420,3528
FerociM.,etal.2012,ExperimentalAstronomy,34,415 NarayanR.,etal.,2003,PASJ,55,L69
GezariS. , HeckmanT. , S. B. Cenko, M. Eracleous, K. Forster, Phinney,E.S.Nature,1989,340,595
T.S.Gonçalves,D.C.Martin,P.Morrissey,S.G.Neff,M.Seib- Rees,M.J.,1988,Nature,333,523
ert,D.Schiminovich,&T.K.Wyder,2009,Ap.J.698,1367 Reis,etal.,2012,Science,337,949
Gezari,S.etal.Nature,485:217 Rossi, E. M. & Begelman, M. C. 2009 Mon. Not. Roy. Astro.
GranotJ.&SariR.,2002,Ap.J.,568,820 Soc.,392,1451
Grupe,D.,Thomas,H.-C.,&Leighly,K.M.1999,Astron.Astro- Sa¸dowskiA.,NarayanR.,McKinneyJ.,TchekhovskoyA.,2014,
phys.,350,L31 Mon.Not.Roy.Astro.Soc.,439,503
Guillochon,J.&Ramirez-Ruiz,E.2013,Ap.J.,767,25 Sari,R,Kobayashi,S.,&Rossi,E.M.2010,Ap.J.,708,605
Hayasaki,K.andStone,N.andLoeb,A.2013,Mon.Not.Roy. SaxtonC.J.,SoriaR.,WuK.,KuinN.P.M.,2012,Mon.Not.Roy.
Astro.Soc.434,909 Astro.Soc.,422,1625
HodgeJ.A.,BeckerR.H.,WhiteR.L.,RichardsG.T.,2013,Ap. ShankarF.,WeinbergD.H.,Miralda-EscudéJ.,2013,Mon.Not.
J.,769,125 Roy.Astro.Soc.,428,421
Kesden,M.,2012,PhRvD,85,4037 Stone,N.andSari,R.andLoeb,A.2013Mon.Not.Roy.Astro.
Khabibullin I., Sazonov S., Sunyaev R., 2014, Mon. Not. Roy. Soc.,435,1809
Astro.Soc.,437,327 StrubbeL.E.&Quataert,E.2009,Mon.Not.Roy.Astro.Soc.,
Komossa,S.,2002,ReviewsinModernAstronomy,15,27 400,2070
Komossa,S.,etal.2008,ApJ,678,L13 Tchekhovskoy,A.,Metzger,B.D.,Giannios,D.&Kelley,L.Z.,
Komossa,S.,&Bade,N.1999,Astron.Astrophys.,343,775 2013,Mon.Not.Roy.Astro.Soc.,accepted.
KrimmH.A.etal.,2011,AstronomerTelegrams,3384 vanVelzenS.,etal.,2011,Ap.J.,741,73
KrimmH.A.etal.,2013,Ap.J.Suppl.,209,14 vanVelzenS.,FrailD.,KördingE.,FalckeH.,2011,Mon.Not.
Levan,A.Jetal.2011,Science,333,199 Roy.Astro.Soc.,417,51
Levan,A.2012,InEuropeanPhysicalJournalWebofConferences, vanVelzenS.,FrailD.,KördingE.,FalckeH.,2013,Astron.As-
39,2005 trophys.,552,A5
Lien A., Sakamoto T., Gehrels N., Palmer D., Barthelmy S., Wang,T.-G.,Zhou,H.-Y.,Komossa,S.,etal.2012,Ap.J.,749,
GrazianiC.,CannizzoJ.,2014,Ap.J.,783,24 115
LiuF.,LiS.,ChenX.,2009,ApJ706,L133 Zauderer,B.A.,Berger,E.,Margutti,R.,Pooley,G.G.,Sari,R.,
LiuF.,LiS.,KomossaS.,2014,ApJ786,103 Soderberg,A.M.,Brunthaler,A.&Bietenholz,M.F.,2013,Ap.
LodatoG.,KingA.R.,&PringleJ.E.,2009,Mon.Not.Roy.Astro. J.,767,152
Soc.,392,332 Zauderer,B.A.,etal.,2011,Nature,476,425
LodatoG.&Rossi,E.M.,2011,Mon.Not.Roy.Astro.Soc.,410,
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