X-ray flare in XRF 050406: evidence for prolonged engine activity

0.3 MB·English

by  P. Romano

Astronomy&Astrophysicsmanuscriptno.romano050406d February5,2008
(DOI:willbeinsertedbyhandlater)
X-ray flare in XRF 050406: evidence for prolonged engine activity
P.Romano1,A.Moretti1,P.L.Banat1,D.N.Burrows2,S.Campana1,G.Chincarini1,S.Covino1,D.Malesani3,
G.Tagliaferri1,S.Kobayashi2,4,B.Zhang5,A.D.Falcone2,L.Angelini6,7,S.Barthelmy6,A.P.Beardmore8,
M.Capalbi9,G.Cusumano10,P.Giommi11,M.R.Goad8,O.Godet8,D.Grupe2,J.E.Hill6,12,J.A.Kennea2,
V.LaParola10,V.Mangano10,P.Me´sza´ros2,D.C.Morris2,J.A.Nousek2,P.T.O’Brien8,J.P.Osborne8,
A.Parsons6,M.Perri9,C.Pagani2,1,K.L.Page8,A.A.Wells8,N.Gehrels6
6
0
0 1 INAF–Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy e-mail:
2 [email protected]
2 DepartmentofAstronomy&Astrophysics,PennsylvaniaStateUniversity,525DaveyLab,UniversityPark,PA16802,USA
n
a 3 InternationalSchoolforAdvancedStudies(SISSA-ISAS),ViaBeirut2-4,I-34014Trieste,Italy
J 4 CenterforGravitationalWavePhysics,PennsylvaniaStateUniversity,104DaveyLab,UniversityPark,PA16802,USA
1 5 DepartmentofPhysics,UniversityofNevada,LasVegas,NV89154-4002,USA
1 6 NASA/GoddardSpaceFlightCenter,Greenbelt,MD20771,USA
7 Department of Physicsand Astronomy, TheJohns Hopkins University,3400 NorthCharlesStreet,BaltimoreMD 21218

2 USA
v 8 X-RayandObservationalAstronomyGroup,DepartmentofPhysics&Astronomy,UniversityofLeicester,LE17RH,UK
3 9 ASIScienceDataCenter,viaG.Galilei,I-00044Frascati(Roma),Italy
7 10 INAF–IstitutodiAstrofisicaSpazialeeFisicaCosmicaSezionediPalermo,ViaU.LaMalfa153,I-90146Palermo,Italy
1
11 AgenziaSpazialeItaliana,Unita`Osservazionedell’Universo,VialeLiegi26,I-00198Roma,Italy
1
12 UniversitiesSpaceResearchAssociation,10211WincopinCircle,Suite500,Columbia,MD,21044-3432,USA
0
6
0 Received:2005September7;accepted2006January3
/
h
Abstract. WepresentobservationsofXRF050406,thefirstburstdetectedbySwiftshowingaflareinitsX-raylightcurve.
p
- Duringthisflare,whichpeaksattpeak ∼210saftertheBATtrigger,afluxvariationofδF/F ∼6inaveryshorttimeδt/tpeak ≪1
o wasobserved.Itsmeasuredfluenceinthe0.2–10keVbandwas∼ 1.4×10−8 ergcm−2,whichcorrespondsto1–15%ofthe
r promptfluence.Wepresentindicationsofspectralvariationsduringtheflare.Wearguethattheproducingmechanismislate
t
s internalshocks,whichimpliesthatthecentralengineisstillactiveat210s,thoughwithareducedpowerwithrespecttothe
a
promptemission.TheX-raylightcurveflattenstoaveryshallowslopewithdecayindexof∼ 0.5after∼ 4400s,whichalso
:
v supportscontinuedcentralengineactivityatlatetimes.ThisburstisclassifiedasanX-rayflash,witharelativelylowfluence
i (∼10−7ergcm−2inthe15–350keVband,E ∼1051erg),asoftspectrum(photonindex2.65),nosignificantfluxabove∼50
X iso
keVandapeakenergyE < 15keV.XRF050406isoneofthefirstexamplesofawell-studiedX-raylightcurveofanXRF.
p
r
WeshowthatthemainafterglowcharacteristicsarequalitativelysimilartothoseofnormalGRBs.Inparticular,X-rayflares
a
superimposedonapower-lawlightcurvehavenowbeenseeninbothXRFsandGRBs.Thisindicatesthatasimilarmechanism
maybeatworkforbothkindsofevents.
Keywords.Gammarays:bursts;X-rays:bursts;X-rays:individuals(XRF050406)
1. Introduction fieldinstruments.TheXRT,whichoperatesinthe0.2–10keV
energy range, can provide ∼ 5′′ positions, while the UVOT,
The Swift Gamma-Ray Burst Explorer (Gehrelsetal. 2004)
whichoperatesinthe1700–6000Åwavelengthrange,canfur-
was successfully launched on 2004 Nov 20. Its payload in- therrefinetheafterglowlocalizationto∼ 0′.′5.Withitsunique
cludes one wide-field instrument, the gamma-ray (15–350
fastre-pointingcapabilitiesSwiftsetouttoinvestigatethevery
keV) Burst Alert Telescope (BAT; Barthelmyetal. 2005a),
earlyphasesofgamma-rayburst(GRB)afterglows,beginning
andtwonarrow-fieldinstruments,theX-RayTelescope(XRT;
asearlyasoneminuteaftertheBATtrigger.Duringtheinitial
Burrowsetal. 2005a) and the Ultraviolet/Optical Telescope
activationandcalibrationphases,whichendedon2005Apr5,
(UVOT; Romingetal. 2005). BAT detects the bursts, calcu-
Swiftdiscovered25GRBs.Thenarrow-fieldinstrumentswere
lates their position to ∼ 1–4′ accuracy and triggers an au-
re-pointed towards seven of them within a few hundred sec-
tonomous slew of the observatory to point the two narrow-
onds,andsuchisthecaseforGRB050406.
Sendoffprintrequeststo:P.Romano
2 P.Romano:X-rayflareinXRF050406
On2005Apr6at15:58:48.40UT,BATtriggeredonGRB rameter (i.e., ∆χ2 = 2.71) unless otherwise stated. Times are
050406(trigger113872;Parsonsetal.2005),andlocateditat referred to the BAT trigger T , t = T − T . The decay and
0 0
RA(J2000) = 02h17m53s,Dec(J2000) = −50◦10′52′′,withan spectralindicesareparameterizedasfollows,F(ν,t) ∝ t−αν−β,
uncertaintyof3arcmin(95%containment;Krimmetal.2005). where F (ergcm−2 s−1 Hz−1)is themonochromaticfluxasa
ν
Thederivedvalueforthetime duringwhich90%oftheburst functionoftimetandfrequencyν;wealsouseΓ=β+1asthe
fluenceisobservedwasT =5±1sinthe15–350keVband.In photonindex,N(E)∝ E−Γ(phkeV−1cm−2s−1).
90
the15–25keVbandthelightcurvepeakhadafast-rise,expo-
nentialdecay(FRED)profile,whileinthe25–50keVband,the
2. Observationsanddatareduction
shapewasmoresymmetric,withthepeakstarting∼2searlier
(Krimmetal.2005).Boththepeakandtime-averagedspectra 2.1.BATobservations
werewellfitbyasimplepower-lawwithatime-averagedspec-
trumphotonindexof2.38±0.34(90%confidence;Krimmetal. Table 1 reports the log of the observationsthat were used for
2005).Thefluenceinthe15–350keVbandwas9.0×10−8erg thiswork.TheBATdatawereanalyzedusingthestandardBAT
cm−2.Thegamma-raycharacteristicsofthisburst,i.e.thesoft- analysissoftwaredistributedwithinFTOOLSv6.0.Theburstis
ness of the observed spectrum and the absence of significant detectedinthefirsttwostandardbands(15–25and25–50keV)
emission above ∼ 50 keV, classify GRB 050406 as an X-ray whilevirtuallynofluxisobservedabove50keV.WefindT90 =
flash (XRF; Heiseetal. 2001). From now on, we shall there- 6.1±1.0sinthe15–150keVband.
forerefertothiseventasXRF050406. TheBATspectrawereextractedoverthefulltimeinterval
Swift executed a prompt slew. The XRT imaged the BAT overwhichtheburstwasdetected(Ttot),intheintervalcovering
field only 84s after the trigger but no bright X-ray source the1-speakTpeak,andfortheT90 andT50 intervals.Response
couldbe detectedwithinthefield ofview.However,a refined matrices were generated with the task batdrmgen using the
on-groundanalysis revealed a previously uncataloguedX-ray latest spectral redistribution matrices. For our spectral fitting
source(Cusumanoetal.2005a;Capalbietal.2005).Fromthe (XSPECv11.3.2)weconsideredthe15–150keVenergyrange.
veryfirstexaminationofthedown-linkeddataitwasclearthat Allspectraarewellfitwithasimplepowerlawwithphotonin-
theafterglowofthisburstwaspeculiar.Indeed,afteraninitial dexΓγ ∼2.65(seedetailsinTable2).Thereisnoevidenceofa
decay, the X-ray count rate began rising, peaking at ≈ 220s, spectralbreakwithintheBATenergyrange,thusconstraining
andsubsequentlydecayingagain(Capalbietal.2005). the peakenergy Ep < 15keV.The indicesare steeper (softer)
Ground-basedobservationsstartedassoonastheburstdis- although consistent with the ones reported by Krimmetal.
coverywasreportedviatheGCNnetwork.TheMagellan/Clay (2005), due to the different energy ranges used for the spec-
telescope imaged the XRT error circle with LDSS-3 in the tralfitting.Nosignificantimprovementsarefoundusingeither
R and i bands and found a single faint source (R = 22.0 ± acutoffpower-laworaBandmodel(Bandetal.1993).The1-s
0.09 mag, 7.8 hr after the burst) located at RA(J2000) = peakphotonfluxwas(2.3+2.8)×10−8ergcm−2s−1(15–350keV
−0.4
02h17m52s.3, Dec(J2000) = −50◦11′15′′ with an uncertainty band), while the fluence was F = (1.0+1.13)×10−7 erg cm−2
−0.36
of ∼ 0′.′5 in each coordinate(Bergeretal. 2005a,b). Similarly (15–350keV band).Thisfluence correspondsto an isotropic-
to XRT, UVOT also imaged the field at the end of the slew equivalentenergyEiso = (1.4+−10..66)×1051 erg(intherestframe
(starting from∼ 88 s after the trigger)and thoughit failed to 52–1204keV)assumingz=2.44±0.36(Schadyetal.2006).
detect the afterglow on-board (Landsmanetal. 2005), subse-
quenton-groundanalysisrevealedasourcewithintheXRTer-
2.2.XRTobservations
rorcircleatthe4.3-(19.0mag),3.0-and2.5-σdetectionlev-
elsintheU, BandV bands,respectively.TheUVOTposition In order to cover the dynamic range and rapid variability ex-
was RA(J2000) = 02h17m52s.2, Dec(J2000) = −50◦11′15′.′8, pectedfromGRBafterglowsandtoproviderapid-response,au-
consistent with the Magellan one. By the time the second tomated observations, XRT was designed to support different
UVOT observation (1.3 hr later) was performed, the source readout modes that optimize the collected information as the
wasnotdetectedintheU band,confirmingitastheafterglow fluxoftheburstdiminishes.TheXRTsupportsfourmajorread-
of XRF 050406.Schadyetal. (2006) obtained an estimate of out modes, one imaging (IM), two timing, Piled-up/Low-rate
z = 2.44± 0.36 from fitting the broad band spectrum (com- Photodiode (PuPD and LrPD) and Windowed Timing (WT),
binedUVOTandXRTdata). andonePhoton-Counting(PC).AdetaileddescriptionofXRT
InthispaperwepresentobservationsofthefirstSwiftburst modescanbefoundinHilletal.(2004).Inthenominaloper-
whereaflareisclearlydetectedinitsX-raylightcurve,during atingstatethemodeswitchingisbasedonthesourcefluxand
whichthesourcecountrateincreasedbyafactorof>∼ 6.This isfullyautomated(autostate)tominimizepile-upinthedata.
featurehadneverbeenobservedbeforeinSwiftdata,andhad TheXRTobservationsofXRF050406startedon2005Apr
rarelybeenobservedbeforeinanyX-rayafterglow(Piroetal. 6at16:00:12UT,only84safterthetrigger,andendedon2005
2005). This paper is organized as follows. In Sect. 2 we de- Apr22,thussummingupatotalnetexposure(inPCmode)of
scribe our observations and data reduction; in Sect. 3 we de- ∼ 163 ks spread over a ∼16d baseline. The monitoringis or-
scribe our spatial, timing and spectral data analysis; in Sect. ganizedin 9 observations(000,001,002,005,006,008,009,
4 we discuss our findings. Finally, in Sect. 5 we summarize 010, 011)and 183 snapshots(continuouspointingsat the tar-
our conclusions. Throughoutthis paper the quoted uncertain- get). This was the first burst to occur after the formal end of
ties are given at 90% confidence level for one interesting pa- the calibration phase (2005 Apr 5), and the first (000) obser-
P.Romano:X-rayflareinXRF050406 3
sources of background are the main reason for the switching
betweenPCandWTmodeevenwhenthesourcecountrateis
below1countss−1.
Throughout the monitoring campaign the CCD tempera-
turewas< −50◦C,withtheexceptionofpartofobservations
002 and 005, where it became as high as −43.5 and −45 ◦C,
respectively; those data were therefore screened out. For our
analysiswefurtherselectedXRTgrades0–12and0–2forPC
and WT data, respectively (according to Swift nomenclature;
Burrowsetal.2005a).
3. Dataanalysis
3.1.Spatialanalysis
Figure1showsthe163ksXRTimageaccumulatedinPCmode
in the 0.2–10 keV energy band. We detected two previously
uncataloguedsourceswithin 1 arcmin of the opticalburst co-
ordinates.The brightestuncataloguedsource, which we iden-
tified as the fading X-ray counterpart of the burst, is present
Fig.1. XRT image of XRF 050406, obtained from the to- inthefirstfourXRT snapshots.Thesourceispiled-upduring
tal ∼163ks PC mode data. The field is centred on the 3′ the initial 500s of PC data. Therefore, to obtain an unbiased
radius BAT error circle. Also shown is the XRT 4′.′2 er- position, we rely on the remainder of the PC data in the first
ror circle, as well as the Magellan (Bergeretal. 2005a) and observation,whichhasanetexposureof49.8ks.We usedthe
UVOT (Roletal. 2005) optical counterpartpositions; the op- xrtcentroidtask(v0.2.7)andfoundthattheafterglowposi-
tical points are so close they cannot be distinguished on this tion isRA(J2000) = 2h17m52s.4,Dec(J2000) = −50◦11′13′.′6.
scale. S2 is a serendipitous source located at RA(J2000) = We estimate its uncertaintyto be 4′.′2 (90%confidencelevel).
02h17m52.s9,Dec(J2000)=−50◦10′36′.′1. This position takes into account the correction for the mis-
alignment between the telescope and the satellite optical axis
(Morettietal. 2005). Figure 1 shows the XRT error circle, as
vation was performed as an automated target (AT) with XRT well as the 3′ BAT error circle (Krimmetal. 2005; 95%con-
inautostate.Therefore,duringobservation000theautomated tainment) and the optical counterpart coordinates determined
mode switching made XRT take an initial 2.5s image (IM at by Magellan (Bergeretal. 2005a) and by UVOT (Roletal.
t = 84s), immediately followed by one PuPD (t = 90s) and 2005). TheXRT coordinatesare23′′ fromthe BAT ones,and
oneLrPD(t = 91s) frame.Thenatt = 92saseriesof5 WT 1′.′6and2′.′8fromtheMagellanandUVOTones,respectively.
frameswas taken until the on-boardmeasured countrate was XRF050406wasdetected(XIMAGEv4.3)inthefirstfoursnap-
low enough for XRT to switch to PC mode (t = 99s). After shotsindividually,butnotfromthe5thon.Thesecondsource,
this,XRTrepeatedlyswitchedbetweenWTandPCmodesbe- S2, is located at RA(J2000) = 02h17m52s.9, Dec(J2000) =
causeofanincreasedbackgroundlevel(seebelow).Sincethe −50◦10′36′.′1andhasaconstantrate(3.8±0.7)×10−4counts
signal-to-noise (S/N) in these late WT frames is low, we did s−1throughouttheobservationcampaign.
notincludetheminouranalysis(Table1).
TheXRTdatawerefirstprocessedbytheSwiftDataCenter
3.2.Temporalanalysis
atNASA/GSFCintoLevel1products(calibratedandquality-
flaggedeventlists).Thentheywerefurtherprocessedwiththe During the first 500s of the XRT observation the intensity of
XRTDAS (v1.4.0) software package written by the Agenzia theafterglowwashighenoughtocausepile-upinthePCmode
Spaziale Italiana (ASI) Science Data Center and distributed data.To accountforthiseffectweextractedthe sourceevents
within FTOOLS v6.0 to produce the final cleaned eventlists. withinanannuluswitha30-pixelouterradius(∼71′′)anda2-
We ran the task xrtpipeline(v0.8.8)applying standard fil- pixelinnerradius.Thesevalueswerederivedbycomparingthe
teringandscreeningcriteria,i.e.,wecutouttemporalintervals observedandnominalPSF.ForthePCdatacollectedafterthe
during which the CCD temperature was higher than −47 ◦C, first500s,theentirecircularregion(30-pixelradius)wasused,
and we removed hot and flickering pixels. These are present instead.Inbothcaseswefurtherdisregardeddatawithinacir-
becausetheCCDisoperatingatatemperaturehigherthanthe cularregioncentredontheserendipitoussourceS2(whichlies
design temperature of −100 ◦C due to a failure in the active within the 30-pixel PC source extraction region) with a 7.17
coolingsystem.Anon-boardeventthresholdof∼0.2keV(un- pixel(17′′)radius.TheWTdatawereextractedinarectangu-
reconstructedpulse-heightPHAS[1]> 80)wasalsoappliedto lar region31 pixels longalong the image strip (and 20 pixels
thecentralpixel,whichhasbeenproventoreducemostofthe wide), which excludes the data from the source S2. The se-
backgroundduetoeitherthebrightEarthlimbortheCCDdark lectedextractionregionscorrespondto∼ 69%(piled-upPC),
current(which depends on the CCD temperature).These two ∼ 95%(nonpiled-upPC),and∼ 94%(WT)oftheXRTPSF.
4 P.Romano:X-rayflareinXRF050406
Fig.2.X-raylightcurveoftheXRF050406afterglowinthe0.2–10keVenergyband.Thecurveisbackground-subtractedand
the time is referredto the BAT trigger,2005Apr 06 at15:58:48.4UT (Parsonsetal. 2005). Thelast pointafter 106s is a 3-σ
upper limit. Inset: details of the first ∼ 1000s, which include data in all XRT modes. The (yellow) diamondsrepresentLrPD
modedatatakenduringthelatterportionoftheslewingphase;the(cyan)triangleistheinitialIMpoint(84safterthetrigger,
see Table 1), the downward-pointingarrow is a LrPD limit (pointing, 91s after the trigger), the (blue) circles are WT mode
data(startingfrom92safterthetrigger),andthe(red)squaresarePCmodedata(startingfrom99safterthetrigger).Thedata
havebeencorrectedforpile-up(whereappropriate)andPSFlosses.Thesolid(red)linerepresentsthebest-fitbrokenpower-law
modeltothelightcurve(excludingtheflare).
Toaccountforthebackground,datawerealsoextractedinPC PC mode data were corrected for the effects of pile-up. We
modewithinacircularregion(radius130′′ = 54.8pixels)and note that, by keeping to the minimum number of counts per
inWTmodewithinarectangularbox(40×20pixels),inloca- timebincriterion,wecreatedseveralbinsduringthefirstsnap-
tionsfar frombackgroundsources.ThemeanPC background shot,butsubsequentlyneededtomergedatabelongingtosnap-
in the 0.2–10 keV band was found to be constant throughout shots 1 and 2 (point at ∼ 4ks), then from snapshots 3 and 4
the observations and, normalized to the PC source extraction (point at ∼ 20ks), and later on from snapshots 5 through 8
region,ithadavalueof∼2.6×10−3countss−1.ThemeanWT (point at ∼ 35ks). Afterwards, we used XIMAGE with the op-
backgroundinthesameenergybandandnormalizedtotheWT tion SOSTA, which calculates vignetting- and PSF-corrected
sourceregionwas∼4.6×10−2countss−1. count rates within a specified box, and the background in a
user-specifiedregion.Toensureuniformitywiththeearlylight
Figure 2 shows the background-subtractedlight curve ex-
curve,thebackgroundwasestimatedinthesameregionasthe
tracted in the 0.2–10 keV energy band, with the BAT trigger
oneusedfortheinitialpartofthelightcurve.Wethusobtained
as origin of time. We considered WT data for the first snap-
a signal-to-noise ratio S/N >∼ 3 (the only exception being the
shot of the first observation, and PC data for all 9 available
point at ∼ 33 ks which has S/N >∼ 2). The last point is a 3-σ
observations(seeTable1).Duringtheinitialphasesoftheaf-
upperlimit.Thislattermethodispreferredfortheconstruction
terglowevolution(t < 4×104s) webinnedthesourcecounts
of the late part of the light curve since it better accounts for
with a minimum of 30 counts per time bin, and dynamically
thebackgroundinalow-countsregime.Wenote,however,that
subtractedthenormalizedbackgroundcountsineachbin.The
P.Romano:X-rayflareinXRF050406 5
extracting the light curve in the same 30-pixel source region
up to the end of the last observation, we obtained fully con-
sistent results, albeit with a noisier light curve. We also note
that the residual contribution of the serendipitous source S2
withinthesourceextractionregionis<∼ 19%oftheS2counts,
whichcorrespondsto <∼ (7±1)×10−5 countss−1. Therefore,
S2 only makes a marginal contribution to the afterglow light
curve,whichamountsto<20%ofthelastpoint.
The light curve clearly shows a complex behaviour, with
apowerlawdecayunderlyingaremarkableflarewhichpeaks
at ≈ 210 s after the BAT trigger (see Fig. 2, inset). To fit the
lightcurvewe usedthe BAT triggerasreferencetime andwe
onlyconsideredspectroscopic-modedataobtainedwhileXRT
was pointing,thus excludingthe early LrPD, the LrPD upper
limitandtheIMpoint.Furtherexcludingthedatatakenduring
the flare (180s < t < 300s), a fit with a simple power law
yields χ2 = 4.32 (12 degrees of freedom, d.o.f.), which is
red
unacceptable.Afitwith abrokenpowerlaw F(t) = Kt−α1 for
t<tbandF(t)= Ktb−α1(t/tb)−α2 fort>tb,wheretbisthetime
ofthebreak,yieldsα =1.58+0.18andα =0.50±0.14,anda
1 −0.16 2
breakat∼4200saftertheBATtrigger.Thislattermodelyields
a good fit (χ2 = 1.20, 10 d.o.f.), a significant improvement
red Fig.3.WTbackground-subtractedlightcurves.(a):Totalband
overthesimplepowerlaw(nullhypothesisprobability=1.7×
(T,0.2–10keV).(b):Softband(S,0.2–1keV).(c):Hardband
10−3,equivalentto3.2σ), butsomeoftheparametersarenot
(H,1–10keV).(d):Ratioofhardtosoftcountrates.
wellconstrained.Alternatively,afitwithtwosmoothlyjoined
powerlaws F(t) = K′[(t/tb)−α1 +(t/tb)−α2] yieldsχ2red = 1.29
(10 d.o.f.) with similar values for the inferred parameters. A (hard, H), as well as the total band, 0.2–10keV. We used the
summaryofthefitstothelightcurvecanbefoundinTable3. sameregionsastheonesdescribedabove,aconstanttimebin-
As a reference, the 0.2–10 keV unabsorbed flux at t is (4± ningof30sanddynamicallysubtractedtheirrespectiveback-
b
1)×10−13ergcm−2s−1(weadoptedacountratetounabsorbed grounds.Figure3showsthethreebackground-subtractedlight
fluxconversionfactorof6.5×10−11ergcm−2count−1,obtained curves,aswellastheratioH/S.Indeed,duringtherisingpor-
fromthebestfitmodelsderivedinSect.3.3)andtheluminosity tionoftheflarethehardbandfluxincreasesbyafactorof>∼ 6
inthe0.7–34.4keVbandis(1.9±0.9)×1046ergs−1. whilethesoftbandfluxonlyincreasesslightly,sothatthespec-
During the flare a rebrightening of the source by a factor trumoftheflarestartsoffharderthantheunderlyingafterglow,
of >∼ 6 in flux was observed between t ∼ 154s and the peak and then evolves into a softer state as its flux decreases; this
at∼ 210s.Boththerisingandthefallingpartoftheflarehad canbeseeninthefollowingtimebin,whenthesoftbandflux
verysteepslopesthat,whenfitwithasimplepowerlaw,yield peaks with a flare to pre-flare flux ratio of ∼ 3.5. This yields
α = −5.8+1.6 and α = 6.7±1.0. When the underly- an indicationofspectralevolutionduringtheflare asa ∼ 3-σ
1,flare −2.1 2,flare
ing power-law afterglow is subtracted, the fit yields α = excess over a constant fit to H/S. It should be noted that this
1,flare
−6.8+2.4andα = 6.8+3.6 andthepeakisat213±7sfrom behaviourisreminiscentofthatobservedinthe promptemis-
−2.1 2,flare −2.0
theBATtrigger.Inallcasestheerrorsaredominatedbytheun- sion(Fordetal.1995),withtheharderbandpeakprecedingthe
certaintyintheplacementoftheflareboundaries.Theflarecan softerbandpeak.
also be characterised,as a simple parametricdescription,as a At t ∼ 1.7 × 105s a second faint bump is observed. Its
Gaussian line. A combined broken power law and Gaussian significanceisnothigh,sinceitisdetectedasa2-σexcessover
model fit yields a peak at 211.1+5.4s (61.4+1.6s in the rest the underlying afterglow. Similar late-time bumps have been
−4.4 −1.3
frame) and a width 17.9+12.3s (χ2 = 1.58, 17 d.o.f.).In this observed in other Swift-detected GRBs (e.g. GRB 050502B;
−4.6 red
casetheratioofthecharacteristictime-scaleandthepeaktime Falconeetal.2006).
isδt/t ∼0.08or0.20,whenusingtheGaussianwidthorits
peak
FWHM(42.2+29.0s),respectively.Ineithercase,δt/t ≪ 1,
−10.8 peak 3.3.Spectralanalysis
whichputssevereconstraintsontheemissionmechanismsthat
canproducetheflare.Weshalladdressthisissueinthediscus- The afterglow of XRF 050406 was very faint, hence it is not
sion section. Integration of the Gaussian best-fitting function possible to performtime-resolvedspectroscopyto distinguish
yieldsanestimateofthefluenceoftheflare,(1.4±1.0)×10−8 the spectral properties of the afterglow proper from the ones
ergcm−2,correspondingtoanenergyof(2.0±1.4)×1050erg. of the flare observed in the light curve. Therefore, we pro-
Thelargeerrorreflectstheuncertaintyontheactualmodelused ceededasfollows.Spectraofthesourceandbackgroundwere
fortheintegrationoftheflare. extractedintheregionsdescribedin§3.2fromthefirstobser-
We also extracted events from the first snapshot WT data vation(000)eventfiles.PCandWTspectrawereextracteddur-
in two more energybands, 0.2–1keV (soft, S) and 1–10keV ingthefirst∼500softhePCobservation(seeTable2fortimes
6 P.Romano:X-rayflareinXRF050406
referredtoT ),whenPCdataare piled-upandwhentheflare 2001),orasan“X-rayrichGRB” (XRR).Thesoftnessofthe
0
is observed in the light curve. We also extracted PC spectra observed spectrum, which is well fit in the 15–150keV band
after the first 500s during the first 4 snapshots. For the latter withasimplepowerlawwithphotonindexΓ =2.65,andwith
γ
we used a circular region with a 10 pixel radius (correspond- nosignificantemissionobservedabove∼ 50keV,impliesthat
ing to ∼ 80% of the XRT PSF) to minimize the background thepeakenergyisbelowtheBATbandpass(E <15keV).The
p
andtobeabletousetheCashstatistics(Cash1979).Ancillary operationaldefinitionofXRFs/XRRs(e.g.Lambetal.2004)is
response files (ARF) were generated with the task xrtmkarf ofafasttransientX-raysourcecharacterizedbyasoftnessra-
withinFTOOLSv6.0usingthelatestARFdistribution(v003). tio SR= log[F(2–30keV)/F(30–400keV)] > 0 for an XRF
These ARFs accountfor different extraction regions and PSF and−0.5<SR<0foranXRR.ExtrapolationoftheBATspec-
corrections.We usedthelatestspectralredistributionmatrices trum,withtheassumptionof E < 2keV,yieldsSR= 0.8+0.5,
p −0.4
(v007).Theenergyrangesadoptedforspectralfittingwere0.5– whichclassifiesthisburstasanXRF. However,abreakinthe
10keVand0.2–10keVforWTandPC,respectively. spectrum may well be present in the 2–15 keV band. In the
Wefirstperformedafitwithanabsorbed(wabsinXSPEC) most conservative case, i.e. assuming no flux below 15 keV,
powerlaw tothe WT data(166counts),whichwererebinned thiseventwouldbeanXRRGRB,withSR=−0.2+0.2.
−0.3
with a minimum of 20 counts per energy bin to allow χ2 fit- The isotropic-equivalent gamma-ray energy of this event
ting within XSPEC. The Hydrogen column was initially kept is E = (1.4+1.6) × 1051 erg (Sect. 2.1), and this effec-
iso −0.6
as a free parameter, and then frozen to the Galactic value tively puts XRF 050406 in the low-energy tail of GRB ener-
(NG = 2.8×1020 cm−2, Dickey&Lockman 1990) when the gies (Bloometal. 2003b). Assuming that the Amati relation
H
fit yielded a value lower than (although consistent with) the (Amatietal. 2002) holds, we can infer a rest-frame Erest ∼
p
Galactic one. The fit was good, χ2 = 1.0 for 6 d.o.f., and 55keV,whichcorrespondstoanobserver-frameE ∼ 15keV.
red p
yielded Γ = 2.11+0.31. We then performeda fit with the same ThisvalueisconsistentwiththenondetectionofE intheBAT
−0.28 p
model to the remainder of the PC data during snapshots 1 energyrange.
through4usingCashstatisticswhichismoreappropriategiven
To date, X-ray afterglows of XRFs have been detected
thelownumberofcounts(21un-binnedcounts)andcalculated
in just a few cases (XRF 011030, XRF 020427: Bloometal.
thegoodnessofthefitvia104 Montecarlosimulations.Thefit
2003a; Levanetal. 2005; XRF 030723: Butleretal. 2004;
wasgoodandyieldedconsistentresults.Wealsoperformedsi-
XRF 040701: Fox 2004; XRF 050315: Vaughanetal. 2006).
multaneousfitstotheWTandPC(60counts)spectraextracted
Thisis one of the firstexamplesof a well-studiedX-ray light
duringthefirst∼ 500s(usingχ2 statistics)andofthePCdata
curveofan XRF. Its maincharacteristicsare notqualitatively
alone(usingCash statistics), also obtainingconsistentresults.
different from those of normal GRBs (Chincarinietal. 2005;
Table2summarizestheresultsofthefits.We notethat,given
Nouseketal.2006).Asobservationsaccumulate,itisbecom-
the current goodness of the XRT calibration (5% systematic
ing clear that these two classes of phenomena share many
uncertaintyforallobservingmodesandgradeselectionsinthe
properties, and both have afterglowswith similar characteris-
0.5–10keVrange;e.g.,Romanoetal.2005),anexcessof N
H tics (Sakamotoetal. 2005). This is a clue that both types of
cannot be excluded and we find a 3-σ upper limit to the to-
events may have a common origin and is supported by re-
tal(Galacticplusintrinsic)Hydrogencolumnalongthelineof
centevidencethatsomeXRFsareassociatedwithsupernovae
sightofN <9×1020cm−2.
H (Soderbergetal.2004;Bersieretal.2005;Fynboetal.2004).
We canthereforeconcludethat,duringthefirst600safter
the burst, which include the X-ray flare observed in the light
curve,themeanphotonindexisΓ=2.1±0.3,andthatthepho- 4.2.X-rayflares:evidenceforprolongedengine
tonindexdoesnotvaryaftertheendoftheflare.However,we activity
dohavecluesregardingthepresenceofspectralevolutiondur-
ingtheflarecomingfromthehardnessratioanalysis(Sect.3.2), The general behaviour of the afterglow of XRF 050406 is a
even though the statistics are not high enough to show it in typical one. The observed X-ray photon index (ΓX = 2.1)
thespectralanalysis.Aswewilldiscusslater(Sect.4.2),other is common among X-ray afterglows (Chincarinietal. 2005;
afterglows with larger amplitude X-ray flares demonstrate a DePasqualeetal. 2005). The light curve shows a break from
strongspectralevolutionoftheflares. arelativelysteepdecay(α1 =1.58)toaflatterone(α2 =0.50).
Itsoverallshapeissimilartotheonetypicallyobservedbythe
XRT(Chincarinietal.2005;Nouseketal.2006),eventhough
4. Discussion theinitialslopeislesssteepthanaverage.
The most striking characteristic of this burst is the strong
4.1.Gamma-rayproperties:similarityofXRFsand
flare in its X-ray light curve, a feature which had never been
GRBs
detectedbySwiftbeforeandhadbeenpreviouslyobservedin
The duration of this burst (T = 5± 1s in the 15–350 keV very few GRBs (GRB 970508,Piroetal. 1999; GRB 011121
90
band) places this burst in the short tail of the long GRB pop- andGRB011211,Piroetal.2005).Thefluenceoftheflareis
ulation (Kouveliotouetal. 1993). Its fluence is relatively low ∼1.4×10−8ergcm−2inthe0.2–10keVband,whichamounts
(1.0 × 10−7 erg cm−2 in the 15–350 keV band) but not un- to∼ 14%oftheobserved(15–350keVband)promptfluence.
usually faint. The gamma-ray characteristics of this burst are Abetterestimateoftheflare-to-promptenergyratiowouldre-
consistent with a classification as an X-ray flash (Heiseetal. quire the knowledge of the prompt spectral energy distribu-
P.Romano:X-rayflareinXRF050406 7
tion (SED). Since the actual peak energy of the prompt SED lution throughout the flare further support this interpretation.
is unknown (E < 15keV), the extrapolationof the BAT flu- The flare appears to be harder than the underlying afterglow,
p
ence to the XRT band is highly uncertain. For plausible val- whichsuggestsadistinctoriginforthisemission.Furthermore,
ues of E , the flare to prompt fluence ratio is in the 1–10% thereareindicationsofspectralevolution,whichshowsthetyp-
p
range. The observed rebrighteningis by a factor of 6 in flux, ical hard-to-soft pattern. Such a behaviour is commonly ob-
presentsa peak at t = 213±7s andtakes place on a very servedinthepromptemissionspikesofGRBs(e.g.Fordetal.
peak
shorttimescale,witharatioofthecharacteristictime-scaleand 1995),whichareproducedininternalshocks.Furtherevidence
the peak time δt/t ≪ 1. Both the rising and the falling oflateengineactivitycomesfromboththeflatpartofthelight
peak
parts of the afterglow-subtracted flare had very steep slopes, curve(α ≈0.5,seeSect.4.3)andpossiblybythepresenceof
2
α ≈ −7andα ≈ 7,assumingthebursttriggerasthe thelate-timebumpobservedatt∼1.7×105s.
1,flare 2,flare
timeorigin. FollowingthediscoveryofaflareintheafterglowofXRF
According to the standard relativistic fireball model, the 050406, initially reported by Burrowsetal. (2005b), many
prompt emission is caused by internal shocks within the ex- others were identified: GRB 050502B (Falconeetal. 2006),
pandingfireball,whiletheafterglowisproducedbythefireball GRB 050724 (Barthelmyetal. 2005b) and GRB 050904
shocking the external medium (external shocks, Piran 1999; (Cusumanoetal. 2005c), just to mention a few. At the time
Zhang&Me´sza´ros 2004). Available models to explain flares of writing (2005Oct), ∼ 50% of the bursts detected by XRT
include refreshed shocks (ReesandMe´sza´ros 1998), external which were immediately re-pointed towards showed flares,
shocks with a clumpy medium (Lazzatietal. 2002) and an- makingflaringquiteacommonbehaviour.Furthermore,allthe
gular inhomogeneities in the outflow (Fenimoreetal. 1999; characteristics of the XRF 050406 flares have now been ob-
Nakaretal. 2003). However, it can be argued (Burrowsetal. served in most flaring GRBs (see Burrowsetal. 2005c for a
2005b,Zhangetal.2005,Nouseketal.2006)thatsuchmodels recentreview).Forexample,highlysignificantspectralevolu-
cannotproducetheobservedlargefluxvariationsδF/Fpeak ≫ tion throughoutthe flare has been reported in GRB 050502B
1 in such short timescales δt/tpeak ≪ 1 (Iokaetal. 2005). (whichwasthebrightestobservedsofar)andGRB050724.In
Similarly, none of the above mechanisms would explain the severalcases the flares presentlargeamplitudesand occur on
steepslopesobservedintheflare.Externalreverseshocks,cre- shorttimescales.Furthermore,severalflaresareoftenobserved
ated when the fireball slows down because of the interaction in thesame event,attimesrangingfrom∼ 100sto 104–105s
with the external medium, are expected to emerge at optical after the burst. Finally, in most cases the afterglow is clearly
andradiowavelengths,hencesynchrotronself-Compton(SSC) presentbefore the onsetof the flare, and hasconsistentdecay
mustbe invokedto produceemission in the X-ray band.This slopeandfluxlevelswithaftertheflare.Thepresentcaseshows
would require carefully balanced conditions (Kobayashietal. thatflaresarepresentbothinXRFsandinGRBs.Sinceflares
2005). arelikelytiedtothecentralengineactivity,thisfindingfurther
Piroetal. (2005) suggested that the X-ray flares observed supportstheideathatasimilarmechanismisatworkforboth
inGRB011121andGRB011211wereduetotheonsetofthe kindofevents(Fan&Wei2005).
afterglow. The steep slopes and the short timescale variabil-
ity can only be accounted for within the thick shell scenario
(Sari&Piran1999).Galli&Piro(2005)successfullymodeled 4.3.TheX-rayafterglowlightcurve
XRF 011030 using this model. In this scenario, the emission
beforeandaftertheflareisduetodifferentprocesses(prompt ThepromptreactionofSwifthasallowedustoobservetheX-
tail and afterglow, respectively), hence a discontinuity in the ray light curves of GRB afterglows starting from a few tens
light curve is generally expected underlying the flare. This is of seconds after the burst explosion. In most cases the X-ray
notthe caseforXRF 050406,wherethesame componentde- light curvesare characterized by an initial steep decay (up to
scribestheX-rayemissionbothbeforeandaftertheflare.Even ∼ 500s) followed by a shallow decay, and then by a steeper
if a fine-tuning may explain this particular event, the lack of decay with a second break normally occurringat a few thou-
a lightcurvebreakis commonto a largefractionofthe flares sandsecondslater(Chincarinietal.2005;Nouseketal.2006).
observedbySwift(Burrowsetal.2005c).Therefore,whilethe The early steep decay seen in the X-ray light curve can be
explanationof flares in terms of the afterglow onset is attrac- explained as the tail of the prompt emission (however, see
tive, it is unlikely to be applicable to the vast majority of the Panaitescuetal. 2005). The few cases where the XRT light
X-rayflaresseenbyXRT. curvelieswellabovetheextrapolationofthepromptemission
Apromisingmechanismtoproducetheflareislateinternal into the X-rayband can be explainedeither by a strong spec-
shocks(Fan&Wei 2005; Zhangetal. 2005; Kingetal. 2005; tral evolutionor by an X-rayflare with the maximumlocated
Pernaetal.2006),whichimpliesthatthecentralengineisstill beforetheXRTobservation(Tagliaferrietal.2005).Thereare
active at t = 213s, even though the prompt emission ended otherinstanceswherethefirststeepdecayisnotobservedatall
after t ∼ 6s. The late-time activity in this case must have (e.g.Campanaetal.2005).
a reduced power with respect to the prompt emission, as the In the case of XRF 050406, however, the initial slope is
relative fluences indicate. Such a mechanism would naturally shallower than the steep values 3 <∼ α <∼ 5 observed in other
explainthesteepriseanddecayslopes.Second,theenergyre- earlyafterglows(Tagliaferrietal.2005).Moreover,thecurva-
quiredtopowertheflarewouldbemuchlowerthanintheother ture relation α = β + 2 (Kumar&Panaitescu 2000; Dermer
scenarios(Zhangetal.2005).Theindicationsofspectralevo- 2004) is not satisfied, even after taking into account the ef-
8 P.Romano:X-rayflareinXRF050406
fects pointedout by Zhangetal. (2005) that would alter such thefirstXRTphasewereduetolateengineactivity,thentheen-
relation.Therefore,we also investigatewhetherthe initialde- ergyinjectioncouldhavebegunmuchearlieranditsemission
cline seen in XRF 050406 is consistent with afterglow emis- wouldhavebeenmasked.
sion.Comparisonofspectralindicesandtemporaldecayslopes Integrationofthelightcurvefromtheonsetoftheflatslope
with theoretical relativistic fireball models (e.g. Table 2 in phaseyieldsF ≈3×10−8(t /7.6×105s)0.5ergcm−2,where
end
Zhangetal. 2005) indicates that the first decay index α = t isthetimeatwhichtheshallowphaseends,forwhichwe
1 end
1.58±0.17andenergyindexβ=1.1±0.3ruleoutfastcooling can only set a lower limit. We note that this depends weakly
models(forwhichtheinjectionfrequencyν exceedsthecool- ontheonsettimeoftheshallowphase,thereforethecalculated
m
ingfrequencyν )forν<ν .Forν>ν ,theα(β)=(3β−1)/2 fluenceiscorrectinbothpresentedscenarios.Forcomparison,
c m m
closure relation is satisfied within the errors and an electron the amount of energy released during the steep phase of the
power-lawdistributionindexp≈2.5isobtained.Thesamere- lightcurve(excludingtheflare)isF ≈2×10−8(t /100s)−0.6
start
lation holds for the slow cooling regime (where ν > ν ) for ergcm−2. We note thatthe shallowphase lasts a considerable
c m
ν > ν (bothwindandISM).Inthiscaseaconsistentsolution time.
c
isalsofoundforν <ν<ν ,althoughwithalarge p≈3.The Zhangetal. (2005) propose three explanations for
m c
ISM environmentis favouredon thebasisofa better satisfied the energy injection mechanism. In the impulsive case
closure relation. In conclusion, the spectral indices and tem- (Sari&Me´sza´ros2000),thecentralengineejectsmaterialwith
poral decay slopes of the first part of the X-ray curve can be awidedistributionofLorentzfactors.Inthiscase,slowermov-
interpretedintermsofrelativisticfireballmodels,eventhough ingshellswillcatchthefireballatalatertime.Wecanestimate
thelargeuncertaintiesassociatedwiththeslopesdonotallow theminimumLorentzfactorasΓmin <∼2(Eiso,50/n0)1/8(1+z)3/8,
ustochooseamongtheavailablemodels. where E = E ×1050 erg is the isotropic-equivalenten-
iso iso,50
An alternative explanationfor the initial XRT emission is ergy, and n is the external medium particle density in units
0
thepresenceofanadditionalflarewhichstartedbeforethebe- of cm−3. This implies that the acceleration process works
ginningoftheXRTobservation,andofwhichweonlyseethe fromultra-tomildly-relativisticvelocities.Withintheputative
decaying part. The superposition of two (and possibly more, Poyintingfluxscenario(Zhang&Kobayashi2005),theenergy
fainter)flareswouldthenmimictheinitialsteeppowerlawde- supply is provided by the transfer of magnetic energy to the
cay.However,thisinterpretationseemslesslikelysincerecent fireball, and the time at which the injection stops is related
SwiftobservationsofX-rayflareswithinthefirstseveralhun- to the ratio σ of the electromagnetic to baryonic kinetic
dred seconds of the prompt emission all had temporal decay energy.Ifthisscenarioiscorrect,wecaninferalowerlimitof
indicesmuchsteeperthantheobservedXRF050406pre-flare σ = (t /t )1−q > 10–100,forq = 0.5–0,wheret < t is
end start start b
index. thestarttimeofinjection.Therefore,aftertheendoftheenergy
At t ∼ 4400 s the XRT light curve breaks to α ≈ 0.5. transferphase,theenergyoftheblast-wavewouldbeincreased
2
Suchaflatdecaycannotbeexplainedintermsofthestandard by a comparable factor. In the third scenario (the prolonged
afterglowmodel.Theonlypossibilitywouldbetoobserve,in energyoutputbythecentralengine,Zhang&Me´sza´ros2001),
thefastcoolingregime,thesegmentwithν < ν < ν (where the end of the injection phase is simply the end of the engine
c m
α = 0.25isexpected,marginallyconsistentwiththeobserved activity. In this case, this activity produces a large amount
value). However, the fast cooling regime is expected to end of energy, particularly so since the radiative efficiency may
much earlier. To maintain the observeddecay unbrokenup to be lower during the late afterglow than during the prompt
∼ 106s,largevaluesoftheequipartitionparametersε andε emission,asisgenerallythecase.Thiswaspreviouslynoticed
e B
orofthe Comptonparameterwouldberequired.We consider byNouseketal.(2006)inasampleofseveralSwiftGRBs.
this possibility quite unlikely. Another possibility is that the ThemonitoringofXRF050406wasdiscontinued22days
angularenergyprofileofthefireballisnottrivial(astructured afterthetrigger.By then,the sourcewasnolongerdetectable
jet), so thatemission comingfromthe (brighter)wingsofthe and only a 3-σ upper limit could be drawn at ≈ 3.6 × 10−4
jetmayincreasetheobservedfluxasthefireballLorentzfactor counts s−1. In order for the afterglow energy not to diverge,
decreases(Panaitescuetal.2005). a further, late break is necessary. One interesting possibil-
An interesting explanation for the shallow-decay phase is ity is that this may be due to seeing the edge of the jet. A
injection of new energy into the fireball through refreshed steepening in the light curve is expected when the fireball
shocks(Sari&Me´sza´ros2000;Zhang&Me´sza´ros2001).For Lorentz factor becomes comparable to the inverse of the jet
thistohappen,theenergyreleaseinsiderefreshedshocksmust half-opening angle. Such a late break is not unexpected for
be sizeable,since the wholefireballdynamicshasto bemod- an XRF. The few XRFs with knownredshift(Soderbergetal.
ified.Assuminganenergyinjectionrate E˙ ∝ t−q,wefindqin 2004; Bersieretal. 2005; Fynboetal. 2004) have a very low
therange0to0.5dependingonthemodeldetails(Zhangetal. isotropic-energy release, and this may be at least in part ac-
2005).Inthismodel,theinitialpartoftheXRTafterglowlight commodated if they have very wide jets. This picture is con-
curvecanbeduetostandardafterglowemissiononlyifthefire- sistent with the result found by Frail et al. (2001; see also
ballevolutionisnotinfluencedatthesestages.Indeed,theen- Ghirlandaetal.2004),whofoundthatlow-energyGRBstend
ergysupplyprovidedbyrefreshedshocksissteadilygrowing, to have wider opening angles. Using the standard formalism
andatthebeginningitcannotalterthefireballdynamics.Inthis (Rhoads 1999; Sarietal. 1999), the jet half-opening angle is
case,thebreakwouldidentifythetimewhenthenew,injected ϑ =16t3/8n1/8(η/0.2)1/8E−1/8 deg,wheret =t ×106sisthe
j j,6 0 iso,50 j j,6
energyiscomparabletothefireballenergy.Onthecontrary,if jetbreaktimeandηistheburstradiativeefficiency.Therefore,
P.Romano:X-rayflareinXRF050406 9
using our lower limit on the jet breaktime tj >∼ 106s, we can Bloom, J. S., Fox, D., van Dokkum, P. G., et al. 2003a, ApJ,
infer a lower limit on the jet half-opening angle of 16 deg. 599,957
This value is at the high end of the distribution of jet angles Bloom,J.S.,Frail,D.A.,&Kulkarni,S.R.2003b,ApJ,594,
(Bloometal.2003b). 674
Burrows, D. N., Hill, J. E, Nousek, J. A., et al. 2005a, Space
Sci.Rev.,120,165
5. Summaryandconclusions
Burrows,D.N.,Romano,P.,Falcone,A.,etal.2005b,Science,
XRF 050406 is classified as an X-ray flash, with fluence ∼ 309,1833
1×10−7ergcm−2 (15–350keV),asoftspectrum(Γγ = 2.65), Burrows,D. N., Romano,P., Godet,O., etal. 2005c,Proc. of
nosignificantfluxabove∼ 50keVandapeakenergyEp < 15 X-RayUniverse2005,astro-ph/0511039
keV.Itsmaincharacteristicsarehowevernotqualitativelydif- Butler,N.R.,Sakamoto,T.,Suzuki,M.,etal.2005,ApJ,621,
ferent from those of normal GRBs. As observations accumu- 884
late,itbecomesclearthatthesetwoclassesofphenomenashare Campana,S.,Antonelli,L.A.,Chincarini,G.,etal.2005,ApJ,
manyproperties,andbothhaveafterglowswithsimilarcharac- 625,L23
teristics. This is a clue that both events may have a common Capalbi, M., Perri, M., Romano, P., et al. 2005, GRB
origin. CoordinatesNetwork,3184
XRF050406isthefirstSwift-detectedburstthatshoweda Cash,W.1979,ApJ,228,939
flare in its X-ray light curve, a feature now found in ∼ 50% Chincarini,G.,Moretti,A.,Romano,P.,etal.2005,ApJ,sub-
of the XRT afterglows. The flare peaked at ∼ 210 s after the mitted,astro-ph/0506453
BAT trigger (∼ 61 s in the rest frame). The best fit of the af- Cusumano,G.,Kennea,J.,Burrows,D.N.,etal.2005a,GRB
terglowdecayisobtainedwithabrokenpowerlawwithα1 = CoordinatesNetwork,3181
1.58±0.17,α2 = 0.50+−00..1143, anda breakat∼ 4400s afterthe Cusumano,G.,Mangano,V.,Angelini,L.,etal.2005b,ApJ,in
BATtrigger.ThemeanphotonindexisΓX =2.1±0.3.During press(astro-ph/0509737)
theX-rayflareafluxvariationofδF/Fpeak ∼ 6inatimescale Cusumano, G., Mangano, V., Chincarini, G., et al. 2005c,
δt/tpeak ≪1isobserved,anditsmeasuredfluenceinthe0.2–10 Nature,inpress
keVbandis∼1.4×10−8ergcm−2[(2.0±1.4)×1050erg],which DePasquale,M.,Piro,L.,Gendre,B.,etal.2005,A&A,sub-
correspondsto 1–15% of the prompt fluence. We argued that mitted,astro-ph/0507708
the flare-producing mechanism is late internal shocks, which Dermer,C.D.2004,ApJ,614,284
impliesthatthecentralengineisstillactiveatt∼210s,though Dickey,J.M.,&Lockman,F.J.1990,ARA&A,28,215
withareducedpowerwithrespecttothepromptemission.We Falcone,A. D., Burrows,D. N., Lazzati,D., etal. 2006,ApJ,
showed possible indications of spectral variations during the inpress,astro-ph/0512615
flare,andaflatteningoftheX-raylightcurveaftert ∼ 4400s Fan,Y.Z.,&Wei,D.M.2005,MNRAS,364,L42
insupportofcontinuedcentralengineactivityatlatetimes. Fenimore,E.1999,ApJ,512,683
SinceXRF050406wasobserved,flareshavebeendetected Fishman,G.J.,&Meegan,C.A.1995,ARA&A,33,415
byXRTinbothX-rayflashesandnormalGRBs,indicatingthat Ford,L.A.,Band,D.L.,Matteson,J.L.,etal.1995,ApJ,439,
flares are linked to some commonpropertiesof both kinds of 307
bursts,andprobablytiedtotheircentralengine. Fox,D.2004,GRBCoordinatesNetwork,2626
Frail,D.A.,Kulkarni,S.R.,Sari,R.,etal.2001,ApJ,562,L55
Acknowledgements. This work is supported at OAB by ASI grant
Fynbo,J.P.U.,Sollerman,J.,Hjorth,J.,etal.2004,ApJ,609,
I/R/039/04,atPennStatebyNASAcontractNAS5-00136andatthe
962
University of Leicester by PPARC. We gratefully acknowledge the
Galli,A.,&Piro,L.2005,A&A,submitted,astro-ph/0510852
contributions of dozens of members of theXRTandUVOT teamat
OAB, PSU, UL, GSFC, ASDC, and MSSL and our subcontractors, Gehrels,N.,Chincarini,G.,Giommi,P.,etal.2004,ApJ,611,
whohelpedmakethisinstrumentpossible. 1005
Ghirlanda, G., Ghisellini, G., & Lazzati, D. 2004, ApJ, 616,
331
References
Heise, J., in’tZand,J., Kippen,R. M., & Woods, P. M. 2001,
Amati,L.,Frontera,F.,Tavani,M.,etal.2002,A&A,390,81 Gamma-rayBurstsintheAfterglowEra,16
Band,D.,Matteson,J.,Ford,L.,etal.1993,ApJ,413,281 Hill, J. E., Burrows, D. N., Nousek, J. A., et al. 2004,
Barthelmy,S.D.,Barbier,L.M.,Cummings,J.R.,etal.2005, Proc.SPIE,5165,217
SpaceSci.Rev.,120,143 Ioka,K.,Kobayashi,S.,&Zhang,B.2005,ApJ,631,429
Barthelmy,S.D.,Chincarini,G.,Burrows,D.N.,etal.2005b, King, A., O’Brien, P. T., Goad, M. R., el al. 2005, ApJ, 630,
Nature,438,994 L113
Berger, E., Oemler, G., & Gladders, M. 2005a, GRB Kobayashi,S.,Zhang,B.,Me´sza´ros,P.,&Burrows,D.N.2005,
CoordinatesNetwork,3185 ApJ,submitted,astro-ph/0506157
Berger,E.,Kulkarni,S.R.,Fox,D.B.,etal.2005b,ApJ,634, Kouveliotou, C., Meegan, C. A., Fishman, G. J., et al. 1993,
501 ApJ,413,L101
Bersier, D., Fruchter, A. S., Strolger, L.-G., et al. 2005, ApJ, Krimm, H., Barbier, L., Barthelmy, S., et al. 2005, GRB
submitted CircularNetwork,3183
10 P.Romano:X-rayflareinXRF050406
Kumar,P.,&Panaitescu,A.2000,ApJ,541,L51
Lamb, D. Q., Donaghy, T. Q., & Graziani, C. 2004, New
AstronomyReview,48,423
Landsman,W.,Hunsberger,S.,Breeveld,A.,etal.2005,GRB
CoordinatesNetwork,3182
Lazzati,D.,Rossi, E.,Covino,S., Ghisellini,G.,& Malesani,
D.2002,A&A,396,L5
Levan,A.,Patel,S.,Kouveliotou,C.,al.2005,ApJ,622,977
Moretti,A.,Perri,M.,Capalbi,M.,etal.2005,A&A,submit-
ted
Nakar, E., Piran, T., & Granot, J. 2003, New Astronomy
Review,8,485
Nousek,J.A.,Kouveliotou,C.,Grupe,D.,etal.2006,ApJ,in
press,astro-ph/0508332
Panaitescu, A., Me´sza´ros, P., Gehrels, N., Burrows, D. N., &
Nousek,J.A., 2005,MNRAS,submitted,astro-ph/0508340
Parsons, A., Barthelmy, S., Cummings, J., et al. 2005, GRB
CoordinatesNetwork,3180
Perna,R.,Armitage,P.J.,&Zhang,B.2006,ApJ,636,L29
Piran,T.1999,Phys.Rep.,314,575
Piro,L.,Costa,E.,Feroci,M.,etal.1999,ApJ,514,L73
Piro,L.,DePasquale,M.,Soffitta,P.,etal.2005,ApJ,623,314
Rees,M.J.&Me´sza´ros,P.1998,ApJ,496,L1
Rhoads,J.E.1999,ApJ,525,737
Rol, E., Schady, P., Hunsberger, S., et al. 2005, GRB
CoordinatesNetwork,3186
Romano, P., Cusumano, G., Campana, S., et al. 2005,
Proc.SPIE,5898,357
Roming,P. W. A., Kennedy,T. E., Mason, K. O., et al. 2005,
SpaceSci.Rev.,120,95
Sakamoto,T., Lamb,D. Q.,Kawai, N., etal. 2005,ApJ,629,
311
Sari,R.,&Piran,T.1999,ApJ,520,641
Sari,R.,Piran,T.,&Halpern,J.P.1999,ApJ,519,L17
Sari,R.,&Me´sza´ros,P.2000,ApJ,535,L33
Schady,P., Mason, K.O., Osbourne,J. P., et al. 2006,ApJ, in
press,astro-ph/0601182
Soderberg,A.M.,Kulkarni,S.R.,Berger,E.,etal.2004,ApJ,
606,994
Tagliaferri,G.,Goad,M.R.,Chincarini,G.,etal.2005,Nature,
436,985
Vaughan,S.,Goad,M.R.,Beardmore,A.P.,etal.2006,ApJ,
inpress,astro-ph/0510677
Zhang,B.,&Me´sza´ros,P.2001,ApJ,552,L35
Zhang, B., & Me´sza´ros, P. 2004, International Journal of
ModernPhysicsA,19,2385
Zhang, B., Fan, Y. Z., Dyks, J., et al. 2005, ApJ, in press,
astro-ph/0508321
Zhang,B.,&Kobayashi,S.2005,ApJ,628,315