Optical observations of GRB 060124 afterglow: A case for an injection break

0.19 MB·English

by  Kuntal Misra

Astronomy&Astrophysicsmanuscriptno.6539˙misra˙manuscript (cid:13)c ESO2008
February5,2008
Optical observations of GRB 060124 afterglow: A case for an
injection break
7
0 KuntalMisra1,D.Bhattacharya2,D.K.Sahu3,RamSagar1,G.C.Anupama4,A.J.Castro-Tirado5,S.S.Guziy5,6
0
andB.C.Bhatt3
2

n
a 1 AryabhattaResearchInstituteofObservationalSciences,ManoraPeak,Nainital263129,India
J e-mail:kuntal,[email protected]

5 2 RamanResearchInstitute,Bangalore560080,India
1 e-mail:[email protected]
3 CenterforResearchandEducationinScience&Technology,Hosakote,Bangalore562114,India

1 e-mail:dks,[email protected]
v 4 IndianInstituteofAstrophysics,Bangalore,560034,India
3
e-mail:[email protected]
1
5 InstitutodeAstrof´ısicadeAndaluc´ıa(IAA-CSIC),POBox03004,18080Granada,Spain
4
e-mail:ajct,[email protected]
1
0 6 NikolaevStateUniversity,Nikolskaya24,54030Nikolaev,Ukraine
7
0
/ Received;accepted
h
p
- ABSTRACT
o
r Aims.Wepresent broadband optical afterglowobservations of along durationGRB060124 using the1.04-mSampurnanand Telescopeat
t
s ARIES,Nainitalandthe2.01-mHCTatIAO,Hanle,includingtheearliestgroundbasedobservationsinRbandforthisGRB.Wedetermine
a
thedecayslopeofthelightcurveatdifferentbandsandexaminetherealityofaproposedjetbreak.
:
v Methods.Weusedatafromourobservationsaswellasothersreportedintheliteraturetoconstructlightcurvesindifferentbandsandmake
i powerlawfitstothem.Thespectralslopeoftheafterglowemissionintheopticalbandisestimated.
X
Results.OurfirstR-bandobservationsweretaken∼0.038dafterburst.Wefindthatallavailableopticaldataafterthisepocharewellfitbya
r
a singlepowerlaw,withatemporalfluxdecayindexα∼0.94.Wedonotfindanyevidenceofajetbreakwithinourdata,whichextendtill∼2d
aftertheburst.TheX-raylightcurve,however,showsadistinctbreakaround0.6day.Weattributethisbreaktoasteepeningoftheelectron
energyspectrumathighenergies.
Conclusions.Weconcludethattheabovemeasurementsareconsistentwiththepictureofastandardfireballevolutionwithnojetbreakwithin
t∼2daysaftertheburst.Thissetsalowerlimitof3×1050ergtothetotalenergyreleasedintheexplosion.
Keywords.gammaraybursts–photometry–observations–temporalindex–spectralindex
1. Introduction BAT energy range is ∼ 7 x 10−6 erg/cm2 in the 15-150 keV
bandafterscalingthefluenceoftheprecursor.
GRB060124(Swifttrigger=178750),alongdurationgamma
Swift-XRTbeganobservingthefield106secaftertheBAT
rayburst,wasdetectedon2006January24atT=15:54:52UT
trigger.AnalysisofthefirstorbitofXRTdatashowsthepres-
bytheSwift-BAT(Hollandetal.2006a).TheBATlightcurve
ence of an X-ray source at a position α = 05h08m27s.27,
shows a precursor from T-3 to T+13 sec, followed by three 2000
δ = +69◦44′25′′.7 which is 2.4 arcmin from the BAT po-
majorpeaksfromT+520toT+550sec,T+560toT+580sec, 2000
sition and 9 arcsec from the optical afterglow candidate re-
whichhasthelargestflux,andT+690toT+710sec(Fenimore
portedbyKann(2006).TheX-raylightcurveintheWindowed
etal.2006).ThetotaldurationoftheGRBis,thus,thelongest
Timing mode shows an initial flat behaviorfollowedby three
everrecordedbyBATSEorSwift.Thefluenceoftheprecursor
emissioninthe15-150keVbandis(4.6±0.5)x10−7erg/cm2. brightflaresafterthefirst100sofobservation(Manganoetal.
2006).
The peak flux in the 15-150 keV band was about 4.5 ± 0.5
counts/cm2/sec at T+570. The estimated total fluence in the An optical afterglow candidate of the GRB 060124 was
discovered by Kann (2006) at α = 05h08m25s.5, δ =
2000 2000
Sendoffprintrequeststo:KuntalMisra +69◦44′26′′.V-bandexposurestaken184and629secafterthe
2 KuntalMisraetal.:OpticalObservationsofGRB060124Afterglow
BATtriggerbySwift-UVOTalso showthepresenceofanop- InthispaperwepresentthebroadbandBVRI photometric
ticaltransient(Hollandetal. 2006b).Noopticalflare wasde- observationsoftheopticalafterglowofGRB060124including
tected at the position of the optical afterglow (Torii 2006). A theearliestobservationsinRbandfromanygroundbasedtele-
low-resolutionspectrumoftheafterglowofGRB060124was scope. A secure photometric calibration has been carried out
obtained by Mirabal and Halpern (2006) on 2006 Jan 25.13. by imaging the Landolt (1992) Standard SA 104 field along
Itspreliminaryanalysisshowsthepresenceofasignificantab- with the GRB 060124field. A brief descriptionaboutthe ob-
sorption feature at 5105 Å, possibly a doublet corresponding servations and data reduction is presented in the next section
to Mg II 2795 Å, 2802 Å (z=0.82) or C IV 1548 Å, 1550 Å whereasthemulti-bandopticallightcurveoftheafterglowfol-
(z=2.30). The presence of doublet feature is later confirmed lowinsection3.Thediscussionsandconclusionsformthelast
by Cenko et al. (2006). However, the Mg II feature has been sectionofthepaper.
ruled out on the basis of their separation and is identified as
CIV feature by Prochaska et al. (2006). They also identified
2. ObservationsandDataReduction
a weak absorption feature consistent with this redshift corre-
spondingtoAlII1670Åandanabsorptionlineat 4000Åcon- Optical observations of the GRB 060124 afterglow were car-
sistent with Lyα absorption and indicating log N(HI) < 20.5. ried out using the 1.04-m Sampurnanand Telescope (ST)
AccordingtoProchaskaetal.(2006)theafterglowspectrumis at Aryabhatta Research Institute of observational sciencES
notable for exhibiting very weak low-ion features (e.g. non- (ARIES), Nainital and the 2.01-m Himalayan Chandra
detections of FeII 1608, OI 1302, CII 1334) and relatively Telescope (HCT) at Indian Astronomical Observatory (IAO),
low HI column density. In this respect, the spectrum of GRB Hanleduring2006January24to26.ACCDchipofsize2048
060124afterglowis similar tothe afterglowsof GRB 050908 × 2048 pixel2 was used at ST covering a field of ∼ 13′ ×
and GRB 021004. Assuming a standard cosmological model 13′ on the sky. The gain and readout noise of the CCD cam-
withH =70km/s/Mpc,Ω =0.3,Ω =0.7andaredshiftof era are 10 e−/ADU and 5.3 electrons respectively. The filters
0 M Λ
z=2.297andafluenceofabout7x10−6erg/cm2,theisotropic- used at ST are the Johnsons BV and Cousins RI. The frames
equivalent gamma ray energy is 8.9 x 1052 erg (Cenko et al. werebinnedin2×2pixel2 toimprovethesignal-to-noisera-
2006). tio of the source. The CCD used at HCT was a 2048 × 4096
The most intense peak of the burst as seen by Swift SITe chip mounted on Himalayan Faint Object Spectrograph
BAT triggered Konus-Wind 558.9 s after the BAT trigger Camera(HFOSC).Thecentral2048×2048regionoftheCCD
(Golenetskii et al. 2006). The Konus-Wind light curve shows was used for imaging which covers a field of 10′ × 10′ on
the presence of the precursors and the three major peaks as the sky. It has a gain of 1.22 e−/ADU and readout noise of
wereseenbySwiftBAT.Golenetskiietal.(2006)makeapre- 4.8 electrons. Filters used are Bessells R and V. The frames
liminary estimate of the total burst fluence in the 20 keV - 2 obtainedusingHCTwereunbinned.Severaltwilightflatfield
MeV energyrangeto be ∼ 2.80× 10−5 erg/cm2/sec. The ma- andbiasframeswereobtainedfortheCCDimagesatboththe
jor peak of GRB 060124 was intense enough to trigger the telescopes. Severalshort exposures,with exposure time vary-
FREGATEinstrumentonHETEII557.7saftertheSwifttrig- ingfrom300secto1800sec,weretakenindifferentfiltersto
gertime(Lambetal.2006). imagetheopticaltransient(OT)ofGRB060124.
GRB060124hasbeenthefirstwellstudiedeventbySwift MIDAS,IRAFandDAOPHOTsoftwareswereusedtopro-
in the prompt and the afterglow emission phase (Romano et cesstheCCDframesinstandardfashion.Thebiassubtracted,
al. 2006).The promptemission was simultaneously observed flat fielded and cosmic ray removed CCD frames were co-
by XRT and UVOT. GRB 990123 was one such event for addedwheneverfoundnecessary.
which the flare in the promptphase was observedby ROTSE Landolt(1992)StandardfieldSA104regionwasimagedto
(Akerlofetal.1999)whichshowedarapiddecline.Thisemis- calibratethefieldofGRB060124.Atmosphericextinctionval-
sioninthepromptphasewasinterpretedasreverseshockemis- uesandthe transformationcoefficientsin B, V, R andI filters
sion. The Konus-Wind light curve resembles the light curves weredeterminedusingthe6standardbrightstarsintheSA104
of two long duration bursts: GRB 041219A (Vestrand et al. region.The6standardstarsintheSA104regioncoverarange
2005)andGRB050820A(Golenetskiietal.2005).TheIRaf- of0.518< (B−V) <1.356incolorand13.484< V <16.059
terglow of GRB 041219A was observed while the GRB was inbrightness.Usingthesetransformation,BVRImagnitudesof
still going on. Optical imaging of GRB 050820A, 5.5 s after 25secondarystarsweredeterminedinGRB060124fieldand
the trigger alert, with the RAPTOR telescope clearly shows theiraveragevaluesarelistedinTable1.Figure1showsthepo-
theemergenceoffaintopticalemissionwhichsuddenlyflares sitionoftheopticalafterglowinRbandandthe25secondary
andfadesawayduringthefirstonehour(Vestrandetal.2006). starsusedforcalibration.Thesestarshaveinternalphotometric
ThepromptopticalemissioninboththeselongdurationGRBs accuracybetterthan0.01mag.The(X,Y)CCD pixelcoordi-
(GRB041219AandGRB050820A)hasbeenattributedtoin- nateswereconvertedtoα ,δ valuesusingtheastromet-
2000 2000
ternal shocks in the ultra-relativistic outflow (Vestrand et al. ric positionsgivenbyHenden(2006).A comparisonbetween
2005,Vestrandetal.2006),thesamesourcethatisthoughtto photometryofHenden(2006)andthatofoursyieldszeropoint
generatethe gammaray emissionin the burst.Theopticalaf- differencesof 0.04± 0.03,0.04± 0.03,0.03± 0.02and 0.04
terglow of GRB 060124 was extensively followed by ground ±0.03inB,V,R,Ifiltersrespectively.Thesezeropointdiffer-
basedtelescopeswiththeearliestobservationsreportedbyour ences are based on the comparison of 14 common secondary
group(Misra2006). starsinthefieldofGRB060124.
KuntalMisraetal.:OpticalObservationsofGRB060124Afterglow 3
Fig.1.AR-bandimageofGRB060124opticalafterglowwith Fig.2. A R-band image of the field of GRB 060124taken on
the1.04-mSampurnanandTelescope(ST)atARIES,Nainital. 2006February16(22daysaftertheburst)withthe2.5-mIssac
Theopticalafterglowisindicatedbyanarrow.Markedarethe Newton Telescope (INT) at La Palma, Spain which does not
25secondarystarsusedforcalibration.NorthisupandEastis showanythingatthepositionoftheopticalafterglow.Northis
totheleft.Theapproximatescaleisshowntowardsthebottom upandEastistotheleft.Theapproximatescaleisshowninthe
rightcorner. figure.
Table1.Theidentificationnumber(ID),(α,δ)forepoch2000,
TheobservationsofthefieldofGRB060124werecarried standardV,(B−V),(V−R)and(R−I)photometricmagnitudes
out towards a later epoch on 2006 February 16 with the 2.5- ofthesecondarystandardsintheGRB060124field
mIssacNewtonTelescope(INT)atLaPalma,Spain.Figure2
showsthezoomedareaofthefieldofGRB060124whichdoes ID α δ V (B−V) (V−R) (R−I)
2000 2000
notrevealanyafterglowcandidateatthelocationoftheoptical (hms) (degms) (mag) (mag) (mag) (mag)
transient. 1 050741 694102 16.99 1.41 0.92 0.83
A full compilation of BVRI magnitudes of the optical af- 2 050759 694148 18.83 1.61 1.19 1.42
terglow differentially calibrated with respect to the secondary 3 050757 694221 15.45 0.99 0.53 0.49
4 050754 694252 17.05 1.08 0.63 0.59
stars numbered 1, 2, 3, 4, 5 and 6 as listed in Table 1 is pre-
5 050808 694112 17.06 0.82 0.45 0.45
sented in Table 2. The photometric magnitudes reported by
6 050811 694151 16.66 0.86 0.49 0.47
Masettietal.(2006)wereconvertedtothepresentphotometric
7 050815 694232 17.12 0.74 0.39 0.41
scaleusingthesecondarystarslistedinTable1.
8 050812 694316 18.08 0.82 0.45 0.44
9 050833 694342 16.78 0.95 0.51 0.46
10 050827 694451 16.83 1.42 0.89 0.77
3. MultibandOpticalLightCurve
11 050825 694514 15.27 0.77 0.42 0.41
12 050835 694510 14.93 0.72 0.39 0.39
Figure3showsthetemporalevolutionoftheGRB060124af-
13 050831 694428 18.09 0.95 0.53 0.46
terglow in BVRI bands based on the data presented here and
14 050850 694343 16.56 0.88 0.49 0.46
those available in the literature. The published magnitudesin
15 050856 694352 16.34 0.86 0.47 0.45
the optical bands were brought to our photometry level us- 16 050915 694351 19.18 1.49 1.04 1.07
ing the 6 secondary standard stars as mentioned in Section 17 050901 694525 15.71 0.69 0.38 0.39
2. The frequency distribution of our data in BVRI bands is 18 050847 694550 17.41 0.67 0.37 0.39
N(B,V,R,I) = (6, 13, 20, 9). It is to be noted here that we 19 050832 694616 17.89 0.75 0.43 0.43
present the earliest ground based R band observations which 20 050825 694629 18.18 0.85 0.48 0.49
have placed important constraints on the overall evolution of 21 050817 694744 16.09 0.93 0.51 0.48
theafterglow.TheX-axishererepresentslog∆t(=t−t )where 22 050811 694716 16.01 0.67 0.38 0.38
0
23 050802 694608 16.64 1.07 0.58 0.57
tisthetimeofobservationandt =2006January24.6331UT
0
24 050758 694444 15.40 0.63 0.35 0.38
istheburstepoch.
25 050754 694340 17.18 0.99 0.56 0.53
Since we havethe R bandobservationsfrom∆t = 0.03to
2.0day,wenoticedthattheRbandfluxdecayoftheafterglow
of GRB 060124 can be well characterized by a single power
4 KuntalMisraetal.:OpticalObservationsofGRB060124Afterglow
a few other GRB afterglowssuch as GRB 000301C(Sagaret
1144
al.2000),GRB021004(Pandeyetal.2003,Bersieretal.2003,
Bjo¨rnsson et al. 2004), GRB 021211 (Li et al. 2003), GRB
1166 050319(Wozniaketal.2005)andGRB051028(Castro-Tirado
et al. 2006). While some of the reported flicker could arise
fromvariableobservingconditions(Wozniaketal2005),most
1188
of the observed flickering appears to be real. Short timescale
variabilitycould,inmanycases,beattributedtodensityvaria-
2200
tionsintheexternalmedium(Wang&Loeb2000).However,in
somecaseslateinjectionofenergyfromthecentralenginepro-
2222 videabetterexplanation(e.g.ZhangandMeszaros2002,Liet
al.2003).Gravitationalmicrolensinghasbeenproposedasthe
2244 cause of achromatic variability in GRB 000301C (Garnavich
et al. 2000),butthis interpretationremainsambiguous.In the
case of GRB 060124,the flickering is observedin all the op-
2266
ticalbandsaswellasinX-rays.Timescalesofthisvariability
areshortcomparedtotheelapsedtimesinceburst,andthereis
2288
--11..66 --11..22 --00..88 --00..44 00 00..44 00..88 11..22 11..66 noevidenceseenofpersistentincreaseinthelightcurvelevel
followingtheflickerepisodes.Thisnatureismoreakintothe
variability expected due to inhomogeneities in the density of
Fig.3. Optical light curve of the GRB 060124 afterglow in
thecircumburstmaterial.
BVRI bands. The light curvesin differentbandsare offset by
a numberas indicated to avoid overlapof the data. Filled cir-
clesrepresentthedatafromthepresentmeasurementswhereas 4. DiscussionsandConclusions
unfilledcirclesrepresentthedatatakenfromtheliterature.The
We present in this paper the broad-band BVRI optical obser-
solidlineindicatestheleastsquarebestfittedvalueofthetem-
vationsusingthe1.04-mSampurnanandTelescopeatARIES,
poral index. Also shown is the upper limit derived using the
Nainital and the 2.01-m HCT at IAO, Hanle during 2006
observationsatalaterepoch.
January 24 to 26. We report the earliest optical observations
in the R band at ∆t < 1 hr. The X-ray light curve showed a
“flaring”promptemission.Thepromptemissionwasalsoob-
lawwrittenas
servedatopticalwavelengthsbutnoflaringactivitywasseen.
The availabledata in the opticalband(Swift UVOT+ ground
F(t)∝t−α basedobservations)isnotsufficienttocomparethepromptop-
ticalemissionwiththatingammarayandX-raybands.
where F(t) is the flux of the optical afterglow at time t since We study the optical afterglowevolutionfrom 0.03 to 2.0
the burst and α is the temporal flux decay index. The above day. The optical afterglow is well characterized with a single
function was fitted to the R band data using the least square powerlaw decaywith a flux decayindexof 0.94± 0.01.The
regression method. This yields the value of α = 0.94 ± 0.01. late INT upper limit shows that any contaminationfrom pos-
We assumed the same power law decay for B, V and I bands siblesteadysourcescoincidentwiththeafterglowisfainterby
and fitted the above function which seems to agree with the atleast2magnitudescomparedto thelastobservedafterglow
observeddata points. The linear least square fit to the data in magnitudeat∼2days.Thisensuresthatsteepeningoftheop-
thedifferentbandsisshowninfigure3. ticallightcurve,ifany,within2dayscouldnotbemaskedby
Our observations extend upto ∆t = 2 days after the burst asteadysourcecontribution.Table3liststhecolorsoftheaf-
includingtheearliestRbanddataat∆t=0.03dayanditdoes terglow at ∆t ∼ 1 and 2 days after the burst. The spectral in-
not show any noticeable steepening in the light curve during dex(β )oftheafterglowofGRB060124intheopticalband
opt
thisentireperiodofobservations. is estimated using the flux normalizationat differentfrequen-
Towards a later epoch on 2006 February 16, the field of cies from the light curves. The estimated value of Galactic
GRB 060124 was observed with the 2.5-m INT at La Palma, Interstellar Extinction in the direction of the burst, using the
Spain.Wedonotseetheopticalafterglowinaco-addedframe reddening map provided by Schlegel et al. (1998) is E(B-V)
of3×600secexposureatthelocationoftheopticaltransient =0.135mag.Theapparentmagnitudes,correctedforGalactic
andthusputan upperlimitof 22.5mag atthe locationof the extinctiononly as the extinctiondue to the host galaxyis un-
afterglow.There is also no sign of any galaxyat the location. known, were convertedto flux using the normalizationin the
Thedisappearanceoftheafterglowisapparentinfigure2when BVRIbandsbyBesseletal.(1998).Thespectrumisdescribed
comparedtofigure1. bya single powerlaw F ∝ ν−β, where F isthe fluxata fre-
ν ν
Both optical and X-ray light curves of GRB 060124 ob- quencyνandβisthespectralindex.Thevalueofβ obtained
opt
tainedfromSwiftshowaremarkableflickeringwhichcanalso inthiswayis0.73±0.08.
beseenatlatetimesinouropticalobservationsatallthepass- Romanoetal.(2006)triedtofittheXRTlightcurvebeyond
bands.Achromaticflickeringinopticalbandshasbeenseenin 104secwithasinglepowerlawwhichgivesafluxdecayindex
KuntalMisraetal.:OpticalObservationsofGRB060124Afterglow 5
ofα=1.36±0.02.Buttheyfindasignificantimprovementin Wei and Lu (2002)proposethatthe multi-wavelengthspectra
thefitwithabrokenpowerlawwithabreaktimet =1.05+0.17 of GRB afterglowsmay notbe made ofexactpowerlaw seg-
b −0.14
×105sec,α =1.21±0.04andα =1.58±0.06(∆α=0.37). ments but with a smooth change in slope and some breaksin
1 2
Romanoetal.(2006)considerthisbreakintheXRTlightcurve theGRBafterglowlightcurvesmaybecausedbysuchcurved
tooshallowtobeinterpretedasajetbreak.TheopticalUVOT spectra.
datafromSwiftisavailableonlyfort<105sandhencecannot Foraninjectionbreak(γ)intheelectronenergyspectrum
i
beusedtoexamineapossiblebreakaroundthisepoch. evolvingasγ ∝ Γq, whereΓisthebulkLorentzfactorofthe
i
Curranetal.(2006)alsomentionabreakintheX-rayand shock(seeBhattacharya2001),wecandescribethefluxevolu-
opticallightcurveat0.77daybuttheydonotquotethequality tionatagivenobservingfrequencyν as
obs
ofthefit.Theyalsomentionthattheachromaticbreakseenin
theX-rayandopticallightcurvesdiffersignificantlyfromthe (t)−23β1
F ∝ ti (1)
utismuealtejemtpboreraalkdwechaicyhraisteasprpeaproernttedinbGyRCBuraraftneregtlaolw.s(2.0T0h6e)laartee νobs 1+(tti)34(1+q)(β2−β1)
unequalinX-rayandopticalbands,andbothareshallowerthan which is a generalisation of the expression of Wei and Lu
expectedinthecaseofaregularjetbreak. (2002) who use the evolution of a regular cooling break for
Withourcoverageofopticaldata,wealsotrytocheckfora whichq=-1/3.β andβ aretheradiationspectralindicesbe-
1 2
possiblejetbreakbyfittingadoublepowerlawtotheobserved lowandabovetheinjectionbreakrespectively.
data.However,combiningtheearlyandlatetimeobservations Based on this model,we havefitted the X-rayand optical
in R band we do not find any convincing evidence for a jet afterglowlightcurveofGRB060124.Foranassumedq=-1/3,
breakaround105sasseenintheX-raylightcurvebyRomano thebestfitvaluesobtainedfortheX-raylightcurvearet =0.55
i
et al. (2006) and there is no considerable steepening seen in ±0.11d,β =0.59±0.07andβ =1.95±0.70withχ2/d.o.f.=
1 2
the light curve till ∼ 2 days after the burst. Using the same 2.1.FitstotheRbandlightcurvewiththesamevaluesoft,β
i 1
values of α , α and t as in the X-ray band (Romano et al. andβ isruledoutatahighconfidencelevelwithaχ2 /d.o.f.
1 2 b 2
2006),we attempt a broken power law fit to the R band light = 225.Thus, the breakwhichis seen in the X-raylightcurve
curve.Thisyieldsaveryhighvalueofχ2 /d.o.f.=269which is not an achromatic break. A good fit with the same β and
1
clearlyrulesoutthepossibilityofasimilarbreakintheoptical β isobtainedfortheRbandopticallightcurvewithχ2/d.o.f.
2
lightcurve.Eventheinclusionofapossiblecontributionfrom = 3.8 if the optical bandbreak time is assumed to be t >> 2
i
a steadysource,consistentwiththeINTupperlimit, doesnot day, resulting in a single power law evolution with slope α =
improvethebrokenpowerlawfitappreciably.Similarlyusing 3β /2.ThefitstotheX-rayandRbandlightcurvesareshown
1
thevaluesreportedbyCurranetal.(2006)fortheopticaldata, infigure4.Thevalueofα=0.89±0.11obtainedthiswayis
ofbreaktime t = 0.77day,α = 0.77andα = 1.30we fit a consistentwiththesinglepowerlawfittothelightcurveofall
b 1 2
broken power law to the R band light curve. This fit is ruled optical bandspresented above. We conclude that the break in
outatahighconfidencelevelwithaχ2 /d.o.f.=15.Thus,we theX-raybandat∼105secmentionedbyRomanoetal.(2006)
concludethattheopticallightcurveiswellcharacterizedbya is not a jet break, but is possibly due to the steepening of the
singlepowerlaw. electron energy spectrum at high energies. The derived value
BasedonthefitsmadetotheXRTlightcurvebyRomano of the high energyradiation spectral index β dependson the
2
et al. (2006) and optical light curve (present work), we see a valueofqforwhichnoindependentmeasurementexistsinthis
breakwhichispresentintheXRTlightcurvebutnotintheop- case since thepassageofthe injectionbreakthroughmultiple
ticallightcurve.Thus,thisisafrequencydependentbreakand frequency bands has not been monitored. The dependence of
is therefore not of dynamical origin. Steepening of this kind derivedβ onthe assumedvalueofq in the range-1/3to 1 is
2
reflectsabreakintheenergydistributionoftheradiatingelec- displayedinfigure5.
trons. Such a break, most commonly, is attributed to the ef- Theexistenceofaninjectionbreakwithintheobservedfre-
fectofsynchrotroncooling.Synchrotroncoolingresultsinthe quencybandshavebeeninferredforseveralotherGRB after-
steepening of the slope of the electron energy distribution by glowstoo(see,forexample,PanaitescuandKumar2002).This
oneandacorrespondingchangeinthelightcurveslopeby0.25 providesanimportantcluetotheparticleaccelerationprocess
(beforethejetbreak).The breakthatisseen in theXRT light operating in these afterglows. The steepening of the injected
curve is however much steeper than 0.25. We tried to fit the electron energy spectrum indicates an upper cutoff, or reduc-
XRTlightcurvebyfixingalightcurveslopeof0.25(similarto tion in efficiency, of the particle acceleration, which may be
thatrequiredbyacoolingbreak)andruleoutsuchafitwitha dueeithertoradiationlosseswithintheaccelerationcycletime
highconfidencelevel(bestfitχ2 /d.o.f.=31).Thesteepening orsimplythelimitedresidencetimeofelectronswithintheac-
observed in the XRT light curve must therefore be attributed celeration zone (Gallant and Achterberg 1999; Gallant et al.
to a different cause. It might be that the energy spectrum of 1999).TheappearanceoftheinjectionbreakintheX-rayband
the injected electrons steepens beyond a certain high energy around ∼ 1 day after the burst implies that the upper cutoff
limit(see,eg.PanaitescuandKumar2001).Suchan“injection Lorentz factor in this case lies in the range 106–108, depend-
break”asitpassesthroughanobservingfrequency,leadsto a ingonthefractionofthetotalenergyinthemagneticfieldand
correspondingbreakintheradiationspectrum,andtoasteep- otherphysicalparametersoftheafterglow.Inthepresentcase,
eningofthelightcurve.Wemaymodelsuchanevolutionusing goodconstraintscanunfortunatelynotbeobtainedonthephys-
a generalform of the expressionused by Wei and Lu (2002). ical parameters, in the absence of extensive multiwavelength
6 KuntalMisraetal.:OpticalObservationsofGRB060124Afterglow
∼2daysaftertheburst,nojetbreakhasoccurredinthisafter-
glow.Whilemanyafterglowsexhibitjetbreakrelativelyearly,
thosewithjetbreakbeyond∼2daysarenotrare.Examplesin-
3.6 cludewellknownafterglowsofGRB 000301C,GRB 011211
and GRB 021004. In fact in a sample of 59 GRB afterglows
2.4 examined by Zeh et al. (2006),five had jet break around ∼ 2
daysandsixhadjetbreakbetween3to8daysaftertheburst.
Thelowerlimittothejetbreaktimementionedabovecan
1.2
be used to put constraints on the jet opening angle and total
energycontentintheafterglow.Usingtheisotropicequivalent
0.0
energyof8.9×1052ergderivedfromγ-rayfluencebyCenko
etal. (2006)anda lowerlimitto the jetbreaktime of2 days,
-1.2
weestimatethelowerlimittothejetopeningangletobe∼4.75
degreefor anassumedambientdensityof 1atom cm−3 andγ
-2.4
-rayefficiencyof0.2.This,inturn,yieldsalowerlimittothe
total energy output of 0.31 × 1051 erg, close to the estimated
-3.6 meanenergyoutputinGRBs(Frailetal.2001).
-1.8 -1.2 -0.6 0.0 0.6 1.2 1.8 It is interesting to note that using the empirical relation
(Ghirlandaetal.2005,Liang&Zhang2005)betweenisotropic
equivalentphotonenergyoutputintheburst,thepeakenergyin
Fig.4. The R band and X-ray light curve of GRB 060124af-
theburstandthejetbreaktime,Romanoetal.(2006)predicted
terglow.ThefilledcirclesrepresenttheX-rayfluxandtheopen
ajetbreaktimeof(2.1±1.2)×105sforthisafterglow,which
circlestheRbandflux.Thesolidlineshowsthebestfitassum-
isconsistentwiththelowerlimittothejetbreaktimediscussed
ingthemodeldescribedbyWeiandLu(2002).
byus.
The above discussion demonstrates that observations of
GRBafterglowscontinuetounveilsurprisingfeaturesintheir
evolutionandrevealaspectsofunderlyingphysicsthatremain
2.2 tobeunderstood.
2 Acknowledgements. We thank the HCT and ARIES observers for
kindly sparing their observing time for these observations. We ac-
1.8 knowledgeK.Viirone(IAC),J.A.Caballero(IAC,ING)andL.Sabin
(ING) for obtaining the INT image on 15/16 February 2006. This
1.6 research has made use of data obtained through the High Energy
Astrophysics ScienceArchive Research Center Online Service,pro-
1.4 videdbytheNASA/GoddardSpaceFlightCenter.TheGCNsystem,
managed and operated by Scott Barthelmy, is gratefully acknowl-
edged.Oneoftheauthors(KM)thanksL.ResmiandBrijeshKumar
1.2
forseveralusefuldiscussionswhiledraftingthemanuscript.Wethank
theanonymousrefereeforconstructivecommentswhichimprovedthe
1
manuscript.
0.8
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8 KuntalMisraetal.:OpticalObservationsofGRB060124Afterglow
Table3.ColorsoftheGRB060124Afterglowatδt=1.04and
1.99day
δt (B−R) (B−V) (V−R) (R−I)
days mag mag mag mag
1.04 0.79±0.05 0.41±0.05 0.37±0.06 0.61±0.06
1.99 0.64±0.08 0.13±0.08 0.51±0.07 0.47±0.09