Table Of ContentAstronomy&Astrophysicsmanuscriptno.Hi071 February5,2008
(DOI:willbeinsertedbyhandlater)
Optical emission from GRB050709: a short/hard GRB in a
⋆
star-forming galaxy
6
0 S.Covino1,D.Malesani2,G.L.Israel3,P.D’Avanzo1,4,L.A.Antonelli3,G.Chincarini1,5,D.Fugazza1,M.L.
0
Conciatore3,M.DellaValle6,F.Fiore3,D.Guetta3,K.Hurley7,D.Lazzati8,L.Stella3,G.Tagliaferri1,M.Vietri9,S.
2
Campana1,D.N.Burrows10,V.D’Elia3,P.Filliatre11,12,N.Gehrels13,P.Goldoni11,12,A.Melandri3,14,S.
n
Mereghetti15,I.F.Mirabel16,A.Moretti1,J.A.Nousek10,P.T.O’Brien17,L.J.Pellizza12,R.Perna8,S.Piranomonte3,
a
J P.Romano1,andF.M.Zerbi1
0
1 1 INAF,OsservatorioAstronomicodiBrera,viaE.Bianchi46,I-23807Merate(Lc),Italy
2 InternationalSchoolforAdvancedStudies(SISSA-ISAS),viaBeirut2-4,I-34014Trieste,Italy
2
3 INAF,OsservatorioAstronomicodiRoma,viadiFrascati33,I-00040MonteporzioCatone(Roma),Italy
v
4 DipartimentodiFisicaeMatematica,Universita`dell’Insubria,viaValleggio11,I-22100Como,Italy
4
4 5 Universita`deglistudidiMilano-Bicocca,DipartimentodiFisica,piazzadelleScienze3,I-20126Milano,Italy
1 6 INAF,OsservatorioAstrofisicodiArcetri,largoE.Fermi5,I-50125Firenze,Italy
9 7 UniversityofCalifornia,Berkeley,SpaceSciencesLaboratory,Berkeley,CA94720-7450,USA
0 8 JILA,UniversityofColorado,440UCB,BoulderCO80309-0440,USA
5 9 ScuolaNormaleSuperiore,piazzadeiCavalieri7,I-56126Pisa,Italy
0 10 DepartmentofAstronomy&Astrophysics,PennsylvaniaStateUniversity,StateCollege,PA16801,USA
h/ 11 LaboratoireAstroparticuleetCosmologie,UMR7164,11PlaceMarcelinBerthelot,F-75231ParisCedex05,France
p 12 Serviced’Astrophysique,DSM/DAPNA,CEASaclay,F-91911Gif-sur-YvetteCedex,France
- 13 NASA,GoddardSpaceFlightCenter,Greenbelt,MD20771,USA
o 14 Universita`degliStudidiCagliari,DipartimentodiFisica,I-09042Monserrato(Ca),Italy
r
t 15 INAF/IASFMilano“G.Occhialini”,viaE.Bassini15,I-20133Milano,Italy
s 16 EuropeanSouthernObservatory-Vitacura,Casilla19001,Santiago19,Chile
a
: 17 X-Ray&ObservationalAstronomyGroup,Dept.ofPhysics&Astronomy,UniversityofLeicester,LeicesterLE17RH,UK
v
i
X
r Abstract.
a Wepresentopticalobservationsoftheshort/hardgamma-rayburstGRB050709,thefirstsucheventwithanidentifiedoptical
counterpart.TheobjectiscoincidentwithaweakX-raysourceandislocatedinsideagalaxyatredshiftz=0.1606±0.0002.
Multiband photometry allowed us to study the broad-band spectral energy distribution. Late-time monitoring places strong
limits on any supernova simultaneous with the GRB. The host galaxy is not of early type. Spectra show that the dominant
stellarpopulationisrelativelyyoung(∼1Gyr),andthatongoingstarformationispresentatalevelof2–3L/L∗ M⊙/yr.Thisis
atleast2ordersofmagnitudelargerthanthatobservedintheellipticalhostsoftheshortGRB050509BandGRB050724.This
showsthatatleastsomeshortGRBsoriginateinayoungpopulation.Short/hardGRBmodelsbasedonthemergerofabinary
degenerate system are compatible withthe host galaxy characteristics, although there isstill thepossibility of a connection
betweenyoungstarsandatleastafractionofsuchevents.
Keywords.Radiationmechanisms:non-thermal–Gammarays:bursts–Gammarays:individualGRB050709
1. Introduction ones(duration<2s).Overthepastyears,greatadvanceshave
beenmadeinunderstandingtheformerclass,thankstotheob-
Gamma-raybursts(GRBs)areshortpulsesofgammaraysoc-
servationoftheiropticalandradiocounterparts.However,until
curringatrandompositionsinthesky.TwoclassesofGRBsare
recently,noopticalemissionfromshortGRBshadbeenidenti-
currentlyknown,characterisedbydifferentdurationsandspec-
fied(Hurleyetal.,2002),leavingfundamentalquestionsabout
tral properties (Kouveliotouetal., 1993). Long GRBs (typi-
theirnature,progenitorsanddistancesunanswered.
callylasting10–100s)areontheaveragesofterthantheshort
Recently,thanksto the Swift andHETE-2satellites, accu-
Sendoffprintrequeststo:S.Covino,e-mail:[email protected] rateandrapidlocalisationsofshortGRBshavebecomeavail-
⋆ Based onobservations carriedout atESOtelescopesunder pro- able, enabling deep, sensitive searches at long wavelengths.
grammesId075.D-0787and075.D-0468. Swift discovered a weak X-ray counterpart to GRB050509B
2 Covinoetal.:GRB050709inastarforminggalaxy
Fig.1.R-bandimageofthefieldofGRB050709,on2005Jul12.41(A)and20.42(B).PanelCshowstheresultofthesubtraction,
evidencingafadingsourcecoincidentwiththeChandracounterpart(circle).Theboxescoveraregion30′′×20′′wide.
(Gehrelsetal.,2005),located∼ 11′′ awayfromabrightellip- Table 1. Observation log and photometry of the transient
tical galaxy at z = 0.2248 (Bloometal., 2005; Hjorthetal., source.Errorsareatthe1σconfidencelevel,whileupperlimits
2005a; Castro-Tiradoetal., 2005). An early-type galaxy areat3σ.DatawerenotcorrectedforGalacticextinction.
(Gorosabeletal., 2005) is also associated with GRB050724
Date Instrument Exp. Filter Magnitude
(Barthelmyetal., 2005)atz = 0.257(Prochaskaetal.,2005),
(UT) (min)
forwhichalso an opticalafterglowwassingledoutinsidethe
2005/07/1209:44 FORS2 6 V 24.38±0.10
host(Bergeretal., 2005a;D’Avanzoetal., 2005). Finally,the
2005/07/1407:21 FORS1 6 V >25.00
lineofsightofGRB050813(Retteretal.,2005)liestowardsa
2005/07/2010:16 FORS1 6 V (reference)
galaxyclusteratz=0.722.Afewellipticalgalaxieswereiden- 2005/07/3002:37 FORS2 9 V >25.20
tifiedinsidetheXRTerrorcircle,againsupportingtheassoci- 2005/07/1209:57 FORS2 5 R 23.83±0.07
ation with early-type galaxies (Gladdersetal., 2005; Berger, 2005/07/2010:07 FORS1 6 R (reference)
2005b;Prochaskaetal.,2005). 2005/07/3002:54 FORS2 50 R >25.00
GRB050709 was discovered by HETE-2 on 2005 Jul 2005/07/1209:32 FORS2 10 I >23.25
9.94209UT(Villasenoretal.,2005).Itspromptemissioncon- 2005/07/1407:32 FORS1 5 I >24.10
sisted of a single pulse lasting 70 ms in the 3–400keV band, 2005/07/1806:38 FORS1 20 I (reference)
followed by a weaker, soft bump ∼ 100 s long. This second 2005/07/3004:10 FORS2 9 I >23.50
episode may be due to the afterglow onset (Villasenoretal.,
2005),ortoflaringactivity(Barthelmyetal.,2005;Kingetal.,
tify variable objects even when blended with nearby sources.
2005; Pernaetal., 2005). In any case, the prompt emission The pointlike object was found to be variable (at the ∼ 10σ
propertiesareconsistentwiththoseofashort/hardGRB.
level),beingundetectablefromJul14onwards.Thisconfirms
Follow-up observations with the Chandra X-ray observa-
the independent finding of Hjorthetal. (2005b). Magnitudes
tory revealed a faint, uncatalogued X-ray source inside the
of the variable source were computed assuming a negligible
HETE-2 error circle (Foxetal., 2005a). At the coordinates
flux in the reference epoch (see Table 1). Photometry of the
α = 23h01m26s.9,δ = −38◦58′39′.′5(0′.′4error),itwas
J2000 J2000 transient was performed by inserting artificial stars of known
coincidentwithapointlikeobjectembeddedinabrightgalaxy
brightnessandcalibratedbyobservingLandoltstandardfields.
(Jensenetal.,2005)atz = 0.16(Priceetal.,2005a).Thevari-
On Jul 30 we took medium-resolutionspectra of the host
abilityofthissourceledPriceetal.(2005b)toproposeitasthe
galaxy.ObservationswerecarriedoutwiththeFORS2instru-
opticalcounterpartofGRB050709.
mentattheVLT-UT1,withthe300Vgrism,coveringthewave-
lengthrange6000–9200Å(6ÅFWHM).Fromthedetectionof
2. Observationsanddataanalysis severalemissionlines,amongthemHα,Hβ,and[OII],wede-
rivedaredshiftz=0.1606±0.0002.Thisisconsistentwiththe
We observed the field of GRB050709 with the ESO Very
resultsofFoxetal.(2005b).Therefore,therest-frame B-band
LargeTelescope(VLT),usingtheFORS1andFORS2instru- luminosityof the hostis1 LB ∼ 3.5×1042 ergs−1 (∼ 0.10L∗,
ments. In our first images, taken on 2005 Jul 12, the point- assuming M∗ = −20.13asdeterminedfromtheSDSSsurvey;
like object reported by Jensenetal. (2005) was clearly visi- B
Blantonetal.,2003)andthecandidateafterglowliesatapro-
blein theV andRbands(Fig.1).Itscoordinatesareα =
J2000 jecteddistanceof≈3.3kpcfromthegalaxycore.
23h01m26s.96,δ = −38◦58′39′.′3(0′.′25error),fullyconsis-
J2000
tentwiththeChandraposition.Theobjectliesinsideabright
galaxy(≈ 1′.′2 away from its nucleus),whose magnitudesare 3. Discussion
V = 21.35±0.07,R = 21.08±0.07,andI = 20.63±0.08.To
Assuming a power-law flux decay (F(t) ∝ t−α), our observa-
searchforbrightnessvariations,wemonitoredthefieldatsev-
tionsconstrainαtobegreaterthan1.0intheV band(3σupper
eral epochs. Data analysis was performedby adopting a sub-
tractiontechnique(Alard&Lupton,1998),wellsuitedtoiden- 1 AssumingacosmologywithΩ =0.3,Ω =0.7,andh =0.71.
m Λ 0
Covinoetal.:GRB050709inastarforminggalaxy 3
Fig.2.Broad-bandspectralenergydistributionoftheafterglow Fig.3. DetailsoftheGRB050709hostgalaxyspectrumclose
ofGRB050709onJul12.4UT.Thebow-tieshapedregionrep- totheHαandHβlines,showingthatthenarrowemissionemis-
resent the simultaneous X-ray spectrum taken from Chandra sionsaresuperimposedonwiderabsorptionfeatures.
(Foxetal., 2005b). The dot-dashed and dotted lines indicate
theextrapolationoftheopticalandX-rayspectra,respectively.
identifies a dominant stellar population ∼ 1 Gyr old (mostly
A-dwarfstars),togetherwithayounger,hottercomponent.As
limit).Thislimitisconsistentwiththeopticaldecayfoundby
indicatedfrom the prominentnebular emission lines, star for-
Foxetal.(2005b)usingHSTdatabetween5and10dafterthe
mation is still present. From the Hα and [OII] emission lines
GRB (see also Hjorthetal., 2005b). A similar limit was also (havingluminosities2.62×1040and2.45×1040ergs−1,respec-
puttotheX-rayafterglowdecay.Flaringactivitywasreported tively),weinferastarformationrateof0.21and0.34M⊙ yr−1
in the X-raylightcurve(Foxetal., 2005b). Giventhe limited (Kennicutt, 1998), which corresponds to ∼ 2–3.5 M⊙ yr−1
availabledata,itisdifficulttosaywhetherasimilarbehaviour
oncenormalisedto L∗.Thisissignificantlylessthanthattyp-
waspresentalsointheopticalband.OurmeasurementsonJul
ically observedin longGRB hostgalaxies(Christensenetal.,
12arenearlysimultaneouswiththefirstChandraobservation, 2004),butmuchlargerthanthatinthehostsofGRB050509B2
sowecanconstructthebroad-bandspectralenergydistribution
(Bloometal., 2005) and GRB050724 (Bergeretal., 2005a),
(Fig.2).TheX-rayspectralindexissimilartothatoflongGRB
byfactorsof>50and>150,respectively.
afterglows(DePasqualeetal.,2003;Nouseketal.,2005).The
Themostpopularmodelforshort/hardGRBsisthemerger
extrapolation of the X-ray spectrum matches the optical flux
of a binary compact object system (e.g. Eichleretal., 1989).
(thatis,theoptical-to-X-rayslopeβ =1.1isconsistentwith
OX Such events can occur in a late-type, star-forming galaxy
theX-rayslopeβ = 1.24±0.35).Thespectrumcorrespond-
X (Belczyn´ski&Kalogera, 2001), and give rise to short GRBs
ing to the optical colours is quite red (β = 2.3± 0.7), but
opt (Perna&Belczyn´ski,2002).Sincethemergingtimescalesmay
givenitslargeuncertaintyitisconsistentwithβ atthe1.7σ
OX beoftheorderof10Myr,smalloffsetsbetweentheexplosion
level.Smalldustextinction(A ≈ 0.2mag;Foxetal.,2005b)
V site andthe galaxycoreare possible.Therefore,GRB050709
wouldmaketheintrinsiccolorbluerandfullyconsistentwith
might have been produced in a tightly bound system, with
the X-ray slope, so that the optical and X-ray emission may
a short merging time, similar to GRB050724 (Bergeretal.,
constituteasinglecomponent.However,thesparsenessofthe
2005a). However, we also note that according to the stan-
datapreventsusfromdrawinganyrobustconclusion.
dard Faber-Jackson relation, the escape velocity from the
Thepropertiesofthehostgalaxyareintriguing.Itscolours GRB050709 host is quite large (about 300 km s−1), so that
are consistent with those of an irregular galaxy at z ≈ 0.2
onlyafractionofbinarysystemsmaybeabletoescapeitspo-
(Fukugitaetal.,1995),andaremuchbluerthanthoseofellip- tential well. In this case, a larger delay (∼ 1 Gyr) would be
ticals(likethoseassociatedwithothershortGRBs).Moreover,
consistentbothwiththeobservedoffsetandwiththeageofthe
hintsofmorphologicalstructureareseeninourbest-seeingim-
older stellar population. A large instantaneous star formation
ages.AcloseinspectionofthespectrumshowsthatboththeHα
rate would not be expected in this case, even if this does not
and Hβ lines have a narrow emission component (FWHM <
poseanyproblemforthemergermodel.
6Å)partiallyfillingawider(∼ 12ÅFWHM)absorptionfea-
ture(Fig.3).Thisclassicalsignature(Dressler&Gunn,1983) 2 However,manybluegalaxieswerelocatedintheXRTerrorcircle.
4 Covinoetal.:GRB050709inastarforminggalaxy
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The presence of pronouncedstar formationactivity in the
437,851
host galaxy of GRB050709, however, prompts us to inves-
Gladders,M.,Berger,E.,Morrell,N.,&Roth,M.2005,GCN
tigate whether this event could be directly related to young
3798
stars. Recently, it was proposedthat shortGRBs may be pro-
Gorosabel,J.,Castro-Tirado,A.J.,Guziy,S.,etal.2005,A&A,
duced by giant flares from soft gamma-ray repeaters (e.g.
inpress(astro-ph/0510141)
Hurleyetal., 2005). However,the luminosityof GRB050709
Hjorth,J.,Sollerman,J.,Gorosabel,J.,etal.2005a,ApJ,630,
would be a factor ∼ 103 larger than that of the giant flare
L117
fromSGR1806-20.Thepromptemissionpropertiesalsomake
Hjorth,J.,Watson,D.,Fynbo,J.P.U.,etal.2005b,Nature,437,
thishypothesisunlikely(Villasenoretal.,2005).Furthermore,
859
our photometry can put strong constraints on the presence of
Hurley, K., Berger, E., Castro-Tirado, A.J., et al. 2002, ApJ,
an unextinguished supernova (SN) exploded simultaneously
567,447
with the GRB (see Fig. 4). Our limits impose a SN >∼ 100
Hurley,K.,Boggs,S.E.,Smith,D.M.,etal.2005,Nature,434,
times fainter than a typical type-Ia SN or a bright hypernova
1098
like SN1998bw. Also fainter events like SN1994I and even
Jensen, B.L., Jo¨rgensen, U.G., Hjorth, J., et al. 2005, GCN
SN1987Aareincompatiblewithourdata.Anassociationwith
3589
a SN seems therefore ruled out for GRB050709 (see also
Kennicutt,R.C.Jr1998,ARA&A,36,189
Hjorthetal.,2005a,b;Foxetal.,2005b).Thepropertiesofthe
King, A., O’Brien, P.T., Goad, M.R., et al. 2005, ApJ, 630,
GRB050709hostare howeverconsistentwiththemodelpro-
L113
posedbyMacFadyenetal.(2005),whichadvocatesacollaps-
Kouveliotou,C.,Meegan,C.A.,Fishman,G.J.,etal.1993,ApJ,
ingneutronstar accretingfroma closenon-compactcompan-
413,101
ion. Such model would also naturally explain the flares ob-
MacFadyen, A.I., Ramirez-Ruiz, E., & Zhang, W. 2005,
servedintheX-raylightcurve.
astro-ph/0510192
Perna,R.,&Belczyn´ski,K.2002,ApJ,570,252
Acknowledgements. DMthanksINAFandtheItalianMIURforsup-
port. This research was supported at OABr and OAR by ASI grant Perna,R.,Armitage,P.J.,&Zhang,B.2005,ApJ,636,L29
I/R/039/04. Weacknowledge theexcellent support of theESOstaff. Nousek,J.A.,Kouveliotou,C., Grupe,D.,etal.2005,ApJ,in
KH is grateful for support under MIT-SC-R-293291 and FDNAG5- press(astro-ph/0508332)
9210.Wealsothanktheanonymousrefereeforher/hisvaluablecom- Price,P.A.,Roth,K.,&Fox,D.W.2005a,GCN3605
mentsandsuggestions. Price, P.A., Jensen, B.L., Jo¨rgensen,U.G., et al. 2005b,GCN
3612
Prochaska, J.X., Bloom, J.S., Chen, H.-W., et al. 2005, ApJ,
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