Identifying the Location in the Host Galaxy of the Short GRB 111117A with the Chandra Sub-arcsecond Position

1 MB·English

by  T. Sakamoto

Identifying the Location in the Host Galaxy of
the Short GRB111117A with the Chandra Sub-arcsecond Position
T. Sakamoto1,2,3,4, E. Troja1,3,5,6, K. Aoki7, S. Guiriec5, M. Im8, G. Leloudas9,18,
3
D. Malesani9, A. Melandri10, A. de Ugarte Postigo9,13, Y. Urata11, D. Xu12, P. D’Avanzo10,
1
0 J. Gorosabel13, Y. Jeon8, R. S´anchez-Ram´ırez13, M. I. Andersen9,19, J. Bai21,22,
2
S. D. Barthelmy3, M. S. Briggs25, S. Foley26, A. S. Fruchter15, J. P. U. Fynbo9,

n N. Gehrels3, K. Huang14, M. Jang8, N. Kawai16, H. Korhonen19,24, J. Mao21,22,23,
a
J J. P. Norris17, R. D. Preece25, J. L. Racusin3, C. C. Th¨one13, K. Vida20 X. Zhao21,22

7

]
E
H ABSTRACT
.
h WepresentoursuccessfulChandraprogramdesignedtoidentify,withsub-arcsecondaccuracy,
p theX-rayafterglowoftheshortGRB111117A,whichwasdiscoveredbySwiftandFermi. Thanks
-
o toourrapidtargetofopportunityrequest, Chandra clearlydetected the X-rayafterglow,though
r noopticalafterglowwasfoundindeepopticalobservations. Thehostgalaxywasclearlydetected
st in the optical and near-infrared band, with the best photometric redshift of z = 1.31+0.46 (90%
−0.23
a
confidence), making it one of the highest known short GRB redshifts. Furthermore, we see an
[
offset of 1.0± 0.2 arcseconds, which corresponds to 8.4± 1.7 kpc, between the host and the

2 afterglow position. We discuss the importance of using Chandra for obtaining sub-arcsecond
v
X-ray localizations of short GRB afterglows to study GRB environments.
4
7 Subject headings: gamma rays: bursts
7
6
5. 1Center forResearchand ExplorationinSpace Science 46,I-23807Merate(LC),Italy
0 and Technology (CRESST), NASA Goddard Space Flight 11Institute of Astornomy, National Central University,
2 Center,Greenbelt, MD20771 Chung-Li32054,Taiwan
1 2JointCenterforAstrophysics,UniversityofMaryland, 12Department of Particle Physics and Astronomy, The
: Baltimore County, 1000 Hilltop Circle, Baltimore, MD
v 21250 WeizmannInstituteofScience,Rehovot 76100,Israel
Xi 3NASA Goddard Space Flight Center, Greenbelt, MD de13laInAstsittruotnoodme´ıaAss/tnro,f´1ıs8i0ca08deGAranndaadlau,c´ıSap(aCinSIC),Glorieta
20771
r 14Academia Sinica Institute of Astronomy and Astro-
a 4Department of Physics and Mathematics, College of physics,Taipei106,Taiwan
Science and Engineering, Aoyama Gakuin University, 5-
15Space Telescope Science Institute, 3700 San Martin
10-1Fuchinobe,Chuo-ku,Sagamihara-shi,Kanagawa252-
Drive,Baltimore,MD21218
5258,Japan
5NASA Postdoctoral Program Fellow, Goddard Space 16DepartmentofPhysics,TokyoInstituteofTechnology,
2-12-1Ookayama,Meguro-ku,Tokyo152-8551, Japan
FlightCenter,Greenbelt,MD20771
6Joint Center for Astrophysics, Department of Astron- 17PhysicsDepartment,BoiseStateUniversity,1910Uni-
versityDrive,Boise,ID83725
omy,UniversityofMarylandCollegePark,MD20742-2421
7SubaruTelescope,NationalAstronomicalObservatory 18TheOskarKleinCentre,DepartmentofPhysics,Stock-
holmUniversity,10691Stockholm, Sweden
ofJapan, 650NorthA’ohokuPlace,Hilo,HI96720
8CenterfortheExplorationoftheOriginoftheUniverse 19Niels Bohr Institute, University of Copenhagen, Ju-
lianeMariesVej30,2100Copenhagen, Denmark
(CEOU),DepartmentofPhysicsandAstronomy,SeoulNa-
tionalUniversity,Seoul,151-747,Korea 20Konkoly Observatory of the Hungarian Academy of
9DarkCosmologyCentre,NielsBohrInstitute, Univer- Sciences, Konkoly Thege u´t 15-17, 1121, Budapest XII,
Hungary
sity of Copenhagen, Juliane Maries Vej 30, 2100 Copen-
hagenØ,Denmark 21Yunnan Astronomical Observatory, Chinese Academy
10INAF-OsservatorioAstronomicodiBrera,viaBianchi
1
1. Introduction andSwift(Gehrels et al. 2004). Theveryfirstlo-
calizationsofshortGRBsimmediatelyprovidedus
Gamma-raybursts(GRBs)aretraditionallydi-
with fundamental clues about their nature. They
vided in two classes based on their duration and
demonstrated that short GRBs are cosmological
spectral hardness: the long duration/soft spec-
eventswithanisotropicequivalentenergyscaleof
trum GRBs, and the short duration/hard spec- 1049-1052erg,thatthey occurindifferentenviron-
trum GRBs (Kouveliotou et al. 1993). The two
ments than long GRBs, and are not associated
classes of bursts further differ in their spectral
with bright Type Ic supernovae (Bloom et al.
lags, the measurement of the delay in the arrival
2006; Prochaska et al. 2006; Covino et al. 2006).
time of the low-energy photons with respect to
Since 2005 the sample of well-localized short
the higher energy ones: long bursts tend to have
GRBs has significantly grown, allowing for a
large positive lags,while shortbursts exhibit neg-
deeper insight into the nature of their progeni-
ligible or negative lags (Norris & Bonnell 2006).
tors. The observed redshift distribution, ranging
The long standing paradigm is that these two
0.11 . z . 1, hints at a progenitor system with
phenomenological classes of GRBs originate from
a broad range of lifetimes (Berger et al. 2007).
different progenitor systems. A preponderance of
Another critical test of the progenitor models is
evidence now links long GRBs with the death of
the observed offset distribution of short bursts
massive stars (Woosley & Bloom 2006, and refer-
(Troja et al.2008;Fong et al. 2010;Church et al.
ences therein), yet the origin of short GRBs re-
2011). The median physical projected offset be-
mains largelyunknown. The commonnotionthat
tween the host center and the short GRB po-
short bursts originate from coalescing compact
sition is ∼5 kpc (Fong et al. 2010), which is
binaries, either neutron star-neutron star (NS-
about five times larger than that of long GRBs
NS) or neutron star-black hole (NS-BH) merg-
(Bloom et al. 2002), andshowsabroaderdisper-
ers (e.g., Eichler et al. 1989; Paczynski 1991;
sion. This is in agreement with the merger sce-
Narayanet al. 1992;Rosswog2005;Rezzolla et al.
nario, as several models NS-NS/NS-BH systems
2011), makes them the most promising tool to
are expected to receive significant kick velocities
aid in the direct detection of gravitational waves
at birth (Bloom et al. 1999; Fryer et al. 1999;
(GWs)byforthcomingfacilitiessuchasAdvanced-
Belczynski et al. 2006), or to dynamically form
LIGO, Advanced-VIRGO or LCGT (KAGRA)
in globularclustersin the outskirtsof their galax-
(e.g., Nissanke et al. 2010). It is therefore of pri-
ies (Grindlay et al. 2006).
mary importance to convincingly corroborate the
Despitethemajorprogressofthelastfewyears,
mergerscenariowitharobustobservationalbasis.
thestudyofshortGRBsandtheirprogenitorshas
Significant progress in understanding the ori-
stillbeensufferingfromtheirlesssecureafterglow
ginofshortGRBshasbeenachievedonlyrecently.
positions and redshifts. Unlike long GRBs, none
Thisadvancewasenabledbythedetectionoftheir
of the redshifts of short GRBs 1 has been di-
afterglowsin2005thankstotherapidpositionno-
rectly measured through afterglow spectroscopy,
ticeandresponsebyHETE-2(Ricker et al. 2003)
and only in the case of GRB060121, a photomet-
ric redshift was derived from the afterglow spec-
ofSciences, Kunming,YunnanProvince,650011, China
tralenergydistribution(de Ugarte Postigo et al.
22KeyLaboratoryfortheStructureandEvolutionofCe-
2006; Levan et al. 2006). This is because the
lestial Objects, Chinese Academy of Sciences, Kunming,
650011, China optical afterglows are significantly fainter than
23Space Science Division, Korea Astronomy and Space those of long GRBs (Nysewander et al. 2009;
Science Institute, 776, Daedeokdae-ro, Yuseong-gu, Dae- Kann et al. 2011). The redshifts of short GRBs
jeon,305-348,RepublicofKorea
are instead measured from spectroscopic observa-
24CentreforStarandPlanetFormation,NaturalHistory
Museum of Denmark, University of Copenhagen, Øster
Voldgade5-7,DK-1350Denmark 1Although the redshift of GRB 090426 (the t90 duration
25Center for Space Plasma and Aeronomic Research, measuredbyBATis1.24)hasbeenmeasuredfromtheab-
sorption spectroscopy (Levesque etal. 2010; Th¨oneetal.
UniversityofAlabamainHuntsville,320SparkmanDrive,
2011), there are growing observational evidences that the
Huntsville,AL35805,USA
progenitorofthisGRBisakintothatoflongGRBs(e.g.,
26Max-Planck-Institut fu¨r extraterrestrische Physik,
Xinetal. 2011;Lu¨ etal. 2010).
Giessenbachstrasse1,85748Garching,Germany
2
tions of the “associated” host galaxy. The like- localized short GRBs. This is the key to distin-
lihood of a spurious association is small when a guish between the different possible short GRB
sub-arcsec position is available. However, if an populations (Sakamoto & Gehrels 2009), which
afterglow is only detected by the Swift X-Ray could arise from a different progenitor and/or en-
Telescope (XRT; Burrows et al. 2005), the prob- vironment.
ability of a chance alignment is higher due to In this paper, we report the first results of our
the larger uncertainty in the localization (2-5′′). Chandra programwhich led to the accurate local-
Unfortunately, the latter scenario represents the ization of GRB 111117A detected by Swift and
majority of cases (∼65% of the Swift short bursts Fermi. GRB 111117A is the 2nd short burst2
sample). in which the Chandra position is crucial for the
A further bias is introduced by the fact that host identification. Our results were leveraged
sub-arcsecond positions are mainly derived from withanintenseground-basedfollow-upcampaign.
optical afterglow detections, which are subject No optical/infraredcounterpartwas found, there-
both to absorption along the line of sight and fore our Chandra localization uniquely provides
density effects. In fact, in the standard fireball theonlyaccuratesub-arcsecondposition. Thepa-
model, the optical brightness depends sensitively per is organized as follows: we introduce GRB
on the density of the circumburst environment 111117Ain§2. Insection§3,wedescribetheanal-
(Kumar & Panaitescu2000). This effect disfavors ysissoftwaresandmethodsusedinthispaper. We
the accurate localizationof shortGRBs occurring report the prompt emission properties in §4, the
in the lower-density galaxy halo or even outside X-ray afterglow properties in §5.1, the deep opti-
their own galaxy, in the intergalactic medium. cal afterglow limits in §5.2, and the host galaxy
Such populations of large-offest short GRBs has properties in §6. We discuss and summarize our
already been suggested by Bloom et al. (2007) results in §7. The quoted errors are at the 90%
and Troja et al. (2008). However being localized confidence level for prompt emission and X-ray
mainly by XRT, their association with the pu- afterglow data, and at the 68% confidence level
tative host galaxy remains uncertain. Increasing for optical and near infrared data unless stated
the sample of large-offset short bursts with sub- otherwise. The reportedopticaland near infrared
arcsecond localization is crucial to discriminate magnitudes are in the Vega system unless stated
whether their progenitors were ejected from their otherwise. Throughoutthe paper,we use the cos-
birthsite,favoringmodels whichpredictNSbina- mological parameters, Ω = 0.27, Ω = 0.73 and
m Λ
ries with large kick velocities and ∼Gyr lifetimes, H = 71 km s−1 Mpc−1.
0
or they were formed from dynamical interactions
in globular clusters (Salvaterra et al. 2010). 2. GRB111117A
In this context, rapid Chandra observations of
On 2011 November 17 at 12:13:41.921UT, the
shortGRB afterglowsrepresentthe criticalobser-
SwiftBurstAlertTelescope(BAT;Barthelmy et al.
vationalgatewaytoovercomethecurrentobserva-
2005a)triggeredandlocalizedtheshortGRB111117A
tional limits. Since 65% of Swift short GRBs are
(Mangano et al. 2011). The Fermi Gamma-ray
detected in X-rays, and only 25% of them are de-
Burst Monitor (GBM; Meegan et al. 2009) also
tectedintheopticalband,X-rayobservationshave
triggered on the burst (Foley et al. 2011). The
a higher probability of detecting the afterglows of
BAT location derived from the ground analysis
short GRBs. The superb angular resolution of
was (R.A., Dec.) (J2000) = (00h 50m 49.4s ,
Chandra allows for a sub-arcsecond localization,
+23◦ 00′ 36′′) with a 90% error radius of 1.8′.
comparable to optical localizations, thus enabling
TheSwift XRT starteditsobservation76.8safter
thesecurehostidentificationandthe precisemea-
the trigger. A fading X-ray source was found at
surement of the GRB projected offset. Further-
the location of (R.A., Dec.) (J2000) = (00h 50m
more, because the X-ray afterglow is less subject
46.22s , +23◦ 00′ 39.2′′) with a 90% error radius
to absorption and density effects, Chandra local-
of 2.1′′ (Melandri et al. 2011a). The Swift UV-
izationsallowustobuildasampleofwell-localized
Optical Telescope (UVOT; Roming et al. 2005)
shortGRBs with limited bias, complementing the
information derived from the sample of optically
2ThefirstonewasGRB111020A (Fongetal. 2012).
3
began the observations of the field 137 s after infrared images was performed using the IRAF4
the trigger,and no optical afterglow was detected softwarepackage. SExtractor5(Bertin & Arnouts
(Oates et al. 2011). 1996), SkyCat Gaia6 and IRAF were used to ex-
The earliest ground observations of the field tract sources and perform the photometry. To
were performed by the Gao-Mei-Gu telescope accomplish consistent photometry for images col-
(GMG) at 1.96 hr after the BAT trigger, and lected by various telescopes, we selected 10 com-
no afterglow was detected within the XRT er- monstarsin the field andperformedrelativepho-
ror circle with an exposure time of 600 s in the tometry. When some of the stars were saturated
R band (Zhao et al. 2011). The Nordic Opti- (especially for a large aperture telescope such as
cal Telescope (NOT) observed the field at 8.9 hr GTC), a subset of these 10 reference stars were
after the burst, and found an optical source in- used. The USNO B-1 R2 magnitude or the SDSS
side the XRT error circle (Andersen et al. 2011), magnitudes were used as the reference magnitude
which was later confirmed to have a possible ex- for the stars. For the near infrared images of
tended morphology by the Magellan/Baade tele- UKIRT and CFHT, we use the reference stars in
scope (Fong et al. 2011), the Gemini-South tele- the 2MASS catalog. The Galactic extinction has
scope (Cucchiara et al. 2011), the GROND tele- beencorrectedusingE(B−V)=0.03magtoward
scope (Schmidl et al. 2011), and the Telescopio the direction to this burst (Schlegel et al. 1998).
Nazionale Galileo (TNG; Melandri et al. 2011b). The log of optical and near infrared observations
The Gran Telescopio CANARIAS (GTC), the presented in this paper are summarized in Table
Subaru telescope, the United Kingdom Infrared 1.
Telescope (UKIRT) and the Canada-France-
Hawai Telescope (CFHT) also collected images 4. Prompt Emission
of the field.
The light curve of the prompt emission is com-
Basedon no clear detection of an optical after-
posedoftwoepisodes: thefirstepisodeshowsmul-
glow of the short GRB111117A, we triggered our
tiple overlapping pulses with a total duration of
ChandraTargetofOpportunity(ToO)observation
0.3 s, and the second episode is composed of two
6 hr after the trigger (Sakamoto et al. 2011b),
pulseswithadurationof0.1s(Figure1). Thedu-
and the observation started 3 days later. The X-
rationisT =464±54ms(1σ error;15-350keV)
90
ray afterglow was clearly detected in 20 ks, ob-
measured using the BAT background-subtracted
taining a sub-arcsecond position of the afterglow
lightcurveusingthemaskmodulation(e.g.,mask-
in X-rays (Sakamoto et al. 2011c).
weighted light curve). This T duration is sig-
90
nificantly shorter than 2 s, which is the stan-
3. Data Analysis
dard classification of short GRBs form BATSE
(Kouveliotou et al. 1993). Furthermore, this du-
HEAsoft version 6.11 and the Swift CALDB
ration is shorter than 0.7 s, which is claimed to
(version 20090130) were used for the Swift BAT
be the dividinglinebetweenlongandshortGRBs
data analysis. The XRT data products were ob-
fortheSwiftsample(Bromberg et al. 2012). The
tained from the automated results available from
hard-to-soft spectral evolution is seen in both the
the UK Swift Science Data Center (Evans et al.
first and the second episode of GRB111117A(see
2007, 2009). CIAO 4.3 and CALDB 4.4.6 were
thehardnessratioplotatthebottompanelofFig-
used for the Chandra data analysis. The Fermi
ure1). Thereisnoindicationofextendedemission
Gamma-rayBurstMonitor(GBM)datawerepre-
pared using the RMFIT software package,3 with (Norris et al. 2011) down to a flux level of ∼2 ×
10−10 erg cm−2 s−1, assuming a power-law spec-
data from three Sodium Iodide (NaI) scintillation
trumwithaphotonindexofα=−2(N(E)∝Eα)
detectors(detectorID6,7and9)andtwoBismuth
in the 14-200 keV band by examining the BAT
Germanate (BGO) scintillation detectors (detec-
skyimagefrom60s(after the spacecraftslewset-
tor ID 0 and 1).
A standard data reduction of optical and near
4http://iraf.noao.edu/
5http://www.astromatic.net/software/sextractor
3http://fermi.gsfc.nasa.gov/ssc/data/analysis/user/
6http://astro.dur.ac.uk/˜pdraper/gaia/gaia.html
4
tled)to950saftertheBATtriggertime(hereafter tion (Band et al. 1993) fit (χ2/d.o.f. = 627/660)
t ). Thespectrallagbetweenthe100-350keV over a CPL fit. The preferential fit to a CPL
0,BAT
and the 25-50 keV band is 0.6 ± 2.4 ms, which modelandthesystematicallyharderphotonindex
is consistent with zero, using the BAT raw light compared to long GRBs are general characteris-
curves (non mask-weighted light curves) by sub- tics of a time-integrated spectrum of short GRBs
tracting a constant background measured around (e.g., Ghirlanda et al. 2009; Ohno et al. 2008).
the burst. In the fluence ratio versus T plane, The fluenceinthe 8-1000keVbandcalculatedus-
90
GRB111117Ais locatedin the sameregionwhere ing the best fit time-integrated spectral parame-
most of the BAT short GRBs are located (Figure ters based on a CPL fit above is 7.3+2.6 ×10−7
−2.1
2), further confirming its short GRB nature. erg cm−2. Due to poor statistics in extracting
The time-integrated spectral properties are in- a spectrum from a very short time window, the
vestigated by performing a joint spectral analy- peakfluxwascalculatedbyscalingtheBATmask-
sis with BAT and GBM data. The spectrum weightedcountrateintoaflux byfoldingthe BAT
is extracted from t + 0.024 s to t + energy response and assuming the best fit time-
0,BAT 0,BAT
0.520 s using batbinevt for the BAT data and integrated spectral parameters in a CPL model.
using the RMFIT software package for the GBM The peak energy flux at the 8-1000 keV band in
data in the same time interval. The BAT en- the 50 ms window starting from t0,BAT + 0.450
ergyresponsefileisgeneratedbybatdrmgen. The s is (3.8±1.2)×10−6 erg cm−2 s−1. The time-
GBMenergyresponsefileswereretrievedfromthe resolvedspectroscopyisdifficulttoperformdueto
HEASARCFermiarchivefortriggerbn111117510. the limited statistics in the data.
Weusethexspecspectralfittingpackagetodothe We search for pre-burst emission by analyzing
joint fit. The energy ranges of 15-150 keV, 8-900 the BAT survey data (detector plane histogram;
keV and 0.2-45 MeV are used for the BAT, the DPH).Approximately4.5hrbeforethebursttrig-
GBM-NaI and the GBM-BGO instruments, re- ger,GRB111117AwasinthefieldofviewofBAT
spectively. The model includes a inter-calibration (26.1◦ fromtheboresightdirection)for∼1ksdur-
multiplicative factor to take into account the cal- ing the observation of the blazar PKS 0235+16
ibration uncertainty among the different instru- (observationID00030880085). Weusebatsurvey
ments. The best fit spectral parameters are sum- script to process the DPH data. The extracted
marized in Table 2. We find that a power-law rates at the location of GRB111117A are cor-
multiplied by an exponential cutoff (CPL) 7 pro- rected to the on-axis rate by applying an off-axis
vides the best representative model of the data. correction based on observations of the Crab. We
The best fit parameters in this model are the find no significant emission during this observa-
power-law photon index αCPL = −0.28+0.31 and tionatthelocationofGRB111117A.The3sigma
−0.26
E =420+170 keV (χ2/d.o.f. = 627/661). The upper limit assuming a power-law spectrum with
peak −110
90% confidence interval of the inter-calibration a photon index of α=−2 is 1.4×10−9 erg cm−2
factor of the GBM detectors is between 0.50 and s−1 at the 14-200 keV band in 300 s exposure.
0.78 which is an acceptable range taking into ac-
count the current spectral calibration uncertainty 5. Afterglow
between the BAT and the GBM. A simple power-
5.1. X-rays
law model yields a significantly worse fit to the
data(χ2/d.o.f. =729/662). Furthermore,thesig- The Swift XRT X-ray afterglow light curve
nificant difference in the power-law photon index can be fit to a simple power-law decay (Figure
the BAT data (−0.52+−00..2242) and the GBM data 4). The spectrum collected in the photon count-
(−1.44+0.06) alone disfavors a simple power-law ing (PC) mode is well described by an absorbed
−0.08
model as the representative model. There is no power-law model. The best fit spectral param-
significant improvement in χ2 using a Band func- eters are a photon index of −2.19+0.36 and an
−0.38
excess N of 1.8+1.1 × 1021 cm−2 (z = 0) as-
H −1.0
7 N(E) ∝ EαCPLexp −E(2+αCPL) , where αCPL is the suming the galactic N at the burst direction of
(cid:16) E (cid:17) H
power-law photon index andpeEakpeak is the peak energy in 3.7×1020 cm−2 (Kalberla et al. 2005). Both the
theνFν spectrum. measuredphotonindexandN forGRB111117A
H
5
are consistent with those of other Swift short betweentheearlyandthelatetimeepochobserva-
GRBs (Kopaˇc et al. 2012; Fong et al. 2012). tionsbyTNGandGTC.WeusetheISISsoftware
The Chandra observation started at 12:39:25 package (Alard & Lupton 1998) to perform the
UT,andendedat18:39:10UT on2011November image subtraction. The early and the late epoch
20 with a total exposure of 19.8 ks. The ACIS observations of TNG and GTC were obtained at
instrument had five CCD chips (S3, S4, S5, I2 t0,BAT + 7.23hr andt0,BAT + 7.89hr,and t0,BAT
and I3) enabled, with the S3 chip as the aiming + 11.4 days and t0,BAT + 14.4 days, respectively.
point for the source. The data were collected in We find no significantemissionatthe Chandra X-
the FAINT mode. The X-ray afterglow is clearly ray afterglowlocationin the subtracted images in
detected in the Chandra observation with 3.9 σ both the TNG and the GTC observations(Figure
significancebywavdetect(sourcenetcountsof8) 5), with 3 sigmaupper limits ofR > 24.1 mag for
withintheXRTerrorcircle. Torefinethe astrom- TNGandr > 24.9(AB)mag forGTC. The TNG
etryoftheChandradata,weapplythesameanal- limitingmagnitudesofthefirstandsecondepochs
ysis method described in Feng & Kaaret (2008). areR>24.7magandR>25.4mag,respectively.
We extract the Chandra image (0.35 - 8 keV) For GTC, the limiting magnitude of the first and
that overlaps with the GTC image (4.4′ ×8.7′). secondepocharer >25.8(AB)magandr >26.1
The astrometry of the GTC image is calibrated (AB) mag, respectively. Those limits are some of
against the SDSS catalog, and its standard devi- the deepest optical limits on short GRBs ever ob-
ation is ∼0.3′′. We run wavdetect with options tained (see upper panel of Figure 6).
of scales=“1.0 2.0 4.0 8.0 16.0” and sigthresh =
4×10−6 to the extracted Chandra image. There 6. Host Galaxy
arefoursourceswhichhaveagoodmatchbetween
The host galaxy of GRB111117Ahas been de-
the images. We then use the geomap task in the
tected in the near infrared and optical bands.
IRAFIMMATCHpackagetofindthebestcoordi-
There is only one near-infrared/optical source lo-
natetransformationbetweentheChandraandthe
cated near the Chandra X-ray afterglow position.
GTC image by fitting those four sources. Finally,
Although the weak nature of the source makes it
we apply geoxytrans task (IRAF IMMATCH
difficult to investigate whether the source is ex-
package) for the originally detected Chandra po-
tendedornot,theopticalfluxofthesourceiscon-
sition using the coordinate transformation calcu-
stantbetween7hrand14daysaftertheburstata
lated by geomap to find the astrometrically cor-
level of ∼1.1 µJy (bottom panel of Figure 6). Us-
rected Chandra afterglow position. The refined
ing the formula providedby Bloom et al. (2002),
afterglow position is shifted by δR.A. = −0.221′′
the probability of finding an unrelated galaxy of
and δDec. = −0.020′′ fromthe position originally
theRmagnitudeof∼23.3withthedistanceof1.0′′
derived by wavdetect. The best Chandra X-ray
is 0.8%. We also investigate the chance probabili-
afterglow position is (R.A., Dec.) (J2000) = (00h
ties of the three nearby objects. The probabilities
50m 46.264s, +23◦ 00′ 39.98′′) with 1 σ statistical
of those objects are between 24% and 42% which
uncertainty of 0.09′′ in right ascension and 0.16′′
are significantly larger than that of the host can-
in declination. When we include the systematic
didate. Although the chance probability of the
uncertainty of 0.3′′, 1 σ error radius of the Chan-
host candidate is non-negligible, the chance of a
drapositionis0.35′′. TheChandrapositioniswell
misidentification of the host galaxy is reasonably
within the XRT 90% error circle (see Figure 3).
small. Therefore, we conclude that the source de-
The combined Swift XRT and Chandra X-ray
tected in the K, J, z, i, r, g, and R bands is the
afterglowlight curve is well fit by a simple power-
host galaxy of GRB111117A(Figure 7).
law with index of −1.25+0.09. As shownin Figure
−0.12 To estimate the redshift of the host galaxy, we
4,theX-rayafterglowofGRB111117Abelongsto
perform a spectral energy density (SED) fit with
a dim population of the Swift short GRBs.
the stellar population model of Maraston (2005).
We use the single stellar populations (SSP) mod-
5.2. Optical
elswithaSalpeterinitialmassfunction(Salpeter
We investigate the possible optical afterglow 1955), solar metallicity which ranges from 0.005
emissionbyusingtheimagesubtractiontechnique
6
Z⊙ to 3.5 Z⊙ (0.005, 0.02, 0.5, 1.0, 2.0 and 3.5 ranges from 10 Myr to 15 Gyr). Although our J
Z⊙), and a red or blue horizontal branch mor- banddatapointshowsarelativelypooragreement
phology. A total number of 269 SED templates withthebestfittemplatewithane-foldingtimeof
ranging in stellar age from 10 Myr to 15 Gyr was 0.1 Gyr and the luminosity weighted mean stellar
applied. The BayesianPhotometric Redshift soft- age of 0.3 Gyr, the fit is still acceptable (reduced
ware (BPZ; Ben´ıtez 2000) is used to fit the data χ2 = 0.91 with 2 d.o.f.). The best fit redshift in
in g, r, and i bands (GTC), z band (GMG), J thiscaseis1.18+0.61. Thefitbecomesworseifthe
−0.21
band (CFHT) and K band (UKIRT) with those e-folding time gets larger. Therefore, our current
SED templates. We find that the best fit SED data also support of an ∼ 0.1 Gyr post-starburst
template corresponds to a solar metallicity, a red galaxy(seebottompanelofFigure9). Wealsofit-
horizontal branch morphology and the luminos- ted the optical-NIR SED using MAGPHYS pack-
ity weighted mean stellar age of 0.1 Gyr with a age (de Cunha et al. 2008) to check the validity
redshift of 1.36+0.45. Our best fit SED template of the fitting result. The MAGPHYS fit includes
−1.18
of SSP model with a solar metallicity and a red an extinction parameter as a part of the fit, but
horizontal branch matches well with other short performs the fitting at a given redshift. An expo-
GRB hosts studied by Leibler & Berger (2010). nentially decayingstar formationrateis assumed.
AsseeninFigure8(top),thereisalesssignificant Theredshiftswereincreasedbyastepof0.05from
low redshiftsolution(z <0.25). We findthat this 0.8 to 1.5. We confirmthat the returnedχ2 is the
low redshift solution is coming from the template smallestintherangeofthez fromtheBPZ,and
ph
withthe youngstellarageof∼10Myr. As wewill a moderate extinction of A = 0.2−0.5 mag is
V
discuss in §7, it is unlikely that the host galaxy found. The best fit solutions give the exponential
hasthe stellarageof∼10Myr. Therefore,to con- time scale of about 1.5 Gyr, with the luminosity
strain the redshift better, we focus on 41 SED weightedmeanstellarage(inr-band)ofafewhun-
templates with solar metallicity and a red hori- dred Myr, and a stellar mass of a few times 109
zontal branch morphology with stellar age from M⊙. These output values are consistent with the
20 Myr to 15 Gyr. The signal-to-noiseis low, and solutionsderivedwiththephotometricredshift. In
there are no clear absorption or emission line fea- summary, based on various SED template fits, we
tures in the spectrum. The continuum is consis- can conclude the following about the host galaxy:
tent with the best fit SED template. The bottom the redshift of the host is ∼1.3 regardless of the
panel of Figure 8 shows the posterior probabil- SED model and the host is either a star-forming
ity distribution of the estimated redshift for this galaxyoftheluminosityweightedmeanstellarage
SED fit. No low redshift solutionis evident in the of 0.1 Gyr and a mass of ∼1×109 M⊙ or a post-
probability distribution. We find the best esti- starburst galaxy. Further deep J or Y band data
matedredshifttobe1.31(90%confidenceinterval are crucial to pin down the host properties.
1.08 < zph < 1.77). The likelihood that the red- A significant offset between the center of the
shiftiscorrectis80%(reducedχ2 ofthefitis0.65 host galaxy and the X-ray afterglow has been
with 2 d.o.f.). The best fit SED template is the found for GRB111117A. The center position of
casewiththeluminosityweightedmeanstellarage the host galaxy has been examined by running
of 0.1 Gyr and a mass of ∼1×109 M⊙. Figure 9 SExtractoronthe secondepochofthe GTC r im-
showsthebestfitSEDwiththephotometricdata, age, our highest quality optical image. The best
and the GTC spectrum with an exposure time of locationofthe hostcenteris (R.A., Dec.) (J2000)
4 × 1800 s. GTC spectroscopy was performed = (00h 50m 46.258s, +23◦ 00′ 40.97′′). The po-
withtheR1000Bgrism,whichhasacentralwave- sition moves by less than half a pixel (< 0.13′′)
lengthof5510˚Aandcoversthe spectralrangebe- bychangingthe detectionthresholdofSExtractor
tween3700and7000˚Awitharesolutionof∼1000 from 1.5 to 3.0 sigma. Therefore, the projected
at 5500 ˚A. To investigate the likelihood of the offset between the center of the host galaxy and
host being a star-forming galaxy, we also exam- the X-ray afterglow is 1.0′′ (δR.A. = 0.083′′ and
ine the SED templates with an exponentially de- δDec. = −0.990′′; see Figure 7). Taking into ac-
cayingstarformationrate(Maraston 2010), with count the statistical error in the X-ray afterglow
an e-folding time of 0.1, 1 and 10 Gyr (stellar age position of 0.18′′ and the statistical error of the
7
host center location of 0.13′′, we estimate the off- 2004),
setwithits errortobe 1.0±0.2arcseconds,which
β (≡log{f (R)/f (3keV)}/log(ν /ν )),
corresponding to a distance of 8.4±1.7 kpc at a OX ν ν 3keV R
redshift of z =1.31.
is estimated to be . 0.78 using the same defini-
tion on the X-ray flux density at 3 keV measured
7. Discussion
at11hrsaftertheburst,andtheopticalafterglow
Ourphotometricredshiftof1.31+0.46(90%con- limit based on the GTC r band. This upper limit
−0.23
ofβ iswithintheallowedrangeofthestandard
fidence)forthehostgalaxyofGRB111117Aisre- OX
afterglowmodelbetween0.5to1.25. Furthermore,
alistic for the followingreasons. First, by plotting
according to Margutti et al. (2012b), the optical
the observed r AB magnitude (r ) of the host
AB
and the radio afterglow limit is consistent with
galaxies of short GRBs as a function of redshift
the external shock model (Granot & Sari 2002)
(Berger 2009), we find that the relatively faint
for a small number density (n . 0.01-0.2 cm−3).
magnitude ofthe host galaxy,r =24.20±0.07,
AB
However, there is a possibility that a significant
is located at the redshift range of > 0.5 (Figure
amount of the optical afterglow flux was extin-
10). Second, we find that the less significant low
guished by the host galaxy. When we fit the X-
redshift solution (z < 0.25) in the photometric
rayafterglowspectrumtoapower-lawmodelwith
redshift estimation (Figure 8) is coming from the
the intrinsic absorption at z = 1.31, the intrinsic
templateswiththeunrealisticyoungstellaragesof
N is estimated to be 7.2+0.7 ×1021 cm−2. As-
∼10 Myr. At z =0.25, if the galaxy is star form- H −0.5
suming a host extinction law similar to the Milky
ing,therewouldbeachanceofseeingemissionsof
Way, A is 4.1 mag (Predehl & Schmitt 1995).
[OII],H-β and[OIII]in the opticalspectrum, yet, V
Therefore, a significant amount of extinction in
we see none of those lines in the GTC spectrum.
the optical flux is expected from the X-ray col-
Furthermore, ∼10Myr is in generaltoo young for
umn density measurement. On the other hand, it
awholegalaxy,asopposedtoaspecificstarform-
isstillnotclearwhetheritispossibletohavesuch
ing region. Therefore the low redshift solution for
high extinction at the outskirts of the host where
the photometric redshift is unlikely to be the case
the X-ray afterglow is indicated. Moreover, the
ofGRB111117A.Therefore,hereafter,wewilldis-
amount of extinction which we derived from the
cusstherest-framepropertiesofGRB111117Aus-
SEDfitofthehostisA =0.2−0.5mag(see§6).
ing our best photometric redshift of 1.31. V
At this stage, the origin of the large column den-
Assuming the redshift of 1.31, the isotropic
sity seen in the X-ray afterglow of GRB 111117A
equivalent γ-ray energy (E ) which is inte-
γ,iso is still remains puzzling.
grated from 1 keV to 10 MeV in the rest frame
is 3.4+5.7 × 1051 erg. The peak energy at the The projected offset between the afterglow lo-
−1.5
cation and the host galaxy center is 8.4 ± 1.7
rest-frame (Esrc ) is 945+455 keV. The 90% er-
peak −310 kpc using the estimated redshift of 1.31. Al-
rors in E and Esrc are taking into account
γ,iso peak though this offset is larger than the median pro-
notonlyastatisticalerrorbutalsoanuncertainty
jectedoffsetof∼5kpcforpreviouslystudiedshort
in the estimated redshift. As shown in Figure 11,
GRBs (Fong et al. 2010), it is within the off-
the E of GRB111117A is located at the high
γ,iso
set distribution of short GRBs. Using the pro-
end of the E distribution of short GRBs and
γ,iso
jected offset of r = 8.4 kpc and the stellar age
at the low end of the E distribution of long
γ,iso
of τ = 0.1 Gyr, the minimum kick velocity, v =
GRBs. Relatively low E and high Esrc com-
γ,iso peak r/τ, is estimated to be ≈ 80 km s−1. The esti-
pared to those of long GRBs make GRB111117A
matedkick velocityis similarto orpossibly larger
inconsistentwiththeEsrc -E (Amati)relation
peak γ,iso than the inferred kick velocity of GRB 060502B
(Amati et al. 2002). Thischaracteristicisconsis-
(Bloom et al. 2007). Using the typical age of 1-
tent with being a short GRB because most of the
10 Gyr in the early-type short GRB hosts such
short GRBs are well known outliers of the Amati
as GRB 050509B, GRB 070809 and GRB 090515
relation (Amati 2006; Nava et al. 2012).
(Bloom et al. 2006; Berger 2010), the minimum
Theoptical-to-X-rayspectralindex(Jakobssonet al. kick velocity is estimated to be ≈ 1-8 km s−1.
In this paper, we have reported the prompt
8
emission, the afterglow and the host galaxy prop- Muchachos of the Instituto de Astrof´ısica de Ca-
erties of short GRB111117A. The prompt emis- narias. This work was supported by Chandra Cy-
sion observed by the Swift BAT and the Fermi cle13grantGO2-13084X.Theresearchactivityof
GBM showed 1) a short duration, 2) no extended AdUP,CCT,RSRandJGissupportedbySpanish
emission, 3) no measurable spectral lag and 4) a researchprojectsAYA2011-24780/ESP,AYA2009-
hard spectrum. All those properties can securely 14000-C03-01/ESP and AYA2010-21887-C04-01.
classify this burst as a short GRB. Although the GL issupportedbythe SwedishResearchCouncil
optical afterglow has not been detected by our through grant No. 623-2011-7117. KV is grate-
deep observations by TNG and GTC, our rapid ful to the Hungarian Science Research Program
Chandra ToO observation provides a sub-arcsec (OTKA) for support under the grant K-81421.
position of the afterglow in X-rays. This Chandra This work is supported by the ”Lendu¨let” Young
position is crucial to identify the host galaxy and Researchers’ Programof the Hungarian Academy
also to measure the significant offset of 1.0′′ be- of Sciences. HK acknowledges the support from
tween the host center and the afterglow location. the European Commission under the Marie Curie
Ourdeepnearinfraredtoopticalphotometrydata IEF Programme in FP7. MI and YJ were sup-
of GMG, TNG, NOT, GTC, UKIRT and CFHT portedbythe he CreativeResearchInitiativepro-
enable us to estimate the redshift of the host to gram,No. 2010-0000712,oftheNationalResearch
1.31. The observation of GRB111117A suggests Foundation of Korea (NRFK) funded by the Ko-
that X-rays are more promising than optical to rea government(MEST).
locate short GRBs with sub-arcsecond accuracy.
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