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The search of the supernova associated to the
Gamma-Ray Bursts GRB 020305
J. Gorosabel , J.P.U. Fynbo† , A.S. Fruchter , A. Levan‡ ,
†
A.J. Castro-Tirado§ , J. Hjorth† , J.M. Castro
, J. Rhoads ,
D. Bersier and I. Burud
Instituto de Astr
de
(IAA-CSIC), Camino Bajo de
, 50, E-18008 Granada,
Spain
†
Niels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30,
DK-2100 Copenhagen Ø, Denmark
3700 San Martin Drive, Baltimore, MD 21218, USA
‡
Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1
7Rh, UK
§
Instituto de Astr
de
(IAA-CSIC), Camino Bajo de
, 24, E-18008 Granada,
Spain
Abstract. We describe the search of the Supernova (SN) associated to the Gamma-Ray Burst
GRB 020305. HST and ground-based multicolour observations are reported. Those observations
revealed a re-brightening 15 days after the GRB and also a Spectral Energy Distribution (SED)
consistent with a combined thermal-synchrotron spectrum. The more natural explanation for the
simultaneous bumps of the lightcurve and the SED is an associated supernova (SN). However, our
data can not unambiguously determine the redshift of the GRB.
INTRODUCTION
The Gamma-Ray Burst GRB 020305 was detected by the HETE-II satellite on March
5.50 UT (Ricker et al. 2002). The high-energy emission as seen by the Interplanetary
network (IPN) consisted in two broad pulses, with a total GRB duration of
280s
(Hurley et al. 2002), placing it in the long-soft burst category. Price et al. (2002) reported
the presence of a transient optical source in the HETE-II/IPN error box in images taken
20 hours after the GRB. Further imaging confirmed the fading behaviour of the
candidate (Lee et al. 2002; Ohyama et al. 2002). Unfortunately, a spectroscopy redshift
has not been secured to date.
OBSERVATIONS
In this paper we present ground and space based optical observations carried out from
11 to 321 days after the burst. We observed the field of GRB 020305 from the ground
with the 2.56-m Nordic Optical Telescope (NOT, equipped with ALFOSC) on 2002
March 16 – March 21, i.e. 11 – 16 days after the GRB. The HST observations were
performed with STIS between April 12 2002 and Jan 20 2003.
The mean astrometric position obtained based on the NOT images is; R.A.(J2000)=
12h 42m 27 963s 0 020s, Dec(J2000)= 14 18 11 45 0 20 . The astrometry is based
on 30 USNO A2.0 stars per image. The errors do not include the systematic uncertainty of the USNO A2.0 catalogue ( 0 25 ; Assafin et al. 2001).
THE AFTERGLOW SED AROUND T
T0
12 DAYS
The optical multicolour imaging carried out during the first NOT observing night (taken
at March 16.95–17.22 UT) allowed us to construct the optical spectral energy distribution (SED) of the afterglow. Furthermore, including the K -band detection on March
14.3 UT reported by Burud et al. (2002) the SED was extended to the near-IR.
The magnitudes have been transformed to flux densities following the conversion
factors given by Fukugita et al. (1995) and Allen (2000) for the optical and near-IR bands
respectively. The fluxes were subsequently corrected for foreground Galactic extinction,
which amounts to E B V
0 053 according to Schlegel et al. (1998). The SED is
formally consistent with a power-law ( 2 d o f 1 7 for 5 d.o.f) with a spectral index
of
0 32 0 12, but there is tentative evidence for deviations away from a powerlaw, most notably an RI-band decrement.
Fig. 1 shows that the SN 1998bw template (Patat et al. 1995) might reproduce the
SED if it is a low redshift (z 0 1). We have also considered a low redshift black body
component plus a power law spectrum, fixing the spectral index at
1, leaving
its amplitude and temperature as free parameters. The inclusion of the black body
component improves substantially the pure power fit ( 2 d o f 0 9 vs. 2 d o f 1 7,
see Fig. 1) and yields T 10600 1000 K for z 0 1. If a higher redshift is assumed
the inferred temperature would be even higher. This shows that if the SED bumps are
due to a SN, then it should show a high photospheric temperature.
R-BAND LIGHTCURVE
The HST magnitudes on April 13.5 UT and June 16.5 UT can be transformed to the Vega
system using the LP and CL filter throughputs implemented in the SYNPHOT package
of IRAF yielding R 24 4 (April 13.5 UT) and R 28 3 (June 16.5 UT). These two
R-band magnitudes have been used for the construction of the late time optical afterglow
lightcurve (see Fig. 2).
Thus, the combination of the NOT and HST data presented here and the data points
in the literature (Lee et al. 2002; Ohyama et al. 2002) allowed to us to construct the
R-band lightcurve of the OA. The data point by Price et al. (2002) has not been included
by lack of information on the calibration, photometric system (their detection is based
on unfiltered images) and the unreported error-bar. The contribution of the host galaxy
(R 25) to the ground based magnitudes (R 22 76) is negligible.
The resulting OA lightcurve (see Fig. 2 is highly inconsistent with a power law decay
fit (F
t ; 2 d o f 65 6). We note, however, that the formally derived power law
decay index of
1 05 0 03 is roughly consistent with the
1 3 value estimated
2
Pure power law fit,
/d.o.f=1.7)
2x10 x SN 1998bw
Power Law + 2x10 x SN 1998bw,
2
Thermal + Power Law ( /d.o.f=0.9)
2
/d.o.f=1.3)
Log (F (milliJy))
Pure power law component
14.2
14.4
14.6
14.8
Log ( (rest frame (Hz))
15
FIGURE 1. The filled squares show the UBV RIK -band data points measured around March 17.0 UT.
The straight thick solid line shows a pure power law fit (
0 32 0 12, 2 d o f 1 7), whereas the
solid thin bumpy curve shows the fitted solution when a SN 1998bw-like component is added. The best
SED solution is obtained with a low redshift (z 0 1) faint SN (the amplitude is only 2% of SN 1998bw,
see the thin long dashed curve at the bottom of the plot), which improves slightly the fit ( 2 d o f 1 3).
Larger redshifts make the SN 1998bw template unable to fit the high UB-band flux, so are not plotted
here. The dot-dashed smooth curve shows the fit obtained at z 0 1 when the contribution of a thermal
spectrum (dotted smooth curve) and a pure power law (
1; long dashed straight line) are added. The
temperature of the black body is T 10600 1000 K. All the fits assume no intrinsic extinction, since
the introduction of it just provides higher values of 2 do f .
by Lee et al. (2002). The poor fit is due to the fast decay between the first two HST
epochs and especially because of the lightcurve bump present 12 – 16 days after the
GRB.
The power law extrapolation of the first two lightcurve data points to the epoch
of the second HST observations yields R 25 69 0 64, which is 2.1 above the
HST measurement (R 28 28 12 07
41 ). This flux overestimation might be indicative of
a lightcurve knee.
14
Lee et al. 2002.
Ohyama et al. 2002
NOT
HST
Broken power law + SN 1994I at z=0.17
Broken power law; 1
2
SN 1994I at z=0.17
Moran et al. 2002, Jelinek et al. 2002
16
18
break
20
22
24
26
28
30
1
10
0 (days)
100
.
FIGURE 2. R-band lightcurve of the GRB 020305 OA and the broken power law fit considering SN
1994I as template. The solid line shows a plausible lightcurve fit solution. The best fit is given at z 0 17,
implying a break time of tbreak 3 8 7 5 days. The dotted line shows the broken power law and the long
dashed curve displays the SN component. As it shown the fit is consistent with the early R-band upper
limits (two empty triangles) given by Jelinek et al. (2002) and Moran et al. (2002)
In order to account for a possible break, the R-band lightcurve was fitted with a
Beuermann et al. (1999) function plus a SN template with variable amplitude. The SN
template used for our study was SN 1994I, successfully used in the past to describe
afterglow lightcurves (GRB 021211, Della Valle et al. 2003; GRB 030723, Fynbo et al.
2004). Fig. 2 shows the possible lightcurve break at tbreak 3 8 7 5 days.
CONCLUSIONS
A natural simultaneous solution for the lightcurve and the SED would be the existence
of a supernova component present 15 days after the GRB. However, our data can not
conclusively discriminate the redshift of the GRB.
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