GCN Circular 1428
Subject
GRB 020531/Candidate Redshift
Event
Date
2002-06-12T11:38:15Z (24 years ago)
From
Shri Kulkarni at Caltech <srk@astro.caltech.edu>
S. R. Kulkarni, R. Goodrich, E. Berger, D. W. Fox, J. S. Bloom and
C. A. Blake report, on behalf of the Caltech-NRAO-CARA collaboration:
N. Butler et al. (GCN 1426) suggested that a fading X-ray source, CXOU
J151455.8-192454 is the afterglow of the short hard burst GRB 020531
(Hurley et al. GCN 1407). Fox, Kulkarni and Weissman (GCN 1427)
identified an object within 1 arcsecond of CXOU J151455.8-192454 and
suggested that the object is the host galaxy of GRB 020531.
On June 12, 2002 (UT) we undertook imaging and spectroscopic
observations with the Echelle Spectrograph & Imager (ESI) on Keck II.
With a seeing of 0.6 arcseconds in the R and I bands we confirm that
the candidate object is indeed extended, with a size of about 1
arcsecond. Next, we obtained four 1800-s spectroscopic exposures
(Echelle mode) and found two features: a broad feature (Gaussian full
width at half maximum of 11.6 A) centered on 7455 Angstrom (A) and a
fainter feature centered around 9725 A. We suggest that the broad
feature is the [O II] 3728.8/3726.0 doublet with an intrinsic velocity
dispersion of 330 km/s and the fainter feature is Hbeta. If these
identifications are correct then the redshift of the candidate host
galaxy is 1.00.
The fluence and peak flux of GRB 020531 over the energy range 50 to 300
keV are 8E-7 erg cm^-2 and 6.4E-7 erg cm^-2 s^-1, respectively (Lamb et
al. 2002; astro-ph/0206151). Assuming, H0=65 km/s/Mpc and flat universe
with Omega-m=0.3, the isotropic energy release (without any k
correction) is 2.4E51 erg and 3.9E51 erg/s.
Lamb et al. (ibid) argue that GRB 020531 is a burst which belongs to
the short duration group. The isotropic energy release of this short
burst is not different from that of the true energy release (i.e.
after accounting for the opening angles of the jets; see Frail et al.
2001, ApJ 562, L55) inferred for the long duration bursts.