GCN Circular 141
Subject
GRB980703: Radio transient
Date
1998-07-09T02:10:20Z (26 years ago)
From
Dale A. Frail at NRAO <dfrail@nrao.edu>
D. A. Frail, (NRAO), S. R. Kulkarni (CIT), J. S. Bloom (CIT), S. G.
Djorgovski and the BeppoSAX GRB Team report:
The radio source identified as a possible afterglow in GCN #128 has
brightened considerably and is exhibiting short-timescale
variability. During a 3.5 hour VLA run on 1998 July 7 the flux density
at 4.86 GHz varied from 1.1 mJy to 0.75 mJy. Variations are also
present at 8.46 GHz but they are smaller than those at 4.86 GHz.
Similar fluctuations have been seen at early times toward previous
radio afterglows and were attributed to interstellar scintillation.
Start Date 1.43GHz flux 4.86GHz flux 8.46GHz flux
(microJy) (microJy) (microJy)
---------------------------------------------------------
Jul. 4.4 UT -- 137 --
Jul. 7.3 UT -- 910 890
Jul. 8.5 UT 120 635 965
---------------------------------------------------------
On 1998 July 8 the spectrum rises with higher frequencies. Such
behavior is uncommon in most extragalactic radio sources but it has
been seen in the early spectra of previous radio afterglows and
attributed to synchrotron self absorption.
The radio source appears unpolarized, with a 3-sigma limit on linear
polarization at 4.8 GHz and 8.46 GHz of ~8%. An improved position is
R.A.=23h59m6.666s, Dec.=+08d35'7.07" (equinox J2000). The position
error is approximately +/-0.05".
The strong [OII] line emission seen toward GRB980703 (GCN#139) raises
the possibility of detecting the host galaxy from radio radiation
produced by ongoing star formation (Condon An.Rev.A&A, 30, 575,
1992). For the assumed distance and star formation rate given in
GCN#139 we estimate that the host galaxy will be 105, 40 and 25
microJy at 1.43, 4.86 and 8.46 GHz, respectively. Thus the measurement
at 1.43 GHz could contain a significant contribution from the host
galaxy.
We note that a redshift of z=0.966 (GCN #137) along with the average
fluxes reported by Galama et al. (GCN #127) implies a lower limit to
the X-ray afterglow isotropic energy release of 9.5e49 ergs over the
course of the 10.6 hour BeppoSAX NFI pointing. Consistent with this
(though it is difficult to translate BATSE count rates to flux and
fluence) the total fluence of the GRB is likely ~0.5 -- 1 x 10^-5
erg/cm^2 implying a total isotropic energy release of ~2 -- 5 x 10^52
erg; this falls between the implied energies of GRB 970508 (Metzger et
al. Nature 387, 878) and GRB 981214 (Kulkarni et al. Nature, 293, 35).
This report may be cited.