The Galactic Extinction Toward GRB 970228 and Its Implications for the Point-Like and Extended Optical Sources

Abstract

The IRAS 100 micron image of the GRB 970228 field shows that the amount of galactic dust in this direction is substantial and varies on arcminute angular scales. From an analysis of the observed surface density of galaxies in the $2.6' \times 2.6'$ HST WFPC image of the GRB 970228 field, we find $A_V = 1.1 \pm 0.10$. From an analysis of the observed spectra of three stars in the GRB 970228 field, we find $A_V = 1.71^{+0.20}_{-0.40}$. This value may represent the best estimate of the extinction in the direction of GRB 970228, since these three stars lie only $2.7''$, $16''$, and $42''$ away from the optical transient. If instead we combine the two results, we obtain a conservative value $A_V = 1.3 \pm 0.2$. This value is significantly larger than the values $A_V = 0.4 - 0.8$ used in papers to date. The value of $A_V$ that we find implies that, if the extended source in the burst error circle is extragalactic and therefore lies beyond the dust in our own galaxy, its optical spectrum is very blue: its observed color $(V-I_c)_{obs} \approx 0.65^{+0.74}_{-0.94}$ is consistent only with a starburst galaxy, an irregular galaxy at $z > 1.5$, or a spiral galaxy at $z > 2$. On the other hand, its observed color and surface brightness $μ_V \approx 24.5$ arcsec$^{-2}$ are similar to those expected for the reflected light from a dust cloud in our own galaxy, if the cloud lies in front of most of the dust in this direction.