Columbia University Astronomy and 
University of Michigan 

MDM Observatory GRB Team

GRB 970815: X-ray Afterglow Location (10" radius)
Palomar 60" I-band: 1997 Aug 18.15
P60 1997 Aug 18
Keck I-band: 1997 Aug 20.28
Keck I 1997 Aug 20
MDM 2.4 m I-band: 2002 Aug 27
2.4m 2002 Aug 27
MDM 2.4 m V-band: 2004 July 12
2.4m 2004 July 12

GCN Circular #2837: GRB 970815: Discovery of Optical Afterglow
A. Soderberg, G. Djorgovski (Caltech), J.P. Halpern (Columbia), and N. Mirabal
(U. Michigan) report:

"Following the belated identification of the X-ray afterglow associated with
GRB 970815 (Mirabal et al., astro-ph/0410616), we re-examined our optical
images at this location.  Within the ROSAT HRI error circle, a point source
is clearly detected in our I-band images (IAUC 6723) taken on 1997 Aug 18.15 UT
(Palomar 60-inch) and Aug 20.28 UT (Keck II-LRIS), with approximate magnitudes
of I = 22.1 +/- 0.1 and I = 22.3 +/- 0.1, respectively.  The source position is

	RA 16:06:53.12,  Dec +81:30:28.5 (J2000)

with an uncertainty of ~0.5 arcsec in each coordinate.  In comparison, an
I-band image taken on 2002 Aug 27 UT with the MDM 2.4-m shows that the
afterglow has since faded below I > 24.  It is also fainter than V > 25.2
on 2004 July 12 UT (Mirabal et al., ref above).

The effective spectral index between the I-band and the contemporaneous 2
keV X-ray flux measured by ASCA on 1997 Aug 18.7-19.9 UT is beta_ox = 0.6
+/- 0.1, which is consistent with the ASCA spectral index beta_x = 0.64
+/- 0.35. This beta_ox is on the border line of those bursts that are
considered optically normal, e.g., De Pasquale et al. 2003, ApJ, 592,
1018, or Jakobsson et al. 2004, ApJ, 617, L21.  Therefore, GRB 970815 may
not have been dark as assumed by Mirabal et al..  Re-analysis of any other
optical observations of this event would be of interest."

Alicia Soderberg
George Djorgovski
Jules Halpern
Nestor Mirabal

Columbia University and MDM Observatory

Keck Observatory

MDM Observatory

Last Modified: January 3, 2005