Gamma-ray Burst 970815

A bright Gamma-Ray Burst was detected with BATSE on August 15, 1997 at 12:07:04.41 (15.5049 UT) at R.A. = 241.83, Decl. = +82.99 degrees. Due to its high ecliptic latitude this GRB was seen in the RXTE all-sky monitor (IAUC 6718): The burst rise was detected in one camera on Aug. 15.50597. At Aug. 15.50623, the cameras were rotated through 6 deg, such that the burst came into the field-of- view of a second camera. Observations with two cameras at different orientations enabled an accurate position (see below). The burst lasted about 130 s and showed a double-peaked structure, reaching a maximum intensity of almost 2 Crab (2-12 keV) in the second peak.

Previous IAU Circulars

• IAUC 6718 Location with ASM/RXTE
  
• IAUC 6721 Optical imaging: no variable down to R = 21.
  
• IAUC 6722 ASCA finds no X-ray source in ASM error box above 1x10E-13 erg/cm^2/s (2-10 keV)
  
• IAUC 6723 no opt. variable > 0.25 mag for V < 21.5; > 0.5 mag for I < 24-24.5; no radio source
• IAUC 6725 no opt. variable > 0.4 mag brighter than R = 19.0 and I = 19.5 on Aug. 17.2 and 27.2
• IAUC 6742 ROSAT source at RXTE error box border (see image below)
• IAUC 6744 upper limits for optical source within ROSAT error circle: Aug. 17, K' > 18; Aug. 20, B > 21.5, R > 22; Aug. 23, B > 22.5, R > 23.

• The RXTE/ASM scanning observations have reduced the GRB error box to less than 10 arcmin: The error box has the following corners: R.A. = 16h06m36s, Decl. = +81o27'36"; 16h10m50s, +81o33'36"; 16h08m10s, +81o31'12"; 16h09m17s, +81o30'00" (equinox 2000.0).
• The image of the 17.1 ksec ROSAT HRI observation with the RXTE error box overplotted. Red circles with numbers are X-ray sources with more than 3sigma confidence, and the green circle denotes the source detected by ASCA (IAUC 6722).
  
• A number of optical images and lightcurves are available on Feng Ma's GRB 970815 page
• astroph/0410616 - Mirabal et al.: The X-ray afterglow of dark GRB 970815: A common origin for GRBs and XRFs?
  Chandra pointings reveals that the ASCA/ROSAT source slightly outside the ASM error box has faded away by a factor of approx. 100. No optical counterpart for this X-ray source down to V > 25.2 on July 12, 2004. The authors therefore suggest this X-ray source to be the X-ray afterglow.
• GCN notice #2837
  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."