Gamma-ray Burst 990308

The GRB on March 8, 1999 05:15:07 UTC with BATSE trigger number #7457 was observed by the RXTE ASM during a 90-s observation. Localization was possible by combination of ASM and Ulysses arcs.

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN notice #275:
  D. A. Smith, A. Levine, and R. Remillard (MIT), on behalf of the RXTE/ASM team at MIT and NASA/GSFC, K. Hurley (UCB), on behalf of the Ulysses GRB team, and S. Barthelmy (NASA/GSFC) report:
  GRB 990308 was detected by the All-Sky Monitor on RXTE during a 90-s observation that began at MJD 51245.218333 (03/08/99 05:14:24 UTC). This burst was also observed by BATSE (Trigger #7457), and the BATSE trigger time was 05:15:07 UTC. At this time, SSC 3 of the ASM (the camera aligned with the rotation axis of the ASM assembly) observed an interval of enhanced count rates that lasted until the end of the observation. The light curve of the event was highly variable, with a mean 1.5-12 keV flux of about 400 mCrab. SSC 3 has lost the use of four of its eight resistive anodes over its the three years of operation, but we could still localize this GRB with the remaining anodes to a 10 deg by 6.6 arcmin error box (full-width at 90% confidence, including estimates of both statistical and systematic effects), centered at R.A. = 182.761832, Decl. = +4.282319 (J2000) and rotated 51.2 deg east from north.
  GRB 990308 was also detected, very weakly, by Ulysses, and it was therefore possible to obtain an IPN annulus by triangulation of burst arrival times at Ulysses and BATSE. This annulus is centered at R.A. = 154.0761 degrees, Decl. = -9.6383 degrees, with a radius of 35.583 degrees, and a 3 sigma half-width of 0.399 degrees. This annulus is based on preliminary data, but we do not expect it to improve much, if at all.
  The IPN annulus and the ASM error box cross each other, yielding a joint error box 48 arcmin by 6.6 arcmin. The corners of this box lie within the BATSE LOCBURST error circle, and their celestial coordinates are:
  

  R.A.(J2000)  Decl. (J2000)
  186.142867     +6.921870
  186.091377     +7.021754
  185.502582     +6.412172
  185.451355     +6.512323
  

  This position was still in the FOV of SSC 3 after its 6-deg rotation, and examination of the instrument response at the center of the joint error box yields a ~4-sigma detection of a ~50 mCrab flux (1.5-12 keV), averaged over the next 90-s ASM observation (56-146 s after the burst trigger). This is an average of less than 2 c/s in the time-series data during this observation, which shows an average total count rate of 27 c/s, so it is not possible to directly measure the burst behavior on any shorter time-scales. By the second observation after the burst (t=152-242 s), the average flux had fallen below 30 mCrab (3-sigma upper limit).
  Figures showing localizations of GRB 990308, as well as the time- series data from the first ASM observation, can be seen on the WWW at http://xte.mit.edu/grb/trig7457.24/web_trig7457_24.html.
• GCN notice #726
  Stephen Holland, Johan Fynbo, Bjarne Thomsen (University of Aarhus), Michael Andersen (University of Oulu), Gunnlaugur Bjornsson (University of Iceland), Jens Hjorth (University of Copenhagen), Andreas Jaunsen (University of Oslo), Priya Natarajan (University of Cambridge, & Yale), and Nial Tanvir (University of Hertfordshire)
  We have obtained 7842 seconds of STIS images with the 50CCD (clear) aperture of the sky where the optical afterglow associated with GRB 990308 was detected. This data was taken as part of the Survey of the Host Galaxies of Gamma-Ray Bursts (Holland et al. GCN 698) approximately 468 days after the burst occurred. Combined images are now available at "http://www.ifa.au.dk/~hst/grb_hosts/data/index.html".
  We estimate that the uncertainty in the location of the optical afterglow in our drizzled STIS image is approximately 1.2 arcsec, mostly due to the uncertainty in transferring the Schaefer et al. (1999, ApJL, 524, L103) coordinates for the afterglow to the STIS image. We do not detect any objects within this error circle, and estimate that our five-sigma limiting magnitude is V ~ 28.5. There are two faint, extended sources located 1.448 arcseconds (1.2 sigma) and 2.228 arcseconds (1.9 sigma) west of Schaefer et al. (1999)'s position for the optical afterglow. Some properties of these objects are given below.

  Object    X       Y       magnitude      FWHM(")  Ellipticity
    1    1081.77  909.91   26.9 +/- 0.1     0.10       0.09
    2    1086.91  879.21   27.0 +/- 0.1     0.11       0.36
  

  The X and Y values are the pixel coordinates on our drizzled image and the magnitude is the STIS 50CCD (clear) AB magnitude measured in an aperture with a diameter of 0.75 arcsec. The resolution limit of the image is ~0.084 arcseconds. There are no other objects within approximately six arcseconds of the Schaefer et al. (1999) position for the optical afterglow, so it is possible that one of these objects is the host galaxy for GRB 990308. A GIF image of the error circle, and the two extended objects, is available at "http://www.ifa.au.dk/~hst/grb_hosts/data/grb990308cd.gif".