GCN Circular 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

This report may be cited.