TITLE: GCN GRB OBSERVATION REPORT NUMBER: 275 SUBJECT: ASM/IPN error box for GRB 990308 (BATSE #7457) DATE: 99/03/14 19:43:32 GMT FROM: Don Smith at MIT 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.