TITLE: GCN GRB OBSERVATION REPORT NUMBER: 665 SUBJECT: RXTE/ASM and IPN localization of GRB 000508B DATE: 00/05/09 22:03:52 GMT FROM: Don Smith at MIT D. A. Smith and A. M. Levine (MIT), K. Hurley (UCB), and S. Barthelmy (NASA/GSFC) report on behalf of the RXTE/ASM teams at MIT and NASA/GSFC and the IPN collaboration: The RXTE All-Sky Monitor has detected emission we associate with GRB 000508B (BATSE #8098). One camera of the ASM was advantageously placed to observe the field around the BATSE LOCBURST position within 200 seconds of the trigger time (19:10:50.88 UTC). A dramatic rise in intensity occured over approximately 10 s, to a peak of ~1.5 Crab (2-12 keV) at roughly 19:13:44, 75 s into a 90-s ASM observation. Analysis of the position histogram data shows evidence for a previously unknown X-ray source in this and the next sequential observation, after the camera was rotated through six degrees. Localizations of these two detections yields an error box in the shape of a long, thin parallelogram centered at R.A. = 253.498d, Decl. = -20.504d (J2000). This position is confirmed by an annulus derived via the IPN through detections by BATSE and Ulysses. The preliminary annulus is 9.67' in width (3sigma), with a radius of 76.512 deg, centered at R.A. = 309.088, Decl. = 35.112 (J2000.0). This annulus intersects the above diamond at the following points, creating a joint error box with area ~46 sq. arcmin.: 253.700d {16h 54m 48s} -20.432d {-20d 25' 55"} 253.643d {16h 54m 34s} -20.380d {-20d 22' 48"} 253.483d {16h 53m 56s} -20.456d {-20d 27' 22"} 253.5383 {16h 54m 09s} -20.511d {-20d 30' 40"} The time of the increase in the count rate in the first ASM observation is consistent with the estimated time that the best-fit location for the GRB rose from behind the Earth's limb, so we attribute the characteristics of the rise and the lack of detectable emission prior to this time to Earth occultation, not to any intrinsic properties of the GRB source. See the web page at http://xte.mit.edu/grb000508b/ for images and further commentary.