GCN Circular 665

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.