GRB 000508B

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.

A. Henden (USRA/USNO) reports on behalf of the USNO GRB team:

We have acquired preliminary shallow BVRc all-sky photometry for
an 11x11 arcmin field that covers the error box for GRB000508B
with the USNOFS 1.0-m telescope on one photometric night.
Stars brighter than V=12 are saturated and should be used with care.
We have placed the photometric data on our anonymous ftp site:
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb000508b.dat
The current photometry has a potential external zero-point
error of about two percent.  The astrometry in this file
is based on linear plate solutions with respect to USNO-A2.0.
The internal errors are less than 100mas.
  Further calibration of this field will be performed if
an optical afterglow is identified.

K. Z. Stanek (CfA), P. M. Garnavich (Notre Dame), P. J. Groot, S. Jha
(CfA) and M. Hicks (JPL) report:

We have observed most of the RXTE/ASM-IPN error-box of GRB 000508B
(GCN 665
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 ) using the FLWO 1.2-meter telescope on Mt. Hopkins and the
KPNO 2.1m telescope. The center of the GRB 000508B error-box is
located at Galactic coordinates (l,b)=(0.48 deg,14.38 deg), and the
Schlegel et al. (1998) Galactic reddening toward this direction is
E(B-V)=0.51 (A_V=1.7 mag). Due to the proximity to the Galactic plane
the stellar field is fairly dense.

At KPNO we obtained four 600 sec R-band images, the first two at UT
May 10 07:30 and the second two at UT May 10 11:33. At FLWO we
obtained two 600 sec R-band images, first one at UT May 10 06:39 and
the second one at UT May 10 09:14. In addition, at FLWO we have
obtained UBVI images for the field. Using DAOPHOT-II (Stetson 1992) on
the FLWO 1.2-meter R-band image we detect about 5,000 objects with
R<22.5 in 11x11 arcmin field.

Visual comparison with the POSS-II red plate reveals no obvious
``new'' objects. Employing the image subtraction code ISIS-2 (Alard
1999) between the two epochs taken at each telescope reveals four
possible variable objects located in the error-box, with the following
coordinates and magnitudes from Henden et al. (GCN 667
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 ):

V1) RA=253.58616  DEC=-20.41683  V=16.75

V2) RA=253.54913  DEC=-20.46479  V=17.33

V3) RA=253.53600  DEC=-20.43531  V=17.72

V4) RA=253.60865  DEC=-20.42111  V=18.24

All of these objects are easily identified on the POSS-II plate, so
most likely NONE of these objects is the optical counterpart to GRB
000508B. Further observations are planned at FLWO.

This message can be cited.

GRB 000508B: Optical observations

B. L. Jensen, H. Pedersen, J. Hjorth (U. of Copenhagen),
J. P. U. Fynbo (U. of Aarhus/ESO),
J. Gorosabel, N. Lund (DSRI,Copenhagen),
H. Kjeldsen, F. Grundahl (U. of Aarhus),
C. Zurita, J. Casares (IAC,Tenerife),
C. Sanchez-Fernandez and A.J. Castro-Tirado (LAEFF, Madrid)
report on behalf of a larger European GRB collaboration:

"We have obtained images of the RXTE/IPN error box
reported by Smith et al. (GCN #665
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 ) with the 2.56-m
Nordic Optical Telescope (NOT+ALFOSC) on La Palma,
and the 1.54-m Danish Telescope (DK-1.54m+DFOSC) at
La Silla, as follows:

Telescope      Date                  Filter/Exp t. FWHM
NOT       2000 May 10.14 UT (t+32h); I:5x300s;     FWHM=0.8"
DK-1.54m  2000 May 10.17 UT (t+33h); R:3x420s;     FWHM=1.5"
DK-1.54m  2000 May 10.38 UT (t+38h); R:3x420s;     FWHM=1.5"
DK-1.54m  2000 May 12.25 UT (t+83h); R:4x420s;     FWHM=1.3"

We have visually compared the combined May 10 2520s R-band
image with red POSS-II exposure. The three sets of May 10
images have been compared internally, both visually
and using automated routines based on DAOPHOT II/ALLSTAR
PSF-photometry. Additionally, we have compared the
combined May 10 and May 12 R-band images, visually and
through automated routines. We do not, in any of the
comparisons, find any significantly variable objects
down to a 3-sigma mag limit of R~22.5. We do not find
significant variability in the objects noted by Stanek
et al. (GCN #668
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 ). An image of the error-box is
posted at http://www.astro.ku.dk/~brian_j/grb/grb000508B "