TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3415
SUBJECT: Chandra observation of GRB 050509b
DATE:    05/05/12 07:21:02 GMT
FROM:    David Burrows at PSU/Swift  <dxb15@psu.edu>

D. N. Burrows, D. Grupe (PSU), C. Kouvelioutou, S. Patel (MSFC), N. Gehrels
(GSFC), P. Meszaros (PSU), B. Zhang (UNLV), and R. Wijers (U. Amsterdam)
report:

We have observed the field of the short burst GRB 050509b (Hurkett et al. 2005,
GCN 3381) with the Chandra ACIS instrument.  The 50 ks observation was centered
at the Swift XRT afterglow position reported by Kennea et al. (2005; GCN 3383).
Within the ACIS S3 detector 20 point sources are found by XIMAGE in our
preliminary analysis.  Two of these sources fall within the revised 8 arcsecond 
radius error circle reported by Rol et al. (2005; GCN 3395).  The coordinates 
and count rates of these sources are:

#5:  12 36 14.0 +28 59 02.5  2.46E-04+/-8.6E-05

#8:  12 36 13.9 +28 59 07.9  1.94E-04+/-7.8E-05 

Source #5 is 2.0 arcseconds from the revised XRT position and is tentatively
identified as the X-ray afterglow.  However, the source intensity is too low for
us to to verify whether this source is fading during our observation.  

Assuming a Crab-like spectrum absorbed by galactic NH (1.5E20), 
this count rate corresponds to a flux of 1.6E-15 cgs (0.2-10 keV). 
Combined with the Swift XRT flux measurement at early times, we estimate a
power-law decay index of approximately -1.0 over the 200 ks interval between 
the two observations.  Confirmation of this afterglow candidate will require
additional Chandra observations to see whether any of these sources are fading.