TITLE:   GCN CIRCULAR
NUMBER:  7704
SUBJECT: GRB 080503: Chandra Observation of X-ray Rebrightening or Flattening
DATE:    08/05/09 22:18:47 GMT
FROM:    Nat Butler at MIT/CSR  <nrbutler@space.mit.edu>

N. R. Butler (UC Berkeley), J. S. Bloom (UCB), W. Li (UCB), A. J. Levan (U.
Warwick), D. A. Perley (UCB),  N. R. Tanvir (U. Leicester), H-W. Chen (U.
Chicago), R. Chornock (UCB), J. X. Prochaska (UCO/Lick), C. D. Bailyn (Yale),
A. Bunker (U. Exeter), R. Chornock (UCB), B. Cobb (Yale), P. B. Hall
(York U.), A. V. Filippenko (UCB), N. Gehrels (NASA/GSFC), D. Burrows (PSU),
K. Glazebrook (Swinburne), J. Granot (U. Hertfordshire), K. C. Hurley (UCB),
D. Kocevski (UCB), B. Metzger (UCB), M. Modjaz (UCB), A. Miller (UCB), W.
Lee (I. de Astronomia), S. Lopez (U. Chile), J. Norris (NASA/Ames), P. E.
Nugent (LBNL), M. Pettini (U. Cambridge), D. Poznanski (UCB), A. L. Piro
(UCB), E. Quataert (UCB), E. Ramirez-Ruiz (UCSC), J. Shiode (UCB), and T.
Sakamoto (GSFC/UMBC) report:

From 2008/05/07 19:18:23 UT to 2008/05/08 04:09:59 UT (4.29 - 4.66 days post
burst), Chandra Director's Discretionary Time observations with ACIS-S were
conducted of the field of GRB 080503 (Mao et al. 2008; GCN7665) for a
total exposure of 29.8 ksec (livetime).  We find a highly significant
detection of one X-ray point source in the XRT error circle (Goad et al.
2008; GCN7669) at:

RA, Dec = 19:06:28.76, +68:47:35.3 +/- 0.5"  (J2000).

The source is 0.2" away from the Gemini optical source (S2) in Perley et al.
(2008; GCN7678), and is 0.7" away from the center of the XRT error circle.
The X-ray source is therefore highly likely to be associated with S2, which
is in turn highly likely to be associated with GRB 080503.  Astrometry is
established using Chandra field sources also detected in the Gemini images.

We detect 40+/-6 counts in the 0.5-8.0 keV band.  There is weak evidence
in favor of variability.  During the first half of the exposure, we measure
21+/-5 cts, as compared to 19+/-4 counts in the second half.  Testing
against the NULL hypothesis of a steady source, we find KS-prob=0.23.

The Chandra flux implies a flattening of the X-ray flux relative to
the rapid decay observed prior to ~13 ksec by the Swift XRT (Guidorzi et al.
2008; GCN7674).  We note also that XRT observations at late time (2.4-3.2
days post burst) exhibit a flux consistent with that of Chandra, yielding a
marginal (~1-sigma) XRT detection.

Minimizing the Cash (1976) statistic, we find the Chandra spectrum to
be acceptably fit by an absorbed powerlaw with photon index Gamma=1.5+/-0.5
and unabsorbed flux Fx(0.3-10 keV) = (1.5+/-0.7) x 10^(-14) erg/cm^2/s.  We
assume Galactic absorption only, as observed for the Swift XRT spectrum
(Guidorzi et al. 2008; GCN7674).

Roughly, the X-ray flux is consistent with an extrapolation of the optical
flux as observed by HST (Bloom et al. 2008; GCN7703), assuming the X-ray
spectral index.  If the optical flux is due to a a Li-Paczynski
"mini"-supernova (1998ApJ...507L..59L) as suggested by Perley et al. (2008;
GCN7678), then the X-ray light curve may be more easily explained by
invoking a separate emission mechanism.  Alternatively, both the optical
and X-ray fluxes may be due to shock energy re-injection or flaring 
occuring on timescales ~1-4 days post burst.

More detailed analyses are ongoing.

We thank Harvey Tananbaum, Patrick Slane, Andrea Prestwich, and the
rest of the Chandra observatory staff for their impressive rapid scheduling
and execution of this observation.

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