TITLE:   GCN CIRCULAR
NUMBER:  7775
SUBJECT: GRB 080503: 2nd Epoch Chandra X-ray Observations
DATE:    08/05/26 23:50:35 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), 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 2008/05/25 18:11:36 UT to 2008/05/26 03:04:28 UT (22.24 - 22.61 days post
burst), Chandra Director's Discretionary Time observations with ACIS-S were
conducted of the field of GRB 080503 (e.g., Mao et al. 2008; GCN 7665) for a
total exposure of 29.8 ksec (livetime).  In a 3-pixel radius source extraction
region centered around the first epoch Chandra source (Butler et al. 2008;
GCN 7704), we find no source counts in the 0.5-8.0 keV band.

We derive a 2-sigma upper limit to the source flux of Fx(0.3-10 keV) < 9.9 x
10^(-16) erg/cm^2/s.  Relative to the first epoch flux ~18 days earlier,
the 2nd epoch flux implies a fade steeper in time than t^(-1.6).  A similarly
steep or steeper fade is implied in the optical band from Gemini and HST
observations of this source (Perley et al. 2008; GCNs 7667, 7678, 7679, 7680,
7695, 7703, 7749).  Combined with a rough agreement in the extrapolated flux
levels assuming a GRB afterglow external shock model (e.g., GCN 7704), the
lightcurve similarity supports an interpretation where the external shock is
responsible for both the X-ray and optical emission.

More detailed analyses are ongoing.

We thank Harvey Tananbaum and the Chandra observatory staff for their
impressive rapid scheduling and execution of this observation.

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