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GCN Circular 9918

Subject
GRB 090915: Swift/UVOT detection of an optical afterglow candidate
Date
2009-09-16T22:01:24Z (15 years ago)
From
Erik Hoversten at Swift/Penn State <hoversten@astro.psu.edu>
E. A. Hoversten (PSU) and T. Sakamoto (NASA/UMBC) report on behalf of  
the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 090915
44647 s after the BAT trigger (Cummings & Markwardt, GCN Circ. 9912).
A faint source is detected with 5.6-sigma confidence in the summed
UVOT observations at the position RA, Dec (J2000) = 238.02016,15.48772
which is equivalent to:

      RA (J2000) = 15:52:04.84
     Dec (J2000) = 15:29:15.8

with an estimated uncertainty of 0.8 arcsec (radius, 90% confidence).
This position is 0.2 arcsec from the UVOT-enhanced XRT position
(Beardmore, et al. GCN Circ. 9914) and the two positions agree within
the error bars.  It is unclear at this point if the source is fading,
but follow-up Swift observations are being planned.

The observed magnitudes using the UVOT photometric system (Poole et
al. 2008, MNRAS, 383, 627) for the initial exposures are:

Filter   T_start(s)   T_stop(s)   Exp(s)          Mag
=======================================================================
white      45492       52117       1647     22.15 +/- 0.24 (4.7 sigma)
v          46338       52837       1583     21.08 +/- 0.39 (2.8 sigma)
u          44647       51275       1647     21.40 +/- 0.43 (2.5 sigma)


The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.05 in the direction of the burst
(Schlegel et al. 1998).
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