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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. 9912Not found
 
 
).
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. 9914Not found
 
 
) 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).