GCN Circular 2139

D. Q. Lamb, M. Nysewander, D. Reichart, D. G. York, J. Barentine, R.
McMillan, J. Dembicky, and B. Ketzeback report on behalf of the ARC
and UNC GRB teams of the FUN GRB collaboration:

We observed the 14 arcminute diameter HETE-2 WXM localization error
circle for GRB030324 (Donaghy et al., GCN 1954
Not found
 
 
, 1955
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) in i* on the
night of March 24 UT, beginning 6.22 hours after the burst (Moran, GCN
2037
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).  We also observed the WXM localization error circle in i* on the
night of April 5 UT, approximately 12 days after the burst.  On both
nights, we took two 1200-second exposures centered at the origin of the
WXM error circle and a mosaic of four overlapping 1200-second exposures
covering the entire WXM error circle.  The limiting magnitude of the
first observation thus ranged from i* = 24.3 (3 sigma) at the center of
the WXM error circle to i* = 23.8 (3 sigma) at the edge of the mosaic.
Similarly, the limiting magnitude of the second observation ranged from
i* = 24.0 (3 sigma) at the center of the WXM error circle to 23.5 (3
sigma) at the edge of the mosaic.

Subtraction of the two images, using the image subtraction routine
ISIS2 (Alard 2000), reveals a single candidate optical afterglow at

RA = +13h 37m 04.418s,  Dec = -00d 15' 33.56"  (J2000).

The candidate optical afterglow is i* = 23.4 +/- 0.2 (S/N = 4.3) in the
first epoch image, using the calibration of Henden (GCN 1860
Not found
 
 
), whereas
no object is detected at the same location down to a limiting magnitude
of i* = 23.9 (2 sigma) in the second epoch image.  The first and second
epoch images, with the location of the candidate optical afterglow
circled, can be found at

http://astro.uchicago.edu/candidate_optical_afterglow.jpg,

and a sky map showing the HETE-2 WXM localization error circle and the
location of the candidate optical afterglow can be found at

http://astro.uchicago.edu/wxm_loc_and_candidate.gif.

We cannot rule out that the candidate is a variable star.  However, a
typical cataclysmic variable or an RR Lyrae variable would have to be
~ 0.5 Mpc away and a typical Cepheid variable would have to be ~2 Mpc
away to be as faint as i* = 23.4.  In either case, the object can not
be associated with the Milky Way galaxy, making the likelihood very
small that the object that we have detected is a variable star.

If this object is indeed the optical afterglow of GRB030324, it would
be the faintest optical afterglow ever detected.

Alard, C. 2000, A&AS, 114, 363