GCN Circular 889

E. Rol, P. Vreeswijk (U. of Amsterdam), R. Strom (U. of Amsterdam,
ASTRON), E. Richards (Hubble fellow at UAH), R. Wijers (SUNY),
L. Kaper (U. of Amsterdam) C. Kouveliotou (MSFC/USRA) report on behalf
of a larger collaboration:

We observed the WFC error circle of GRB001109 (Gandolfi, GCNs #878
Not found
 
 
,
#879
Not found
 
 
) with the Westerbork Synthesis Radio Telescope at 4.8 GHz, on
Nov. 11.6, 13.7 and 19.7 UT. The flux density detection limits for
these three epochs are 0.043 mJy, 0.048 mJy and 0.059 mJy,
respectively. Within the NFI error circle (Amati et al., IAUC #7519),
we do not detect any sources down to a 4 sigma limit for the first two
epochs, and 5 sigma for the last epoch.

Assuming a low value for the spectral slope of an extra-galactic radio
source of -0.5, the reported VLA 8.4 GHz flux density of 0.24(3) mJy
(Taylor et al., GCN #880
Not found
 
 
), implies a 4.8 GHz flux of about 0.32 mJy on
Nov. 9.8 UT. Then this source has decreased by at least a factor of
about two in flux density (from 0.32 mJy to below our 4 sigma limit of
0.17 mJy on Nov. 11.6) between 0.4 days and 2.2 days after the
burst. If the radio source is an extragalactic object of constant
flux, then our measurement requires its spectral index to be +0.6 or
steeper, which is well outside the usual range -1 to -0.5 for
extragalactic sources.

Therefore, it is more likely that the source has actually faded,
making this source the probable counterpart of GRB001109. The radio
behavior is then reminiscent of the short radio flare counterparts,
like those of GRB970828 and GRB990123. Assuming a radio spectral index
during the fading phase of -0.8, the temporal decay has to be faster
than t**-0.5.  This is consistent both with a reverse-shock and
forward-shock origin of the radio emission.

Further radio measurements are required to exclude or confirm that 
the radio source is a variable extragalactic object of some kind.

We thank the WSRT staff for their assistance.