GCN Circular 6019

H. A. Krimm (GSFC/USRA),  D. N. Burrows (PSU), M. Chester (PSU),
D. Grupe (PSU), V. Mangano (INAF-IASFPA), F. Marshall (GSFC),
J. Nousek (PSU), B. Sbarufatti (INAF-IASFPA), N. Gehrels (GSFC)
report on behalf of the Swift Team

GRB070110 has been identified as a "Swift Burst of Interest"
(Krimm et al, GCN 6014
Not found
 
 
) due to the remarkable irregular light curve
seen in X rays, and the long duration and slow fading of the optical
emission.  We report that the U band emission has now faded below
the  UVOT threshold of detectability (roughly 23rd magnitude),
so the UVOT can no longer follow this GRB, but we project that the
optical brightness will continue to slowly fade, making it a possible
candidate for late-time optical detection.  We are continuing
observations with the XRT and we encourage ground-based observers
to attempt measurements of the afterglow, if they are capable of
detecting an afterglow to our estimate of V=22.7 over the next five days.

The UVOT light curves (Figure 4 in GCN Report 26.3
http://gcn.gsfc.nasa.gov/reports/report_26_3.pdf )  show
power-law decays with the following indices.  V and B bands
(T+1ksec to T+200 ksec): -0.5; U band (T+ 10 ksec to T+500 ksec):
-0.7.  Converting to magnitudes using the zero points for the
filters yields the following fits:
Vmag = 21.2 + 1.25*log10(t)
Bmag = 21.9 + 1.25*log10(t)
Umag = 21.8 + 1.75*log10(t).
where the time, t, is measured in days from the trigger.  For example
at 7:00 UT on Jan. 20, 2007, the predicted V magnitude is 22.4.

The X-ray light curve (see Figure 2 in GCN Report 26.3) is decaying
very slowly and in fact the three most recent data points suggest a
slight upward trend.  Fitting the data after the steep drop in flux
(from T+35 ksec to T+800 ksec), but excluding the big rise at
around T+65 ksec, the best fit decay index is -0.6 +/- 0.1.  If the
source follows this decay law we predict a count rate in the XRT
(0.3-10.0 keV) of 1.5 X 10^-3 cts/sec on Jan. 22, 2007.  If it
continues to decay at this rate, the source will continue to be bright
enough to be observed with the XRT during the month of February 2007.

We note that this source (RA/dec:   0h 03m 39.27s, -52s 58' 26.9",
epoch 2000.0) is currently observable from an observatory at
-29 degrees latitude and is currently above 30 degrees elevation
for approximately 2 hours after astronomical twilight.  This time
will be only about one hour by the end of January.