TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5194
SUBJECT: GRB 060526: Break in X-ray light curve
DATE:    06/06/01 18:54:41 GMT
FROM:    David Burrows at PSU/Swift  <dxb15@psu.edu>

A. Moretti (INAF-OAB), D.N. Burrows (PSU) , S. Campana (INAF-OAB),
G. Chincarini (UNIMIB, INAF-OAB), and G. Tagliaferri (INAF-OAB)
report on behalf of the Swift XRT team:

We have analyzed six days of Swift XRT data from GRB 060526
(Campana et al., GCN5162), with a total exposure of 70ks.  After the
flaring episodes between 200s and 400s interval from the trigger,
already reported in Campana et al. (GCN 5168), the light curve
presents a shallow decay with slope -0.7(+-0.1) until (1.1+-0.3)e5
seconds after the trigger (1.2+-0.3 days). Superimposed on the
shallow decay a small bump is present at 7 ks.

At (1.1+-0.3)e5 seconds the light curve breaks to a much steeper slope
of (-2.8+-0.1).  We note that this slope is very similar to the optical decay
slope of the last five data points obtained by the RTT150 telescope
(Khamitov et al., GGNs 5186, 5189, and 5193, slope ~ -2.75), and that the
X-ray break time also seems to correspond roughly to a break in the
general trend of the optical light curve (e.g., Thoene et al., GCN 5179;
Kann and Hoegner, GCN 5182; Kann and Thoene, GCN 5187;
although the slopes reported in these circulars are shallower than
those seen in the XRT light curve and in the last 5 RTT150 data points).
On the other hand, the optical light curve is quite complex
(Halpern, Amstrong, and Mirabel, GCNs 5176, 5188).

If interpreted as a jet break, the jet opening half-angle for this burst is
about 4.7 degrees x (Eiso/10^53)^(-1/8) x (n/10 cm^-3)^(1/8).
Using the total BAT fluence, we obtain Eiso = 2.3e52 ergs
(15-150 keV observed frame), which would correspond to 5.6 degrees
for the jet opening angle.  We did not extrapolate Eiso into the usual
energy band due to the limited energy range of the BAT and the single
power law fit to the BAT spectrum, but we expect that the jet angle
should be in the range of 4-5 degrees.  A lower density would reduce
this angle.

This Circular is an official product of the Swift XRT Team.