[vsnet-grb-info 5921] GRB 080319B: Potential Jet Break Observed by Swift-XRT and UVOT

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7567
SUBJECT: GRB 080319B: Potential Jet Break Observed by Swift-XRT and UVOT
DATE:    08/04/07 19:44:51 GMT
FROM:    Judith Racusin at PSU  <racusin at astro.psu.edu>

J.L. Racusin (PSU), S.R. Oates (UCL-MSSL), P. Schady (UCL-MSSL), S. T.
Holland (CRESST/USRA/GSFC), D.N. Burrows (PSU), report on behalf of the
Swift XRT and UVOT teams:

We have analyzed the first 19 days of Swift XRT data from GRB 080319B
(Racusin et al. GCN 7427), with a total exposure time of 200 ks.  The
light curve can be fit by a triple broken power-law with initial decay
slope of 1.54+/-0.01, breaking at 2790+/-664 s to a slope of 1.85+/-0.05,
breaking again at 41.4+/-9.0 ks to a slope of 1.17+/-0.06, and finally
breaking at 1.04+/-0.43 Ms to a slope of 2.9+/-2.3.

If this last break is interpreted as a jet break, the jet opening
half-angle is 8 degrees x (n/10 cm^-3)^(1/8).  We obtained Eiso=1.3x10^54
ergs (25 kev - 7 MeV) from Golenetskii et al. (GCN 7482), assuming
cosmological parameters of H_0=70 km/s/Mpc, Omega_M=0.3, Omega_Lambda =
0.7.  The corresponding beaming corrected energy is 1.3 x 10^52 ergs.

The uncertanties in the post-jet break slope and time are large.
Therefore, the evidence for a break is preliminary and further
observations will be required to confirm it.

Late-time UVOT white filter observations are also suggestive of a break at
approximately the same time as the X-ray break.  However, further
observations are needed to confirm the break because the afterglow flux is
near the UVOT detection limit.

We strongly encourage additional late-time optical follow-up to further
test for achromaticity and constrain the possible jet break.

This circular is an official product of the Swift Team.