[vsnet-grb-info 2771] GRB 060123: Swift refined analysis of an unusual GRB

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4608
SUBJECT: GRB 060123: Swift refined analysis of an unusual GRB
DATE:    06/01/27 22:37:00 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <jayc at milkyway.gsfc.nasa.gov>

J. Cummings (GSFC/ORAU), V. Mangano (INAF-IASFPA),
L. Angelini (GSFC), L. Barbier (GSFC), S. Barthelmy (GSFC), 
D.N. Burrows (PSU), M. Chester (PSU), E. Fenimore (LANL), 
N. Gehrels (GSFC), D. Hullinger (UMD), J. Kennea (PSU), 
H. Krimm (GSFC/USRA), V. La Parola (INAF-IASFPA), 
A. Levan (U. Leicester), C. Markwardt (GSFC/UMD), 
T. Mineo (INAF-IASFPA), J. Norris (GSFC), P. O'Brien (U. Leicester), 
D. Palmer (LANL) A. Parsons (GSFC), E. Rol (U. Leicester), 
P. Romano (INAF-OAB), T. Sakamoto (GSFC/ORAU), G. Sato (ISAS), 
J. Tueller (GSFC),
on behalf of the Swift team:

Using Swift-BAT survey data from T-23 to T+877 sec and target of 
opportunity (ToO) Swift-XRT observations, we report further
analysis of Swift-BAT GRB 060123 (Palmer, et al., GCN 4584).   

T90 was 900 +- 100 seconds (estimated one-sigma error), with multiple
peaks but an overall gradual profile.  Peak flux was 
0.04 +- 0.01 photons/cm2/sec in the 15-150 keV band.

The prompt emission time-averaged spectrum is well fitted by a simple 
power law model.  The power law index is 1.9 +- 0.6.  The fluence in 
the 15-150 keV band was (3.0 +- 1.2) x 10^-7 erg/cm2. 

A Swift ToO observation started at 19:14:14 on Jan 24th, 2006 
(75.1 ks after the event).  XRT detected a point source within the
BAT error box (Palmer et al, GCN 4584).  Subsequent ToO observations 
revealed that, after an initially flat phase, this source is steadily
decaying with a slope of -2.2 +/- 0.4.  The initial flat phase 
extended to ~1.1 x 10^5 sec.  Further observations are ongoing.

The average XRT spectrum accumulated in the time interval from 
75.1 ksec to 291 ksec after the event (20 ksec of net exposure) was 
fitted, using Cash statistics, with an absorbed power law.  The best 
fit photon index is 2.1 +/- 0.2.  The spectrum shows evidence of 
absorption at the level of (2 +/- 1)e21 cm^-2, in excess of the 
estimated Galactic value (1.6e20 cm^-2, Dickey & Lockman 1990).  The 
average unabsorbed 0.2-10 keV flux is 7.0e-13 ergs cm^-2 s^-1.

All the quoted errors are at the 90% confidence level except as
noted.

This was an unusual event.  It was very long and very gradual, having
the longest duration and weakest peak flux of any BAT burst.  The GRB
nature of the event would be uncertain on the basis of the BAT data
alone, but is highly probable given the fading XRT light curve.  The 
flat decay phase extends very late for a GRB.  Followup observations
are strongly encouraged.