GCN Circular 9272
Subject
GRB 090426: Swift/BAT spectral lag results
Date
2009-04-27T20:43:47Z (16 years ago)
From
Hans Krimm at NASA-GSFC <hans.krimm@nasa.gov>
T. N. Ukwatta (GWU/GSFC), N. Gehrels (GSFC), H. A. Krimm
(CRESST/GSFC/USRA), T. Sakamoto (GSFC/UMBC),
S. D. Barthelmy (GSFC), M. Stamatikos (GSFC/ORAU)
(for the Swift-BAT team):
For GRB 090426 (Cummings et al GCN 9254), the spectral lag analysis
of the data from T-1 sec to T+2 sec yields a lag of -6 +/- 45 msec
for the 25-50 to 50-100 keV bands using light curve binnings of
16, 32 and 64 msec. With redshift of 2.609 (Levesque et al GCN 9264)
this lag value transforms to 1.7 +/- 12.5 ms in the GRB
rest frame. The lag is consistent with zero but has relatively large
uncertainty.
Furthermore, the observed T90 (1.2 +/- 0.3 sec; Swift/BAT 15-350 keV,
Sato et al., GCN 9263) and rest frame T90 (0.33 +/- 0.08 sec)
values suggest a short-hard burst. However, the spectral fit
to the BAT data (Sato et al., GCN 9263) indicates a relatively
soft spectrum (a power-law fit to the spectrum gives index
of 1.93 +/- 0.22).
In summary, the observed and rest frame T90 values for GRB 090426
point strongly toward a short-hard burst classification.
The spectral lag is consistent with zero, and the spectrum is
marginally softer than typical for a short-hard burst.
Our conclusion is that this burst could be either short or long,
but the BAT data leads toward a classification as short.