GRB 090307
GCN Circular 8945
Subject
GRB 090307: Swift-BAT refined analysis of trigger 345551
Date
2009-03-07T17:07:38Z (16 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
P. A. Evans (U Leicester), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
Using the data set from T-60 to T+303 sec from recent telemetry downlinks,
we report further analysis of posible GRB 090307 (trigger #345551)
(Evans, et al., GCN Circ. 8943). The BAT ground-calculated position is
RA, Dec = 245.007, -28.647 deg, which is
RA(J2000) = 16h 20m 01.7s
Dec(J2000) = -28d 38' 48.2"
with an uncertainty of 3.3 arcmin, (radius, sys+stat, 90% containment).
We note that this position is based on a weak peak in the image domain (5.5 sigma).
This is lower than our regular detections because this is a Subthreshold
trigger. The partial coding was 87%.
The mask-weighted light curve shows a weak pulse starting at ~T-7 sec and
ending at ~T+20 sec. T90 (15-350 keV) is 22 +- 8 sec (estimated error
including systematics).
The time-averaged spectrum from T-7.4 to T+16.6 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.08 +- 0.39. The fluence in the 15-150 keV band is 2.6 +- 0.6 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+5.57 sec in the 15-150 keV band
is 0.2 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/345551/BA/
Given the weak nature of this event and the as yet unconfirmed nature
of this trigger by the XRT follow-up observations, we can not rule out
that this trigger is due something other than a GRB. There are further
XRT follow-up observations in the observing plan.
GCN Circular 8946
Subject
GRB 090307: Swift-XRT afterglow confirmation and analysis
Date
2009-03-08T13:54:58Z (16 years ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT observed the field of GRB 090307 (BAT trigger 345551; GCN
Circs. 8943, 8945) for 4.6 ks, from 135 s to 80 ks after the BAT
trigger. All of the data were taken in Photon Counting (PC) mode. The
X-ray source reported by Evans et al. (GCN Circ. 8943) is seen to fade,
and is therefore identified as the afterglow, confirming the GRB nature
of this BAT trigger. The source has an enhanced XRT position: RA, Dec =
244.9944, -28.6333 which is equivalent to:
RA (J2000): 16 19 58.64
Dec (J2000): -28 37 59.9
with an uncertainty of 2.1 arcsec (radius, 90% confidence). The light
curve shows a power-law decay, with an index 0.7 (+0.2, -0.1).
The PC mode spectrum can be fitted with an absorbed power-law, with a
photon index of 2.21 (+0.72, -0.60) and an absorbing column of 3.5
(+2.7,-1.1) e21 cm-2, in excess of the Galactic value of 1.3e21 cm-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.1 x 10-11 (7.6 x 10-11) erg cm-2
count-1.
If the light curve continues to decay with alpha=0.7, the XRT count-rate
at T+48 hrs will be 0.002 counts s-1, corresponding to an observed
(unabsorbed) 0.3-10 keV flux of 7.38e-14 (1.4e-13) erg cm-2 s-1.
The results of the automatic XRT analysis are available online at
http://www.swift.ac.uk/xrt_products/00345551.
This circular is an official product of the Swift-XRT team.
GCN Circular 8959
Subject
GRB 090307: GROND Upper Limits
Date
2009-03-09T13:45:36Z (16 years ago)
From
Christian Clemens at MPE <cclemens@mpe.mpg.de>
A. C. Updike (Clemson University), C. Clemens, J. Greiner and P. Afonso (MPE
Garching) report on behalf of the GROND team:
We observed the field of GRB 090307 (Swift trigger 345551; Barthelmy et al.,
GCN #8945) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla Observatory
(Chile).
Observations started at 05:06 UT on March 7, 80 min after the GRB trigger, and
continued for 66 min. They were performed at an average seeing of 1.2" and at
an average airmass of 1.8.
We do not detect a source within the Swift-XRT error circle reported by Evans
et al. (GCN #8946) down to (all in AB system)
g' > 23.3,
r' > 23.7,
i' > 23.2,
z' > 22.9,
J > 21.7,
H > 21.2 and
K > 20.3.
The given limits are derived based on calibrating the images against GROND
zeropoints and 2MASS field stars and are not corrected for the Galactic
foreground extinction corresponding to a reddening of E_(B-V)= 0.528 in the
direction of the burst (Schlegel et al. 1998).