GRB 070920B

J. L. Racusin (PSU), S. D. Barthelmy (GSFC),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC),
S. T. Holland (CRESST/GSFC/USRA), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
C. Pagani (PSU), D. M. Palmer (LANL), M. Stamatikos (NASA/ORAU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 21:04:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070920B (trigger=291728). 
The BAT on-board calculated location is 
RA, Dec 0.144, -34.819 which is 
   RA(J2000)  = 00h 00m 35s
   Dec(J2000) = -34d 49' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 35 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Because Swift is in the process of returning to normal operations, 
automatic slewing to GRBs is currently disabled.  Therefore, there 
are no prompt XRT or UVOT observations of this burst. 

Burst Advocate for this burst is J. L. Racusin (racusin AT astro.psu.edu). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

A. de Ugarte Postigo, J. Gorosabel, M. Jelinek,
A.J. Castro-Tirado (IAA-CSIC, Granada),
D. Perez-Ramirez (U. Jaen), R. Corradi (ING, La Palma),
T. Liimets (ING, La Palma and Tartuu Obs.) report:

We have observed the field of GRB070920B (Racusin et al.,
GCNC 6808) using the 2.5m NOT telescope (+ALFOSC) at
Roque de los Muchachos Observatory in V, R and I bands
(180 s each) at high airmass. The mean time of the
observations was 21.0486 Sep. 2007 (UT), ~4.1 hours after
the burst.  We do not detect any bright afterglow candidate
within the Swift/BAT error box.
However, we do see a red, stationary object (detected in R
and I bands) slightly over the DSS2 limit that does not seem
to have its correspondent in the DSS2 images. The
coordinates of this object are (J2000 +/- 0.5"):

R.A.: 00:00:34.56
Dec.: -34:49:08.4

Further observations are needed to confirm wether this could
be the optical afterglow of GRB 070920B.

A finding chart comparing with the DSS2 can be found at:

www.iaa.es/~deugarte/GRBs/070920B/GRB070920B.jpg

This message can be quoted.

S.D. Barthelmy (GSFC), L. Barbier (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), J.L. Racusin (PSU),
T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070920B (trigger #291728)
(Racusin, et al., GCN Circ. 6808
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 ).  The BAT ground-calculated position is
RA, Dec = 0.127, -34.844 deg  which is 
   RA(J2000)  =  00h 0m 30.5s 
   Dec(J2000) = -34d 50' 39" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted lightcurve shows a symmetrical peak starting at ~T-20 sec,
peaking at ~T+1 sec, and ending at ~T+17 sec.  T90 (15-350 keV) is
20.2 +- 0.2 sec (estimated error including systematics).
 
The time-averaged spectrum from T-11.2 to T+13.4 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.67 +- 0.58, 
and Epeak of 41.3 +- 4.5 keV (chi squared 45.21 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 6.6 +- 0.5 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T-0.49 sec in the 15-150 keV band is
0.8 +- 0.1 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.00 +- 0.10 (chi squared 67.16 for 57 d.o.f.).  All the quoted errors
are at the 90% confidence level.

[GCN OPS NOTE(21sep07): Per author's request, the 'B' was added to the GRB
name in the first sentence.]

E. Berger (Princeton,OCIW) and W. Krzeminski (LCO) report:

"We observed the BAT error circle of GRB 070920b (GCN 6808
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 ) with the Swope 
40-inch telescope at Las Campanas Observatory starting on 2007 Sep. 21.22 
UT (8.2 hours after the burst).  A total of 30 minutes were obtained in 
R-band in mediocre seeing conditions (~1.5").  A comparison to DSS reveals 
no new objects within the revised BAT error circle (GCN 6811
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 )."

B. E. Cobb, part of the larger SMARTS consortium, reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical imaging of the error region of GRB 070920b
(Racusin et al. GCN 6808
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 ) with a mid-exposure time of ~4.4 hours
post-burst (2007-09-21 01:29 UT).  The total summed exposure
time amounted to 36 minutes in I, but images were taken under poor
weather conditions.

Comparison to the DSS reveals no new sources in the
refined GRB error region (Barthelmy et al. GCN 6811
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 ).
Our I-band limiting magnitude is I~20.8 (calibrated using
several USNO-B1.0 stars).

Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 070920B detected by SWIFT
(trigger 291728) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 2.7h after the GRB trigger
(beginning of the night). The elevation of the field increased from
from 27 degrees above horizon and weather conditions
were good but the moon (phase=0.58) was only at 68 degrees
of the field, limiting the detectivity.

We co-added a series of 8 x 180s unfiltered exposures.
We do not detect any OT in the refined position
provided by Barthelmy et al. (GCNC 6811) nor the source
reported by de Ugarte Postigo et al. (GCNC 6809):

t0+2.7h to t0+3.6h : R > 18.2

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction
(which is very low along the line of sight).

This message may be cited.

J. L. Racusin (PSU) reports on behalf of the Swift-XRT team:

We have analysed 9.7 ks of Swift XRT data for GRB 070920B (Racusin et al.,
GCN 6808
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 ) beginning 64 ks after the BAT trigger. The data are entirely in
Photon Counting mode, and reveal a faint uncatalogued X-ray source at RA,
Dec= 0.13029, -34.85284 degrees, which is equivalent to:

RA (J2000) =  00h 00m 31.27s
Dec(J2000) = -34d 51' 10.2"

with an estimated uncertainty of 8.0 arcsec (radius, 90% containment
including boresight uncertainty). This is 33 arcsec from the refined BAT
position (Barthelmy et al. GCN 6811
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 ). The source has a count rate of
2.1e-3 +/- 0.6e-3 counts s-1. At present we are unable to determine
whether the source is fading, however observations are ongoing.

This circular is an official product of the Swift-XRT team.

H. Swan (U Mich), E.S. Rykoff (UCSB), F. Yuan (U Mich), report on
behalf of the ROTSE collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia,
responded to GRB 070920B (Swift trigger 291728; J. L. Racusin, GCN
6808
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 ), producing images beginning 8.5 s after the GCN notice time. An
automated response took the first image at 21:05:16.5 UT, 44.1 s after
the burst, under partly cloudy conditions. We took 10 5-sec, 10 20-sec
and 386 60-sec exposures. These unfiltered images are calibrated
relative to USNO A2.0 (R).  We lost internet contact with ROTSE-IIIc
shortly after the GCN trigger, thus we were unable to retrieve the burst
data until Sept. 25.

Comparison to the DSS (second epoch) reveals no new sources within the
3-sigma Swift/BAT error circle or the XRT error circle , (J. L. Racusin,
GCN 6820
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 ) for both single images and coadding into sets of 10; the field
is not crowded. Individual images have limiting magnitudes ranging from
12.8-17.8; we set the following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
21:05:16.5   21:05:21.5         5     15.7           44.1       N
21:05:16.5   21:06:34.5        78     16.7           44.1       Y
21:29:06.7   21:34:44.6       337     17.8         1474.3       Y