GRB 070920, GRB 070920A

A.O. Jaunsen (Univ. Oslo), J. Gorosabel (IAA-CSIC), J.P.U. Fynbo, D. 
Malesani, J. Sollerman, Ch. Gall (DARK), T. Morel (Leuven), M.D. 
Stritzinger (DARK), A. Somero, J. Telting, C. Villforth (NOT), report on 
behalf of a larger collaboration:

We observed the field of GRB 070920A (Grupe et al., GCN #6805
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 ) in the R 
band with the Nordic Optical Telescope on 2007 September 20.209-20.221 
UT (1.0-1.3 hr after the GRB). Comparison images were taken on 
September 20.216 and again on October 15.135 UT with the same 
telescope, instrumentation and filter. Our images cover 96% of the 
revised BAT error circle(Barbier et al., GCN #6807
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 ), missing a small 
region at the northern boundary.

At a mean time of 1.15 hr after the GRB, we do not detect any 
variable object brighter than R=24.3 (calibrated against USNO-B1.0, R1 
magnitudes). There is no evidence for an associated SN on Oct. 15 down 
to the same limit.

Galante N. (MPI/Muenchen), Garczarczyk M. (MPI/Muenchen),
Gaug M. (IAC/Tenerife), Bastieri D. (U. Padova), Longo F. (INFN/Trieste)
and Scapin V. (U. Udine), for the MAGIC Collaboration

The MAGIC Imaging Atmospheric Cherenkov Telescope performed a follow-up
observation of the SWIFT/BAT burst GRB070920A (GCN circular 6805
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 ,
Grupe D., et al.). We received the GCN alert at T0+539s. The data taking
by MAGIC started at T0+690s, 2.5 minutes after the alert income.
We observed the sky positon for 36min spanning a zenith angle
in the range 53-50 degrees.

No evidence for VHE gamma-ray emission above the analysis threshold of
about 250 GeV was found. A preliminary analysis, taking advantage
for the first time of the fast 2GHz digitalization from the
new data acquisition system, and for the hypothesis of
steady emission and assumption of a differential photon
spectral index of -2.5, yields the following 95% CL differential flux
upper limits (inc. 30% systematic error on the efficiency):

E(225-300 GeV)  : 7.1 x 10^(-8) erg/cm^2
E(300-400 GeV)  : 4.6 x 10^(-8) erg/cm^2
E(400-1000 GeV) : 2.2 x 10^(-8) erg/cm^2
E(1-10 TeV)     : 0.53 x 10^(-8) erg/cm^2

The upper limits correspond to the time period between T0+690s and
T0+2490s. We can also exclude emission of a constant flux during our observation
in any 100s time bin higher than:

E(225-300 GeV)  : 1.9 x 10^(-7) erg/cm^2
E(300-400 GeV)  : 1.0 x 10^(-7) erg/cm^2
E(400-1000 GeV) : 2.4 x 10^(-8) erg/cm^2

This message can be cited.

L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), D. Grupe (PSU),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
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-119 to T+300 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070920 (trigger #291614)
(Grupe, et al., GCN Circ. 6805
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 ).  The BAT ground-calculated position is
RA, Dec = 100.968, 72.250 deg which is 
   RA(J2000) =  06h 43m 52.2s 
   Dec(J2000) = 72d 14' 59" 
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted lightcurve shows a single peak (with structure) starting
at ~T-15 sec, broadly peaking at ~T+25 sec, and ending at ~T+140 sec.
T90 (15-350 keV) is 56 +- 1 sec (estimated error including systematics).
 
The time-averaged spectrum from T+15.1 to T+75.0 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.69 +- 0.21.  The fluence in the 15-150 keV band is 5.1 +- 0.7 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+30.45 sec in the 15-150 keV band
is 0.3 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

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

We imaged the field of GRB 070920 detected by SWIFT
(trigger 291614) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 544.7s after the GRB trigger
(6.8s after the notice). The elevation of the field increased from
from 57 degrees above horizon and weather conditions
were excellents.

The date of trigger : t0 = 2007-09-20T04:00:14.112

The first image is 90.0s exposure in tracking mode:
t0+544.7s to t0+634.7s : R > 18.2

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon=142.6283 lat=+25.1560
and the galactic extinction in R band is 0.3 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

D. Grupe (PSU), N. Gehrels (NASA/GSFC), F. E. Marshall (NASA/GSFC),
C. Pagani (PSU), D. M. Palmer (LANL), J. L. Racusin (PSU),
G. Sato (GSFC/ISAS), M. Stamatikos (NASA/ORAU), M. C. Stroh (PSU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 04:00:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070920 (trigger=291614). 
The BAT on-board calculated location is 
RA, Dec 100.932, +72.227 which is 
   RA(J2000) = 06h 43m 44s
   Dec(J2000) = +72d 13' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is often the case with 64-second
image triggers, the BAT light curve shows no obvious
variation in the count rate. 

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 D. Grupe (grupe 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.)