GRB 070913

C. B. Markwardt (CRESST/GSFC/UMD), S. D. Barthelmy (GSFC),
J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA),
K. L. Page (U Leicester), D. M. Palmer (LANL),
M. Stamatikos (NASA/ORAU) and T. N. Ukwatta (GSFC/GWU) report on
behalf of the Swift Team:

At 00:36:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070913 (trigger=290843). 
The BAT on-board calculated location is RA, Dec 228.832, -24.342,
which is:
   RA(J2000)  = 15h 15m 20s
   Dec(J2000) = -24d 20' 30"
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 5 sec.  The peak count rate
was ~1000 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 C. B. Markwardt (craigm AT milkyway.gsfc.nasa.gov). 
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.)

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

We imaged the field of GRB 070913 detected by SWIFT
(trigger 290843) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 87.3s after the GRB trigger
(69.4s after the notice). The elevation of the field decreased from
from 35 degrees above horizon and weather conditions
were good.

The date of trigger : t0 = 2007-09-13T00:36:44.064

The first image is 30.0s exposure in tracking mode.
We do not detect any OT with a limiting magnitude of:
t0+87.3s to t0+117.3s : R > 17.8

We co-added a series of exposures:
t0+123.6s to t0+576.5s : R > 18.4

Further observations are in progress.

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

N.B. Galactic coordinates are lon=340.2109 lat=+27.9232
and the galactic extinction in R band is 0.4 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

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

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 070913
(Markwardt et al. GCN 6782
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 ) with a mid-exposure time of
(2007-09-13 01:11 UT, ~34 minutes post-burst). Optical imaging in B,V,R
and I has a field of view of 6'x6' and, therefore, covers the entire GRB
error circle.  IR imaging in J, H, K and Y has a smaller field
of view and covers a region totalling 3 square arcminutes in
the middle of the quoted error region.  Several dithered images
were obtained in each filter, with total summed exposure times
of 180s in each of BRIYJK and 120s in each of H and V.

Preliminary visual comparison of the optical images to the DSS
and the IR images to 2MASS frames does not reveal any new sources
to a limiting magnitude of I>18.2 and J>15.8 (calibrated using USNO-B1.0 
and 2MASS stars).

[GCN OPS NOTE (13sep07): Per author's request, the following statement was added:
"Due to the recalculated BAT position position (#6785), the SMARTS observations
actually do not cover any of the GRB error region for this burst."]

C. B. Markwardt (CRESST & NASA/GSFC), S. D. Barthelmy (NASA/GSFC),
on behalf of the Swift BAT Team, report:

As noted in GCN #6781
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 , currently Swift is calibrating
its new gyroscope configuration for the roll axis.

In GCN Circ #6782
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 , we quoted an error circle radius of 3 arcminutes
for the BAT position for GRB 070913.  This radius was incorrect.  The
correct value was distributed in the Notice (radius ~9.97 arcmin).
We apologize for this error and any inconvenience this may have caused.

In ground analysis, we have now applied the roll axis correction, and
the new best position is, RA, Dec 228.734, -24.290, which is:
   RA(J2000)  =  15h 14m 56s
   Dec(J2000) = -24d 17' 23"
with an uncertainty of 3 arcmin radius, 90% containment, including
systematic uncertainty).  This is a shift of 6.2 arcmin from the
position quoted in GCN Circ #6782
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 .

G. Sato (GSFC/ISAS), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. Ukwatta (GWU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+523 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 070913 (trigger #290843)
(Markwardt, et al., GCN Circ. 6782
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 , 6785
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 ).  The BAT ground-calculated 
position is
RA, Dec = 228.737 -24.278 deg which is
    RA(J2000) = 15h 14m 57s
    Dec(J2000) = -24d 16' 42"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 51%.

The mask-weighted light curve shows a single peak of about 4 seconds duration.
T90 (15-350 keV) is 3.2 +- 0.3 sec (estimated error including systematics).

The time-averaged spectrum from T-2.4 to T+1.2 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.61 +- 0.19.  The fluence in the 15-150 keV band is 2.5 +- 0.3 x 
10^-07 erg/cm2.
The 1-sec peak photon flux measured from T-0.34 sec in the 15-150 keV band
is 1.4 +- 0.2 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 analyzed our set of unfiltered images described
in GCNC 6783 taking account for the new position provided
by Sato et al. (GCNC 6786). On the two first images,
we find a fading optical transcient candidate for the
GRB 070913 afterglow at 1.29 arcmin from the BAT position
(which is within the 1.7 arcmin radius of BAT).

TAROT afterglow candidate position:

RA(J2000) = 15h 15m 01.3s
Dec(J2000) = -24d 17' 35"

with an uncertainty of 1 arcsec radius.

t0+87s  to t0+117s : R ~ 17.5
t0+124s to t0+154s : R ~ 18
t0+288s to t0+378s : R > 18

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

Screen copies are available at URL:
http://www.cesr.fr/~klotz/grb070913

This message may be cited.

A.P. Beardmore, K.L. Page (U. Leicester) and C. B. Markwardt 
(CRESST/GSFC/UMD) report on behalf of the Swift-XRT team.

The Swift-XRT began observing the field of the BAT GRB 070913 (trigger
#290843) at 2007-09-13T21:30, 75.2 ks after the BAT trigger (Markwardt
et al., GCN 6782
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 ). In 8 orbits of photon counting mode data totalling
7.8 ks we find a faint, uncatalogued source inside the refined
BAT error circle (Sato et al., GCN 6786
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 ) at RA, Dec (J2000)
228.75715, -24.29302, which is

RA(J2000)  =  15h 15m 01.7s
Dec(J2000) = -24d 17' 34.9"

with an uncertainty of 6.5 arcsec (radius, 90 percent confidence,
including boresight uncertainties). The source is 5.7 arcsec away
from the TAROT optical afterglow candidate reported by Klotz et al.
(GCN 6787
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 ). The source has an observed 0.3-10.0 keV count rate of 
(1.4+/-0.5)e-3 count/s.

We note the presence of a second faint X-ray source with a count rate of 
(1.3+/-0.5)e-3 count/s at RA, Dec (J2000) 228.70481, -24.21053, i.e.

RA(J2000)  =  15h 14m 49.2s
Dec(J2000) = -24d 12' 37.9"

with an uncertainty of 6.6 arcsec (radius, 90 percent confidence,
including boresight uncertainties). This is 4.4 arcmin from the
refined BAT position, and outside its 1.7 arcmin error radius.

It is impossible to say whether either of the sources are fading at
this stage, though the association of the first one with the fading 
optical object makes it the most likely candidate for the GRB X-ray 
afterglow.

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