GRB 070911

B. E. Cobb, 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 070911
(Krimm et al. GCN 6777
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 ) with the following mid-exposure times:
         2007-09-13 07:10:30 UT -  2.05 days post-burst
         2007-09-15 06:42:00 UT -  4.03 days post-burst
         2007-09-22 05:22:21 UT - 10.98 days post-burst
For each observation, total summed exposure times amounted to
36 minutes in I and 30 minutes in J.

A fading afterglow is detected in both I and J within the X-ray afterglow
error region (Stratta et al. 6779). This afterglow has the following 
coordinates and magnitudes:

RA/DEC: 01:43:14.4 -33:29:02.5

days post
burst           I-magnitude (or limit)  J-magnitude (or limit)
----------------------------------------------------------------
  2.05           20.9 +/- 0.1            19.8 +/- 0.2
  4.03           21.9 +/- 0.2           >19.5 +/- 0.1
10.98          >22.3 +/- 0.1           >19.7 +/- 0.1

The I-band photometry is calibrated to a number of secondary stars in the
field of GRB 070911.  The magnitude of these secondary stars was
derived using Landolt standard star observations obtained
during the first two epochs, which were photometric night at CTIO.
The J-band images are calibrated based on several 2MASS stars in the 
field.

The afterglow's behavior between the first two epochs is consistent with a
decay rate of alpha = -1.4+/-0.3.  Interestingly, no J-band afterglow was
detected by Butler et al. (GCN 6780
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 ) to J > 16.5 only 3 hours after
the burst. This indicates that a period of rebrightening might have 
occurred between 3 hours and 2 days post-burst, though steady decay
is only ruled out at the ~1 sigma level.

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), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. Ukwatta (GWU)
on behalf of the Swift-BAT team,

M. Ohno, T. Uehara, T. Takahashi, Y. Fukazawa, C. Kira (Hiroshima U.),
Y. E. Nakagawa, K. Yamaoka, S. Sugita (Aoyama Gakuin U.),
Y. Terada, T. Tamagawa (RIKEN), K. Morigami, N. Kodaka, K. Onda,
M. Tashiro, M. Suzuki, Y. Urata, A. Endo (Saitama U.), T. Enoto,
R. Miyawaki, K. Nakawaza, K. Makishima (Univ. of Tokyo), E. Sonoda,
M. Yamauchi, S. Maeno, H. Tanaka, R. Hara (Univ. of Miyazaki),
M. Kokubun, M. Suzuki, T. Takahashi (ISAS/JAXA), S. Hong (Nihon U.),
on behalf of the Suzaku-WAM team,

report:

We performed the Swift/BAT, and Suzaku/WAM joint fit spectral
analysis of GRB 070911 (Swift-BAT trigger #290624; Krimm, et al.,
GCN Circ. 6777
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 ).  The time interval of the spectral data for each
instrument is chosen from T0(WAM)-56 to T0(WAM)+149 sec where
T0(WAM) is the trigger time of WAM at 05:58:20.069 UTC.
The energy ranges which we used in the joint spectral analysis
are 14-150 keV and 100-1000 keV for Swift/BAT and Suzaku/WAM
respectively.  The spectral data of two instruments are fit
with the spectral model multiplied by the constant factor to
take into account the systematic uncertainties in the response
matrices of each instrument.

The spectrum is well fit with a power-law with exponential cutoff
model (dN/dE ~ E^{alpha} * exp(-(2+alpha)*E/Epeak)).  The constant
factors of each instrument agree within 10%.  No systematic residual
from the best fit model is seen in the spectral data of each instrument.
The best fit spectral parameters are: alpha = -1.6 +- 0.1 and
Epeak = 170 (-50/+280) keV (chi2/dof = 59/74).  The energy fluence
in the 15-1000 keV band calculated by a power-law with exponential
cutoff model for this 205 sec interval is 2.1 (-0.2/+0.4) x 10^-5 erg/cm2
(assuming the constant factor of the BAT is fixed to 1).

All the quoted errors are at the 90% confidence level.

Y. E. Nakagawa, K. Yamaoka, S. Sugita (Aoyama Gakuin U.), Y. Terada,
T. Tamagawa (RIKEN), M. Ohno, T. Uehara, T. Takahashi, Y. Fukazawa,
C. Kira (Hiroshima U.),  M. Tashiro, Y. Urata, A. Endo, K. Onda,
M. Suzuki, N. Kodaka, K. Morigami (Saitama U.), T. Enoto, R. Miyawaki,
K. Nakawaza, K. Makishima (Univ. of Tokyo), E. Sonoda, M.Yamauchi,
S. Maeno, H. Tanaka, R. Hara (Univ. of Miyazaki), M. Kokubun, M. Suzuki,
T. Takahashi (ISAS/JAXA), S. Hong (Nihon U.),
on behalf of the Suzaku WAM team, report:

The long GRB 070911 (Swift/BAT trigger #290624 ; Krimm et al., GCN 6777
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 ;
Palmer et al., GCN 6778
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 ) triggered the Suzaku Wide-band  All-sky Monitor (WAM)
which covers an energy range of 50 keV - 5 MeV at 2007-09-11 05:58:20 UT (=T0).

The observed light curve shows a multi-peaked structure lasting from T0-160s
to T0+120s, with a duration (T90) of about 182 seconds. The fluence in
100 - 2000 keV was 1.4 (+0.3, -0.3) * 10^-5 erg/cm^2. The 1-s peak flux
measured from T0+0s was 1.1 (+0.3, -0.2) photons/cm^2/s in the same energy
range.

Preliminary result shows that the time-averaged spectrum from T0-160s to 
T0+120s is well fitted by a single power-law with a photon index of
1.9 (+0.3, -0.4) (chi^2/d.o.f = 8.7/16).

All the quoted errors are at statistical 90% confidence level,
in which the systematic uncertainties are not included.

The light curves for this burst are available at:

http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html

N. Butler, D. Starr, J. S. Bloom (UCB) report:

In simultaneous J, H, and Ks band observations of the GRB 070911  
field (Krimm et al.; GCN 6777
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 ) starting on September 11 08h53m33s UT,
2 hrs, 56 min after the burst, we find no new sources in the XRT error  
region (Stratta et al.; GCN 6779
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 ).  We derive the following limiting 
magnitudes: J > 16.5 mag, H > 16 mag, and Ks > 16 mag.

This message may be cited.

Giulia Stratta (ASDC), Matteo Perri (ASDC) and David
Burrows (PSU) report, on behalf of the Swift XRT team:

We have analyzed the first 9 orbits of Swift-XRT
data on GRB 070911 (GCN 6777
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 , Krimm et al.) for a
total of 6.8 ks of exposure taken in Photon Counting
mode starting 14.5 hours after the burst.
This is the first BAT GRB after Swift safe-hold recovery.
Swift-XRT detected an uncatalogued, fading source
at the following coordinates
RA,Dec 25.8094,-33.4842 which is

  RA(J2000)  = 01h 43m 14.26s
  Dec(J2000) = -33d 29' 03.1"

with an estimated error radius of 6 arcseconds
(90% confidence). This error radius has been increased
to take into account the uncertainties in the pointing
calibration during Swift's reactivation phase.
This position is 0.5' from the BAT refined position
(GCN 6778
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 , Palmer et al.). We note however that XRT position
uncertanties are currently being re-validated.

The 0.3-10 keV X-ray light curve from T0+14.5 h to T0+27.3 h
can be fit with a simple power law model.
The best fit decay index is alpha=-1.5+/-0.2

The 0.3-10.0 keV X-ray spectrum with 6.8 ks of integration
time, is well fit by an absorbed power law with a
photon index of 2.2 +/- 0.3 and a total column density of
NH=(1.3+/-0.5)e21 cm**-2 (chi2/dof=0.8/6).
The Galactic column density in the direction of the source
is 2.4e20 cm**-2 (Dickey & Lockmann 1990).
The 0.3-10 keV observed flux is 1.9e-12 erg cm**-2 s**-1
(unabsorbed: 2.8e-12 erg cm**-2 s**-1).
We note that the XRT CCD is now operating with a raised
substrate voltage, and thus has a different gain.
The new gain is not yet well calibrated, and so our spectral
results are only preliminary.

Assuming the X-ray emission continues to decline at the same
rate, we predict a 0.3-10 keV count rates of 0.02 cts/s 2 days
after the burst trigger, that corresponds to a flux of
1.3e-12 erg cm**-2 s**-1.

This is an official product of the Swift XRT team.

D. Palmer (LANL),  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), A. Parsons (GSFC),
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-299 to T+303 sec from the recent telemetry downlink,
we report further analysis of BAT GRB  (trigger #290624)
(Krimm, et al., GCN Circ. 6777
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 ).  The BAT ground-calculated position is
RA, Dec = 25.819, -33.484 deg which is
    RA(J2000) = 1h 43m 16.5s
    Dec(J2000) = -33d 29' 3"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 88%.

The mask tagged light curve starts to rise at around T-70 sec and 
continues above
baseline until  T+160 sec.  The burst contains multiple strong peaks 
in a cluster
from T-5 to T+55 sec, then a drop in flux and another cluster of peaks from
T+85 to T+145 sec.The first cluster of peaks has a harder spectrum 
than the second.
T90 (15-350 keV) is 162 +- 10 sec (estimated error including systematics).

The time-averaged spectrum from -73.0 to  +158.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.72 +- 0.04.  The fluence in the 15-150 keV band is 1.2 +- 0.02 x 
10^-05 erg/cm2.
The 1-sec peak photon flux measured from T+36.13 sec in the 15-150 keV band
is 3.9 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

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

At 05:57:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070911 (trigger=290624).  Swift automatic slews are
currently disabled, as we are currently recovering from a safehold. 
The BAT on-board calculated location is 
RA, Dec 25.799, -33.473 which is 
   RA(J2000) = 01h 43m 12s
   Dec(J2000) = -33d 28' 21"
with an uncertainty of 4 arcmin (radius, 90% containment, including 
systematic uncertainty, however there is an uncalibrated boresight
shift due to operations with a different set of gyros than we have
used previously).  The BAT light curve showed a complex multi-peaked
structure with a duration of about 150 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~36 sec after the trigger. 

Because we are in the process of resuming normal operations,
there is no automatic follow-up observation by
the XRT and UVOT. 

Burst Advocate for this burst is H. A. Krimm (krimm 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.)