GRB 100915A

O. M. Littlejohns (U Leicester), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), A. Rowlinson (U Leicester),
M. H. Siegel (PSU) and M. Stamatikos (OSU/NASA/GSFC) report on behalf
of the Swift Team:

At 01:31:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100915A (trigger=434178).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA, Dec 315.648, +65.667 which is 
   RA(J2000) = 21h 02m 36s
   Dec(J2000) = +65d 39' 60"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is a BAT image-only trigger with significance
reported on-board of 8.67 sigma.  Ground processing of the BAT scaled map
image indicates a slightly weaker, 6.4 sigma excess.  As is typical for 
such triggers, there is nothing obvious in the raw light curve.  We will 
need to wait for event-by-event data on the ground to clarify the nature 
of the light curve. 

The XRT began observing the field at 01:33:17.7 UT, 131.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
315.6937, 65.6731 which is equivalent to:
   RA(J2000)  = 21h 02m 46.50s
   Dec(J2000) = +65d 40' 23.0"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 71 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.27
x 10^21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 5.89e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 140 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.51. 

Burst Advocate for this burst is O. M. Littlejohns (oml2 AT star.le.ac.uk). 
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.)

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 5278 s of XRT Photon Counting mode data and 9 UVOT
images for GRB 100915A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 315.69440, +65.67303 which is equivalent
to:

RA (J2000): 21h 02m 46.66s
Dec (J2000): +65d 40' 22.9"

with an uncertainty of 1.6 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

O. M. Littlejohns (U Leicester) reports on behalf of the Swift-XRT team:

We have analysed 8.5 ks of XRT data for GRB 100915A (Littlejohns et al.
GCN Circ. 11277
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 ), from 138 s to 18.8 ks after the BAT trigger. The data
comprise 95 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Osborne et al. (GCN. Circ 11279).

The light curve can be modelled with a series of
power-law decays. The initial decay index is alpha=0.9 (+0.6, -1.8). At
T+320 s the decay steepens to an alpha of 2.4 (+0.6, -0.3). The light
curve breaks again at T+1058 s to a decay with alpha=0.66 (+0.16,
-5.16), before a final break at T+6126 s after which the decay index is
1.6 (+0.8, -0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.37 (+0.21, -0.20). The
best-fitting absorption column is 3.8 (+1.3, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 2.3 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.74 (+/-0.18) and a
best-fitting absorption column of 4.6 (+1.0, -0.9) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 5.5 x 10^-11 (8.2 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
1.6, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x
10^-14 (8.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00434178.

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

W. H. Baumgartner (GSFC/UMBC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), O. M. Littlejohns (U Leicester),
C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 100915A (trigger #434178)
(Littlejohns, et al., GCN Circ. 11277
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 ).  The BAT ground-calculated position is
RA, Dec = 315.664, 65.676 deg, which is 
   RA(J2000)  =  21h 02m 39.4s 
   Dec(J2000) = +65d 40' 32.3" 
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 54%.
 
The mask-weighted light curves shows two broad peaks, the first starting
at ~T-90sec, peaking at ~T+5 sec, and ending at ~T+90 sec.  The second,
weaker peak starts at ~T+110, peaks at ~T+170 sec and ends at ~T+190 sec.
T90 (15-350 keV) is 200 +- 31 sec (estimated error including systematics).
 
The time-averaged spectrum from T-36.3 to T+72.2 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 2.88 +- 0.00, 
and Epeak of 265.2 +- 0.0 keV (chi squared 66.1 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.5 +- 0.2 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+8.76 sec in the 15-150 keV band is
0.5 +- 0.3 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.50 +- 0.23 (chi squared 73.1 for 57 d.o.f.).  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/434178/BA/

C. A. Swenson (PSU) and O. M. Littlejohns (U Leicester) report on behalf
of the Swift/UVOT team.

The Swift/UVOT began settled observations of the field of
GRB 100915A 123s after the BAT trigger (Littlejohns et al.,
GCN Circ. 11277
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 ).  No new source is found at the enhanced
XRT position (Osborne et al., GCN Circ. 11279
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 ).

The 3-sigma uppder limits for the finding chart (fc) and
summed exposures are reported below:

FILTER      T_start(s)    T_stop     Exposure      Mag/3UL
===========================================================
white fc        140          290        147        >21.46
white           142        11637       1619        >22.83
v               123        13122       1057        >20.98
b               557         7343        467        >21.43
u fc            300          550        246        >20.68
u               300         7161        963        >21.52
uvw1            680         6956        491        >20.87
uvm2           1256        18902       2787        >21.79
uvw2            606        12544       1152        >21.50

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.51 (Schlegel et al.,
1998, ApJS, 500, 525).  The photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).

T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC) 
(i.e. the Swift-BAT team):

We would like to make corrections to the BAT refined analysis of GRB 100915A 
(GCN Circ. 11281
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 ).  The BAT time-averaged spectrum is best fit by a simple 
power-law (PL) model rather than a power-law with an exponential cutoff (CPL) 
model.  We believe a low signal to noise in the observed spectrum of this burst 
causes a significant improvement in chi2 by a CPL model over a PL model.  We 
would like to correct the 3rd paragraph of GCN Circ. 11281
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  to:

"The time-averaged spectrum from T-36.3 to T+72.2 sec is best fit by a simple 
power-law model.  The power law index of the time-averaged spectrum is
1.50 +- 0.24.  The fluence in the 15-150 keV band is 1.5 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+8.76 sec in the 15-150 keV band
is 0.5 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. "

We apologize for possible inconvenience.  We would like to thank Eleonora Troja 
for pointing this issue.

T. Sakamoto (UMBC/GSFC), D. Donato (UMCP/GSFC), N. Gehrels (GSFC),
T. Okajima (JHU/GSFC), Y. Urata (NCU), C.A. Wallace (FGCU)

We observed the field of GRB 100915A detected by Swift
(trigger #434178; Littlejohns et al., GCN Circ. 11277
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 ) with the 14-inch
Goddard Robotic Telescope (GRT) located at the Goddard Geophysical
and Astronomical Observatory (http://cddisa.gsfc.nasa.gov/ggao/).

A total 423 images of 5 sec (200 images), 30 sec (80 images), 
60 sec (120 images) and 100 sec (23 images) exposures were taken in 
the R filter starting from September 15 01:33:32 (UT) about 146 
seconds after the trigger (70 seconds after the BAT position notice) 
and stopped on September 15 05:49:30 (UT).  We do not detect 
the optical afterglow both in the individual images and the stacked 
image, which only combined good quality 418 images, inside the XRT 
position (Osborne et al., GCN #11279
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 ).  The estimated five sigma 
upper limit of the combined image (total exposure of 12500 sec) is 
~19.8 mag using the USNO-B1 catalog.

D. Kuroda (OAO, NAOJ),  H. Hanayama, T. Miyaji, J. Watanabe (IAO, NAOJ),
K. Yanagisawa (OAO, NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 100915A (Littlejohns et al., GCN 11277
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 )
with the optical three color (g', Rc and Ic) CCD camera attached
to the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

The observation started on 10 10:46:01 UT (~9.2 h after the trigger).
We did not find any new point source within the enhanced XRT error
circle (Osborne et al., GCNC 11279) in all the three bands.

Three sigma upper limits of the OT are listed below. We used GSC2.3
catalog for flux calibration.

T0+[day]   MID-UT   T-EXP[sec]    g'     Rc     Ic
------------------------------------------------------
0.40450    11:13:34   2760.0    >22.0  >21.5  >21.0
0.56696    15:07:30   4560.0    >22.5  >21.8  >21.1
------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

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

We imaged the field of GRB 100915A detected by SWIFT
(trigger 434178) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 97s after the GRB trigger
(20s after the notice). The elevation of the field decreased from
47 degrees above horizon and weather conditions
were good.

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
Time sampling is 10 seconds / pixel.
We do not detect any OT with a limiting magnitude of:
t0+97s to t0+157s : R > 17.2

We co-added a series of exposures:
t0+174s to t0+394s : R > 19.0

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

This message may be cited.