GRB 091026

C. B. Markwardt (CRESST/GSFC/UMD), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), D. Grupe (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
E. A. Hoversten (PSU), J. A. Kennea (PSU), V. Mangano (INAF-IASFPA),
J. Mao (INAF-OAB), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), P. Roming (PSU), B. Sbarufatti (INAF-IASFPA),
M. H. Siegel (PSU) and M. Stamatikos (OSU/NASA/GSFC) report on behalf
of the Swift Team:

At 13:11:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091026 (trigger=373871).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 276.583, -86.121 which is 
   RA(J2000)  =  18h 26m 20s
   Dec(J2000) = -86d 07' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a FRED pulse
with a duration of about 20 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 13:12:48.9 UT, 78.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 276.59337, -86.11056 which
is equivalent to:
   RA(J2000)  = 18h 26m 22.41s
   Dec(J2000) = -86d 06' 38.0"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 37 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 in excess of the Galactic value (8.36e+20
cm^-2, Kalberla et al. 2005), with an excess column of 3.6
(+2.72/-2.29) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 3.42e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 87 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
0.13. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt AT 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.)

P.A. Evans and A.P. Beardmore (U. Leicester) report on behalf of the 
Swift-XRT Team:

Using the full set of promptly downlinked XRT event mode data totalling 
760 s in Photon Counting mode, and 2 UVOT images, we find an enhanced 
XRT position for GRB 091026 (Markwardt et al. GCN Circ. 10081
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 ) of RA, 
Dec = 276.5725, -86.1115 (degrees) which is equivalent to:

RA  (J2000.0) = 18 26 17.41
DEC (J2000.0) = -86 06 41.5

with an uncertainty of 2.1 arcsec (radius, 90% containment).

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

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

Using 512 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 091026, 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 = 276.57076, -86.11163 which is equivalent
to:

RA (J2000): 18h 26m 16.98s
Dec (J2000): -86d 06' 41.9"

with an uncertainty of 1.9 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.

P.A. Evans, J.P. Osborne (U. Leicester)  and C. B. Markwardt (CRESST/GSFC/UMD)
report on behalf of the Swift-XRT team:

We have analysed 2.2 ks of XRT data for GRB 091026 (Markwardt et al. GCN
Circ. 10081
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 ), from 85 s to 6.8 ks after the BAT trigger. The data
comprise 326 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Evans et al. (GCN. Circ 10084).

The light curve shows three flares in the first snapshot, with peaks
at T0+180 s, T0+340 s and T0+~890 s. The orbital gap interrupts the
final flare, making its peak time somewhat uncertain. If these flares
are disregarded (i.e. fitting only the data from 85 s to 150 s and 2 ks to
6.8 ks), the underlying light curve can be modelled with an initial
power-law decay with an index of alpha=3.40 (+0.31, -0.26), followed by
a break at T+149 s to an alpha of 0.98 (+0.07, -1.7).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+/-0.08). The
best-fitting absorption column is 2.93 (+0.28, -0.27) x 10^21 cm^-2, in
excess of the Galactic value of 8.4 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.01 (+0.18, -0.17)
and a best-fitting absorption column of 3.5 (+0.7, -0.6) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.6 x 10^-11 (7.6 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
0.98, the count rate at T+24 hours will be 0.018 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x
10^-13 (1.4 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00373871.

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

P.A. Curran (MSSL-UCL) and C.B. Markwardt (CRESST/GSFC/UMD)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 091026 87 s 
after the BAT trigger (Markwardt et al., GCN 10081
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 ). No optical afterglow 
consistent with the enhanced XRT position (Evans et al., GCN 10084
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 ) is 
detected in the initial UVOT exposures. Preliminary 3-sigma upper limits 
using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for 
the first finding chart (FC) exposures and subsequent exposures are:


Filter          T_start(s)   T_stop(s)    Exp(s)    Mag
-----------------------------------------------------------
white (FC)      87           237          147       >21.00
u (FC)          300          550          246       >20.36

white           87           12292        950       >22.09
v               629          5959         236       >19.63
b               555          6778         235       >20.62
u               300          6573         462       >20.72
w1              678          6369         236       >20.09
m2              826          6164         216       >19.85
w2              605          16097        561       >20.70

The values quoted above are not corrected for the Galactic extinction due 
to the reddening of E(B-V) = 0.13 in the direction of the burst (Schlegel 
et al. 1998).

T. N. Ukwatta (GWU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+243 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 091026 (trigger #373871)
(Markwardt, et al., GCN Circ. 10081
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 ).  The BAT ground-calculated position is
RA, Dec = 276.657, -86.113 deg which is 
   RA(J2000)  =  18h 26m 37.7s 
   Dec(J2000) = -86d 06' 47.2" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 36%.
 
The mask-weighted light curve shows a single FRED-like peak which starts
to rise around T-5 sec, peaks near T+0 sec, and has a long tail extending 
to about T+120 sec.  T90 (15-350 keV) is 41.6 +- 7.2 sec (estimated error 
including systematics).
 
The time-averaged spectrum from T-3.7 to T+47.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.53 +- 0.11.  The fluence in the 15-150 keV band is 1.8 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-0.18 sec in the 15-150 keV band
is 2.2 +- 0.3 ph/cm2/sec.  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/373871/BA/

Robert Filgas (Tautenburg Obs.), Adria Updike (Clemson University), Arne
Rau, and Jochen Greiner (both MPE Garching) report on behalf of the GROND
team:

We observed the field of GRB 091026 (Swift trigger 373871; Markwardt et
al., GCN #10081
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla
Observatory (Chile).

Observations started at 00:48 UT on Oct 27, 2009, 11.6 hours after the GRB
trigger.  They were performed at an average seeing of 2" and at an average
airmass of 1.94.

We do not detect a source within the Swift-XRT error circle reported by
Evans et al. (GCN #10084
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 ) down to

g' > 24.8 mag,

r' > 24.2 mag,

i' > 23.9 mag,

z' > 23.7 mag,

J > 22.2 mag,

H > 21.3 mag and

K > 20.2 mag.

Magnitudes are given in the AB system and calibrated against USNO as well
as 2MASS field stars, and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of E_(B-V) = 0.13 mag
in the direction of the burst (Schlegel et al. 1998).

N. Ohmori, E. Sonoda, K. Kono, H. Hayashi, A. Daikyuji, 
Y. Nishioka, K. Noda, M. Yamauchi (Univ. of Miyazaki),
M. Ohno, M. Suzuki, M. Kokubun, T. Takahashi (ISAS/JAXA),
K. Yamaoka, S. Sugita (Aoyama Gakuin U.), Y. E. Nakagawa,
T. Tamagawa (RIKEN), S. Hong (Nihon U.), N. Vasquez (Tokyo Tech.),
Y. Hanabata, T. Uehara, T. Takahashi, Y. Fukazawa (Hiroshima U.),
W. Iwakiri, M. Tashiro, Y. Terada, A. Endo, K. Onda,
T. Sugasahara (Saitama U.), Y. Urata (NCU),
T. Enoto, K. Nakazawa, K. Makishima (Univ. of Tokyo),
on behalf of the Suzaku WAM team, report:

The long GRB 091026 (Swift-BAT trigger #373871; Markwardt et al.,
GCN Circ. 10081
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 ) triggered the Suzaku Wide-band All-sky 
Monitor (WAM) which covers an energy range of 50 keV - 5 MeV
at 13:11:29.812 UT (=T0).

The observed light curve shows a multi-peaked structure, starting at
T0-1 s and ending at T0+13 s, with a total duration (T90) of about 9 s.
The fluence in 100 - 1000 keV was 1.47 (-0.40, +0.29) x 10^6 erg/cm^2.
The 1-s peak flux measured from T0+1.5 s was 0.59 (-0.30, +0.19)
photons/cm^2/s in the same energy range.

Preliminary result shows that the time-averaged spectrum from
T0-1 s to T0+13 s is well fitted by a single power-law with 
 a photon index of 1.86 (-0.25, +0.33) (chi^2/d.o.f. = 77/50).
 
All the quoted errors are at statistical 90% confidence level,
in which the systematic uncertainties are not included.

The light curves for this burst will be available at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html

K. Nishimoto, T. Sako (Nagoya Univ.) on behalf of the MOA Collaboration,
M. Kurita, T. Nagayama (Nagoya Univ.) on behalf of the IRSF Collabration
report:

We searched for a NIR afterglow of GRB091026 (Markwardt et al., GCN
10081
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 ) starting from 18:43:32.3 UT on 2009 Oct 26 (5.5 hours after the
burst) with the SIRIUS on the IRSF 1.4m telescope at SAAO in South Africa.
In images of a 300sec exposure with J, H and Ks filter, we did not find
any object within the error circle of the Swift XRT source position
(Evans. et al., GCN 10084
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 ).

A 5 sigma upper limits are the followings.

J > 18.35
H > 17.39
K > 16.70

This photometry was done by using the DoPhot and calibrated against the
2MASS cataloged stars