GRB 081126

R. Margutti (Univ Bicocca&OAB), A. P. Beardmore (U Leicester),
P. J. Brown (PSU), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC),
C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
J. Mao (INAF-OAB), F. E. Marshall (NASA/GSFC), C. Pagani (PSU),
P. Romano (INAF-IASFPA), M. Stamatikos (NASA/ORAU),
T. N. Ukwatta (GSFC/GWU), L. Vetere (PSU) and H. Ziaeepour (UCL-MSSL)
report on behalf of the Swift Team:

At 21:34:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081126 (trigger=335647).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 323.542, +48.709 which is 
   RA(J2000) = 21h 34m 10s
   Dec(J2000) = +48d 42' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a total duration of about 40 sec.  The two peaks are about
30 seconds apart. The peak count rate was ~7000 counts/sec (15-350 keV), 
at ~2 sec after the trigger. 

The XRT began observing the field at 21:35:16.0 UT, 65.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 323.5150,
48.7097 which is equivalent to:
   RA(J2000)  = 21h 34m 3.60s
   Dec(J2000) = +48d 42' 34.9"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 64 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
4.13e+21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 7.07e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White   
filter  starting 75 seconds after the BAT trigger. No credible   
afterglow candidate has  been found in the initial data products. The
 2.7' x 2.7' sub-image covers 100% of  the XRT error circle. The   
estimated 3-sigma limiting magnitude is white > 21 mag.  No 
correction has   been made for the large, but uncertain extinction 
expected.  There is   a high proper motion 2MASS source inside the 
XRT error circle. 

Burst Advocate for this burst is R. Margutti (raffaella.margutti AT brera.inaf.it). 
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.)

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

We imaged the field of GRB 081126 detected by SWIFT
(trigger 335647) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 20.6s after the GRB trigger
(6.9s after the notice). The elevation of the field decreased
from 41 degrees above horizon and weather conditions
were very good.

The date of trigger : t0 = 2008-11-26T21:34:10

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We do not detect any OT at the XRT position (Margutti et al. 2008
GCNC 8554) with a limiting magnitude of:
t0+20.6s to t0+80.6s : R > 16.6

The second image is 30.0s exposure in tracking mode:
t0+86.7s to t0+116.7s : R > 18.0

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

N.B. Galactic coordinates are lon= 92.9 lat= -2.3
and the galactic extinction in R band is 2.1 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

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

Using 2152 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 081126, 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 = 323.5147, +48.7098 which is equivalent
to:

RA (J2000): 21h 34m 3.54s
Dec (J2000): +48d 42' 35.2"

with an uncertainty of 1.8 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
http://www.swift.ac.uk/xrt_positions/Goad.pdf), the current algorithm is an
extension of this method.

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

G. Sato (ISAS), 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), R. Margutti (U Bicocca&OAB), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-120 to T+183 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 081126 (trigger #335647)
(Margutti, et al., GCN Circ. 8554
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 ).  The BAT ground-calculated position is
RA, Dec = 323.526, 48.714 deg, which is 
   RA(J2000)  =  21h 34m 06.3s 
   Dec(J2000) = +48d 42' 51.0" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 67%.
 
The mask-weighted light curve shows a small peak starting at ~T-30 sec,
peaking at ~T-18 sec, and returning almost to zero at T-7 sec, at which time
the second peak starts to rise.  It peaks at ~T+1.5 sec and reachs a maximum
at ~T+7 sec.  The third peak peaks at ~T+31.5 sec and ends at ~T+100 sec.
T90 (15-350 keV) is 54 +- 4 sec (estimated error including systematics).
 
The time-averaged spectrum from T-21.4 to T+45.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.27 +- 0.06.  The fluence in the 15-150 keV band is 3.3 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.75 sec in the 15-150 keV band
is 3.7 +- 0.2 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/335647/BA/

Andreev M., Sergeev A.,  (Terskol Branch of Institute of Astronomy),  A. 
Pozanenko (IKI) on behalf of larger GRB follow up collaboration report:

We observed   the field of GRB 081126 detected by SWIFT (trigger 335647, 
Margutti et al GCN8554
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  ) with the Z-600  telescope of Mt. Terskol 
observatory.  A set of  90 s unfiltered exposures was taken starting (UT) 
Nov. 26 22:07:20.  In a combined image of  3x90 s we detect a new object 
not presented in DSS2(R), in near vicinity to the object of USNO-A2.0 
1350-14391154. A preliminary astrometry of the new object based on 
USNO-A2.0  is RA, Dec (J2000) = 21 34 03.56   +48 42 38.62 with 
uncertainty of 1", which is formally outside of  enhanced XRT position 
(Osborne et al GCN 8556
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 ).  It is not clear at this moment relation of the 
new object with GRB  081126.

A combined image can be found at 
http://grb.rssi.ru/GRB081126/GRB081126_z600.jpg

S. T. Holland (CRESST/GSFC/USRA) and R. Margutti (Univ Bicocca & OAB)
report on the behalf of the Swift UVOT team:

       The Swift/UVOT observed the field of GRB 081126 starting 56 s
after the BAT trigger (Margutti, et al. 2008, GCN Circ. 8554
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 ).
Settled exposures started at T+75 s.  We do not find any new source,
relative to the DSS or 2MASS, or any variable source inside the
UVOT-enhanced XRT error circle (Osborne, et al. 2008, GCN Circ. 8556
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 ).
Preliminary 3-sigma upper limits for detecting a source in the
co-added images, using a 2.5 arcsecond radius circular aperture, are

Filter   T_start   T_stop   Exp(s)     Mag
------------------------------------------
  white        75      225     147    >21.3
     u        289      420     129    >20.1

     v       5161     5361     197    >19.7
     b       4546     6183     393    >20.9
     u       3133     5976     326    >20.7
   uvw1      5572     5771     197    >20.0
   uvm2      5366     5566     197    >19.8
   uvw2      4957     6398     199    >20.0
  white      4751     6388     393    >21.8
------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight corresponding to a
reddening of E_{B-V} = 0.78 mag (Schlegel, et al., 1998, ApJS, 500,
525).  All photometry is on the UVOT flight system described in Poole
et al. (2008, MNRAS, 383, 627).

      The 2MASS point source 21340333+4842361 is located 3.2 arcsec
north of the centre of the XRT error circle.  Our photometry is
slightly contaminated by an uncatalogued source approximately 2.6
arcsec northwest of the centre of the XRT error circle.  This source
appears to be extended and may be the host galaxy of GRB 081126.
Preliminary magnitudes are

Filter     Mag  Err
----------------------------------------
     v    19.2   0.3
     b    20.2   0.2
     u   >20.9        3-sigma upper limit
   uvw1  >20.1        3-sigma upper limit
   uvm2  >19.9        3-sigma upper limit
   uvw2  >20.1        3-sigma upper limit
  white   20.2   0.2
----------------------------------------

The detection in the b filter, combined with the lack of a detections
in the u and UV filters, is consistent with this source having a
redshift of approximately 2.8 < z < 4.5.  This corresponds to a
physical separation of approximately 18 proper kpc assuming a
cosmology with (H_0,Omega_matter,Omega_Lambda) = (70,0.3,0.7).

R. Margutti (Univ. Bicocca & INAF-OAB) reports on behalf of the Swift 
XRT team:

We have analysed the first 5 orbits of Swift-XRT data obtained for GRB 
081126
(Margutti et al., GCN Circ. 8554
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 ), covering 100 s of Windowed Timing
(WT) and 9.1 ks of Photon Counting (PC) mode data, respectively, between 77
and 24 ks after the trigger.
The UVOT-enhanced XRT position was given by Osborne et al. in GCN Circ. 
8556
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 .

The light-curve can be modelled by a double broken power-law,
with an initial decay of alpha_1= 1.2 +/- 0.2, a flat decay
of alpha_2= 0.6 +/- 0.1 and a final value of alpha_3= 2.1 +/- 0.2.
The two break times are around 200 and 9000 s, respectively.

A spectrum extracted from WT data can be modelled with an
absorbed power-law, with Gamma = 1.9 +/- 0.2 .
A spectrum extracted from PC data in the time interval  3-30 ks post 
trigger
can be modelled with an absorbed power-law, with Gamma = 2.0 +/- 0.1 and
a column density of NH = (5.0 +/- 0.7)x10^21 cm^-2, which is compatible with
the Galactic value in this direction (4.1x10^21 cm^-2).
The observed (unabsorbed) 0.3 -10 keV flux over this time
interval is 7.1x10^-12 (1.2x10^-11) erg cm^-2 s^-1.
For both spectra there is no strong evidence for intrinsic NH.
Uncertainties are given at 90% confidence.

If the light-curve continues to decay with alpha ~ 2, the count rate 24
hours after the burst is estimated to be 2.7x10^-3 count s^-1, which
corresponds to an observed flux of 1.4x10^-13 erg cm^-2 s^-1.

The results of the xrt automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00335647.

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

S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin,
D. Frederiks, P. Oleynik, D. Svinkin, M. Ulanov,
and T. Cline, on behalf of the Konus-Wind team report:

The long GRB 081126 (Margutti et al., GCN 8554
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 , Sato et al. GCN 8557
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 ) 
triggered Konus-Wind at T0=77649.857 s UT (21:34:09.857).

The burst light curve shows a precursor at ~T0-20 s and two main pulse 
at ~T0-3 s and ~T0+28 s with a total duration of ~60 s.

As observed by Konus-Wind the burst
had a fluence of 9.91(-1.46, +1.57)x10^-6 erg/cm2,
and a 256-ms peak flux measured from T0+1.808 s
of (1.65 +/- 0.36)x10^-6 erg/cm2/s
(both in the 20 keV - 2 MeV energy range).

The time-integrated spectrum of the burst
(from T0 to T0+41.216 s) is well fitted (in the 20 keV-2 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.82(-0.26, +0.32) ,
and Ep = 315(-71, +123)  keV (chi2 = 68.0/61 dof).
Fitting by GRBM (Band) model yields only an upper limit on
the high energy photon index: beta < -2.16.

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

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.ru/LEA/GRBs/GRB081126_T77649/

S. T. Holland (CRESST/GSFC/USRA), P. A. Evans (U Leicester),
F. E. Marshall (NASA/GSFC), P. Schady (MSSL), and R. Margutti (U
Bicocca & OAB) report on the behalf of the Swift UVOT team:

      We confirm the optical afterglow of GRB 081126 reported by
Sergeev & Pozanenko (2008, GCN Circ. 8558
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 ). A re-examination of the
UVOT data shows that there is a fading source at

      RA (J2000)  21:34:03.59  =  323.51496 (deg)
     Dec (J2000) +48:42:38.3   =  +48.71064 (deg)

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence,
statistical + systematic).  This is consistent with the XRT error
circle.  Preliminary photometry and 3-sigma upper limits for this
source are

Filter   T_start   T_stop   Exp(s)     Mag   Err
------------------------------------------------
  white        75      225     147     18.82 0.09
     u        289      420     127     18.17 0.13

     v       5161     5361     197     19.29 0.32
     b       4546     4746     197     20.20 0.28
     u       5778     5976     197    >20.4
   uvw1      5572     5771     197    >20.0
   uvm2      5366     5566     197    >19.6
   uvw2      4957     5156     197    >20.0
  white      4751     4951     197     20.33 0.21
------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight corresponding to a
reddening of E_{B-V} = 0.78 mag (Schlegel, et al., 1998, ApJS, 500,
525).  All photometry is on the UVOT photometric system described in
Poole et al. (2008, MNRAS, 383, 627).  The detection in the u filter,
combined with the lack of detections in the UV filters, is consistent
with the afterglow having a redshift of approximately 2.4 < z < 3.8.
However, this result is uncertain due to the large Galactic reddening
along the line of sight to this source.

      The source identified in Holland & Margutti 2008, (GCN
Circ. 8559
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 ) as being possibly extended is consistent with the
catalogued position of the 2MASS point source 21340333+4842361.  The
separation between this 2MASS source and the afterglow is 3.2 arcsec.
Since the photometry of the 2MASS source and the afterglow are
consistent with both sources having at the same redshift it is
possible that the 2MASS source is the host galaxy.  Further
observations are encouraged to confirm or reject this hypothesis.

      Holland & Margutti 2008, (GCN Circ. 8559
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 ) misidentified the
optical afterglow as as the 2MASS point source 21340333+4842361.  We
apologize for any confusion caused by the misidentification in the
previous UVOT Circular on GRB 081126.

T. Sakamoto (UMBC/GSFC), Y. Urata (Saitama U), D. Donato (ORAU/GSFC), 
N. Gehrels (GSFC), T. Okajima (JHU/GSFC), T. N. Ukwatta (GWU/GSFC) 

We observed the field of GRB 081126 detected by Swift (trigger #335647)
with the 14-inch Goddard Robotic Telescope (GRT) located at the 
Goddard Geophysical and Astronomical Observatory 
(http://cddisa.gsfc.nasa.gov/ggao/).  

Five set of 30s and 60s exposures were taken in the R filter starting 
from Nov. 27 03:58:18 (UT).  We do not detect any OT at the XRT 
position (Osborne et al. 2008 GCNC 8556).  The estimated 3-sigma 
upper limit of the combined image is ~17.5 mag using the USNO-B (r) catalog.

J. Skvarc and H. Mikuz on behalf of PIKA observing program at Crni Vrh
Observatory.

We confirm variability of the source identified by Sergeev and Pozanenko
(GCN Circ. 8558
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 ).  The object was observed from Crni Vrh Observatory with
a 60 cm telescope using R filter and a CCD in about 5 arc-second seeing
conditions.  First 60 s exposure started at 2008-11-26 21:35:32 UT.  All
twenty exposures were of 60 s.  The table contains the time since the
Swift GRB detection to the middle of exposure in seconds, R magnitude and
formal 1-sigma magnitude error estimate.

Time     Mag     Err.
 [s]        [R]    [1-sigma]
----------------------------------
 112.1  17.02  0.02
 182.9  16.75  0.02
 253.9  16.81  0.02
 325.7  16.85  0.02
 396.5  16.93  0.02
 468.0  17.03  0.02
 538.8  17.07  0.02
 609.6  17.25  0.02
 680.4  17.25  0.02
 751.1  17.78  0.03
 821.5  17.64  0.03
 892.5  17.44  0.03
 963.3  17.79  0.03
1034.3  17.79  0.03
1105.1  17.65  0.03
1176.8  17.76  0.03
1247.6  17.77  0.03
1318.5  17.71  0.03
1389.3  17.77  0.03
1460.0  18.50  0.05

N. Gehrels (NASA-GSFC) on behalf of the Swift team

Swift observations of GRB 081127 have been terminated in favor of
the more interesting GRB 081126.  The observations of GRB 081126
will continue through November 29 and possibly longer.

Poonam Chandra (RMC) and Dale A. Frail (NRAO) report on
behalf of the Caltech-NRAO-Carnegie GRB Collaboration:

"We used the Very Large Array to observe the field of view toward
optically bright GRB 081126 (GCN 8554
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 ) at a frequency of 8.46 GHz
on 2008 Nov 28.97 UT. The GRB radio afterglow is undetected and
the peak radio flux at the UVOT position (GCN 8564
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 ) is 24 � 64 uJy.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."

P. N. Bhat (UAH) and A.J. van der Horst (NASA/ORAU)
report on behalf of the Fermi GBM Team:

"At 21:34:09.06 UT on 26th November 2008, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB081126 (trigger 249428050 / 081126899) which was
also detected by the SWIFT-BAT (Margutti et al. 2008, GCN Circular 8554
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 )
 
The angle from the Fermi LAT boresight is 18 degrees.

This burst was also independently detected by INTEGRAL SPI-ACS and
Konus/Wind (Golenetskii et al., 2008, GCN Circular 8562
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 ).

The GBM light curve consists of two pulses separated by
about 30 s with almost no emission in between. There is
also a weak untriggered emission at about T0-20 s.
The burst duration (T90) is about 56 s (8-1000 keV).
The time-averaged spectrum of the first pulse from T0 s to T0+11 s is
well fit by a Band function with Epeak = 192 +/- 74 keV,
alpha = -0.3 +/- 0.4, and beta = -1.6 +/- 0.1.(chi squared 547 for 487
d.o.f.) 

Similarly the second pulse from about 20 s to 40 s is also
well fit by a Band function with Epeak = 162 +/- 77 keV,
alpha = -0.3 +/- 0.5, and beta = -1.6 +/- 0.1.
(chi squared 539 for 487 d.o.f.). The time averaged spectrum of the
entire burst is well fit by a Band function with Epeak = 176 +/- 84 keV,
alpha = -0.3 +/- 0.5, and beta = -1.7 +/- 0.1. (chi squared 464 for 487
d.o.f.)

The event fluences (8-1000 keV) in the two pulses are
(2.7 +/- 0.8)E-07 erg/cm^2 and (1.9 +/- 0.8)E-07 erg/cm^2
respectively. The 1-sec peak photon fluxes in the 8-1000 keV
band in the two pulses are 1.3+/- 0.4 ph/s/cm^2 and 1.0 +/- 0.4
ph/s/cm^2 respectively. The total event fluence (8-1000 keV) is
(1.5 +/- 0.6)E-07 erg/cm^2. The 1-sec peak photon fluxes in the 8-1000 keV
band in the two pulses are 0.7+/- 0.3 ph/s/cm^2
 

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

M. Andreev, A. Sergeev, (Terskol Branch of Institute of Astronomy), A. 
Pozanenko (IKI) on behalf of larger GRB follow up collaboration report:

We observed the field of GRB 081126  (Margutti et al GCN 8554
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 ) with the 
Z-600 telescope of Mt. Terskol observatory. Series of unfiltered   exposures 
were taken between Nov.26 (UT) 21:48:45 - 22:34:35. A photometry of the 
afterglow (Andreev et al, GCN 8558
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 , Holland et al, GCN 8564
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 ) against 
USNO-B1.0 (R2) field stars is following

T0+   Exp.      Mag.  Err.
mid     s

 940  10�15     18.6  0.2
1203  10x30     18.8  0.2
1635   5x90     19.3  0.3
2375   7x90     19.8  0.3
2920   5x90     20.1  0.3

The photometry is compatible with a power law decay of a light curve with 
index of ~ -0.66.