GRB 081121

F. Yuan (U Mich), W. Rujopakarn (Steward) report on behalf of the ROTSE 
collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, 
responded to GRB 081121 (Swift trigger 335105). The first image was at 
20:36:30.0 UT, 57.2 s after the burst (8.9 s after the GCN notice time). 
The unfiltered images are calibrated relative to USNO A2.0. We detect a 
11.6 magnitude, fading source with coordinates:

     05:57:06.1      -60:36:10.3    (J2000), with positional uncertainty 
of 1" or better

start UT    	mag     mlim(of image)
----------------------------------
20:36:29.9     11.6     15.7 


This source is not visible in DSS (second epoch), 2MASS or the MPChecker 
database.

A jpeg image is available at 
http://www.rotse.net/images/gsb335105_3c00_img.jpg Note that the object 
marked 4 is the candidate in question.

Continuing observations are in progress.

S. R. Oates (UCL-MSSL), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), N. Gehrels (NASA/GSFC),
O. Godet (U Leicester), S. T. Holland (CRESST/USRA/GSFC),
E. A. Hoversten (PSU), H. A. Krimm (CRESST/GSFC/USRA),
C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester),
C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Schady (MSSL-UCL), M. Stamatikos (NASA/ORAU),
R. L. C. Starling (U Leicester), L. Vetere (PSU) and
H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 20:35:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081121 (trigger=335105).  Due to an Earth limb constraint
slew could not immediately slew to this burst. The BAT on-board 
calculated location is 
RA, Dec 89.264, -60.626 which is 
   RA(J2000) = 05h 57m 03s
   Dec(J2000) = -60d 37' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single FRED
structure with a duration of about 15 sec, with overlying spikes. 
There may be continued low-level emission out to 100 seconds. 
The peak count rate was ~4000 counts/sec (15-350 keV), 
at ~1.5 sec after the trigger. 

Due to an observing constraint, Swift will not slew until 
T0+45.2 minutes. There will be no XRT or UVOT data until
this time. 

Burst Advocate for this burst is S. R. Oates (sro AT mssl.ucl.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.)

S. R. Oates  and P. Schady (MSSL-UCL) report on behalf of the Swift  
UVOT team:

The Swift/UVOT began observing the field of GRB081121 starting 2816  
seconds
after the BAT trigger (Oates et al., GCN 8537
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 ) with a White finding  
chart exposure
of 150 seconds. There is a candidate afterglow in the 2.7'x2.7' sub- 
image at

RA(J2000) =	05:57:06.19 = 89.27581
DEC(J2000) = -60:36:10.3 = -60.60286

with a 90%-confidence error radius of about 0.62 arc sec. This  
position is 0.7
arc sec. from the center of the XRT error circle and is consistent  
with the position
of the optical afterglow found by ROTSE-IIIc (Yuan el al., GCN 8536
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 ).

The estimated magnitude is 17.98 with a 1-sigma error of about 0.04. No
correction has been made for the expected extinction corresponding to  
E(B-V) of 0.05.

T. Sakamoto (GSFC/UMBC), 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), S. R. Oates (UCL-MSSL),
D. M. Palmer (LANL), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-119 to T+271 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 081121 (trigger #335105)
(Oates, et al., GCN Circ. 8537
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 ).  The BAT ground-calculated position is
RA, Dec = 89.282, -60.612 deg, which is 
   RA(J2000)  =  05h 57m 07.7s 
   Dec(J2000) = -60d 36' 42.1" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 5%.
 
The mask-weighted light curve shows an approximately square-shaped pulse
with a trailing spike.  The pulse starts at ~T-10 sec and ends at ~T=15 sec.
There is a hint of low-level precursor emission starting before T-119 sec
(the beginning of the saved event-by-event data).
T90 (15-350 keV) is 14 +- 2 sec (estimated error including systematics).
 
The time-averaged spectrum from T-2.8 to T+12.9 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.43 +- 0.54, 
and Epeak of 123 +- 69 keV (chi squared 44.9 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 4.1 +- 0.3 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+6.94 sec in the 15-150 keV band is
4.4 +- 1.0 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.21 +- 0.12 (chi squared 52.0 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/335105/BA/

S. Loew, T. Kruehler and J. Greiner (all MPE Garching) report on behalf of 
the GROND team: 


We observed the field of GRB 081121 (Swift trigger #335105; Oates et al., 
GCN #8537
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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 00:30 UT on November 22nd, 2008, 3.9 hr after the 
burst at a high airmass of 2.35, and are continuing. 


We detect the optical afterglow reported by Yuan et al. (GCN #8536
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 ) and 
Oates & Schady (GCN #8538
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 ) clearly in all filters. 


At a midtime of 3.9454 hours after the burst, we estimate the following 
preliminary magnitudes (all in AB system) in stacked images of 4 x 35s 
integration in g'r'i'z' and 24 x 10s integration in JHK: 


g' = 19.1
r' = 18.8
i' = 18.6
z' = 18.5
J = 18.4
H = 18.2
K = 18.1 


with typical errors of 0.2 mag, which are mostly due to a preliminary 
calibration using GROND zeropoints in g'r'i'z' and 2MASS field stars in JHK.

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

Using 939 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 081121, 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 = 89.2757, -60.6028 which is equivalent
to:

RA (J2000): 05h 57m 6.17s
Dec (J2000): -60d 36' 10.1"

with an uncertainty of 1.7 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.

E. Berger (Harvard) and M. Rauch (Carnegie) report:

"We obtained spectroscopic observations of the optical afterglow of GRB
081121 (GCNs 8536
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 ,8537
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 ) with the LDSS3 spectrograph on the Magellan/Clay
6.5-m telescope starting on 2008 Nov 22.16 UT.  The spectra cover the
wavelength range 3800-6500A.  We detect Ly-alpha absorption, as well as
aborption features corresponding to SiII, CII, SiIV, CIV, FeII, and AlII
at a redshift of z=2.512, which we consider the redshift of the burst.
Further analysis is in progress."

O. Godet (U Leicester) and S. Oates (UCL-MSSL) on behalf the Swift-XRT 
team:

The Swift-XRT started observing the field of GRB 081121 (trigger number
335105, Oates et al., GCN Circ. 8537
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 ) at 2008-11-21 20:35:32 UT, 2.8 ks 
after the trigger. The best XRT position is the UVOT-enhanced position 
reported by Goad et al. (GCN Circ. 8541
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 ). This position is consistent with 
the optical positions reported by the Swift-UVOT and ROTSE III (Oates et 
al., GCN Circ. 8538
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 ; Yuan et al., GCN Circ. 8536
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 ).

The X-ray light curve presently spans 4.2 ks of photon counting (PC) mode 
data from T+2.8 ks to T+16 ks. The light curve shows an initial decay with 
a slope of 1.11 +0.06/-0.05 up to at least ~T+17 ks, followed by a steep 
decay up to ~T+20 ks and then a flatter decay with a slope of 0.25 
+0.62/-0.13.

The PC X-ray spectrum from the same interval can be well fit by an absorbed
power-law with a photon index of 1.99 +0.10/-0.09 and a column density of 
(9.7 +2.2/-2.0) x 10^20 cm^-2 (the Galactic value is 4.0 x 10^20 cm^-2 in 
the direction of the burst).  The observed 0.3-10.0 keV flux is (7.1 
+/-0.4) x 10^-11 erg cm^-2 s^-1 which corresponds to an unabsorbed flux of 
(9.0 +0.2/-0.5) x 10^-11 erg cm^-2 s^-1.

If the burst continues on the current plateau phase, the predicted count 
rate at T+1 day would be approximately 1.2 x 10^-1 count s^-1.

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

S. R. Oates (MSSL-UCL) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 081121,
2816s after the BAT trigger (Oates et al., GCN Circ. 8537
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 ). We detect the
optical afterglow in the white, v, b, u filters and marginally in the
uvw1 filter at the position:

RA(J2000.0)  =   5:57:06.15
DEC(J2000.0) = -60:36:10.0

with an estimated uncertainty of 0.6 arcsec (radius, 90% confidence).
This position is consistent with the UVOT-enhanced XRT position
and the position reported by ROTSE-IIIC (Yuan et al., GCN Circ. 8536
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 ).

The marginal detection of the optical afterglow in uvw1, and the lack
of a detection in the uvm2 and uvw2 filters is consistent with a redshift
of z=2.512 reported by Magellan (Berger and Rauch, GCN Circ. 8542
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 ).

The magnitudes and 3 sigma upper limits are reported below:

Filter    T_start (s) T_stop  Exposure      Mag/3sig UL
---------------------------------------------------------
white       2816       2965     147       17.93 +/- 0.04
v           2972       3172     197       17.58 +/- 0.08
b           3792       3992     197       18.26 +/- 0.07
u           3587       3787     197       17.62 +/- 0.06
uvw1        3383       3582     197       20.60 +/- 0.48 (2.3-sigma)
uvm2        3177       4813     393       > 20.09
uvw2        4203       11200    790       > 20.78
---------------------------------------------------------

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.05 mag (Schlegel et al.,
1998, ApJS, 500, 525).  The photometry is on the UVOT flight system
described in Poole et al. (2008, MNRAS, 383,627).

Colleen A. Wilson-Hodge (NASA/MSFC) and Valerie Connaughton (UAH)
report on behalf of the Fermi GBM Team:

"At 20:35:27.5 UT on 21 November 2008, the Fermi Gamma-Ray Burst Monitor
triggered on GRB 081121 (trigger 248992528/ 081121.858). This burst
triggered Swift-BAT 4 seconds later (Oates et al 2008, GCN 8537
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 ) and an
optical counterpart was detected with ROTSE-III (Yuan et al. 2008,
GCN 8536
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 ). The burst direction was 135 degrees from the Fermi pointing-axis
so the Fermi GBM sensitivity to this GRB is limited. Further analysis
is necessary to provide meaningful spectral information about this burst.

Final spectral analysis results will be published in the GBM GRB Catalog."

B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained several epochs of optical/IR imaging of the
error region of GRB 081121 (GCN 8537
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 , Oates et al.).  Each
epoch consisted of several dithered images in each filter, with
total summed exposure times of 180s in each of BRIJK and 120s
in each of H and V.

At a mid-exposure time of 2008-11-22 02:02 (5.4 hrs post-burst),
the GRB afterglow (GCN 8536
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 , Yuan et al.) is detected with the
following magnitudes:
B = 19.34 +/- 0.04
R = 18.61 +/- 0.04
I = 18.21 +/- 0.04
J = 17.21 +/- 0.15
H = 17.05 +/- 0.15
K = 16.75 +/- 0.15

Observations were obtained under non-photometric conditions.  In optical,
these preliminary magnitudes are calibrated against several USNO-B1.0
stars in the field, so there is likely an additional photometric
calibration error of ~0.3 magnitudes.  In the IR, calibration is against 
2MASS stars.

Between 5.4 hrs and 8.7 hours post-burst, the afterglow decays
with an approximate optical decay rate of alpha~-1 (where
afterglow flux is proportional to t^alpha).

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 081121 (Swift-BAT trigger #335105:
Oates et al., GCN 8537
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 , Sakamoto et al. 8539) triggered
Konus-Wind at T0=74131.435 s UT (20:35:31.435).

The burst light curve shows a multipeaked structure with a total 
duration of ~18 s.

As observed by Konus-Wind the burst
had a fluence of 1.79(-0.31, +0.37)x10^-5 erg/cm2,
and a 256-ms peak flux measured from T0+16.368 s
of 2.60(-0.63, +0.70)x10^-6 erg/cm2/s
(both in the 20 keV - 7 MeV energy range).

The time-integrated spectrum of the burst
(from T0 to T0+16.640 s) is well be fitted (in the 20 keV - 7 MeV
range) by GRB (Band) model for which:
the low-energy photon index is alpha = -0.77(-0.14, +0.15),
the high energy photon index beta = -2.51(-0.66, +0.31),
the peak energy Ep = 248(-32, +38) keV (chi2 = 92.3/79 dof).

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

Assuming z = 2.512 (Berger & Rauch, GCN 8542
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 ) and a standard
cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.27, Omega_\Lambda =
0.73, the isotropic energy release E_iso ~2.7x10^53 erg, the peak
luminosity (L_iso)_max ~ 1.4x10^53 erg/s, and Ep_rest ~870 keV.

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB081121_T74131/

Aquib Moin (Curtin Institute of Radio Astronomy / Australia Telescope
National Facility), Steven Tingay (Curtin Institute of Radio Astronomy),
Chris Phillips (Australia Telescope National Facility), Gregory Taylor
(University of New Mexico), Mark Wieringa (Australia Telescope National
Facility) and Ralph Martin (Perth Observatory) report:

We observed the SWIFT-UVOT refined position of the GRB 081121 optical
afterglow (GCN 8544
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 ) at 4.800 and 4.928 GHz with the Australia Telescope
Compact Array (ATCA) between 01:15:05 UT, November 24, 2008 and 20:35:30
UT, November 25, 2008.

We did not detect a radio source at the optical afterglow position of
the GRB 081121 (GCN 8544
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 ). The data at 4.800 and 4.928 GHz were merged
and the radio flux density at the afterglow position found out to be
0.106 +/- 0.208 mJy/beam (1-sigma).

The Australia Telescope Compact Array (/ Parkes telescope / Mopra
telescope / Long Baseline Array) is part of the Australia Telescope
which is funded by the Commonwealth of Australia for operation as a
National Facility managed by CSIRO.

See the 4.800 & 4.928 GHz combined image at:

http://cira.ivec.org/dokuwiki/doku.php/grb/grb081121_field_image

E. Gorbovskoy, V. Lipunov, V.Kornilov, A.Belinski, N.Shatskiy, N.Tyurina, 
D.Kuvshinov, P.Balanutsa
Sternberg Astronomical Institute, Moscow State University

A. Tlatov, A.V. Parhomenko Kislovodsk Solar Station of the Pulkovo Observatory

V.Krushinski, I.Zalognikh
Ural State University, Kourovka


S.Yazev, K.Ivanov
Irkutsk State University

One of the four  MASTER Very Wide Field Cameras located at Kislovodsk
(http://apollo.sai.msu.ru/, D=50 mm, 4x1000 square degrees, 35'' per pix)
has observed  Fermi  error box  (Trig Num 251059717) with 5s 
exposures during all night without time gap between images.
The M31 (ANdromeda) is inside Fermi error box.

The message may be cited.

mailto: lipunov@sai.msu.ru

E. Gorbovskoy, V. Lipunov, V.Kornilov, A.Belinski, N.Shatskiy, N.Tyurina, 
D.Kuvshinov, P.Balanutsa, V.V.Chazov
Sternberg Astronomical Institute, Moscow State University

A. Tlatov, A.V. Parhomenko Kislovodsk Solar Station of the Pulkovo Observatory

V.Krushinski, I.Zalognikh
Ural State University, Kourovka


S.Yazev, K.Ivanov
Irkutsk State University

One of the four  MASTER Very Wide Field Cameras located at Kislovodsk
(http://apollo.sai.msu.ru/, D=50 mm, 4x1000 square degrees, 35'' per pix)
has observed  Fermi  error box  (Trig Num 251059717) with 5s exposures during 
all night without time gap between images.
The synhronous with GRB time image with M31 (Andromeda) are available

http://observ.pereplet.ru/images/GRB081215A/film_grb.html

We have dosen alert images on the new wide field MASTER telescope (FOW 8 
square degrees up to 17 mag), that it just tested now 
(http://www.pereplet.ru/lipunov/257.html#257). 
The message may be cited.

mailto: lipunov@sai.msu.ru