GRB 080218B

P. Schady (MSSL-UCL), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
P. A. Evans (U Leicester), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL),
T. Sakamoto (NASA/UMBC), G. Sato (GSFC/ISAS),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
D. E. Vanden Berk (PSU) and P. A. Ward (MSSL-UCL) report on behalf of
the Swift Team:

At 23:57:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080218B (trigger=303631).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 177.951, -53.100 which is 
   RA(J2000) = 11h 51m 48s
   Dec(J2000) = -53d 06' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 12 sec, up to an
interruption in the available data caused by a TDRSS gap. 
The peak count rate was  ~1300 counts/sec (15-350 keV), 
at ~0 sec after the trigger. 

Using prompt downlinked data starting 931 s after the BAT trigger,
we find a bright X-ray source located at RA, Dec 177.9560, -53.0960
which is
   RA(J2000)  = 11 51 49.44
   Dec(J2000) = -53 05 45.7
with an uncertainty of 3.0 arcsec (radius, 90% containment). 
This location is 18 arcseconds from the BAT on-board position,
within the BAT error circle. 

UVOT took a finding chart exposure of 99 seconds with the White
(160-650 nm) filter starting 551 seconds after the BAT trigger.  No
afterglow candidate has been found in the initial data products.  The
UVOT image covers 100% of the XRT error circle and has an estimated
3-sigma limiting magnitude of white = 20.6.  No correction has been
made for the expected extinction corresponding to a reddening of
E(B-V) = 0.17 mag. 

Burst Advocate for this burst is P. Schady (ps 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.)

J. French (UCD Dublin), 
M. Jelinek  and Petr Kubanek (IAA CSIC Granada) and 
Antonio de Ugarte Postigo (ESO Santiago)

on behalf of the Watcher collaboration report:

"The  0.4 m telecope  Watcher,  located  at  Boyden
observatory,  South Africa,  followed the Swift GRB
080218B (Schady et al. GCN 7314
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 ).

10s unfiltered (limmag = 16.0) images were obtained
starting 60.4s after the GRB (18.3s after receiving
the notification).

We  do  not detect  any  new source in  single  nor
combined images within the BAT or XRT errorbox."

This message may be cited.

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 080218b
(Schady et al. GCN 7314
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 ) with a mid-exposure time of ~3.1 hours
post-burst (2008-02-19 03:03 UT).  Total summed exposure times amounted
to 18 minutes in I and 15 minutes in J.

No source is detected at the position of the X-ray
afterglow (Schady et al. GCN 7314
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 ) to approximate
limiting magnitudes of I > 21.1 and J > 18.7. Magnitudes
are calibrated using USNO-B1.0 values for I, and 2MASS
values for J.

A. Rossi (Tautenburg Obs.), J. Greiner, A. K"upc"u Yoldas, and A. Yoldas
(MPE Garching) report for the GROND team:

We observed the field of GRB 080218B, Swift trigger 303631 (Schady et al. 2008,
GCN Circ. 7314
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 ) simultaneously in grizJHK with GROND (Greiner et al. 2008,
astro-ph/0801.4801) mounted at the 2.2m MPI/ESO telescope at La Silla (Chile).
Observations started at 00:37, 379 min after the GRB. At this time, the GRB 
location was becoming visible at 20 degrees above horizon.

Through thin cirrus, we do not detect any source inside the XRT error.
We estimate the following limits: g'>24.2, r'>24.2, i'>23.7, z'>23.4,
J>21.6, H>20.6, K>19.5, calibrated against USNO-B and 2MASS stars.
These values are not corrected for the substantial foreground extinction.

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

Using 311 s of overlapping XRT Photon Counting mode and UVOT
V-band data for GRB 080218B, 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 = 177.95739, -53.09712 which is equivalent
to:

RA (J2000): 11h 51m 49.77s
Dec (J2000): -53d 05' 49.6"

with an uncertainty of 2.4 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).

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

S. Covino, P. D'Avanzo, L.A. Antonelli, D. Fugazza, L.  Calzoletti,  
S. Campana, G.  Chincarini, M.L. Conciatore, S. Cutini, V. D'Elia, F.  
D'Alessio, F. Fiore, P. Goldoni, D. Guetta,  C. Guidorzi, G.L.  
Israel, E. Maiorano, N. Masetti, A. Melandri, E.J.A. Meurs, L.  
Nicastro, E. Palazzi, E. Pian, S. Piranomonte, L.  Stella, G.   
Stratta, G. Tagliaferri, G. Tosti, V. Testa, S.D. Vergani,  F. Vitali  
report on behalf of the REM team:

The robotic 60-cm REM telescope located at La Silla (Chile) observed  
automatically the field of the GRB 080218B (Schady et al., GCN 7314
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 ).  
A preliminary analysis of the first set of H and K-band exposures  
does not reveal any afterglow candidate inside the refined XRT error  
box (Osborne et al., GCN 7320
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 ). The 3sigma upper limits of our images  
are H > 13.7 about 2 minutes after the burst and K > 12.6 about 8  
minutes after the burst (calibrated against the 2MASS catalog).

Further analysis is in progress.

P. Schady (MSSL-UCL) reports on behalf of the Swift-UVOT team

The Swift-UVOT observed the field of GRB080218B starting 551 s after the 
BAT trigger (Schady et al., GCN 7314
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 ). No new source is detected within 
the refined XRT error circle (Osborne et al., GCN 7320
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 ) in any of the UVOT 
co-added observations down to the following 3-sigma upper limits:

Filter  Tmid (s) Exp (s)  Upper Limit
wh      4178     420        > 21.09
v       2955     494        > 19.87
b       4126     283        > 20.24
u       4098     294        > 19.98
uvw1    3984     294        > 19.77
uvm2    3868     313        > 19.67
uvw2    6056     281        > 19.91

where Tmid is the weighted mid time of the co-added observations. The 
values quoted above are not corrected for the expected Galactic extinction 
corresponding to a reddening of E(B-V)= 0.17 mag in the direction of the 
burst (Schlegel et al. 1998),

P.A. Evans, K.L. Page, A.P. Beardmore, J.P. Osborne (U Leicester) and P.
Schady (MSSL-UCL) report on behalf of the Swift-XRT team:

We have analysed the first four orbits of XRT data obtained for GRB
080218B (Schady et al., GCN Circ. 7314
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 ), totalling 4.6 ks in Photon
Counting (PC) mode. Partly due to the proximity of Swift to the SAA, the
observations did not start until 930 s after the trigger.  The
UVOT-enhanced XRT position was given in Osborne et al. (GCN Circ.
7320
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 ).

The light curve shows a simple power-law decay, with an index of
alpha=0.92 (+0.13/-0.11). A spectrum formed from all available data can
be modelled with an absorbed power law, with gamma=2.46 (+0.53/-0.47)
and a column density nH=5.17(+0.24/-0.18)e21 cm^-2, significantly in
excess of the Galactic value of 1.35e21 cm^-2 (Kalberla et al. 2005).
Given the excess absorption, the relation given in Grupe et al (2007)
suggests that this burst lies at a redshift <3.3 (90% confidence). The
0.3-10 keV observed (unabsorbed) flux during the interval T0+1--25 ks is
5.72e-12 (1.48e-11) erg cm^-2 s^-1.

If the light curve continues to decay at alpha=0.92, the XRT count rate
at T0+24 hours will be 4.0e-3 count s^-1, which is equivalent to an
observed (unabsorbed) 0.3-10 keV flux of 2.29e-13 (5.92e-13) erg cm^-2
s^-1.

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

H. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), C. Markwardt (GSFC/UMD),
K. McLean (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
T. Sakamoto (GSFC/UMBC), G. Sato (GSFC/ISAS), P. Schady (MSSL-UCL),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-198 to T+962 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080218 (trigger #303631)
(Schady, et al., GCN Circ. 7314
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 ).  The BAT ground-calculated position is
RA, Dec = 177.927, -53.086 deg, which is 
   RA(J2000)  =  11h 51m 42.4s 
   Dec(J2000) = -53d 05' 11" 
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 10%.
 
The mask-weighted light curve shows a roughly triangular peak starting
at ~T-3 sec, peaking at T+1 sec, and ending at ~T+7 sec.
T90 (15-350 keV) is 6.2 +- 1.2 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.8 to T+6.6 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.11 +- 2.44, 
and Epeak of 23.6 +- 14.6 keV (chi squared 49.02 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 5.1 +- 1.0 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T+0.28 sec in the 15-150 keV band is
3.1 +- 0.6 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.63 +- 0.29 (chi squared 55.36 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/303631/BA/
 
We note that the fluence ratio in a simple power-law fit between the
25-50 keV band and the 50-100 keV band is 1.55.  This fluence ratio is larger
than 1.32 which can be achieved in the Band function of alpha=-1.0, beta=-2.5,
and Epeak=30 keV.  Thus, preliminary analysis shows that Epeak of the burst
is very likely around or below 30 keV.  Therefore the burst can be classified
as an X-ray flash (e.g. Sakamoto et al. ApJ in press, arXiv:0801.4319).

Paul Vreeswijk, Johan Fynbo, Bo Milvang-Jensen, Daniele Malesani,
Jens Hjorth (DARK/NBI), Pall Jakobsson (U. Hertfordshire),
Andreas Jaunsen (U. Oslo), Nial Tanvir (U. Leicester) report
on behalf of a larger collaboration:

The Swift localization of GRB 080218B (Schady et al., GCN 7314
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 ) was
observed with FORS2 mounted at the VLT Antu telescope on Cerro Paranal
in Rapid-Response Mode, and with SOFI at the NTT on La Silla. Starting
at Feb 19, 1:03 UT (roughly 1 hour after the burst) VLT/FORS2
performed the imaging sequence R-I-z-V-B-R (30-150s per filter)
through thin cirrus, at an airmass of 2.5, and with an effective
seeing of 1.5". NTT/SOFI imaged the afterglow location at an airmass
of 1.3-1.5 and in 1.4" seeing conditions in Ks-J-H-Ks (10 min. per
filter), starting at Feb 19, 2:52 UT.

No object is detected within the enhanced XRT error circle (Osborne et
al., GCN 7320
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 ) down to the following limiting magnitudes (2 sigma):
B>22.1, V>22.7, R>22.9, I>22.6, J>20.6, H>20.1 and Ks>19.4.  The
corresponding upper limit on the optical to X-ray spectral slope,
beta_OX, is 0.24, rendering the burst dark according to the definition
proposed by Jakobsson et al. (2004, ApJ, 617, L21). A spectrum of an
object just outside the XRT error circle at R.A. 11:51:49.25,
decl. -53:05:50.1 (J2000), shows it to be a Galactic star of type M6V.

We acknowledge the excellent support of ESO's night astronomers at La
Silla Paranal Observatory, and in particular that of Linda
Schmidtobreick, Colin Snodgrass, Stephane Brillant and Lorenzo Monaco.

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-BAT refined position of the GRB 080218b (GCN 7325
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 )
at 4.8 and 8.456 GHz with the Australia Telescope Compact Array (ATCA)
on May 31, 2008 between 04:24:45 UT and 10:15:15 UT.

We did not detect a radio source at the BAT position of the GRB 080218b
(GCN 7325
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 ). The radio flux density at the GRB position is -0.286 +/-
0.252 mJy at 4.8 GHz, and -0.350 +/- 0.271 mJy at 8.456 GHz (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 field images at:

http://cira.ivec.org/dokuwiki/doku.php/grb/grb_080218b_field_image_at_4.8_ghz

http://cira.ivec.org/dokuwiki/doku.php/grb/grb_080218b_field_image_at_8.456_ghz