GRB 050528

S.T. Holland (GSFC/USRA), D. Band (GSFC/UMBC), S. Barthelmy (GSFC),
N. Gehrels (GSFC), M. Goad  (Leicester U.), C. Gronwall (PSU),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
D. Palmer (LANL), P. Roming (PSU) on behalf of the Swift team

At 04:06:45 UT on May 28, 2005 the BAT instrument on the Swift
spacecraft triggered on (trigger = 130679) and located possible
GRB 050528.  The BAT on-board calculated location is RA 353.529d
{+23h 34m 07s} (J2000),  +45.944d {+45d 56' 37"} (J2000) with
an uncertainty of 4 arc minutes (radius, 3-sigma, including systematic
uncertainty).

Due to the temporary lack of BAT lightcurves and engineering work on the
XRT, there is no additional information on this burst at this time.
We can not say if this is a real GRB or not.  The full data set from
the next available Malindi downlink will be available at approximately
10:00 UT.   A burst validity statement will be made at that time.

A. Monfardini, A. Gomboc, C. Guidorzi, C.G. Mundell, I. A. Steele,
C.J. Mottram, D. Carter, R.J. Smith, M.F. Bode (Liverpool JMU) report:

"The Liverpool Telescope reacted to the SWIFT trigger 130679. The
automatic detection mode resulted in a non-detection. We remind that our
field-of-view is 4.6 arcmin wide.

The earlier, robotic upper limit is R>17.2 at 2.5 minutes after the
possible GRB reported time (limited by observing conditions).

Further observations have been performed, and quick inspection reveals no
obvious candidates. Analysis is in progress.

This message can be cited."

Martin Jelinek, Alberto J. Castro-Tirado, Antonio de Ugarte
Postigo, Stanislav Vitek, Sergey Guziy (IAA CSIC Granada,
Spain), Petr Kubanek, Rene Hudec, Martin Nekola, (ASU AV CR
Ondrejov, Czech Republic) Petr Pata, Martin Bernas (CVUT
Praha, Czech Republic), and Tomas de J. Mateo Sanguino (Univ.
Huelva, Spain)

report

Robotic telescope BOOTES-1B located at CEDEA in El Arenosillo,
Suthern Spain, has observed the error box of GRB050528
starting 71s after the GRB trigger. We see a bright variable
object at 

23:34:42.93 +45:56:14.4

Further analysis is in progress.

Martin Jelinek, Alberto J. Castro-Tirado, Antonio de Ugarte
Postigo, Stanislav Vitek, Sergey Guziy (IAA CSIC Granada,
Spain), Petr Kubanek, Rene Hudec, Martin Nekola, (ASU AV CR
Ondrejov, Czech Republic) Petr Pata, Martin Bernas (CVUT
Praha, Czech Republic), and Tomas de J. Mateo Sanguino (Univ.
Huelva, Spain)

enunciate sadly

that the optical candidate reported by ourselves in previous
GCN (Jelinek et al., GCN3498
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 ) is in fact a star, which is not
present in GSC2 catalogue. This mislead me (MJ) and although
doing a double-check using DSS, I have missed the star, thus
convincing myself about reality of the object.

We apologize to the comunity for a false alert.

Anyhow: The BOOTES-1B has observed the errorbox starting 71s
after the trigger in a light twilight, setting the limit to
the possible GRB counterpart to V>13.8 and I>13.0 during the
first 60s after the beginning of our observation.

S. Barthelmy (GSFC), S.T. Holland (GSFC/USRA), N. Gehrels (GSFC),
M. Goad (Leicester U.), C. Gronwall (PSU), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), P. Roming (PSU)
on behalf of the Swift team

The putative Swift-BAT burst GRB050528 (Trigger 130679 at 04:06:45 UT)
(GCN 3496
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 ) is still uncertain.  Intermittant problems with the transfer
of data from Malindi to the US have prevented a final determination
of the validity of this BAT trigger.  The file transfer problem
is being worked, and when we get the data from the trigger time interval,
we will issue an update.  We apologize for the delay and any inconvenience
in follow-up observing programs.

J. Cummings (GSFC/NRC), L. Barbier, S. Barthelmy (GSFC),
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD),
H. Krimm (GSFC/USRA), C. Markwardt, (GSFC/UMD), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC), G. Sato (ISAS), M. Suzuki (Saitama),
T. Takahashi (ISAS), J. Tueller (GSFC)
on behalf of the Swift-BAT team:

The full data set has been delivered (GCN 3501
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 ).
Analysis of the data has allowed us to determine that Trigger 130649
at 04:06:45 UT (GCN 3496
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 ) is a real GRB.  The ground-analysis position
is RA,Dec 353.515,+45.938 (J2000) with an uncertainty of 3 arcmin
(radius, 90%, statistical plus systematic).

The lightcuve shows a single smooth peak lasting ~12 sec with an ~1-sec spike
riding on top towards the end of the event.  It starts ~T-8 and ends at T+4 sec.
T90 is 10.8 sec.

The burst spectrum is consistant with a simple power-law with a photon index
of 2.44 +-0.25 and a flux of 11.8 +-0.1 ph/cm2/sec (15-350 keV, 90% c.l.).
This is a very soft event with essentially no emission above 100 keV.
The fluence is (5.1+-0.1)x10^-7 ergs/cm2 (15-350 keV, 90% c.l.).

A. Monfardini, A. Gomboc, C. Guidorzi, C.G. Mundell, I. A. Steele,
C.J. Mottram, D. Carter, R.J. Smith, M.F. Bode (Liverpool JMU) report:

"As a result of 1 hour monitoring of the center of the possible GRB050528
field (4.6x4.6 arcmin FoV) we provide the following rough limits to update
our previous GCN3497
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 :

R > 20  at 30 minutes mean epoch
I > 19  at 35 minutes mean epoch

Observations started 2.5 minutes after the GRB under non-ideal conditions.

This message can be cited."

S. Barthelmy (GSFC) on behalf of the Swift-BAT team:

The peak flux quoted in GCN 3502
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  is wrong.  The peak flux
should be 1.5+-0.3 ph/cm2/sec (not 11.8).

M. Goad (U. Leicester), J. Kennea (PSU), and D. N. Burrows (PSU) report on
behalf of the Swift XRT team:
 
We have analysed Swift XRT data from the soft burst GRB050528
(GCN3496
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 , GCN3502
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 ).  The XRT was in an engineering mode at the time of the
burst and did not begin observations until it was switched back into its
normal observing mode.
Analysis of 15965 seconds (9 orbits) of Photon Counting mode data starting
14.15 hours after the BAT trigger reveals 3 faint sources within the BAT 3
arcmin error circle.
 
The source coordinates are :
 
RA(J2000)   23 34 03.7
Dec(J2000) +45 58 18.9
 
RA(J2000)   23 33 52.1
Dec(J2000) +45 57 40.2
 
RA(J2000)   23 34 03.2
Dec(J2000) +45 56 03.3

with an estimated uncertainty of 7 arcsecs radius.
 
Sources 1, 2 and 3 are located 122.1, 146.1 and 14.1 arcsecs from the
BAT position respectively.
 
Assuming a Crab like spectrum, photon index 2.1, and column density 1.2e21
cm-2 the observed count rates correspond to unabsorbed fluxes of 4.9e-14,
4.2e-14 and 3.2e-14 erg/cm2/s (0.5-10.0 keV), with an uncertainty of
1.2e-14 erg/cm2/s.
 
Due to the faintness of the sources, we cannot determine at this time
whether any of the sources are fading.