GRB 050309

GRB 050309 : Lulin R-band observation

H.Hsieh, R.Mann (IfAUH)
K.Y. Huang, W.H. Ip (NCU), Y. Urata, T. Tamagawa (RIKEN),
Y. Qiu (BAO), Y.Q. Lou(THCA) on behalf of the East Asian collaboration
report:

 We observed the SWIFT/BAT error region for GRB 050309 (trigger
no. 107873) using Lulin 1-m telescope at Taiwan. We imaged a single
R-band 300 sec exposure at 11.14 UT (~ 25 mins after the burst
). Compare with DSS-II image, no new object was found. The limiting
magnitude is about 19 mag ( S/N=3 , with USNOB1.0 catalogue).


This message may be cited.

S. Barthelmy (GSFC), D.Burrows (PSU), L. Barbier (GSFC), J. Cummings 
(GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD), 
J. Kennea (PSU), H. Krimm (GSFC/USRA), D. Malesani (SISSA), C. Markwardt 
(GSFC/UMD), A. Moretti (INAF-OAB), K. Page (U. Leicester), D. Palmer 
(LANL), A. Parsons (GSFC), P. Romano (INAF-OAB), T. Sakamoto (GSFC), G. 
Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC)
on behalf of the Swift-BAT/-XRT teams:

Further analysis of Swift data on BAT trigger #107873 has led to an 
ambiguous situation regarding the reality of this marginal BAT trigger.

The Swift-BAT triggered (#107873) at 10:43:21 UT.  The trigger occurred on 
the rising edge of the SAA.  Ground analysis has confirmed the very low 
image-domain detection significance of this trigger.  A visual inspection 
of the mask-tagged light curve also shows very marginal emission.
As a result, a retraction GCN Notice was issued earlier today.  We are now
ammending that Notice.

The BAT position was within the Swift Earth limb constraint at the time of 
the burst, so Swift did not execute a prompt automated slew.  The spacecraft 
executed a delayed slew to the BAT location at about 11:33 UT and XRT 
observations began at 11:35:54 UT.  Two orbits of data were collected by 
the XRT, for a total of 4157s of exposure.  Preliminary analysis indicates 
that there are four X-ray sources in the XRT field of view, two of which 
fall inside the BAT error circle.  These are:

Source #1: RA(J2000) = 12 10 29.1, Dec(J2000) = +77 37 04.8. This is a very 
weak source (about 0.006 cps).  It appears to be roughly constant in 
intensity, but the source is so faint that we cannot determine with 
confidence whether or not it is varying in flux.

Source #2: RA(J2000) = 12 09 37.2, Dec(J2000) = +77 35 56.5.  This source
is brighter (about 0.01 cps) and may be fading.  Again, we cannot be certain 
with the data currently in hand.

The other two sources are a bright star and a very faint object well 
outside the BAT error circle.  The estimated uncertainty in the XRT 
positions is 6 arcseconds radius.

A 20 ks Target of Opportunity observation has been initiated to provide a 
second set of data beginning at about 19:45 UT.  This should help us 
determine the likelihood that either XRT source is related to the BAT trigger.

In light of these XRT observations, the BAT team cannot rule out the 
possibility that this trigger represents a threshold-level GRB.

Derek B. Fox and S. Bradley Cenko report on behalf of the
Caltech-NRAO-Carnegie GRB collaboration:

"Examination of a coadded R-band image from our P60 observations for
the BAT localization region of GRB050309 (Barthelmy et al, GCN 3082
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 ),
taken at a mean epoch of 10:55 UT (12 minutes after the burst),
confirms our earlier report (Cenko & Fox, GCN 3080
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 ) of the absence of
new optical sources to the limit of the Digitized Sky Survey and our
own observations.  We estimate this limit to be R < 20.5 mag as
calibrated against the USNO B-1.0 catalog magnitudes of bright stars
in the region.

This limit applies to XRT source #2 (GCN 3082
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 ), where no emission is
seen.  The position of XRT source #1 is 6-arcsec (twice the FWHM in
our images) from a bright star and USNO B-1.0 catalog object having
R=12.98 mag.  No extension of the catalog star is apparent in our
images."

P.R. Wozniak, S. Evans, W.T. Vestrand, R. White, J. Wren (LANL)
on behalf of the RAPTOR team. 

The RAPTOR A telescope responded to Swift-BAT trigger 107873 and began
optical imaging of the burst location in 6.7 seconds from the time
of receiving the alert (2.3 min. after GRB time). Photometry at the
locations of the two X-ray candidate counterparts to GRB 050309 reported
by Barthelmy et al. (GCN 3082
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 ) shows no optical emission in a series of
10 second exposures, each with the limiting magnitude R=15.0. A stack of
the first 10 images with the mean time of observation 4.2 min. after the
GRB time had a magnitude limit of R=16.1.

V. Tilvi, L. Shamir, W. Pereira, D. Cordell, V. Muzzin, M. Merlo & 
R. Nemiroff (Michigan Tech)  report on behalf of the greater Night Sky 
Live Collaboration

The NightSkyLive.net CONCAM2 operating on Mauna Kea has recorded
fisheye images including the location of GRB050309, being sensitive
to the GRB location starting about 28 minutes before Swift trigger.  No bright
optical transient was found consistent with the GRB location on any frame.  
A 170-second exposure including the GRB region was in progress during 
the trigger time and prompt optical emission can be limited as being more 
faint than unfiltered optical CCD magnitude 3.8.  The MK CONCAM image 
at the time of trigger is here: 
http://nightskylive.net/mk/mk050309/mk050309ut104239a.jpg

A table of observation times and approximate limiting magnitudes follows:

Date            From (UT)    To (UT)       m_limit     Remarks
2005-03-09  10:15:03       10:17:53     2.5           28 minutes before trigger
2005-03-09  10:19:00       10:21:50     2.8           Before trigger
2005-03-09  10:22:56       10:24:46     3.0           Before trigger
2005-03-09  10:26:53       10:28:43     3.1           Before trigger
2005-03-09  10:30:47       10:32:37     3.2           Before trigger
2005-03-09  10:38:41       10:40:31     3.5           Before trigger
2005-03-09  10:42:39       10:44:29     3.8           During!  Swift trigger: 10:43:21 UT
2005-03-09  10:46:34       10:48:24     4.0           After trigger
2005-03-09  10:50:32       10:52:22     4.0           After trigger
2005-03-09  10:54:29       10:56:19     4.0           After trigger