GRB 090305

S. T. Holland (CRESST/GSFC/USRA) and A. P. Beardmore (U Leicester)
report on the behalf of the Swift UVOT team:

     The Swift/UVOT observed the field of GRB 090305 starting 78 s
after the BAT trigger (Beardmore, et al. 2009, GCN Circ. 8932
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 ).
Settled exposures started at T+96 s.  We do not find any new source,
relative to the DSS, USNO-B1.0, or 2MASS, or any variable source at
the position of the GMOS-S afterglow (Cenko, et al. 2009, GCN
Circ. 8933
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 ).  Preliminary 3-sigma upper limits for detecting a source
in the white finding chart, and the co-added images, using a 2.5
arcsecond radius circular aperture, are

Filter   T_start   T_stop   Exp(s)     Mag
------------------------------------------
white        96      198      100   >20.6

v        3503   11,765     1034   >20.6
b        4323   15,980     1278   >21.8
u        4118     5755      393   >20.6
uvw1       3913     5550      393   >20.6
uvm2       3708     5345      393   >20.3
uvw2       4734   17,549     1723   >21.6
white      4528   16,892     1982   >22.7
------------------------------------------

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} = 0.21 mag (Schlegel, et al., 1998, ApJS, 500,
525).  All photometry is on the UVOT photometry system described in
Poole et al. (2008, MNRAS, 383, 627).

    The GMOS-S afterglow is located in the wings of the B = 13.72
mag USNO-B1.0 star 0584-0391760.  UVOT photometry at this location is
contaminated by this star.
Please update my e-mail address in your Address Book to <Stephen.T.Holland@nasa.gov 
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A.P. Beardmore, K. L. Page, P.A. Evans (U. Leicester), C. Pagani, J. Kennea,
D.N. Burrows (PSU) report on behalf of the Swift-XRT team :

The Swift-XRT began observing the field of the short GRB 090305
(Beardmore et al. GCN Circ. 8932
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 ) in photon counting mode at
2009-03-05T05:21:37, 103.4 s after the trigger.  The first orbit of
data lasted only 90 s before the spacecraft slewed to another target
because of an observing constraint.

The default data processing does not reveal any X-ray source during
the first orbit (103.4 to 193.5 s post trigger), either within the BAT
refined error circle (Krimm et al. GCN Circ. 8936
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 ) or at the location
of the optical afterglow (Cenko et al. GCN Circ. 8933
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 ) - the latter
occurring close to a bad-column on the XRT CCD.  However, by relaxing
the default screening criteria we find 8 counts in a 10 pixel radius
circle extraction region at the location of the optical afterglow,
compared with a predicted background level of 1.0 count (where an
annulus with radii 15 to 100 pixels was used to sample the local
background level), implying a greater than 99.99 percent confidence
detection (Kraft, Burrows, and Nousek 1991, Ap. J., 374, 344). The
clustering of the counts, their proximity to the location of the
optical afterglow and their detection significance allow us to
conclude the afterglow was detected by the XRT at this time. The
estimated count rate is 0.112 +0.051 -0.041 count/s (where a
systematic error of 5 percent has been included in the 1 sigma error
estimate to account for the PSF correction uncertainty).  Applying a
canonical GRB count to observed flux conversion factor of 5e-11 erg
cm^-2 count^-1 (for a photon index of 1.98, Evans et al. 2009, MNRAS,
submitted. arXiv:0812.3662, http://www.swift.ac.uk/xrt_spectra), we
estimate the observed 0.3-10 keV flux was (5.5 +2.6 -2.0)e-12 
erg cm^-2 s^-1.

A further 2.05 ks of data was taken in orbit two, from 3.92 to 5.97 ks
after the trigger, where the optical afterglow position was located
away from the XRT CCD bad column. This showed the source was no longer
detected and a 3 sigma upper limit to the count rate of 7.0e-3 count/s
was obtained.  Using all of the available data after the first orbit
(i.e. presently an exposure of 12.64 ks from 3.92 to 46.1 ks post
trigger) reduces the 3 sigma upper limit to 1.7e-3 count/s.  The X-ray
data suggest the source faded with a decay slope of at least ~0.8.

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

H. A. Krimm (GSFC/USRA),  S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL),
N. Gehrels (GSFC), C. B. Markwardt (GSFC/UMD), K. McLean (GSFC/UMD),
D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
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-61 to T+242 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 090305 (trigger #345127)
(Beardmore et al., GCN 8932
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 ).  The BAT ground-calculated position is
RA, Dec = 241.764, -31.572 deg which is
  RA(J2000)  =  16h 07m 03.5s
  Dec(J2000) = -31d 34' 20.9"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
This position is 1.3 arcmin from the optical afterglow reported by
Cenko et al. (GCN Circ 8933
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 ). The partial coding was 50%.

The lightcurve consists of two pulses, the first about 0.08 seconds
long and the second about  0.28 seconds long.  T90 (15-350 keV) is
0.4 +- 0.1 sec (estimated error including systematics).  There was
very little emission seen in the BAT low energy band, 15-25 keV.

The time-averaged spectrum from T+0.0 to T+0.4 sec is best fit by a
simple power-law model.  The power law index of the time-averaged
spectrum is 0.86 +- 0.33.  The fluence in the 15-150 keV band is
7.5 +- 1.3 x 10^-08 erg/cm2.  The 1-sec peak photon flux measured from
T-0.29 sec in the 15-150 keV band is 1.9 +- 0.4 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/345127/BA/

E. Berger (Harvard) and D. Kelson (OCIW) report:

"We imaged the BAT error circle of the short GRB 090305 (GCN 8932
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 ) with
IMACS on the Magellan/Baade 6.5-m telescope.  Observations were taken in R
band on 2009 March 5 at 06:48 and 09:11 UT (88 and 231 min after the
burst, respectively).  We confirm the existence of the fading source
reported by Cenko et al. from Gemini South observations (GCN 8933
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 ) and
consider this source to be the afterglow of GRB 090305."

S. B. Cenko, B. E. Cobb, D. A. Perley, and J. S. Bloom (UC Berkeley)
report on behalf of a larger collaboration:

We have imaged the field of the short GRB 090305 (Beardmore et al., GCN
8932
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 ) with GMOS mounted on the Gemini South telescope beginning
approximately 27 minutes after the burst.  Observations were taken in
the Sloan g', r', and i' filters and cover nearly the entire BAT error
circle.

Inside the BAT error circle, we find a fading point source at the location
(J2000.0):

   RA: 16:07:07.61
   Dec: -31:33:21.9

Calibrating with respect to the USNO-B1 object at 16:07:06.58, -31:33:37.9
(R2=18.99), we measure an initial magnitude of R ~ 23.3.  The source fades
by approximately 1.1 mag over the next two hours, corresponding to a
power-law decay index of ~ 0.6 (assuming no contribution from an
underlying host galaxy).  At this point we tentatively consider this
source the afterglow of GRB090305, although we await confirmation from the
XRT.

We wish to thank the staff at Gemini South, in particular Peter Pessev,
for performing our target of opportunity observations.

A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC),
S. T. Holland (CRESST/USRA/GSFC), C. B. Markwardt (CRESST/GSFC/UMD),
C. Pagani (PSU), K. L. Page (U Leicester) and D. M. Palmer (LANL)
report on behalf of the Swift Team:

At 05:19:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090305 (trigger=345127).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA, Dec 241.756, -31.567 which is 
   RA(J2000) = 16h 07m 02s
   Dec(J2000) = -31d 34' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single short spike
with a duration of about 0.4 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 05:21:23.8 UT, 92.7 seconds after
the BAT trigger. No source was detected in the promptly available XRT
data in an exposure time of 92.8 seconds before Swift slewed away. We are 
waiting for the full dataset to detect and localise the
XRT counterpart. 


UVOT took a finding chart exposure of 102 seconds with the White filter
starting 96 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. Because part of the 2.7'x2.7'
sub-image was not received, the overlap with the BAT error circle is uncertain. 
The overlap is at most 25%. The coverage of the BAT error circle by the 8'x8'
region for the list of sources generated on-board is uncertain because the
large number of sources filled the available telemetry. No correction has been
made for the expected extinction corresponding to E(B-V) of 0.22. 




We note the presence of a ROSAT survey source, 1RXS J160703.2-313251,
within the BAT error circle.  Further data are required to determine
whether this object is related to the burst. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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.)