GRB 090308

P. A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC),
R. Margutti (Univ Bicocca&OAB), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
C. Pagani (PSU), K. L. Page (U Leicester), A. M. Parsons (GSFC),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-IASFPA),
R. L. C. Starling (U Leicester), M. C. Stroh (PSU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 18:01:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090308 (trigger=345777).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 183.526, -48.833 which is 
   RA(J2000) = 12h 14m 06s
   Dec(J2000) = -48d 49' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is an image trigger, but even so
the BAT light curve shows a weak peak at T+20 to T+35 sec. 
The peak count rate was ~350 counts/sec (15-350 keV), at ~25 sec
after the trigger. 

The XRT began observing the field at 18:03:30.4 UT, 126.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 183.5017,
-48.8169 which is equivalent to:
   RA(J2000)  = 12h 14m 0.41s
   Dec(J2000) = -48d 49' 00.9"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 81 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (8.54e+20
cm^-2, Kalberla et al. 2005), with an excess column of 1.8
(+1.51/-1.36) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 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, there is no overlap with the XRT error circle. 
The 8'x8' region for the list of sources generated on-board covers the entire
XRT error circle. No correction has been
made for the expected extinction corresponding to E(B-V) of 0.11. 

Burst Advocate for this burst is P. A. Evans (pae9 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.)

C. B. Markwardt (GSFC/UMD), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), P. A. Evans (U Leicester), E. E. Fenimore (LANL),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), 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-60 to T+243 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 090308 (trigger #345777)
(Evans, et al., GCN Circ. 8948
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 ).  The BAT ground-calculated position is
RA, Dec = 183.545, -48.754 deg, which is 
   RA(J2000)  =  12h 14m 10.7s 
   Dec(J2000) = -48d 45' 13.8" 
with an uncertainty of 3.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 89%.
 
The mask-weighted light curve shows a single peak starting at ~T-10 sec,
peaking at ~T+30 sec, and ending at ~T+65 sec.  T90 (15-350 keV) is
48 +- 22 sec (estimated error including systematics).
 
The time-averaged spectrum from T+4.0 to T+52.0 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.33 +- 0.37.  The fluence in the 15-150 keV band is 2.6 +- 0.6 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+27.50 sec in the 15-150 keV band
is 0.1 +- 0.0 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/345777/BA/

P. A. Evans and K.L. Page (U Leicester) report on behalf of the 
Swift-XRT team:

We have analysed 4.4 ks of XRT data for GRB 090308 (Evans et al. GCN
Circ. 8948
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 ), from 130 s to 12.2 ks after the BAT trigger. The data
comprise 55 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. Using 460 s of PC mode data and 1 UVOT image, we
find an enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 183.50120,
-48.81710 which is equivalent to:

RA (J2000): 12 14 0.28
Dec(J2000): -48 49 01.6

with an uncertainty of 1.6 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.63 (+0.16, -0.13), followed by a break at T+830 s to an
alpha of 1.2 (+0.5, -1.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.93 (+0.28, -0.26). The
best-fitting absorption column is 2.86 (+1.01, -0.88) x 10^21, in excess
of the Galactic value of 8.5 x 10^20 cm^-2 (Kalberla et al. 2005). The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.5 x 10^-11 (6.8 x 10-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 2.0 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 x 
10^-15 (1.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00345777.

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

F.E. Marshall (NASA/GSFC) and P. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 0900308 starting 132 s after
the BAT trigger (Evans et al., GCN Circ. 8948
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 ). No optical afterglow
is detected in the initial UVOT exposures at the position
of the X-ray afterglow (Evans and Page, GCN Circ. 8956
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 ).
Three-sigma upper limits using the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627) for the first finding chart (FC)
exposure and subsequent exposures are:

Filter          T_start(s)   T_stop(s)      Exp(s)       Mag 

white (FC)         132          282          147         >21.1
white              570        11390         1102         >22.2
v                  620        17423         1101         >20.6
b                  545         6455          382         >20.8
u                  290         6281          639         >20.7
uvw1               669         6077          399         >20.6
uvm2               644        18019          798         >20.8
uvw2               595        12237         1044         >21.2

The values quoted above are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.11 in the direction of the burst 
(Schlegel et al. 1998).

S. B. Cenko, A. N. Morgan, D. A. Perley, N. R. Butler, and J. S. Bloom (UC
Berkeley) report on behalf of a larger collaboration:

We have imaged the field of GRB090308 (Evans et al.; GCN 8948
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 ) with GMOS
mounted on the Gemini South telescope beginning at approximately 1:38 UT
on the night of 9 March 2009 (~ 7.6 hours after the burst).  Images were
taken in the g', r', i', and z' filters for a total of 300 s in each band.

We find no significant sources inside the revised XRT error circle (Evans
et al.; GCN 8956
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 ) to the following limits: g' > 23.8; r' > 24.2; i' >
23.6; z' > 22.5. These limits were calculated using reference stars from
the USNO-B1 catalog and applying the filter transformations of Jordi,
Grebel, and Ammon (A&A 460, 2009).

We do, however, note the presence of an extremely red point source located
several arcseconds southwest of the refined XRT position, and immediately
outside the X-ray position provided by Butler (AJ 133, 2007; see
http://astro.berkeley.edu/~nat/swift/xrt_pos.html).  The coordinates of
this source are (J2000.0):

	RA: 12:13:59.89
	Dec: -48:49:03.5

This object is detected only in the r', i', and z' filters with
approximate magnitudes: r' = 23.6; i' = 21.6; z' = 20.2 (uncorrected for
Galactic extinction).  While we consider the association unlikely, further
observations are required to determine if this is the afterglow of
GRB090308.

We wish to thank the staff at Gemini Observatory for undertaking these
observations.

A. C. Updike (Clemson University), C. Clemens, J. Greiner and P. Afonso
(MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 090308 (Swift trigger 345777; Evans et al.,
GCN #8948
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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 00:01 UT on March 9th, 6 hours after the GRB
trigger for 2 hours, and were followed by a second epoch beginning at
07:44 UT and lasting 30 min. They were carried out at an average seeing of
1.7".

The red object just outside the enhanced XRT error circle noted by Cenko
et al. (GCN #8965
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 ) is detected in both epochs with no sign of fading.
Therefore, we conclude this is not the afterglow of GRB 090308.

Based on 30 min of our first epoch, we do not detect a source within the
Swift-XRT error circle reported by Evans & Page (GCN #8956
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 ) down to (all in
AB system)

 g' > 22.7,
 r' > 22.9,
 i' > 22.6,
 z' > 22.3,
 J > 21.1,
 H > 20.6 and
 K > 19.6

The given limits are derived based on calibrating the images against GROND
zeropoints and 2MASS field stars and are not corrected for the Galactic
foreground extinction corresponding to a reddening of E_(B-V)= 0.114 in the
direction of the burst (Schlegel et al. 1998).