GRB 090904A

S. B. Pandey (U Mich), W. Rujopakarn (Steward), W. Zheng (U Mich) report on
behalf of the ROTSE collaboration:

ROTSE-IIId, located at the Turkish National Observatory at Bakirlitepe,
Turkey, responded to GRB 090904A (Swift trigger 361830; Perri M., GCN
9879
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 ), producing images beginning 9.5 s after the GCN notice time. An
automated response took the first image at 01:02:31.6 UT, 84.7 s after the
burst, and during the gamma-ray emission, under fair conditions. We took 10
5-sec, 10 20-sec and 10 60-sec exposures. These unfiltered images are
calibrated relative to USNO A2.0 (R). Imaging is on going.

Comparison to the DSS (second epoch) reveals no new sources within the
3-sigma Swift/BAT error circle or the XRT error circle, for both single
images and coadding into sets of 10. Individual images have limiting
magnitudes ranging from 15.8-16.6; we set the following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
01:02:31.5   01:02:36.5         5     15.8           84.6       N
01:04:48.1   01:09:27.0       278     17.8          221.2       Y

D. Malesani (DARK/NBI), P. Jakobsson (U. Iceland), N. Tanvir
(U. Leicester) and I. Ilyin (AIP) report on behalf of a larger
collaboration:

We observed the field of GRB 090904A (Perri et al., GCN 9879
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 )
with the Nordic Optical Telescope equipped with StanCam.
Observations were carried out on 2009 Sept 4.17 (3.1 hr after
the GRB) in the R- and I-bands. No objects are detected inside
the XRT error circle to a limit of R > 23. However, just outside
the error circle at

RA(J2000)  =  06:43:32.33
Dec(J2000) = +50:12:12.4

there is an object with R = 22 +/- 0.2 mag (calibrated against
several nearby USNO-B1 stars). If weather permits, further
observations are planned tonight to estimate whether the object
is fading.

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

Using 1030 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 090904A, 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 = 100.88052, +50.20366 which is equivalent
to:

RA (J2000): 06h 43m 31.33s
Dec (J2000): +50d 12' 13.2"

with an uncertainty of 2.0 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) and Evans
et al. (2009, MNRAS, 397, 1177).

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

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

The automatic enhanced XRT position (GCN 9885
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 ) was incorrect and should 
be disregarded. The position was obtained by averaging all the PC data 
from the first orbit, however the degree of pile up in this period 
changes by much more than is normal (see the  light curve at: 
http://www.swift.ac.uk/xrt_curves/00361830/). As a result, the PSF of 
the image did not match any of our calibrated PSFs. We apologise for any 
confusion.

The best XRT position available is the enhanced SPER position, which was 
distributed as an XRT Position Update GCN notice about at 01:27 UT. This 
position is RA, Dec = 100.8858, 50.2038, which corresponds to

RA(J2000.0) =  06h 43m 32.59s
Dec (J2000.0) = +50d 12' 13.6"

with an uncertainty of 2.3 arcsec (radius, 90% confidence).

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

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), M. Perri (ASDC),
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 the recent telemetry downlink,
we report further analysis of BAT GRB 090904A (trigger #361830)
(Perri, et al., GCN Circ. 9879
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 ).  The BAT ground-calculated position is
RA, Dec = 100.855, 50.235 deg, which is 
   RA(J2000)  =  06h 43m 25.2s 
   Dec(J2000) = +50d 14' 06.9" 
with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.
 
The mask-weighted light curve shows low level emission starting at ~T-40 sec,
with a small peak at ~T+5 sec. Then there was a larger peak at ~T+50 sec,
and then the largest peak (FRED-like) starting at ~T+123 sec, peaking at
~T+128 sec.  Riding on that tail is a peak ~T+173.  The event is over
in the BAT energy band at ~T+240 sec.  T90 (15-350 keV) is 122 +- 10 sec
(estimated error including systematics).
 
The time-averaged spectrum from T+29.3 to T+186.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.01 +- 0.10.  The fluence in the 15-150 keV band is 3.0 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+127.07 sec in the 15-150 keV band
is 1.9 +- 0.2 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/361830/BA/

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team.

With 6 ks of XRT data, and 5 UVOT images from recent data downlinks we 
have generated an enhanced XRT position for GRB 090904A, which does not
include the problematic first image and is thus reliable.

The enhanced position is RA, Dec = 100.88480, +50.20300 which is
equivalent to:

RA (J2000): 06h 43m 32.35s
Dec (J2000): +50d 12' 10.8"

with an uncertainty of 1.6 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) and Evans
et al. (2009, MNRAS, 397, 1177).

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

[GCN OPS NOTE(04sep09):  Per operator's desire, the Subject-line
was changed from "090404A" to "090904A". F.Marshall noticed this.]

M. Perri, V. D'Elia, G. Stratta (ASDC) report on behalf of the
Swift-XRT team:

We have analysed the first 12 ks of XRT data for GRB 090904A (Perri et
al.,
GCN Circ. 9879
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 ), from 134 s to 29.6 ks after the BAT trigger. The enhanced
XRT position for this burst was given by Evans (GCN. Circ. 9891).

During the first Swift orbit, the light-curve shows a steep decay
with a large superimposed flare peaking at about T+300 s. Starting
from T+1000 s, the curve is well described by a broken power-law model
with temporal decay indices alpha1=0.2+/-0.1, alpha2=0.9+/-0.3 and a
temporal break at around T+12 ks.

A spectrum extracted from the PC mode data from T+460 s to T+29.6 ks can
be fitted with an absorbed power-law model with a photon spectral index of
2.46 (+0.16,-0.26) and an absorption column density of
1.03 (+0.66,-0.64)e21 cm-2 in excess of the Galactic value of
9.3e20 cm-2 (Kalberla et al. 2005). The counts-to-observed (-unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum is
3.3e-11 (6.2e-11) erg cm-2 count-1. Errors are given at the
90% confidence level.

If the light curve continues to decay with a power-law index of
0.9, the count rate at T+24 hours will be 6e-3 count s-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of
2.0e-13 (3.7e-13) erg cm-2 s-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00361830.

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

T. A. Pritchard (PSU), C. A. Swenson (PSU) and M. Perri (ASDC) report on 
behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of GRB 090904A,  141 seconds 
after the
BAT trigger (M. Perri et al., GCN Circ. 9879
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 ). We do not detect any 
source at the
UVOT-enhanced XRT position (M.R. Goad, GCN Circ. 9885
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 ).

The 3-sigma upper limits for the exposures are:

Filter   T_start(s)   T_stop(s)   Exp(s)  Mag (3-sigma upper limit)
-------------------------------------------------------------
white      141      	1189       341      > 21.57
v          629       	1078        58      > 18.72
b          555          1177        58      > 19.59
u          299       	1152       285      > 20.43
uvw1	   679		1127	    58      > 19.11
uvm2	   827		847	    19      > 17.61
uvw2	   605		1054	    58      > 19.17
-------------------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction corresponding to a reddeningalong the line of sight of
E_(B-V) = 0.1 mag (Schlegel et al., 1998, ApJS, 500, 525).  All 
photometry is on the
UVOT photometric system described in Poole et  al. (2008, MNRAS, 383, 627).

Daniele Malesani (DARK/NBI), Pall Jakobsson (Univ. Iceland), and Ilya 
Ilyin (AIP), report on behalf of a larger collaboration:

We observed again the field of GRB 090904A (Perri et al., GCN 9879
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 ) with 
the NOT equipped with StanCam. Observations were carried out in the R 
band, for a total exposure time of 30 min, with a mean time September 
6.218 UT (2.176 days after the GRB trigger).

The object identified in our previous epoch of imaging (Malesani et al., 
GCN 9884
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 ) has clearly faded and is hardly detectable, if at all. A faint 
source is visible at its position, with magnitude R = 24.0 +- 0.3. If 
real, the inferred decay slope is alpha = 0.55 +- 0.12, assuming F(t) 
propto t^-alpha. The value of alpha is shallower than usually found at 
late times, and may indicate some host contribution or an unusual light 
curve.

The position of the transient is consistent with the latest available 
UVOT-enhanced XRT position (Goad et al., GCN 9885
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 ; see also 
http://www.swift.ac.uk/xrt_positions/index.php). We thus identify the 
object as the optical afterglow of GRB 090904A.

A finding chart comparing the two epochs can be found at the following URL:

http://www.astro.ku.dk/~malesani/GRB/090904A/GRB090904A_finder_NOT.jpg