GRB 090628

V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU), J. A. Kennea (PSU),
D. M. Palmer (LANL), B. Sbarufatti (INAF-IASFPA),
M. Stamatikos (NASA/ORAU) and M. A. Stark (PSU) report on behalf of
the Swift Team:

At 21:20:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090628 (trigger=355942).  Swift could not immediately 
slew to the burst.  The BAT on-board calculated location is 
RA, Dec 237.050, -15.977 which is 
   RA(J2000)  =  15h 48m 12s
   Dec(J2000) = -15d 58' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 5 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Due to an Earth limb constraint, a prompt slew was not possible.  
Swift will execute a delayed slew after 2656 s.  No XRT or UVOT 
data will be available until the slew occurs. 

The series GCN Notices for this burst are corrupted 
with many (but not all) bogus values.  For the moment,
please disregard the original series of notices.  We may
issue Updated position notices when the information becomes available. 

Burst Advocate for this burst is V. Mangano (vanessa AT ifc.inaf.it). 
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.)

B. Sbarufatti, V. Mangano (INAF-IASF PA)
report on behalf the Swift XRT team:

Analysis of preliminary data from the BAT trigger 355942 (GRB090628, 
Mangano, et al., GCN Circ 9585
Loading...
 
 )
reveals an uncatalogued point source at RA, Dec = 237.05355, -15.98577 
which is equivalent to:

RA(J2000) = 15h48m12.85s
Dec(J2000) = -15:59:08.79

with an estimated uncertainty of 6.5 arcseconds radius (90% confidence).
Analysis is ongoing and an improved analysis will be reported as soon as
more data will be available.

This is an official product of the Swift XRT team.

E. A. Hoversten and V. Mangano (INAF-IASFPA) report on behalf of the Swift UVOT
team:

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 2790 seconds after the BAT trigger. No credible afterglow candidate
has been found in the initial data products. The 2.7'x2.7' sub-image covers
100% of the XRT error circle. The typical 3-sigma upper limit has been about
19.6 mag. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. The list of sources is typically complete to
about 18 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.18.

P. Kubanek (IAA-CSIC & GACE), N. Morales (IAA-CSIC), J.L. Ortiz
(IAA-CSIC), A. de Ugarte Postigo (ESO, INAF/OAB), J. Gorosabel (IAA-CSIC),
A.J. Castro-Tirado (IAA-CSIC), M. Jelinek (IAA-CSIC), report on behalf of
a larger collaboration:

We carried out R-band observations of the GRB 090628 XRT position
(Sbarufatti et al., GCN 9586
Loading...
 
 ) with the 1.23m Calar Alto telescope. The
observations were done on June 28.9486-29.0225 UT (1.43-3.20 hours post
GRB) with an exposure time of 3120s. A quick inspection of the XRT error
box reveals no object brighter than R ~ 22.

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

Using 3973 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 090628, 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 = 237.05272, -15.98489 which is equivalent
to:

RA (J2000): 15h 48m 12.65s
Dec (J2000): -15d 59' 05.6"

with an uncertainty of 1.9 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, arXiv:0812.3662).

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

T. Kruehler (MPE Garching), S. Klose (Tautenburg) and J. Greiner (MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 090628 (Swift trigger #355942; Mangano et
al., GCN #9585
Loading...
 
 ) 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 02:06 UT on June 29th, 2009, 4.8 hr after the
burst and were performed under poor seeing conditions. In stacked images
of 75 min total integration time in griz and 60 min in JHK, we do not
detect any object within the enhanced XRT error circle (Goad et al., GCN
#9590
Loading...
 
 ), down to the following limiting magnitudes (all in the AB system):

g' > 23.7
r' > 24.0
i' > 23.0
z' > 22.8
J > 21.8
H > 21.1
K > 20.5

These upper limits were derived using the GROND zeropoints and 2MASS field
stars as reference and are not corrected for the Galactic foreground
extinction corresponding to a reddening of E(B-V)=0.18 mag (Schlegel et
al., 1998).

We note the presence of a r'~23.5 mag source ~5 arcsec north east of the
center of the enhanced XRT error circle, which given the large offset is
very unlikely to be related to the GRB.

C. B. Markwardt (GSFC/UMD), 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), V. Mangano (INAF-IASFPA), D. M. Palmer (LANL),
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-240 to T+649 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 090628 (trigger #355942)
(Mangano, et al., GCN Circ. 9585
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 237.038, -15.969 deg, which is 
   RA(J2000)  =  15h 48m 09.0s 
   Dec(J2000) = -15d 58' 09.0" 
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 63%.
 
The mask-weighted light curve shows a weak 1-sec wide precursor peak
at ~T-7.5 sec that then returns almost to background levels, and the
main peak starting at ~T-0.3 sec, peaking at ~T+1 sec, and ending 
at ~T+6 sec.  T90 (15-350 keV) is 20.1 +- 4.6 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-25.0 to T+4.0 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.36 +- 0.20.  The fluence in the 15-150 keV band is 7.0 +- 0.8 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.05 sec in the 15-150 keV band
is 1.5 +- 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/355942/BA/

M. H. Siegel (PSU) and V. Mangano (INAF-IASFPA) report on behalf
of the Swift/UVOT team.

The Swift/UVOT began observing the field of GRB 090628 2790s after
the BAT trigger (Mangano et al., GCN Circ. 9585
Loading...
 
 ).  Data summed
from the first orbit does not reveal a source at the refined
position of the X-ray afterglow (Goad et al., GCN Circ.
9590
Loading...
 
 ).

The 3-sigma upper limits for the finding chart (fc) and
summed exposures are reported below:

FILTER     T_start(s)  T_stop    Exposure     Mag/3UL
=============================================================
white fc     2791       2941        147       > 20.26
white        3361       3560        196       > 20.72

v            3771       3971        196       > 19.23

b            3155       3355        196       > 20.16
b            4591       4672         79       > 19.64

u            2950       3149        196       > 19.63
u            4386       4586        196       > 19.82

uvw1         4181       4381        196       > 19.65

uvm2         3976       4176        196       > 19.33

uvw2         3566       3766        196       > 19.62
=================================================================

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.05 (Schlegel et al.,
1998, ApJS, 500, 525). The photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).

V. Mangano, B. Sbarufatti (INAF-IASF PA) report on behalf
the Swift XRT Team:

We have analyzed 19 ks of PC data of the candidate X-ray
afterglow of GRB 090628 (Mangano et al, GCN Circ. 9585
Loading...
 
 )
spanning the time interval from T+2796 s to T+101.7 ks.

The best position available for the source is the
enhanced Swift-XRT position from Goad et al. (GCN Circ. 9590
Loading...
 
 ):
RA, Dec = 237.05272, -15.98489 which is equivalent to

RA (J2000): 15h 48m 12.65s
Dec (J2000): -15d 59' 05.6"

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The 0.3-10 keV XRT light curve shows an initial flat or rising
phase followed by a steep decay. The best fit with a broken
power-law model is insensitive to the initial rise slope,
of the order of 0.4, possibly consistent with zero,
and provides a break at 14+/-4 ks after the trigger
and a post-break decay slope of -1.3+/-0.3.
The source is rather faint, and at this rate it will
reach a level of 8.0e-4 counts/s at T+48 hours.
All quoted errors are at the 90% confidence level.

The average spectrum can be fitted by an absorbed
power-law model with Cash statistics. Best fit spectral
parameters are NH < 2.0e21 cm^-2 (consistent with Galactic
absorption of 8.7e20 cm^-2, Kalberla et al. 2005)
and photon index = 1.4 +/- 0.3 (Cstat(dof): 87.6(113)).
The 0.3-10 keV observed(unabsorbed) average flux is
5.0e-13(5.5e-13) erg cm^-2 s^-1.

The rate to flux conversion factor is 8.0e-11.

The results of the XRT team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00355942

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