GRB 120728A

J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC),
M. M. Chester (PSU), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (UCL-MSSL), C. B. Markwardt (NASA/GSFC),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL), M. H. Siegel (PSU) and
T. N. Ukwatta (MSU) report on behalf of the Swift Team:

At 22:25:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120728A (trigger=529021).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 137.100, -54.437, which is 
   RA(J2000)  =  09h 08m 24s
   Dec(J2000) = -54d 26' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a peak with structure
with a total duration of about 20 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 22:27:38.8 UT, 147.8 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 137.09412, -54.43783 which
is equivalent to:
   RA(J2000)  = 09h 08m 22.59s
   Dec(J2000) = -54d 26' 16.2"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 12 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 consistent with the Galactic value of 5.18
x 10^21 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
148 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. Because of the density of catalogued stars, further analysis
is required to report an upper limit for any afterglow in the sub-image. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. Because of the density of catalogued stars, further analysis is
required to report an upper limit for any afterglow in the region. No
correction has been made for the large, but uncertain extinction expected. 

Burst Advocate for this burst is J. R. Cummings (jayc AT milkyway.gsfc.nasa.gov). 
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.)

D. A. Kann, S. Klose (both TLS Tautenburg), and J. Greiner (MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 120728A (Swift trigger 529021; Cummings et
al., GCN 13525
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 ) simultaneously in g'r'i'z'JHKs with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla
Observatory (Chile).

Observations started at 23:05 UT on 2012-07-28, 41 minutes after the GRB
trigger. Observations were obtained in twilight, at high (2.5) airmass and
bad seeing (2".7 in the r' band).

Based on an exposure of 9.4 minutes in the optical and 16 minutes in the
NIR we do not detect any source within the Swift/XRT error circle at
coordinates RA, Dec(J2000)= 09:08:22.69, -54:26:17.7, error 3".6. (SPER
data)

At the edge of the error circle, we find two sources. One, at RA (J2000)
= 09:08:23.20, Dec. (J2000) = -54:26:18.8, is a star also visible in the
DSS. The other may be marginally detected in the DSS, and likely consists
of two sources, centered at (+/- 0".5) RA (J2000) = 09:08:22.82, Dec.
(J2000) = -54:26:13.1 and RA (J2000) = 09:08:22.55, Dec. (J2000) =
-54:26:14.2. We are unable to make any statements about the variability
of these objects at this time, but given the crowded field this close to
the Galactic plane, it is likely neither is associated with the GRB.

Preliminary limiting magnitudes (AB mags) are:

  g' > 21.6
  r' > 21.9
  i' > 21.3
  z' > 21.1
  J  > 19.5
  H  > 19.9
  Ks > 19.3.

Magnitudes are derived based on GROND zeropoints (g'r'i'z') and 2MASS
stars (JHKs). No correction for the high Galactic reddening along the
line of sight (E(B-V)=0.57 mag; Schlegel et al. 1998) has been applied.

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

Using 4986 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 120728A, 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 = 137.09469, -54.43760 which is equivalent
to:

RA (J2000): 09h 08m 22.73s
Dec (J2000): -54d 26' 15.4"

with an uncertainty of 1.7 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.

V. D'Elia (ASDC), A. Maselli  (INAF-IASFPA), M.C. Stroh (PSU), D.N.
Burrows (PSU), J.A. Kennea (PSU), P.A. Evans (U. Leicester), O.M.
Littlejohns (U. Leicester), J.P. Osborne (U. Leicester), V. Mangano
(INAF-IASFPA) and J.R. Cummings report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 120728A (Cummings  et al.
GCN Circ. 13525
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 ), from 161 s to 24.9 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Osborne et al. (GCN. Circ 13528).

The late-time light curve (from T0+5.1 ks) can be modelled with  a
power-law decay with a decay index of alpha=0.98 (+/-0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.67 (+0.25, -0.10). The
best-fitting absorption column is  consistent with the Galactic value
of 5.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.4 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     0 (+2.2, -0) x 10^20 cm^-2
Galactic foreground: 5.2 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.67 (+0.25, -0.10)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00529021.

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

S. R. Oates (MSSL-UCL) and J. R. Cummings (NASA/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 120728A
148 s after the BAT trigger (Cummings et al., GCN Circ. 13525
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 ).
No optical afterglow consistent with the XRT position
(Osborne et al., GCN Circ.13528
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

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

u_FC               148          398          246         >19.5
v                  456        17454         1356         >20.6
b                  404        24174         2261         >22.3
u                  148        23261         2438         >21.0
w1                 505        19107         1200         >20.8
m2                 480        18359         1357         >21.0
w2                 430        24878         1735         >21.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.53 in the direction of the burst
(Schlegel et al. 1998).

C. B. Markwardt (GSFC), 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), D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (OSU), J. Tueller (GSFC),
T. N. Ukwatta (MSU) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 120728A (trigger #529021)
(Cummings, et al., GCN Circ. 13525
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 ).  The BAT ground-calculated position is
RA, Dec = 137.085, -54.437 deg which is
    RA(J2000)  =  09h 08m 20.3s
    Dec(J2000) = -54d 26' 12.4"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 13%.

The mask-weighted light curve shows a broad FRED-like peak of total duration
~25 seconds with three subsidiary peaks superimposed.  The
T90 (15-350 keV) is 22.06 +- 2.59 sec (estimated error including systematics).

The time-averaged spectrum from T+0.56 to T+26.46 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.79 +- 0.13.  The fluence in the 15-150 keV band is 2.4 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+12.12 sec in the 15-150 keV band
is 2.4 +- 0.5 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/529021/BA/

D. A. Kann, S. Klose (both TLS Tautenburg), and J. Greiner (MPE Garching)
report on behalf of the GROND team:

We observed again the field of GRB 120728A (Swift trigger 529021; Cummings
et al., GCN 13525
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 ) simultaneously in g'r'i'z'JHKs with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla
Observatory (Chile).

Compared to our first epoch (Kann et al., GCN 13526
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 ), seeing conditions
significantly improved to 1".5 in the r' band. We obtained 7 minutes of
optical observations and 12 minutes of NIR observations centered 1.035 days
after the GRB. We achieve limiting magnitudes which are 0.5 mag deeper in
all optical bands compared to our first epoch, but shallower in the NIR.

The enhanced Swift-XRT position (Osborne et al., GCN 13528
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 ) does not include
the bright DSS source in the south-east anymore, but still overlaps with the
two northern sources (Kann et al., GCN 13526
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 ). Upon close inspection, the
eastern source is also detected in the DSS, whereas the western source is
not. This source is furthermore not present anymore in our second epoch,
which we confirm by image subtraction.

We therefore propose this source to be the optical afterglow of GRB 120728A,
and derive a position:

RA  (J2000) =  09:08:22.62,
DEC (J2000) = -54:26:14.5

with an error of 0".8.

The preliminary magnitude is r' = 20.80 +/- 0.16 (statistical) +/- 0.27
(systematic) against the USNO-B1.0 catalog in the first epoch, 0.0377 days
after the GRB.

Shaolin Xiong (UAH) reports on behalf of the Fermi GBM Team:

"At 22:25:13.737 UT on 28 July 2012, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 120728A (trigger 365207115 / 120728934)
which was also detected by the Swift/BAT (Cummings et al. 2012, GCN 13525
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 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight is 110 degrees.

The GBM light curve consists of multiple pulses
with a duration (T90) of about 32.8 s (50-300 keV).
The time-averaged spectrum from T0-1.0 s to T0+23.6 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.66 +/- 0.10 and
the cutoff energy, parameterized as Epeak, is 119.80 +/- 30.80 keV.

The event fluence (10-1000 keV) in this time interval is
(5.29 +/- 0.27)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+13.12 s in the 10-1000 keV band
is 3.58 +/- 0.28 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."