GRB 100728A

J. K. Cannizzo (NASA/UMBC), W. H. Baumgartner (GSFC/UMBC),
P. A. Evans (U Leicester), J. M. Gelbord (PSU),
S. T. Holland (CRESST/USRA/GSFC), H. A. Krimm (CRESST/GSFC/USRA),
O. M. Littlejohns (U Leicester), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), C. Pagani (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
A. Rowlinson (U Leicester), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC) and R. L. C. Starling (U Leicester)
report on behalf of the Swift Team:

At 02:18:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100728A (trigger=430151).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 88.743, -15.262 which is 
   RA(J2000) = 05h 54m 58s
   Dec(J2000) = -15d 15' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of at least 200 sec, although the burst was
already in progress when the spacecraft slewed to the original position.  
The peak count rate was ~11,000 counts/sec (15-350 keV), at ~25 sec after
the trigger. 

The XRT began observing the field at 02:19:41.0 UT, 76.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 88.7610, -15.2561 which is equivalent to:
   RA(J2000)  = 05h 55m 02.63s
   Dec(J2000) = -15d 15' 21.9"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). This
location is 64 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 0.1 s image was 2.01e-08 erg cm-2 s-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 86 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. 

Burst Advocate for this burst is J. K. Cannizzo (cannizzo 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.)

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

Using 5085 s of XRT Photon Counting mode data and 8 UVOT
images for GRB 100728A, 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 = 88.75839, -15.25531 which is equivalent
to:

RA (J2000): 05h 55m 2.01s
Dec (J2000): -15d 15' 19.1"

with an uncertainty of 1.4 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.

A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team: 

"At 02:17:30.61 UT on 28 July 2010, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 100728A (trigger 301976252 / 100728095), 
which was also detected by the Swift-BAT (Cannizzo et al. 2008, GCN 11004
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 )

The GBM on-ground location is consistent with the Swift position.
 
The angle from the Fermi LAT boresight is 58.6 degrees.

The GBM light curve shows a multi-peaked structure 
with a duration (T90) of about  162.9 +/- 0.9 s (50 - 300 keV). 
The time-averaged spectrum from T0-8.192 s to T0+191.491 s is 
well fit by a power law function with an exponential
high energy cutoff.  The power law index is -0.76 +/- 0.01 and
the cutoff energy, parameterized as Epeak, is 353.7 +/- 6.7 keV
(Castor C-STAT 3540 for 727 d.o.f.) .

The event fluence (8-1000 keV) in this time interval is 
(1.291 +/- 0.008)E-04 erg/cm^2. The 1.024-sec peak photon flux measured 
starting from T0+80.2 s in the 8-1000 keV band is 10.8 +/- 0.2 ph/s/cm^2.

A Band function fits the spectrum equally well (Castor C-STAT 3537 for 726
d.o.f.) with Epeak= 344.3 +9.1/-7.9 keV, alpha = -0.75 +/- 0.01 and 
beta = -3.04 +0.23/-0.57. 

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

D. A. Perley, C. R. Klein, A. N. Morgan, and W. Li (UC Berkeley) report 
on behalf of the KAIT follow-up team:

The Katzman Automatic Imaging Telescope (KAIT) responded to GRB 100728A 
and began taking data starting less than one minute after the trigger. 
A moderately bright optical transient not present in DSS survey imaging 
is visible in the initial set of exposures at the following position:

RA  =  02:56:13.48
dec = +00:16:51.8
(J2000)

We measure I = 18.7 +/- 0.3 mag at 10:33:51 UT (t = 95 secs after the 
BAT trigger)

In addition, we also manually slewed the 1.0 meter Nickel telescope and 
began taking data starting at 6.6 minutes after the trigger; the 
afterglow is also well-detected and is seen to fade in subsequent 
exposures.  Follow-up is continuing.

K. Ivarsen, J. Haislip, D. Reichart, A. LaCluyze, J. Moore, A. Foster, R.
Egger, A. Oza, M. Schubel, A. Trotter, J. A. Crain, and M. Nysewander
report:

Skynet observed the Swift/BAT localization of GRB 100728A (Swift trigger
#430172) with four of the 16" PROMPT telescopes at CTIO beginning 45
seconds after the trigger in BVRI. We detect the afterglow in all filters.

At 50 seconds after the trigger, its magnitude is I = 14.62 +0.08 -0.07
(calibrated to 10 SDSS stars), R = 15.24 +0.07, -0.06 (calibrated to 7
SDSS stars), V = 15.78, +0.08, -0.07 (calibrated to 8 SDSS stars), B =
16.54 +0.21, -0.17 (calibrated to 2 SDSS stars). SDSS transformations were
applied using Jester et al., 2005, ApJ, 130, 873.

PROMPT's BVRI light curve of the first 4.3 minutes after the trigger,
calibrated to SDSS stars, can be found here:
http://skynet.unc.edu/grb/grb100728a.png. Further observations were
limited by morning twilight at PROMPT.

P. A. Evans (U. Leicester) and J. K. Cannizzo (NASA/UMBC) report on
behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 100728A (Cannizzo et al. GCN
Circ. 11004
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 ), from 66 s to 23.4 ks after the BAT trigger. The data
comprise 1.3 ks in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Beardmore et
al. (GCN. Circ 11005).

The light curve can be modeled with a broken power-law decay, with a
series of flares superimposed during the first kilosecond. The
underlying decay begins with a decay index of 1.6 (+/-0.1), breaking at
T+410 sec to a decay index of 1.07 (+0.03, -0.04). The decay then breaks
again at T+11600 sec to a final decay of 1.82 (+/-0.2). Nine discrete
flares can clearly be identified, with peak times of T+ 88, 111, 189,
252, 296, 383, 445, 516, 659 sec. These all occurred while the XRT was 
in Windowed Timing mode.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.022 (+/-0.022). The
best-fitting absorption column is 3.69 (+0.09, -0.08) x 10^21 cm^-2, in
excess of the Galactic value of 1.0 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.05 (+/-0.09) and a
best-fitting absorption column of 3.7 (+/-0.3) x 10^21 cm^-2. The counts
to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from
this spectrum is 4.5 x 10^-11 (7.6 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.82, the count rate at T+24 hours will be 0.024 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.8 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00430151.

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

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

The Swift/UVOT began settled observations of the field of
GRB 100728A 86s after the BAT trigger (Cannizzo et al., GCN 11004
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 ).
We do not detect a new source at the enhanced Swift XRT position
(Evans et al. GCN 11005
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 ).

The preliminary 3-sigma upper limits for the finding chart exposures  
(FC)
and summed images are:

Filter   T_start   T_stop   Exp(s)  Mag (3-sigma upper limit)
-------------------------------------------------------------
white (FC)  86      236      147        >20.54
white       578     7432     657        >21.16
u     (FC)  299     548      246        >19.82
u           702     7021     490        >20.04
v           628     11695    1218       >20.06
b           554     7226     510        >20.37
uvw1        677     13337    1228       >20.62
uvm2        1255    12600    1337       >20.69
uvw2        604     7554     447        >20.19
-------------------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction corresponding to a reddening along the line of
sight of E_(B-V) = 0.18 mag. All photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).

T. N. Ukwatta (GWU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. K. Cannizzo (NASA/UMBC),J. R. Cummings (GSFC/UMBC),
E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), M. Stamatikos (OSU), J. Tueller (GSFC),
(i.e. the Swift-BAT team):

Using the data set from T-130 to T+963 sec from the recent telemetry 
downlink,
we report further analysis of BAT GRB 100728A (trigger #430151)
(Cannizzo, et al., GCN Circ. 11004
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 ).  The BAT ground-calculated position is
RA, Dec = 88.753, -15.259 deg which is
  RA(J2000)  =  05h 55m 00.7s
  Dec(J2000) = -15d 15' 33.0"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 81%.

The BAT light curve is quite complex with a long series of peaks and flares
extending from approximately T-100 sec to T+750 sec.  Detected emission
began around T-100 sec, rising to a first peak at T-40 sec.  The burst
continued as a series of at least ten overlapping peaks with the strongest
peak centered at T+30 sec.  There was a strong set of BAT peaks centered
at T+120 sec, corresponding with the second flare detected by the XRT
(Evans & Cannizzo, GCN Circ 11014
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 ).  This was followed by a series of 
weaker
flares out to T+750 sec, at times which appear to correspond to flares
detected by the XRT. The burst triggered just after the spacecraft settled
following a slew to a pre-planned target, so it is possible that there was
some very early emission (before T-120 sec) which was not detected. T90
(15-350 keV) is 198.5 +- 12.7 sec (estimated error including systematics).

The time-averaged spectrum from T-84.3 to T+334.0 sec is best fit by a
simple power-law model.  The power law index of the time-averaged spectrum
is 1.18 +- 0.02.  The fluence in the 15-150 keV band is 3.8 +- 0.0 x 10^-5
erg/cm2. The 1-sec peak photon flux measured from T+28.26 sec in the 15-150
keV band is 5.1 +- 0.2 ph/cm^2/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/430151/BA/

F. Olivares, T. Kruehler, R. Filgas, J. Greiner, M. Nardini (all MPE
Garching) and A. Nicuesa (TLS Tautenburg) report on behalf of the GROND
team:

We observed the field of GRB 100728A (Swift trigger 430151; Cannizzo et
al., GCN #11004
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 ; A. von Kienlin, GCN #11006
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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 MPI/ESO telescope at La Silla Observatory (Chile).

Observations started at 09:26 UT, 7 hours after the GRB trigger and were
terminated at 10:32 UT by the GRB 100728B trigger. They
were performed at an average seeing of 1.3" and at an average airmass of
2.5.

We found a single, very faint point source in the H band within the
enhanced Swift-XRT error circle reported by Beardmore et al. (GCN #11005
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 ) at

RA (J2000.0) = 05:55:02.01
DEC (J2000.0) = -15:15:20.4

with an uncertainty of 0.5" in each coordinate.

Based on 24 min of total exposures in g'r'i'z' JHK, we
estimate preliminary magnitudes and upper limits (all in AB system) of

g' > 23.3
r' > 23.9
i' > 23.5
z' > 23.5
J > 22.0
H = 21.2 +- 0.3
K > 20.3

Given magnitudes and upper limits are obtained by calibrating the images
against GROND zeropoints as well as 2MASS field stars and are not
corrected for the expected Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.18 mag in the direction of the burst
(Schlegel et al. 1998).

These magnitudes imply very red colors of the afterglow of J-H > 0.8
and r-H > 2.7. Together with the significant excess absorption seen by
the XRT (Evans & Cannizzo, GCN 11014
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 ), and the lack of prompt detections
in the optical and UV filters (Oates & Cannizzo, GCN 11016
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 ), these values
are suggestive of a highly extinguished optical/NIR afterglow. The K band
upper limit is not constraining even for the steepest SED slopes and
consistent with this hypothesis.

S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf
of the Konus-Wind team, report:

The long GRB 100728A
(Swift/BAT trigger 430151: Canizzo et.al, GCN 11004
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 )
triggered Konus-Wind at T0=8300.008s UT (02:18:20.008)

The burst light curve shows a complex structure with
a total duration of ~200 s. There is a hint of a weaker
emission lasting at least to T0+290 s.
The emission is seen up to several MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB100728_T08300/

As observed by Konus-Wind the burst
had a fluence of 1.95(+/-0.35)x10-4 erg/cm2,
and a 256-ms peak flux, measured from T0+27.392s,
of 4.2(+/-0.7)x10-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+137.216 s) is best fit
in the 20 keV - 10 MeV range by the GRB (Band)
model, for which:
the low-energy photon index alpha = -0.65 (-0.09, +0.09),
the high energy photon index beta = -2.5 (-0.3, +0.2),
the peak energy Ep = 305(-25, +27)keV (chi2 = 88/84 dof).

The spectrum at the maximum count rate
(measured from T0+24.832 to T0+31.488 s) is best fit
in the 20 keV - 10 MeV range by the GRB (Band)
model, for which:
the low-energy photon index alpha = -0.47 (+/-0.15),
the high energy photon index beta = -2.6 (-0.3, +0.2),
the peak energy Ep = 390(-48, +63)keV (chi2 = 84/84 dof).

All the quoted results are preliminary.
All the quoted errors are at the 90% confidence level.

P. Tsai, Y. Urata, H.M. Lin (NCU), T. Uehara, Y. Hanabata, T. Takahashi, Y.
Fukazawa (Hiroshima U.), N. Vasquez (Tokyo Tech.) K. Yamaoka (Aoyama Gakuin U.),
S. Sugita (Nagoya U.), Y. Terada, M. Tashiro, W. Iwakiri, K. Takahara, 
T. Yasuda (Saitama U.), M. Ohno, M. Suzuki, M. Kokubun, 
T. Takahashi (ISAS/JAXA), Y. E. Nakagawa, T. Tamagawa (RIKEN), 
N. Ohmori, A. Daikyuji, Y. Nishioka, M. Yamauchi (Univ. of Miyazaki), 
K. Nakazawa, K. Makishima (Univ. of Tokyo),
S. Hong (Nihon U.), on behalf of the Suzaku WAM team, report:


The long GRB 100728A (Fermi/GBM trigger #301976252; von Kienlin et al., GCN
#11006
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 ; Swift/BAT trigger #430151; Cannizzo et al., GCN 11004
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 ) triggered the
Suzaku Wide-band  All-sky Monitor (WAM) which covers an energy range of 50
keV - 5 MeV at 2010-07-28 02:17:32.68 UT (=T0).

The observed light curve shows a multi-peaked structure starting at T0-5s,
ending at T0+198s, with a duration (T90) of about 162 seconds. The fluence
in 100 - 1000 keV was 9.09 (-1.55, +0.07) x 10^-5 ergs/cm^2. The 1-s peak
flux measured from T0+80s was 4.66 (-0.78, +0.03) photons/cm^2/s in the same
energy range.

Preliminary result shows that the time-averaged spectrum from
T0-2s to T0+180s is well fitted by a power-law with exponential cutoff
model:
  dN/dE ~  E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
  alpha       0.76 (+/-0.11), and
  Epeak       378 (-14, +13) keV (chi^2/d.o.f. = 34.4/22).

All the quoted errors are at statistical 90% confidence level.

The light curves for this burst are available at:

http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html

T. Kruehler (DARK), J. Greiner (MPE) and D. A. Kann (TLS) report:

We performed deep observations of the field of GRB 100728A
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405). GRB 100728A triggered Swift (Cannizzo et al.,
GCN 11004
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 ), GBM (von Kienlin, GCN 11006
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 ), Konus-Wind (Golenetskii
et al., GCN 11021
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 ) and Suzaku WAM (Tsai et al., GCN 11037
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 ).

Our late observations were performed on 2010-11-10, 105 days
after the trigger, and consisted of dithered images with a total of
4427 s exposure in g'r'i'z' and 3600 s in JHK under clear sky
conditions and a seeing of 0.8".

Inside the enhanced X-ray error-circle
(http://www.swift.ac.uk/xrt_positions), we detect an extended
object with an r'-band AB magnitude of r' = 25.0 +/- 0.2.

Its barycentric coordinates are:

RA (J2000) = 05:55:01.98
Dec. (J2000) = -15:15:19.7

with uncertainties of 0.3" in each coordinate.
Registering the early afterglow images (Olivares et al., GCN 11020
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 )
against the late host frame, we derive an offset between galaxy
center and afterglow of approximately 0.4".

A spectrum of this galaxy was taken on 2012-11-15 with the ESO VLT
UT2 equipped with the X-shooter spectrograph, covering the
wavelength range 3000-25000 AA. The seeing was 1.2" and the
total exposure of our spectrum was 2400 s. In the VIS and NIR arm,
we detect emission lines, which we interpret as being due to
[O II](3729) and Halpha at a common redshift of z = 1.567.

The bright X-ray emission (Evans et al., GCN 11014
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 ), coupled with the
faint and red optical/NIR afterglow (Olivares et al., GCN 11020
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 ) is
characteristic of dust-extinguished events. Fitting the available
afterglow data with synchrotron models, the visual extinction along
the GRB sightline is constrained to 1.5 mag < A_V < 3 mag at z = 1.567.