GRB 100727A

S. T. Holland (CRESST/GSFC/USRA) and
D. C. Morris (GWU/GSFC)
report on the behalf of the Swift UVOT team:

    The Swift/UVOT observed the field of GRB 100727A starting 47 s
after the BAT trigger (Morris, et al., 2010, GCN Circ. 10998
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 ).
Settled observations started at 65 s.  We do not find any new source,
relative to the DSS, USNO-B1.0, or 2MASS at the position of the XRT
afterglow (Osborne, et al. 2010, GCN Circ. 11000
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 ).  Preliminary
3-sigma upper limits for detecting a source in the finding charts, and
in the co-added images, using a 2.5 arcsecond radius circular
aperture, are

Filter      T_start    T_stop    Exp(s)      Mag
------------------------------------------------
white (fc)       65       215       147    >20.8
                856      1006       147    >21.2
    u (fc)      278       527       246    >20.5

    v           609    18,409      1414    >20.9
    b           533    24,918      1215    >21.7
    u           278    24,189      2335    >21.7
 uvw1           658    29,885      2289    >21.9
 uvm2           633    28,542      2390    >21.8
 uvw2           584    17,496      1415    >21.8
white            65      7075       805    >22.1
------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight corresponding to a
reddening of E_{B-V} = 0.05 mag (Schlegel, et al., 1998, ApJS, 500,
525).  All photometry is on the UVOT photometry system described in
Poole et al. (2008, MNRAS, 383, 627).

B. Sbarufatti (INAF OAB/IASF PA), V. Mangano (INAF/IASF PA)
and D. C. Morris (GWU/GSFC) report on behalf of the Swift-XRT
team:

We have analysed 9.8 ks of XRT data for GRB 100727A (Morris et al.
GCN Circ. 10998
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 ), from 48 s to 39.8 ks after the BAT trigger.
The data comprise 185 s in Windowed Timing (WT) mode (the first
7 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 Osborne et al. (GCN. Circ 11000). The light curve can
be modeled with a doubly broken power-law decay. The initial decay
index is alpha=2.6 (+/-0.1). A bright flare starts at T+190 s and
peaks at T+247 s. The light curve breaks at T+450 s entering a
plateau phase with alpha=0.0 (+0.06 -0.08). The second break is
observed at T+6.5 ks, followed by a decay index
alpha=0.64 (+0.12-0.09)

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.8 (+/-0.14). The
best-fitting absorption column is  4.2 (+/-0.3) x 10^21 cm^-2, in
excess of the Galactic value of 5.1 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.6 (+/-0.2)
and a best-fitting absorption column of 3.1 (+/-0.5) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 9.7 x 10^-11 (2.3 x 10^-10) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
0.64, the count rate at T+24 hours will be 2.8 x 10^-2 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of
2.7 x 10-12 (6.4 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/00430094.

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

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

"At 05:42:22.00 UT on 27 July 2010, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 100727A (trigger 301902143 / 100727238),
which was also detected by the SWIFT-BAT (Morris et al. 2010, GCN 10998
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 )
The GBM on-ground location is consistent with the Swift position.
 
The angle from the Fermi LAT boresight is 95.1 degrees.

The GBM light curve consists of single peak
with a duration (T90) of about 24 +/- 3 s (50-300 keV). 
The time-averaged spectrum from T0-6.144 s to T0+12.288 s is 
best fit by a simple power law function with 
index -1.71 +/- 0.04 (Castor C-STAT 785 for 473 d.o.f.).

The event fluence (8-1000 keV) in this time interval is 
(2.03 +/- 0.14)E-06 erg/cm^2. The 1.024-sec peak photon flux measured 
starting from T0-4.096 s in the 8-1000 keV band 
is  1.6 +/- 0.2  ph/s/cm^2.

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

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

Using the data set from T-240 to T+617 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 100727A (trigger #430094)
(Morris, et al., GCN Circ. 10998
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 ).  The BAT ground-calculated position is
RA, Dec = 154.187, -21.417 deg which is
    RA(J2000)  =  10h 16m 44.8s
    Dec(J2000) = -21d 25' 02.7"
with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 98%.

The burst began with weak precursor activity starting at T-80 sec
and peaking at T-40 sec.  The main pulse, a mostly featureless
FRED profile, began at T-2 sec and continued to approximately
T+50 sec. T90 (15-350 keV) is 84 +- 14 sec (estimated error
including systematics).

The time-averaged spectrum from T-82.0 to T+29.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.81 +- 0.13.  The fluence in the 15-150 keV band is 1.2 +- 0.1 x 10-6 erg/cm2.
The 1-sec peak photon flux measured from T+1.33 sec in the 15-150 keV band
is 1.2 +- 0.1 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/430094/BA/

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

Using 4198 s of XRT Photon Counting mode data and 8 UVOT
images for GRB 100727A, 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 = 154.17739, -21.39042 which is equivalent
to:

RA (J2000): 10h 16m 42.57s
Dec (J2000): -21d 23' 25.5"

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

S. Kobara, K. Omori, T. Sumi, D. Suzuki, H.Naito and T. Sako (STE Lab. , Nagoya Univ.)
on behalf of the MOA Collaboration report :

We searched for an optical afterglow of GRB100727A (GCN 10998
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 , D. C. Morris et al.)
starting from 07:12:3 UT on 2010 July 27 ( 90 minutes after the burst) 
with the MOA-II 1.8m telescope at Mt.John observatory in New Zealand.
In a single image of a 300 sec exposure with a wideband Red filter (center 
wavelength ~ 750nm and FWHM ~ 250nm), we did not find any object
within the error circle of the Swift XRT source position (GCN 10998
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 , D. C. Morris et al.
).
A 3 sigma upper limit is set in the I magnitude at 21.6 mag.   

This photometry was done by using the DoPhot and calibrated against the 
USNO-B1.0 catalog stars, and not corrected for the Galactic extinction.

D. C. Morris (GWU/GSFC), M. M. Chester (PSU), J. M. Gelbord (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASFPA),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 05:42:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100727A (trigger=430094).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 154.166, -21.364 which is 
   RA(J2000) = 10h 16m 40s
   Dec(J2000) = -21d 21' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 50 sec, with possible additional
activity around T+175.  The peak count rate
was ~3300 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 05:43:14.4 UT, 56.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 154.17823, -21.39002 which
is equivalent to:
   RA(J2000)  = 10h 16m 42.78s
   Dec(J2000) = -21d 23' 24.1"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 100 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.  We cannot determine whether the source is
fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
5.09e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.29e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 65 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. 
Data from the list of sources generated on-board are not available at this
time. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.05. 

Burst Advocate for this burst is D. C. Morris (david.c.morris AT 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.)