GRB 130807A

A. Melandri (INAF-OAB), D. N. Burrows (PSU),
P. A. Evans (U Leicester), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC) and R. L. C. Starling (U Leicester)
report on behalf of the Swift Team:

At 10:25:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130807A (trigger=565651).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 269.801, -27.613 which is 
   RA(J2000) = 17h 59m 12s
   Dec(J2000) = -27d 36' 44"
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 at least 30 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 10:27:08.2 UT, 85.0 seconds after
the BAT trigger. Although a bright and variable source is apparent 
in the promptly downloaded light curve, the instrument did not
centroid on the source in the initial 2.5-s promptly downloaded
image, and the XRT stayed in WT mode for the remainder of the 
short first snapshot until Swift slewed away from the target. 
We therefore will have to wait until the second orbit to 
localize the XRT counterpart. 

UVOT took a finding chart exposure of 166 seconds with the U filter starting
145 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 25% of the
BAT 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 BAT
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. 

Due to the proximity of this source to the Galactic center 
(3.3 degrees from the center) it is more likely that this is a 
Galactic source rather than a cosmological GRB.  Determination
of the nature of this source will require the full downlinked data. 

Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.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.)

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (NASA/GSFC/ORAU),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (MSU) (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 130807A (trigger #565651)
(Melandri, et al., GCN Circ. 15082
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 ).  The BAT ground-calculated position is
RA, Dec = 269.801, -27.616 deg, which is
    RA(J2000)  =  17h 59m 12.4s
    Dec(J2000) = -27d 36' 56.7"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 92%.

Based on the currently available event data, the mask-weighted light 
curve shows an initial FRED-like peak starting at T-10 sec, peaking at 
T=0 sec and decaying to background by T+50 sec.  This is followed by at 
least two more weaker peaks, one from roughly T+90 sec to T+130 sec and 
the second beginning at T+160 sec and continuing until the data cuts off 
at T+243 seconds.  The two later peaks are coincident with flares seen 
in the Swift/XRT data.  We note also that the Swift satellite slewed 
away from the source location at ~T+300 seconds, while an XRT flare was 
still ongoing. Since we do not have all of the data, we cannot determine 
T90 at this time.

The time-averaged spectrum from T-12.18 to T+163.82 sec is best fit by a 
power law with an exponential cutoff.  This fit gives a photon index 
0.31 +- 0.89,
and Epeak of 75.9 +- 35.2 keV (chi squared 56.96 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.2 +- 0.2 x 10^-06 
erg/cm2
and the 1-sec peak flux measured from T-4.68 sec in the 15-150 keV band is
0.4 +- 0.1 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.46 +- 0.17 (chi squared 63.40 for 57 d.o.f.).  All the quoted errors
are at the 90% confidence level.

The current analysis of the BAT data does not allow us to distinguish 
between the interpretations of this event as either a GRB or galactic 
transient. These results will be updated as more event data becomes 
available.

Examination of archival images from the Swift/BAT transient monitor 
through 3:03 UT, August 6, 2013 (~31.5 hours before the trigger) shows 
no sign of earlier emission from the location of trigger 565651.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/565651/BA/

A. Melandri (INAF/OAB), D.N. Burrows (PSU), P. Romano (INAF-IASFPA),  
H.A. Krimm (GSFC/USRA), J.P. Osborne, P.A. Evans (U. Leicester), N.  
Gehrels (GSFC) report on behalf of the Swift team.


We have analysed 14.4 ks of XRT data for GRB 130807A (Melandri et al.  
GCN Circ. 15082
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 ), from 88 s to 80 ks after the  BAT trigger. The XRT  
position for this burst is RA, Dec = 269.8007, -27.6159 which is  
equivalent to:

RA (J2000): 17h 59m 12.16s
Dec (J2000): -27d 36' 57.5"

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

In the first orbit (WT mode) the source is initially detected at a  
rate of ~10 XRT count/s, rising to ~100 c/s at the end of the first  
orbit. During this period the spectrum can be fit by an absorbed power- 
law model with a photon index of 1.56 +/- 0.08. The source showed an  
absorption column of (8.6 +/- 1.0) x 10^21 cm^-2, slightly in excess  
with respect to the Galactic value of 6 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 8.0 x 10^-11 (1.2 x 10^-10) erg  
cm^-2 count^-1.

In the second orbit (PC mode) the source had become much fainter  
(~0.03 c/s), and much softer, with a photon index of 2.3 (+0.5,  
-0.4).The source still showed an absorption column (9.0 +/- 5.0 x  
10^21 cm^-2) consistent with the Galactic value. The counts to  
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from  
this spectrum  is 5.0 x 10^-11 (1.7 x 10^-10) erg cm^-2 count^-1.  
Starting with the second orbit, the light curve rose from a count rate  
of ~0.015 c/s after ~4 ks to a maximum of ~0.06 c/s at ~10 ks (with  
slope of -1.5 +/- 0.1). After that it decayed with a slope of 0.9 +/-  
0.1, reaching the count rate of ~0.012 c/s at ~75 ks.

The light curve behaviour (at late times) is unusual, but not  
unprecedented, for a GRB. The BAT and XRT spectral indices are similar  
to those observed for GRB 060124 (BAT spectrum gamma = 1.89 +/- 0.19,  
XRT-WT spectrum gamma = 1.40+/-0.01, XRT-PC spectrum gamma =  
2.1+/-0.1; Romano et al., 2006, A&A, 456, 917) and for the transient  
supermassive black hole Swift J1644+57 (Burrows et al., 2011, Nature,  
476, 421), but inconsistent with what generally observed for a  
Supergiant Fast X-ray Transient. The dynamical range of GRB 130807A  
(~3300) is similar to what observed for GRB 060124 or for a typical  
SFXT, but very different from what was observed for Swift J1644+57.

The source was not detected in the BAT transient monitor (15-50 keV)  
before 3:03 UT, August 6, 2013, the most recent monitor epoch  
currently available. Events like Swift J1644+57 or SFXTs are usually  
detected in X-rays for several days before the trigger, Swift J1644+57  
behaved similarly. It is therefore likely that BAT trigger 565651 was  
a somewhat unusual GRB rather than a Galactic or extragalactic  
transient, in spite of its proximity to the Galactic center.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/possible_GRBs/00565651/source1

M. De Pasquale (UCL-MSSL) and A. Melandri (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 130807A
145 s after the BAT trigger (Melandri et al., GCN Circ. 15082
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 ).
No optical source consistent with the XRT position (Melandri et al.,
GCN Circ. 15085
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 ) is detected in the 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 summed exposures are:

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

white_FC          3572         3722          147         >20.3
white             3572         5573          541         >21.1
v                 4348         5984          393         >19.3
b                 3732         5369          393         >20.3
u_FC               145          311          163         >19.7
u                  145         5164          360         >19.9
w1                4759         4959          197         >19.6
m2                4552         6078          284         >19.7
w2                4143         5780          393         >20.2

The magnitudes in the table are not corrected for the strong Galactic
extinction corresponing to the reddening of E(B-V) = 2.03 in the
direction of the burst (Schlegel et al. 1998).

M. De Pasquale (UCL-MSSL) reports on behalf of the Swift/UVOT team:

In the previous GCN Circ. 15086
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  "GRB130807A: Swift/UVOT observations",
the sentence "No optical source" should be changed into "No uncatalogued
optical source".

I apologize for the mistake.