GRB 131103A

J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC),
F. E. Marshall (NASA/GSFC), C. Pagani (U Leicester),
K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of
the Swift Team:

At 22:07:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131103A (trigger=576562).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 348.954, -44.670 which is 
   RA(J2000) = 23h 15m 49s
   Dec(J2000) = -44d 40' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 20 sec with possible
emission on a longer timescale.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:08:42.0 UT, 76.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 348.92161, -44.63816 which
is equivalent to:
   RA(J2000)  = 23h 15m 41.19s
   Dec(J2000) = -44d 38' 17.4"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 141 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 in excess of the Galactic value (1.11 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 4
(+2.09/-1.83) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.31e-09 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
0.01. 

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.N. Burrows (PSU), J.A. Kennea (PSU), V. Mangano (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester), B.
Sbarufatti (INAF-OAB/PSU), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC) and
J.R. Cummings report on behalf of the Swift-XRT team:

We have analysed 9.1 ks of XRT data for GRB 131103A (Cummings  et al.
GCN Circ. 15440
Loading...
 
 ),  from 82 s to 25.5 ks after the  BAT trigger. The
data comprise 793 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
348.9213, -44.6377 which is equivalent to:

RA (J2000): 23 15 41.11
Dec(J2000): -44 38 15.7

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The late-time light curve (from T0+5.9 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.16 (+/-0.26).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.76 (+0.27, -0.25). The
best-fitting absorption column is  4.0 (+0.8, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 1.1 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.15 (+0.18, -0.17)
and a best-fitting absorption column of 3.5 (+0.7, -0.6) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.0 x 10^-11 (7.2 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.5 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^20 cm^-2
Excess significance: 8.9 sigma
Photon index:	     2.15 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
1.16, the count rate at T+24 hours will be 6.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-13 (4.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00576562.

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

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

Using 7277 s of XRT Photon Counting mode data and 8 UVOT
images for GRB 131103A, 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 = 348.91855, -44.64060 which is equivalent
to:

RA (J2000): 23h 15m 40.45s
Dec (J2000): -44d 38' 26.1"

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.

N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), 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 131103A
87 s after the BAT trigger (Cummings et al., GCN Circ. 15440
Loading...
 
 ).
A decaying source consistent with the XRT position  (Goad et al. GCN Circ. 15442
Loading...
 
 )
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  23:15:40.55 = 348.91894 (deg.)
    Dec (J2000) = -44:38:25.4  = -44.64040 (deg.)
with an estimated uncertainty of 0.60 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

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

white               87          237          147         19.41 +/- 0.13
white            19290        19893          599         21.45 +/- 0.17
v                  629        12543         1173        >20.0
b                  554        25509         1664         21.64 +/- 0.23
u                  299          549          246         18.93 +/- 0.13
w1                 678        24073         1116        >20.6
w2                6741         6941          197        >19.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

Z. Cano (U Iceland), S. Schulze (PUC, MCSS), D. Malesani (DARK/NBI), P. Jakobsson (U Iceland), D. J. Watson (DARK/NBI), N. R. Tanvir (U Leicester), D. Xu, J. P. U. Fynbo (DARK/NBI) and M. Hayes (Stockholm University) report on behalf of a larger collaboration:

We observed the field of GRB 131103A (Cummings et al., GCN Circ. 15440
Loading...
 
 ) with the 2.5-m Nordic Optical Telescope (NOT) equipped with ALFOSC, and subsequently with the ESO VLT using the optical acquisition camera of the X-shooter spectrograph, as well as with the HAWK-I near-infrared imager. A brief summary of our observations is reported below. For the NOT observation, the object was very low on the horizon (airmass = 4.56), hence we could only achieve a poor S/N.

At the position of the UVOT afterglow (Kuin et al., GCN Circ. 15443
Loading...
 
 ), a source is detected in all our images. It is clearly elongated in both R and J as seen in the VLT data, and we thus suggest that this is the host galaxy, though some afterglow contribution cannot be excluded. We note that a marginally significant object is visible in the DSS (R band) at the same position.

Instrument    Time (hr)  Filter  Magnitude
NOT/ALFOSC    1.29       R       20.07 +/- 0.12
VLT/X-shooter 4.48       R       20.48 +/- 0.01
VLT/HAWK-I    5.94       J       19.63 +/- 0.02

While the comparison of the two R-band magnitudes provides some evidence for fading, we note that both R-band measurements have an uncertain calibration, since no common star is available between the ALFOSC and X-shooter images. The two measurements are therefore tied to different USNO stars, which can introduce an error of ~0.1-0.2 mag. Specifically, we used the R1 magnitudes of stars 0453-0803362 and 0453-0803335 for ALFOSC and X-shooter, respectively. The HAWK-I observation was calibrated against five nearby 2MASS stars.


We kindly acknowledge excellent support by the observing staff at the VLT, in particular Claudia Reyes, Dimitri Gadotti, Julien Girard and Roger Wesson.

[GCN OPS NOTE(12nov13):  Per author's request, the observing time for the VLT/X-shooter
was changed from 5.84 to 4.48.]

D. Xu, D. Malesani (DARK/NBI), S. Schulze (PUC, MCSS), A. de Ugarte
Postigo (IAA-CSIC, DARK/NBI), P. Jakobsson, Z. Cano(U Iceland), N. R.
Tanvir (U. Leicester), J. P. U. Fynbo, D. Watson (DARK/NBI), P.
Goldoni (APC, CEA/Irfu), S. Vergani (CNRS/GEPI), R. Wijers (U
Amsterdam) report on behalf of the X-shooter GRB collaboration:

We have obtained further observations of the afterglow of GRB 131103A
(Cummings et al., GCN Circ. 15440
Loading...
 
 ; Cano et al., GCN 15445
Loading...
 
 ) using the
ESO VLT equipped with the X-shooter spectrograph, covering the
wavelength range 3000-20000 AA.

The spectrum was secured on Nov. 5.2 with a total exposure time of
4x600 s. We detect a rich emission line spectrum from the underlying
host galaxy at a common redshift z = 0.5955. We also detect strong
absorption lines from FeII and
MgII at the same redshift in the blue part of the spectrum implying
that there is significant afterglow light superimposed on the host
light.

We acknowledge excellent support from the observing staff at Paranal,
in particular Dimitri Gadotti, Roger Wesson, and Claudia Reyes.

[GCN OPS NOTE(05nov13):  Per author's request, the typo in the redshift value
was changed from 0.599 to 0.5955.]

S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (NASA/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+243 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 131103A (trigger #576562)
(Cummings, et al., GCN Circ. 15440
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 348.948, -44.645 deg which is 
   RA(J2000)  =  23h 15m 47.5s 
   Dec(J2000) = -44d 38' 42.3" 
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 30%.
 
The mask-weighted light curve shows a 2 or 3 overlapping peaks starting
at ~T-8 sec, peaking at ~T+0 sec, and ending at ~T+15 sec.
T90 (15-350 keV) is 17.3 +- 4.8 sec (estimated error including systematics).
 
The time-averaged spectrum from T-9.5 to T+13.5 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.97 +- 0.19.  The fluence in the 15-150 keV band is 8.2 +- 1.0 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.62 sec in the 15-150 keV band
is 1.5 +- 0.3 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/576562/BA/