GRB 131018A

A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
P. D\'Avanzo (INAF-OAB), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), S. T. Holland (STScI), J. A. Kennea (PSU),
A. Y. Lien (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), C. Pagani (U Leicester),
K. L. Page (U Leicester), P. Romano (INAF-IASFPA), M. H. Siegel (PSU)
and R. L. C. Starling (U Leicester) report on behalf of the Swift
Team:

At 12:47:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131018A (trigger=574935).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 98.462, -19.882 which is 
   RA(J2000) = 06h 33m 51s
   Dec(J2000) = -19d 52' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a broad single peak
structure with a duration of about 100 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~56 sec after the trigger. 

The XRT began observing the field at 12:49:41.4 UT, 112.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
98.4716, -19.8961 which is equivalent to:
   RA(J2000)  = 06h 33m 53.19s
   Dec(J2000) = -19d 53' 45.9"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 60 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 2.03
x 10^21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 4.45e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 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
0.26. 

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.)

Arne Henden (AAVSO), as part of a larger collaboration, reports:

We observed the field of GRB131018A using the Mt. John University Observatory 
Optical Craftsman 61cm telescope (part of AAVSOnet), with observations starting 
at 12:55:24 UT (7.6 minutes after the Swift trigger) under poor seeing 
conditions and bright moonlight.  We do not see any optical afterglow at the XRT 
position to a limiting magnitude of R=17 (using APASS calibrations).

[GCN OPS NOTE(18oct13): The "A' suffix was added to the GRB name.]

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

Using 1872 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 131018A, 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 = 98.47133, -19.89620 which is equivalent
to:

RA (J2000): 06h 33m 53.12s
Dec (J2000): -19d 53' 46.3"

with an uncertainty of 1.9 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. B. Cenko (NASA/GSFC) and D. A. Perley (Caltech) report on behalf of a
larger collaboration:

We have observed the location of the Swift GRB131018A (Melandri et al.,
GCN 15349
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 ) with the robotic Palomar 60 inch telescope.  Two 60 s frames
were obtained before the telescope closed due to the imminent sunrise.  We
find no new sources within the enhanced XRT error circle (Goad et al., GCN
15352
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 ).  Using nearby point sources from SDSS for photometric calibration,
we the following upper limits:

r' > 20.1 mag at 12:56 UT (dt = 8.6 min)
i' > 19.5 mag at 12:58 (dt = 10.0 min)

T. N. Ukwatta (MSU), 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), A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (GSFC),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sakamoto (AGU),
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-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 131018A (trigger #574935)
(Melandri, et al., GCN Circ. 15349
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 ).  The BAT ground-calculated position is
RA, Dec = 98.473, -19.897 deg which is
  RA(J2000)  =  06h 33m 53.4s
  Dec(J2000) = -19d 53' 48.5"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a single long peak starts at ~T+21 sec
and ends at ~T+118 sec. T90 (15-350 keV) is 73.22 +- 18.97 sec
(estimated error including systematics).

The time-averaged spectrum from T+21.10 to T+117.74 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.24 +- 0.14.  The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+59.41 sec in the 15-150 keV band
is 0.5 +- 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/574935/BA/

WeiKang Zheng, Alexei V. Filippenko, Adam Morgan (UC Berkeley), and
S. B. Cenko (NASA/Goddard Space Flight Center) report on behalf of the
KAIT GRB team:

The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, responded to GRB 131018A (Melandri et al., GCN 15349
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 )
starting at 12:50:47 UT, 179 s after the burst. Observations were
performed with an automatic sequence in the V, I, and clear(roughly R)
filters, and the exposure time was 20 s per image. Comparing to the SDSS
image, we do not detect new sources in our single images or in coadds of
5 images within the enhanced XRT error circle (Goad et al., GCN 15352
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 ).
There is a faint stellar object (r = 22.2 mag) in the SDSS image 1.8"
from the enhanced XRT position, but it is beyond our limiting magnitudes
of R ~ 18.7 for single images and R ~ 19.6 for 5 coadded images.

V. D'Elia (ASDC), A. Maselli  (INAF-IASFPA), J.A. Kennea (PSU), V.
Mangano (PSU), M.C. Stroh (PSU), A.P. Beardmore (U. Leicester), P.A.
Evans (U. Leicester), B.P. Gompertz (U. Leicester) and A. Melandri
report on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for GRB 131018A (Melandri  et al.
GCN Circ. 15349
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 ),  from 119 s to 25.2 ks after the  BAT trigger. The
data comprise 171 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN. Circ 15352).

The light curve can be modelled with an initial power-law decay with an
index of alpha=4.24 (+0.17, -0.15), followed by a break at T+549 s to
an alpha of 0.30 (+/-0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.06 (+0.13, -0.12). The
best-fitting absorption column is  4.3 (+/-0.3) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.16 (+0.18, -0.17)
and a best-fitting absorption column of 3.1 (+/-0.6) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.8 x 10^-11 (6.7 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.1 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 2.0 x 10^21 cm^-2
Excess significance: 2.9 sigma
Photon index:	     2.16 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
0.30, the count rate at T+24 hours will be 0.040 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-12 (2.7 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/00574935.

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

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 131018A
99 s after the BAT trigger (Melandri et al., GCN Circ. 15349
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 ).
No optical afterglow consistent with the XRT position
(Goad et al. GCN Circ. 15352
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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

white_FC           120          270          147         >21.3
u_FC               279          528          246         >20.5
white              120        13702         1588         >22.4
v                   99         7600          581         >20.3
b                  534        19468         2174         >22.0
u                  279        18703         2368         >21.9
w1                 657        17790         1382         >21.3
m2                 632         7805          568         >20.5
w2                 584         7395          568         >20.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.26 in the direction of the burst
(Schlegel et al. 1998).

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB),
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara
(ORAU/GSFC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús
González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and
Harvey Moseley (GSFC) report:

We observed the field of GRB 131018A (Melandri, et al., GCN 15349
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 ) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the
1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional on
Sierra San Pedro Mártir from 2013/10 19.34 to 2013/10 19.53 UTC (19.36 to
23.83 hours after the BAT trigger), obtaining a total of 2.58 hours
exposure in the r and i bands and 1.09 hours exposure in the Z, Y, J, and H
bands.

We detect no new sources within the enhanced Swift-XRT error circle (Goad,
et al., GCN 15352
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 ).  In comparison with the SDSS DR9 and 2MASS, we obtain
the following upper limits (3-sigma):

  r > 23.30
  i > 23.18
  Z > 22.43
  Y > 22.05
  J > 21.70
  H > 21.27

The SDSS source present in the XRT error circle (see also, Zheng, et al.,
GCN 15355
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 ) is well-detected, apparently non-extended, and is measured to
have the following magnitudes:

  r     22.06 +/- 0.11
  i     20.73 +/- 0.05
  Z     20.10 +/- 0.05
  Y     19.63 +/- 0.05
  J     19.64 +/- 0.06
  H     19.18 +/- 0.06

The above magnitude are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.