GRB 151031A

T.Fujiwara, T. Yoshii, Y. Saito, Y. Tachibana, Y. Yano, 
Y.Ono, S.Harita, Y.Muraki, Y. Yatsu, and N. Kawai (Tokyo Tech)

report on behalf of the MITSuME collaboration:

We searched for the optical counterpart of GRB 151031A (E. Sonbas et al., GCN Circular #18538
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 ) with the 
optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm
telescope of Akeno Observatory, Yamanashi, Japan.

The observation started on 2015-10-31 16:18:19 UT (10.5 h after the burst).
We did not find any new point source within XRT circle in all three bands.

We obtained following limits for the magnitudes.

T0+[sec] MID-UT T-EXP[sec] g' Rc Ic
----------------------------------------------------------
37669  17:00:37  4440  > 18.8  > 18.9  > 18.3
----------------------------------------------------------
T0+ : Elapsed time after the burst 
T-EXP: Total Exposure time 
We used GSC2.3 catalog for flux calibration.

S. Schmidl (TLS Tautenburg), J. Graham, and J. Greiner (both MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 151031A (SWIFT trigger 662330; Sonbas et al, GCN 18538
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at ESO La Silla Observatory (Chile).

Observations started on November 1, 2015, at 06:42 UT, 1.04 days after the GRB trigger (due to being clouded out the previous night). They were performed at an average seeing of 1.52" and at an average airmass of 1.02.

We detect the source reported by Malesani et al. (GCN 18540
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 ) and Kuin et al. (GCN 18546
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 ).

Based on total exposures of 33 minutes in g'r'i'z' and 72 minutes in JHK, we measure the following preliminary magnitudes and upper limits (AB magnitude system):

g' = 22.1 +/- 0.1 mag,
r' = 21.9 +/- 0.1 mag,
i' = 21.7 +/- 0.1 mag,
z' = 21.5 +/- 0.1 mag,
J  = 21.6 +/- 0.3 mag,
H  = 20.7 +/- 0.3 mag, and
K  > 19.8 mag.

Given magnitudes and upper limits are calibrated 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) = 0.03 mag in the direction of the burst (Schlegel et al. 1998).

D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ),
S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech)
report on behalf of MITSuME and OISTER collaboration:

We observed the field of GRB 151031A (Sonbas et al., GCNC 18538)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.

The observation started on 2015-10-31 15:54:27 UT (10.1 h after
the burst). We could not detect the previously reported afterglow
(Malesani et al., GCNC 18540; Kuin and Sonbas, GCNC 18546) in
all the three bands.

Three sigma upper limits of the OT are listed below.
We used GSC2.3 catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'    Rc     Ic
-----------------------------------------------------
0.49617    17:44:59    6000.0   >18.6  >18.9  >18.5
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

N. P. M. Kuin (UCL-MSSL) and E. Sonbas (Adiyaman Univ.) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151031A
439 s after the BAT trigger (Sonbas et al., GCN Circ. 18538
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 ). A source
at the VLT position (Malesani et al, GCN Circ. 18540
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 ) and consistent with
the XRT position (Evans et al. GCN Circ. 18541
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 ) is detected in the initial
UVOT exposures.

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              439         1020          314         20.63 +/- 0.16
v                  595         8257          627        >20.1
b                  695         7642          588        >20.2
u                  670         7437          430         20.34 +/- 0.28
w1                 645         8677          635        >20.0
m2                 620         8462          627        >20.0
w2                 746         8053          607        >21.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.03 in the direction of the burst
(Schlegel et al. 1998).

A. Maselli  (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU), T.G.R. Roegiers (PSU), P.A. Evans (U. Leicester), S.L. Gibson
(U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and E. Sonbas report on behalf of the Swift-XRT
team:

We have analysed 7.5 ks of XRT data for GRB 151031A (Sonbas et al. GCN
Circ. 18538
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 ), from 443 s to 25.5 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Evans et al. (GCN Circ. 18541
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.56 (+/-0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.13 (+0.22, -0.21). The
best-fitting absorption column is  9.4 (+3.8, -3.3) x 10^21 cm^-2, at a
redshift of 1.167, in addition to the Galactic value of 2.6 x 10^20
cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum  is 3.2 x
10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.6 x 10^20 cm^-2
Intrinsic column:    9.4 (+3.8, -3.3) x 10^21 cm^-2 at z=1.167
Photon index:	     2.13 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
0.56, the count rate at T+24 hours will be 0.010 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.3 x
10^-13 (4.7 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/00662330.

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

T. N. Ukwatta (LANL),S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU),
E. Sonbas (Adiyaman Univ.), M. Stamatikos (OSU), (i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 151031A (trigger #662330)
(Sonbas, et al., GCN Circ. 18538
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 ).  The BAT ground-calculated position is
RA, Dec = 83.189, -39.125 deg which is
    RA(J2000)  =  05h 32m 45.3s
    Dec(J2000) = -39d 07' 28.3"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 67%.

The mask-weighted light curve shows a single FRED-shaped peak, peaking at T+2 sec
and returning to background by T+7 sec. T90 (15-350 keV) is 5.00 +- 2.24 sec
(estimated error including systematics).

The time-averaged spectrum from T+0.30 to T+6.30 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.41 +- 0.18.  The fluence in the 15-150 keV band is 3.2 +- 0.3 x 10^-7erg/cm2.
The 1-sec peak photon flux measured from T+1.30 sec in the 15-150 keV band
is 1.7 +- 0.2 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/662330/BA/

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

Using 436 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 151031A, 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 = 83.19596, -39.12134 which is equivalent
to:

RA (J2000): 05h 32m 47.03s
Dec (J2000): -39d 07' 16.8"

with an uncertainty of 2.2 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.

D. Malesani (DARK/NBI), N. R. Tanvir (Univ. Leicester), T. Kruehler 
(MPE), A. de Ugarte Postigo (IAA-CSIC and DARK/NBI), H. Flores 
(GEPI/Paris obs), D. Xu (NAOC/CAS), J.P.U. Fynbo (DARK/NBI), report on 
behalf of the X-shooter GRB collaboration:

We observed the field of GRB 151031A (Sonbas et al., GCN 18538
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 ) with the 
ESO Very Large Telescope UT2 (Kueyen) equipped with the X-shooter 
spectrograph. Observations were carried out using the automatic "rapid 
response mode" (RRM), and started immediately following the reception of 
the XRT position notice.

In the acquisition images, we detect a single source consistent with the 
position of the XRT afterglow, at coordinates (J2000):

RA = 05:32:47.11
Dec = -39:07:19.1

We measure for this object R = 20.4 +- 0.1 (calibrated against three 
nearby USNO stars) in an image taken on Oct 31.2565 UT, that is 18.8 min 
after the GRB.

A sequence of spectra with increasing exposure time (3 to 32 min) was 
secured covering the wavelength range 3,000 - 25,000 AA. From the 
analysis of the first 8 min data, several absorption features can be 
seen on top of the object continuum, which we interpret as due to Ca II, 
Mg I, Mg II and Fe II in absorption at a common redshift of z = 1.167. 
In the visible arm, an emission feature is also seen, corresponding to 
[O ii] at a consistent redshift, which we thus consider to be the 
redshift of GRB 151031A. Analysis of the full data set is ongoing.

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Thomas Rivinius, Marcela Espinoza, and Claudia Cid.

E. Sonbas (Adiyaman Univ.), V. D'Elia (ASDC), L. Izzo (URoma/ICRA),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 05:50:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151031A (trigger=662330).  Swift slewed to the burst after
a 6 minute delay due to viewing constraints. 
The BAT on-board calculated location is 
RA, Dec 83.199, -39.092 which is 
   RA(J2000) = 05h 32m 48s
   Dec(J2000) = -39d 05' 28"
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 10 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 05:57:43.9 UT, 433.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 83.19532, -39.12049 which is equivalent
to:
   RA(J2000)  = 05h 32m 46.88s
   Dec(J2000) = -39d 07' 13.8"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 103 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 2.62
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 439 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 none of
the XRT error circle. 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 0.03. 

Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.com). 
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.)