GRB 151023A

P. D'Avanzo (INAF-OAB), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
C. Pagani (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. G. R. Roegiers (PSU),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 13:43:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151023A (trigger=661253).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 271.019, -8.323, which is 
   RA(J2000)  =  18h 04m 05s
   Dec(J2000) = -08d 19' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 15 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 13:44:19.5 UT, 74.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 270.9847, -8.3165 which
is equivalent to:
   RA(J2000)  = 18h 03m 56.33s
   Dec(J2000) = -08d 18' 59.4"
with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 124 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 4.10
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 796 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 1.42. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo 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.)

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

Using 1446 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 151023A, 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 = 270.98458, -8.31640 which is equivalent
to:

RA (J2000): 18h 03m 56.30s
Dec (J2000): -08d 18' 59.0"

with an uncertainty of 2.3 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.

F. Knust (MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner 
(MPE Garching) report on behalf of the GROND team: 

We observed the field of GRB 151023A (Swift trigger 661253; D'Avanzo
et al., GCN #18450
Loading...
 
 ) simultaneously in g'r'i'z'JHK with GROND (Greiner
et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO
La Silla Observatory (Chile). 
Observations started at 23:52 UT on Oct. 23rd 2015, 10 hrs after the
GRB trigger, and are continuing. They were performed at an average
seeing of 1.6" and at an average airmass of 1.6  
We found a single point source within the 2.3" Swift-XRT error circle
reported by Evans et al. (GCN #18459
Loading...
 
 ) at 

RA (J2000.0) = 18:03:56.244 
Dec. (J2000.0) = -08:18:57.42

with an uncertainty of 0.2" in each coordinate. 
Based on the first 4.4 min of total exposures in g'r'i'z' and 4.0 min
in JHK, we estimate preliminary magnitudes (all in AB system) of 

g'> 22.1 mag,
r' = 20.2 +/- 0.1 mag,
i' = 19.1 +/- 0.1 mag,
z' =18.4 +/- 0.1 mag,
J = 17.6 +/- 0.2 mag,
H = 17.0 +/- 0.1 mag, and
K = 17.3 +/- 0.2 mag.

Given magnitudes 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)=1.38
mag in the direction of the burst (Schlegel et al. 1998).         		 	   		  

[GCN OPS NOTE(24oct15):  The suffix "A" was added to the Subject-line.]

M. H. Siegel (PSU), B. G. Balzer (PSU) and P. D'Avanzo (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151023A
796 s after the BAT trigger (D'Avanzo et al., GCN Circ. 18450
Loading...
 
 ).
No optical afterglow consistent with the XRT position
(D'Avanzo et al. GCN Circ. 18450
Loading...
 
 ) 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           796          946          147         >19.6
white              796         2140          411         >20.0
v                 1130         2191          136         >18.5
b                 1229         2116          117         >19.3
w1                1354         1374           19         >18.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.42 in the direction of the burst
(Schlegel et al. 1998).

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. Melandri
(INAF-OAB), V. D'Elia (ASDC), D.N. Burrows (PSU), T.G.R. Roegiers
(PSU), L.M. McCauley (PSU), C. Pagani (U. Leicester) and P. D'Avanzo
report on behalf of the Swift-XRT team:

We have analysed 7.4 ks of XRT data for GRB 151023A (D'Avanzo et al.
GCN Circ. 18450
Loading...
 
 ), from 756 s to 35.7 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. 18459
Loading...
 
 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.32 (+0.27, -0.21).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.7 (+1.1, -0.8). The
best-fitting absorption column is  2.7 (+1.8, -1.4) x 10^22 cm^-2, in
excess of the Galactic value of 4.1 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.9 x 10^-11 (2.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+1.8, -1.4) x 10^22 cm^-2
Galactic foreground: 4.1 x 10^21 cm^-2
Excess significance: 2.7 sigma
Photon index:	     2.7 (+1.1, -0.8)

If the light curve continues to decay with a power-law decay index of
1.32, the count rate at T+24 hours will be 2.5 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-14 (6.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00661253.

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

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (GSFC/STScI), 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), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report:

We observed the field of GRB 151023A (D'Avanzo, et al., GCN 18450
Loading...
 
 ) 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 2015/10 24.09 to 2015/10 24.14 UTC (12.35 to 13.66 hours after the BAT trigger), obtaining a total of 1.07 hours exposure in the r, i, and z bands.

We detect the GROND source (Knust, et al., GCN 18461
Loading...
 
 ) at a similar brightness level.  In comparison with the USNO-B1 catalog, we find:

   r 20.27 +/- 0.04
   i 19.10 +/- 0.02
   z 18.29 +/- 0.09

These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB.

We note that this source is present in the DSS at a comparable brightness level and may not, therefore, be the GRB afterglow.

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

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),  J. R. Cummings (GSFC/UMBC),
P. D'Avanzo (INAF-OAB), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL) (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 151023A (trigger #661253)
(D'Avanzo, et al., GCN Circ. 18450
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 271.012, -8.289 deg which is
  RA(J2000)  =  18h 04m 02.9s
  Dec(J2000) = -08d 17' 19.7"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 83%.

The mask-weighted light curve shows a single-pulse structure that starts at ~ T-1 s,
peaks at ~T0, and ends at ~ T+10 s. T90 (15-350 keV) is 10.66 +- 4.41 sec
(estimated error including systematics).

The time-averaged spectrum from T-0.65 to T+12.16 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.24 +- 0.24.  The fluence in the 15-150 keV band is 2.6 +- 0.4 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.03 sec in the 15-150 keV band
is 1.1 +- 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/661253/BA/