GRB 181126A

E. Sonbas (Adiyaman Univ.), S. D. Barthelmy (GSFC),
P. D'Avanzo (INAF-OAB), N. P. M. Kuin (UCL-MSSL),
A. Melandri (INAF-OAB), K. L. Page (U Leicester) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 09:54:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181126A (trigger=873539).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 152.351, -29.679, which is 
   RA(J2000)  =  10h 09m 24s
   Dec(J2000) = -29d 40' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows two peaks
with a total duration of about 2 sec.  The peak count rate
was ~5500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 09:55:23.1 UT, 75.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 152.34775, -29.68611 which is
equivalent to:
   RA(J2000)  = 10h 09m 23.46s
   Dec(J2000) = -29d 41' 10.0"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 27 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 79 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
0.07. 

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.)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 181126A, we find an
enhanced XRT position of the afterglow: RA, Dec: 152.3477, -29.6875
which is equivalent to:
   RA (J2000)  = 10 09 23.46
   Dec (J2000) = -29 41 15.0
with an uncertainty of 2.5 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/873539.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

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

Using 804 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 181126A, 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 = 152.34848, -29.68747 which is equivalent
to:

RA (J2000): 10h 09m 23.64s
Dec (J2000): -29d 41' 14.9"

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.

E. Bissaldi (Politecnico & INFN Bari)
reports on behalf of the Fermi GBM Team:


"At 09:54:09.94 UT on 26 November 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 181126A (trigger 564918854/ 181126413),
which was also detected by the Swift-BAT (Sonbas et al. 2018, GCN 23453
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 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi-LAT boresight
at the GBM trigger time is 111 degrees.

The GBM light curve consists of bright single pulse
with a duration (T90) of about 0.6 s (50-300 keV).
The time-averaged spectrum from T0-0.1 s to T0+0.4 s is
adequately fit by a Band function with Epeak = 90 +/- 25 keV,
alpha = -0.1 +/- 0.6, and beta = -2.1 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(4.8 +/- 0.5)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0-64 ms in the 10-1000 keV band
is 10.5 +/- 1.7 ph/s/cm^2.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

N. R. Tanvir (U. Leicester) and W. Fong (Northwestern Univ.)
report on behalf of a larger collaboration:

We observed the field of Swift GRB 181126A (Sonbas et al.
GCN 23453
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 ) with the Gemini North telescope on Mauna Kea,
beginning 2018-11-26 14:11 UT, approximately 4.3 hr post-burst.
Images were obtained in the z filter using GMOS-N with total
exposure of 15x60s.

No credible afterglow is visible at the revised X-ray position
(Beardmore et al. GCN 23455
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 ) to a 3-sigma magnitude z(AB)=25.4,
uncorrected for Galactic extinction.

We thank Mirko Simunovic and Christy Cunningham for
efficiently executing these observations.

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A. Kennea
(PSU) and E. Sonbas report on behalf of the Swift-XRT team:

We have analysed 9.3 ks of XRT data for GRB 181126A (Sonbas et al. GCN
Circ. 23453
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 ), from 82 s to 29.9 ks after the  BAT trigger. The data are
entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Beardmore et al. (GCN Circ. 23454
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.95 (+/-0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.7 (+0.8, -0.6). The
best-fitting absorption column is  9.7 (+6.5, -4.2) x 10^21 cm^-2, in
excess of the Galactic value of 8.1 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 3.3 x 10^-11 (1.2 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.7 (+6.5, -4.2) x 10^21 cm^-2
Galactic foreground: 8.1 x 10^20 cm^-2
Excess significance: 3.4 sigma
Photon index:	     2.7 (+0.8, -0.6)

If the light curve continues to decay with a power-law decay index of
0.95, the count rate at T+24 hours will be 9.5 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.2 x
10^-14 (1.1 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/00873539.

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

A. A. Breeveld (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 181126A
80s after the BAT trigger (Sonbas et al., GCN Circ. 23453
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 ).
No optical afterglow consistent with the XRT position (Beardmore et al.,
GCN Circ. 23455
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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            80          230          147         >20.4
white               80         1181          342         >20.5
v                  622         1069           58         >18.0
b                 1147         1167           20         >18.2
u                  292         1142          285         >20.1
w1                 671         1118           58         >20.2
m2                 646         1093           58         >19.0
w2                 597         1044           58         >18.4

The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.07 in the direction of the
burst (Schlegel et al. 1998).

K. De (Caltech) reports on behalf of a larger collaboration

At 2018-11-26 14:13 UT, 4.32 hours after GRB 181126A (GCN 23453
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 ), we observed the location of the XRT counterpart (GCN 23455
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 ) with the MOSFIRE instrument on the Keck I telescope. With a total exposure time of 416 seconds, no afterglow is detected to a 3-sigma Ks-band limit of 23.6 AB mag.

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), J. P. Norris (BSU),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), E. Sonbas (Adiyaman Univ.),
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 181126A (trigger #873539)
(Sonbas et al., GCN Circ. 23453
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 ).  The BAT ground-calculated position is
RA, Dec = 152.350, -29.698 deg which is
  RA(J2000)  =  10h 09m 24.0s
  Dec(J2000) = -29d 41' 52.9"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 68%.

The mask-weighted light curve shows two short pulses. The first pulse starts
at ~ T-0.1 s, peaks at ~T0, and ends at ~T+0.2 s. The second pulse starts
at ~T+1.6 s, peaks at ~T+1.8 s, and ends at ~T+2.2 s. T90 (15-350 keV) is
2.09 +- 0.12 sec (estimated error including systematics).

The time-averaged spectrum from T-0.07 to T+2.16 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index -0.13 +- 0.75,
and Epeak of 83.8 +- 24.2 keV (chi squared 66.64 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 3.1 +- 0.4 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T+1.22 sec in the 15-150 keV band is
2.7 +- 0.3 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.22 +- 0.15 (chi squared 79.71 for 57 d.o.f.).  All the quoted errors
are at the 90% confidence level.

Using the 8-ms binned light curve, the spectral lag for the 100-350 keV to
25-50 keV bands is 26 (+19, -15) ms for the first pulse, and 10 (+21, -13) ms
for the second pulse. These lag values are intermediate between short and
long GRBs, but also are consistent with short GRBs when considering the
large error bars. In addition, no extended emission is found.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/873539/BA/

L. Rhodes, D. Williams (Oxford), R.P. Fender (Oxford/UCT), A.J. van der
Horst (GWU), A. Horesh (HUJI), K. Mooley (NRAO/Caltech) and P.A. Woudt
(UCT) report on behalf of the ThunderKAT collaboration:

���We observed the position of the GRB 181126A afterglow at 1.3 GHz with
the new MeerKAT radio telescope on November 28, from 3.55 to 4.72 UT,
i.e. 1.76 days after the burst (GCN 23453
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 ). We do not detect a radio source
at the position of the X-ray counterpart (GCN 23455
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 ). The rms noise in the
map around that position is 30 microJy per beam.

These observations were taken as part of the ThunderKAT Large Survey
Project, a 5-year program for observations of transients with MeerKAT.
We would like to thank the staff at the South African Radio Astronomy
Observatory (SARAO) for scheduling and obtaining these observations.���