GRB 170126A

D. Kocevski (NASA/MSFC), S. D. Barthelmy (GSFC),
A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), A. D'Ai (INAF-IASFPA),
S. W. K Emery (UCL-MSSL), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 11:30:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170126A (trigger=735188).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 263.579, -64.757 which is 
   RA(J2000)  =  17h 34m 19s
   Dec(J2000) = -64d 45' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single FRED-like peak
structure with a duration of about 15 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 11:32:09.4 UT, 87.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 263.63341, -64.76046 which is
equivalent to:
   RA(J2000)  = 17h 34m 32.02s
   Dec(J2000) = -64d 45' 37.7"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 84 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 131 seconds with the White filter
starting 90 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	17:34:31.95 = 263.63311
  DEC(J2000) = -64:45:37.0  = -64.76027
with a 90%-confidence error radius of about 0.62 arc sec. This position is 0.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.12 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.10. 

Burst Advocate for this burst is D. Kocevski (dankocevski AT gmail.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.)

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

Using 2613 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 170126A, 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 = 263.63308, -64.76064 which is equivalent
to:

RA (J2000): 17h 34m 31.94s
Dec (J2000): -64d 45' 38.3"

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.

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


"At 11:30:41.54 UT on 26 January 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170126A (trigger 507123046 / 170126480),
which was also detected by the Swift/BAT (Kocevski et al. 2017, GCN 20530
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 ).
The GBM on-ground location is consistent with the Swift position.

The trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux
of the GRB. This ARR was accepted and the spacecraft slewed
to the GBM in-flight location. The initial angle from the
Fermi-LAT boresight to the BAT location is 46 degrees.

The GBM light curve consists of a single peak
with a duration (T90) of about 14 s (50-300 keV).
The time-averaged spectrum from T0 to T0+14 s is
adequately fit by a power law function with an exponential
high-energy cutoff. The power law index is -1.13 +/- 0.04 and
the cutoff energy, parameterized as Epeak, is 260 +/- 21 keV.
A Band function fits the spectrum equally well
with Epeak = 187 +/- 23 keV, alpha = -1.02 +/- 0.06 and
beta = -2.08 +/- 0.11.

The event fluence (10-1000 keV) in this time interval is
(9.5 +/- 0.3)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1 s in the 10-1000 keV band
is 12.3 +/- 0.4 ph/s/cm^2.


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

F. E. Marshall (NASA/GSFC) and D. Kocevski (NASA/MSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170126A
91 s after the BAT trigger (Kocevski et al., GCN Circ. 20530
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 ).
A fading source consistent with the enhanced XRT position
(Beardmore et al. GCN Circ. 20531
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 )
is detected in the initial UVOT exposure.

The preliminary UVOT position is:
    RA  (J2000) =  17:34:31.97 = 263.63319 (deg.)
    Dec (J2000) = -64:45:37.3  = -64.76035 (deg.)
with an estimated uncertainty of 0.45 arc sec. (radius, 90% confidence).

Preliminary detection 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               91          222          129         18.10 +/- 0.05
white            11432        12036          590        >19.7
v                 1907         2273           58        >17.7
b                 1832         6164          132        >20.1
u                 1807         6084          255        >19.3
w1                1783         5879          255        >19.8
m2                1931         2297           58        >20.0
w2                1882         2248           58        >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.10 in the direction of the burst
(Schlegel et al. 1998).

D.N. Burrows (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), V. D'Elia (ASDC), A. Cholden-Brown (PSU), S. J.
LaPorte (PSU) and  report on behalf of the Swift-XRT team: report on
behalf of the Swift-XRT team:

We have analysed 7.0 ks of XRT data for GRB 170126A, from 73 s to 40.7
ks after the  BAT trigger. The data comprise 68 s in Windowed Timing
(WT) mode (the first 7 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.30 (+0.06, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.52 (+0.33, -0.28). The
best-fitting absorption column is  2.0 (+2.5, -1.3) x 10^21 cm^-2,
consistent with the Galactic value of 7.0 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 4.5 x 10^-11 (5.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     2.0 (+2.5, -1.3) x 10^21 cm^-2
Galactic foreground: 7.0 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.52 (+0.33, -0.28)

If the light curve continues to decay with a power-law decay index of
1.30, the count rate at T+24 hours will be 9.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.2 x
10^-14 (4.9 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/00735188.

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

V. Sharma, D. Bhattacharya and V. Bhalerao (IUCAA), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data showed clear detection of GRB170126A (Swift detection: D. Kocevski et al., GCN Circ. 20530
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 ) in quadrant C and D in the 40-200 keV energy range. The light curve shows single peak structure with main peak at 11:30:42 UT, coincident with Swift trigger. The measured peak count rate is 185.7 counts/sec above the background in combined data of four quadrants, with a total 1008.2 counts. The local mean background count rate was 336.3 counts/sec. Using cumulative rates, we measure a T90 of 26.9 secs.

It was clearly detected in the CsI anticoincidence detector (Veto) also as bright detection in the 100-500 keV energy range.

CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

A.Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 170126A
(Swift-BAT detection: Kocevski et al., GCN 20530
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 ;
Fermi-GBM observation: Bissaldi, GCN 20533
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 ;
AstroSat CZTI detection: Sharma et al., GCN 20537
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 )
triggered Konus-Wind at T0=41440.253 s UT (11:30:40.253).

The burst light curve shows a single pulse
started at ~T0-0.5 s with a total duration of ~28 s.
The emission is seen up to 2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170126_T41440/

As observed by Konus-Wind, the burst
had a fluence of 8.25(-0.81,+0.94)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+0.992 s,
of 2.87(-0.85,+0.87)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+16.640 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.98(-0.22,+0.24)
and Ep = 188(-27,+40) keV (chi2 = 54/59 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.8
(chi2 = 54/58 dof)

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model
with  alpha = -1.03(-0.17,+0.18)
and Ep = 206(-27,+37) keV (chi2 = 40/58 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.4
(chi2 = 40/57 dof)

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

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

Using the data set from T-240 to T+200 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 170126A (trigger #735188)
(Kocevski, et al., GCN Circ. 20530
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 ).  The BAT ground-calculated position is
RA, Dec = 263.623, -64.755 deg which is
   RA(J2000)  =  17h 34m 29.4s
   Dec(J2000) = -64d 45' 18.9"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 4%.

The mask-weighted light curve shows a single-peaked structure that starts
at ~T-1 s, peaks at ~T+1 s, and ends at ~T+11 s. T90 (15-350 keV) is
9.5 +- 1.6 sec (estimated error including systematics).

The time-averaged spectrum from T-0.8 to T+10.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.54 +- 0.14.  The fluence in the 15-150 keV band is 2.8 +- 0.3 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.31 sec in the 15-150 keV band
is 7.3 +- 1.1 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/735188/BA/