GRB 250118B

A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the HEND/Mars Odyssey team,

A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
and T. Cline on behalf of the Konus-Wind team,

E. Burns on behalf of the IPN,

E. Bozzo and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,

S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, and A. Tohuvavohu
on behalf of the Swift-BAT team,

and

W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr,
and A.S. Gardner on on behalf of the GRS-Odyssey GRB team,
report:

The long-duration GRB 250118B
was detected by Konus-Wind, INTEGRAL (SPI-ACS), Swift (BAT),
and Mars-Odyssey (HEND) at about 13107 s UT (03:38:27).
The burst was outside the coded field of view of the BAT.

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  108.006 (07h 12m 01s) -29.557 (-29d 33' 25")
 Corners:
  114.190 (07h 36m 46s) -29.989 (-29d 59' 21")
  112.319 (07h 29m 17s) -30.334 (-30d 20' 03")
  101.717 (06h 46m 52s) -28.434 (-28d 26' 02")
  103.724 (06h 54m 54s) -28.453 (-28d 27' 11")
 ---------------------------------------------
The error box area is 4.5 sq. deg, and its maximum
dimension is 11.0 deg (the minimum one is 28.7 arcmin).
The Sun distance was 128 deg.

This localization may be improved.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250118_T13107/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.

The Konus-Wind time history and spectrum will be given
in a forthcoming GCN Circular.

A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 250118B
(IPN triangulation: Kozyrev et al., GCN 39017
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triggered Konus-Wind at T0=13107.725 s UT (03:38:27.725).

The burst light curve shows a multipeaked structure,
which starts at ~T0-5.8 s and has a total duration of ~9.4 s.
We note a weaker and softer pulse seen in the same KW detector
before the main pulse, from ~T0-58.8 s to ~T0-55.9 s.
Since this pulse was detected by KW only, its relation
to GRB 250118B cannot be currently confirmed.
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/GRB250118_T13107/

As observed by Konus-Wind, the burst
had a fluence of 3.49(-0.35,+1.30)x10^-6 erg/cm^2
and a 64-ms peak flux, measured from T0-0.244 s,
of 2.97(-0.60,+1.22)x10^-6 erg/cm^2/s
(both in the 20 keV - 10 MeV energy range).

Since a major part of the burst emission
was detected before the trigger time, the spectral analysis
was performed using the KW 3-channel light curve data.

Modelling the KW 3-channel time-integrated spectrum
(measured from T0-5.837 s to T0+3.552 s)
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep),
yields alpha = -1.55(-0.14,+0.18) and Ep = 287(-95,+256) keV.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.