GRB 221025A

Arimoto M. (Kanazawa University) Pillera R. (Politecnico and INFN
Bari), and Maheso D. (Johannesburg Univ.) report on behalf of the
Fermi-LAT team:

At 00:58:18.42 on October, 25, 2022 Fermi-LAT detected high-energy emission
from GRB 221025A, GBM trigger 221025041 / 688352303 at 00:58:18.42 UT.

The best LAT on-ground location is found to be

RA, Dec =  133.4, 11.0 (J2000)

with an error radius of 1.3 deg (90 % containment, statistical error only).
This was 45 deg from the LAT boresight at the time of the trigger.

The data from the Fermi-LAT show a significant increase in the event
rate that is spatially
and temporally correlated with the trigger with high significance.


The 100 MeV - 1 GeV photon flux in the time interval 0-200 s after
the GBM trigger is (2.1 +/- 0.6)E-05 ph/cm2/s.
The estimated integrated photon index above 100 MeV is -2.3 +/- 0.3.
The highest-energy photon is a 1.3 GeV event with 93% probability
which is observed 3 seconds after the GBM trigger.

A Swift ToO has been requested for this burst.


The Fermi-LAT point of contact for this burst is
Dimakatso Jeannett Maheso (d.j.maheso@gmail.com).

The Fermi-LAT is a pair conversion telescope designed to
cover the energy band from 20 MeV to greater than 300 GeV.
It is the product of an international collaboration
between NASA and DOE in the U.S. and many scientific
institutions across France, Italy, Japan and Sweden.

J. Wood (NASA/MSFC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 00:58:18 UT on 25 October 2022, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 221025A (trigger 688352303 / 221025040).
which was also detected by Fermi-LAT (Arimoto M. et al. 2022, GCN 32843
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 ).
The Fermi GBM on-ground location is consistent with the Fermi-LAT position.

The on-ground calculated location, using the GBM trigger
data, is RA = 128.9, DEC = +2.2 (J2000 degrees,
equivalent to 08h 36m, +02d 12'), with an uncertainty
of 6.8 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which we have characterized as a core-plus-tail model, with 90% of
GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32]).

The angle from the Fermi LAT boresight is 48.5 degrees.

The GBM light curve consists of a single pulse with a duration (T90)
of about 0.4 s (50-300 keV). The time-averaged spectrum
from T0+0.0 to T0+0.8 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.4 +/- 0.1 and the cutoff energy,
parameterized as Epeak, is 1680 +/- 345 keV.

The event fluence (10-1000 keV) in this time interval is
(1.7 +/- 0.1)E-06 erg/cm^2. The 64-ms peak photon flux measured
starting from T0+0.3 s in the 10-1000 keV band is 13.3 +/- 0.9 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

R. Gopalakrishnan (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. 
Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao 
(IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat 
CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 
2021, JApA, 42, 73) showed the detection of a short GRB 221025A which 
was also detected by Fermi-GBM (Wood et al,. GCN 32851
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 ) and Fermi-LAT 
(Arimoto et al,. GCN 32843
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 ).

The source was clearly detected in the 20-200 keV energy range. The 
light curve peaks at 2022-10-25 00:58:18.6 UTC. The measured peak count 
rate associated with the burst is 904 (+212, -102) counts/s above the 
background in the combined data of all quadrants, with a total of 313 
(+62, -63) counts. The local mean background count rate was 435 (+8, 
-12) counts/s. Using cumulative rates, we measure a T90 of 0.45 (+0.6, 
-0.2) s.

It was also detected in the CsI anticoincidence (Veto) detector in the 
100-500 keV energy range. Due to the intrinsic 1 s binning of veto data, 
we cannot reliably estimate a T90 from it.

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, URSC, IUCAA, SAC, 
and PRL. The Indian Space Research Organisation funded, managed, and 
facilitated the project.

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

A. Goldstein, M. S. Briggs, C. Wilson-Hodge,
and E. Burns on behalf of the Fermi GBM team,

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

The short-duration GRB 221025A
(Fermi-LAT detection: Arimoto et al., GCN Circ. 32843
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 ;
Fermi-GBM detection: Wood and Meegan, GCN Circ. 32851
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 ;
AstroSat-CZTI detection: Gopalakrishnan et al., GCN Circ. 32853
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 )
was detected by Fermi (GBM trigger 688352303), INTEGRAL (SPI-ACS),
Konus-Wind, AstroSat (CZTI), and GECAM-B (trigger tn221025_005818_fb)
at about 3498 s UT (00:58:18).

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
  ---------------------------------------------
   RA(2000), deg                 Dec(2000), deg
  ---------------------------------------------
  Center:
   134.373 (08h 57m 29s) +13.068 (+13d 04' 07")
  Corners:
   133.601 (08h 54m 24s) +10.772 (+10d 46' 20")
   133.259 (08h 53m 02s) +10.806 (+10d 48' 20")
   135.172 (09h 00m 41s) +15.363 (+15d 21' 46")
   135.522 (09h 02m 05s) +15.330 (+15d 19' 48")
  ---------------------------------------------
The error box area is 1.6 sq. deg, and its maximum
dimension is 5.03 deg (the minimum one is 19.44 arcmin).
The Sun distance was 78 deg.

This localization may be improved.

The IPN localization is consistent with, but reduces the area of,
the Fermi-GBM final localization (glg_healpix_all_bn221025040_v00).

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB221025_T03498/IPN/

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