GRB 250118A

The Fermi GBM team reports the detection of a likely SHORT GRB

At 18:31:11 UT on 18 Jan 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250118A (trigger 758917876.966464 / 250118772).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 263.8, Dec = 38.8 (J2000 degrees, equivalent to J2000 17h 35m, 38d 47'), with a statistical uncertainty of 10.7 degrees.

The angle from the Fermi LAT boresight is 14.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250118772/quicklook/glg_skymap_all_bn250118772.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250118772/quicklook/glg_healpix_all_bn250118772.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250118772/quicklook/glg_lc_medres34_bn250118772.gif

Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: 

Swift/BAT did not localize GRB 250118A onboard (T0: 2025-11-18T18:31:11.97 UTC, Fermi GCN 38976
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 ) 

The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). 

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground. 

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 13.4 in a 0.128 s analysis time bin, starting at T0 - 0.064 s. 

Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)

The 90% credible area is 7,422 deg2 and the 50% credible area is 1,652 deg2.
The integrated probability inside the coded field of view is <1%. 

The NITRATES skymap is consistent with the Fermi localization reported in GCN 38976
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 . The joint NITRATES+Fermi 90% credible area is 565 deg2 and the 50% credible area is 111 deg2 

A plot of the probability skymap can be viewed here:

[skymap_plot](https://guano.swift.psu.edu/trigger_report?id=758917906/#:~:text=Probability%20Skymap)

The probability skymap file can be downloaded from the link here

[skymap_fits_file](https://guano.swift.psu.edu/files/758917906/0_n_PROBMAP)

Instructions on how to read and manipulate this map can be found here:

https://guano.swift.psu.edu/documentation

More details about this burst can be found on the trigger report page here:

https://guano.swift.psu.edu/trigger_report?id=758917906

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. 

A live reporting of Swift/BAT event data recovered by GUANO can be found at: 
https://www.swift.psu.edu/guano/

P. Veres (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 18:31:11.97 UT on 18 January 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250118A (trigger 758917876/250118772).
which was also detected by Swift-BAT/GUANO (Ronchini et al. 2025, GCN 39004
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 ).
The Fermi GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight is 14 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 0.3 s (50-300 keV). The time-averaged spectrum
from T0-0.13 to T0+0.06 s is best fit by
a simple power law function with index -1.18 +/- 0.06.

The event fluence (10-1000 keV) in this time interval is
(4.1 +/- 0.4)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0-0.064 s in the 10-1000 keV band is 7.4 +/- 0.9 ph/s/cm^2.

A power law function with an exponential high-energy cutoff gives an acceptable fit to the spectrum.
The power law index is -0.73 +/- 0.22 and the cutoff energy,
parameterized as Epeak, is 1100 +/- 500 keV.

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/"