GRB 250228A

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

"At 07:23:46.16 UT on 28 February 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250228A (trigger 762420231/250228308).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 94.90, Dec = -46.81 (J2000 degrees, equivalent to J2000 6h 19m, -46d 48'),
with a statistical uncertainty of 11.91 degrees.

The angle from the Fermi LAT boresight is 27 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250228308/quicklook/glg_skymap_all_bn250228308.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/bn250228308/quicklook/glg_healpix_all_bn250228308.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250228308/quicklook/glg_lc_medres34_bn250228308.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 250228A onboard (T0: 2025-02-28T07:23:46.16 UTC, Fermi GCN 39547
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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 8.03 in a 0.128 s analysis time bin, starting at T0 - 0.096 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 14,714 deg2 and the 50% credible area is 5,217 deg2.
The integrated probability inside the coded field of view is <1%. 

The joint NITRATES+GBM localization has a 90% credible area of 1171 deg2 and a 50% credible area of 304 deg2

A plot of the probability skymap can be viewed here:

[skymap_plot](https://guano.swift.psu.edu/trigger_report?id=762420261/#:~:text=Probability%20Skymap)

The probability skymap and joint skymap files can be downloaded from the links here

[skymap_fits_file](https://guano.swift.psu.edu/files/762420261/0_n_PROBMAP)

[joint_skymap_fits_file](https://guano.swift.psu.edu/files/762420261/0_n_JOINTMAP)

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=762420261

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/

U. Pathak (IIT Bombay), M. Dafčíková (Masaryk U.), and C. Meegan (UAH) report 
on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 07:23:46.16 UT on 28 February 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250228A (trigger 762420231/250228308).
which was also detected by Swift/BAT-GUANO (Ronchini et al. 2025, GCN 39674
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 ).
The Fermi GBM on-ground location is consistent with the Swift/BAT position.

The angle from the Fermi LAT boresight is 27 degrees.

The GBM light curve consists of a weak pulse from a single emission episode
with a duration (T90) of about 0.6 s (50-300 keV). The time-averaged spectrum
from T0-0.5 to T0+0.3 s is best fit by
a simple power law function with index -1.45 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(2.0 +/- 0.3)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 4.3 +/- 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/"