GRB 240415B

J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E.
Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), D.N. Burrows (PSU) and P.A. Evans (U. Leicester) reports on
behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Swift/BAT-detected burst GRB 240415B, collecting 3.4 ks of Photon
Counting (PC) mode data between T0+86.1 ks and T0+115.2 ks. 

Four uncatalogued X-ray sources have been detected consistent with
being within 493 arcsec of the Swift/BAT position, however none of them
is above the RASS limit or shows definitive signs of fading. Therefore,
at the present time we cannot identify which, if any, is the afterglow.
Details of these sources are given below:

Source 2:
  RA (J2000.0):  132.2001  =  08:48:48.03
  Dec (J2000.0): -27.0714  =  -27:04:17.2
  Error: 6.4 arcsec (radius, 90% conf.)
  Count-rate: (9.5 [+10.4, -8.3])e-4 ct s^-1   
  Distance: 374 arcsec from Swift/BAT position.

Source 3:
  RA (J2000.0):  132.3176  =  08:49:16.23
  Dec (J2000.0): -27.1553  =  -27:09:19.0
  Error: 6.6 arcsec (radius, 90% conf.)
  Count-rate: (2.6 [+1.6, -1.3])e-3 ct s^-1   
  Distance: 306 arcsec from Swift/BAT position.
  Flux: (1.32 [+0.83, -0.68])e-12 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 6:
  RA (J2000.0):  132.3018  =  08:49:12.44
  Dec (J2000.0): -27.1242  =  -27:07:27.0
  Error: 5.5 arcsec (radius, 90% conf.)
  Count-rate: (3.4 [+1.6, -1.3])e-3 ct s^-1   
  Distance: 200 arcsec from Swift/BAT position.
  Flux: (4.6 [+2.2, -1.8])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 7:
  RA (J2000.0):  132.4111  =  08:49:38.66
  Dec (J2000.0): -27.0674  =  -27:04:02.8
  Error: 5.6 arcsec (radius, 90% conf.)
  Count-rate: (3.6 [+1.8, -1.5])e-3 ct s^-1   
  Distance: 301 arcsec from Swift/BAT position.
  Flux: (8.8 [+4.5, -3.6])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Three uncatalogued sources were also detected too far from the GRB
position to be likely afterglow candidates.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021684.

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

Sheng-Lun Xie, Shao-Lin Xiong report on behalf of the GECAM team:

GECAM-B was triggered in-flight and on-ground by a short burst, GRB 240415B, at 2024-04-15T03:10:49.150 UTC (T0), which was also observed by Swift/BAT (GCN #36125
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 ). 

According to the GECAM-B light curve, this burst consists of roughly a pulse with a total duration of ~250 ms. 

The GECAM-B ground calculated location (J2000) is:
Ra: 156.77 deg   
Dec: -20.78 deg
Err: 11.16 deg (1-sigma, statistical only)
This GECAM-B localization is consistent with that of Swift/BAT within the error.

We note that these results are very preliminary. Refined analysis will be reported later.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor(GECAM) mission originally consists of two microsatellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the Swift/BAT GRB 240415B. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021683

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Swift/BAT event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

UVOT and XRT observations have been requested and approved. 

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

Swift/BAT did not localize GRB 240415B onboard (T0: 2024-04-15T03:10:49.351 UTC, GECAM trigger 328).

The GECAM 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), detects the burst in a 0.256 s analysis time bin starting at T0 - 0.128 s with a sqrt(TS) of 16.2.
An arcminute localization is found with DeltaLLHOut of 26.8 and a DeltaLLHPeak of 9.4.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is
RA, Dec = 132.317, -27.07 deg which is
   RA(J2000)  = 08h 49m 16.0s
   Dec(J2000) = -27d 04′ 12.0″
with an estimated uncertainty of 5 arcmin radius.

Follow-up observations from other instruments are requested.
XRT and UVOT will not follow-up until March 18 due to planned observatory downtime.

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/