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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17419, 17420
Message-Id: <20250926071114.5027B28809AE@server.astronomerstelegram.org>
Date: Fri, 26 Sep 2025 03:11:14 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17419							     ATEL #17419

Title:	Fermi-LAT Gamma-ray Detection of the Nova V7994 Sgr
Author:	G. Marti-Devesa (University and INFN Trieste), Paul Fauverge
		(University of Bordeaux and CNRS), Pierre Jean (IRAP, Toulouse), C.
		C. Cheung (NRL) on behalf of the Fermi Large Area Telescope Collaboration
Queries:	guillem.marti-devesa@ts.infn.it
Posted:	25 Sep 2025; 21:25 UT
Subjects:Gamma Ray, >GeV, Nova, Transient

The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray
Space Telescope, has detected gamma-ray emission from a source positionally
consistent with V7994 Sgr (TCP J18035290-3127298), discovered by J. Seach
on 2025 Sep 21.381 UT at 10.2 mag (CBET #5612).

Preliminary analysis of LAT data from 2025-09-23 14:33 to 2025-09-25 04:15
results in a >5-sigma significance detection with a (E >100 MeV) flux of
(3.5 +/- 1.3) x 10^-7 photons cm^-2 s^-1 and a photon index = 1.9 +/- 0.2
(statistical uncertainties only). The highest-energy photon, with an energy
of 12.9 GeV and an association probability of 99.8%, was recorded on 2025-09-23
at 20:04:51. The new source does not correspond to a previously known source
or to a plausible extragalactic counterpart, but is consistent with the
optical position of the nova (R.A. = 18h03m52s.90 deg, Dec. = -31d27'29".8
deg, J2000).

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray
monitoring of this source will continue. In consideration of the ongoing
activity of this source, we encourage multi-wavelength observations. For
this source the Fermi LAT contact person is C.C. Cheung (Teddy.Cheung@nrl.navy.mil).

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.

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Password Certification: Guillem Marti-Devesa (guillem.marti-devesa@ts.infn.it)
https://www.astronomerstelegram.org/?read=17419
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ATEL #17420							     ATEL #17420

Title:	Narrow, kiloJansky FRB detected by the DSA-110
Author:	Pranav Sanghavi (Center for Astrophysics | Harvard &amp; Smithsonian),
		Liam Connor (Center for Astrophysics | Harvard &amp; Smithsonian),
		Vikram Ravi (California Institute of Technology) on behalf of the
		Deep Synoptic Array-110 (DSA-110) collaboration
Queries:	pranav.sanghavi@cfa.harvard.edu
Posted:	26 Sep 2025; 00:27 UT
Subjects:Radio, Transient, Fast Radio Burst

The Deep Synoptic Array (DSA-110) is a radio interferometer at the Owens
Valley Radio Observatory (OVRO). The array consists of 96 4.65-m dishes
automatically steerable in elevation, observing on the meridian. It conducts
a real-time FRB search over a 187.5 MHz bandwidth centered at 1405 MHz
by forming 512 coherent, fan-shaped search beams. For this observation,
81 core antennas and 12 outriggers were online.

On 2025 September 24 at 2025-09-24T04:08:23.8272 UTC, the DSA-110 real-time
system detected FRB 20250924A, a bright, apparent non-repeater. The burst
was initially detected with the modest signal-to-noise ratio (S/N) of 19,
but on closer inspection, the pulse was found to be so bright that it saturated
the bit-depth of the synthesized search-beam data. After forming a beam
at the correct position and without RFI excision, the S/N was found to
be ~2000. FRB 20250924A was bright enough to be detected in all individual
4.65 m antennas with S/N ~ 20. The peak flux density was 1.9 kJy in the
leading 60 microsecond sub-pulse. The fluence integrated over the multicomponent
FRB is 185 Jy ms. The event had an optimal dispersion measure of 405.25
pc/cm^3.

Using the saved voltage data, we performed an interferometric localization
of the burst. The burst was localized at RA, Dec = (20h31m06.38s, +53d50m56.4s;
J2000). Localizations for the full DSA-110 array are being commissioned,
but initial checks suggest an arcsecond-level localization precision for
this event. The Galactic line-of-sight dispersion measure in this direction
is ~155 pc/cm^3 (from the NE2001 model), suggesting the event is extragalactic.

The NE2001 DM value results in an extragalactic DM of roughly 200-230 pc/cm^3,
depending on assumptions about the Milky Way's halo contribution. For an
extragalactic DM of ~215 pc/cm^3, the redshift is expected to be less than
0.21 with 90% confidence. This assumes the P(DMex | z) distribution from
Connor et al. (2025). At the median redshift of z=0.10 from P(DMex | z),
the energy released by FRB 20250924A is nearly 5e31 erg/Hz.

Multi-wavelength follow-up observations are encouraged as are deep radio
follow-up observations to search for repeat bursts.

The incoherent beam dynamic spectrum of  FRB 20250924A: https://sites.astro.caltech.edu/~vikram/ATel_files/frb20250924a.png
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Password Certification: Pranav Sanghavi (pranav.sanghavi@cfa.harvard.edu)
https://www.astronomerstelegram.org/?read=17420
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