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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel 17421, 17422, 17423
Message-Id: <20250927071806.AC9E72880805@server.astronomerstelegram.org>
Date: Sat, 27 Sep 2025 03:18:06 -0400 (EDT)

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ATEL #17421							     ATEL #17421

Title:	NuSTAR follow-up of the X-ray transient EP250916a
Author:	G. K. Jaisawal, J. Chenevez (DTU Space), G. Illiano (INAF-OAB),
		A. Sanna (University of Cagliari), Q. C. Zhao (IHEP)
Queries:	gaurava@space.dtu.dk
Posted:	26 Sep 2025; 08:18 UT
Subjects:X-ray, Transient

The X-ray transient EP250916a was discovered by the Einstein Probe/WXT
on 2025 September 16 (ATel #17395). Follow-up observations with EP/FXT
(GCN #41861), SVOM/MXT (ATel #17396), and Swift/XRT (ATel #17397) confirmed
rapid brightening, with the spectrum consistently described by an absorbed
power-law with photon index in the range 1.6-2.0 and hydrogen column density
of (3-4) &times; 1021 cm-2.

We observed EP250916a with NuSTAR on 2025 September 24 between 11:25 and
22:15 UTC for a total exposure of 21 ks. The source was detected at R.A.
= 268.239 deg, Dec. = -35.323 deg (J2000), consistent with the position
reported by Swift/XRT (ATel #17397). The 3-79 keV light curve shows an
average count rate of ~12 cts/s per focal plane module. No X-ray bursts
or flares were observed during the observation. A preliminary search for
pulsations and quasi-periodic oscillations in the 0.01-400 Hz range yielded
no significant candidates.

The broadband 3-70 keV NuSTAR spectrum can be described by an absorbed
power-law model. Preliminary best-fit parameters are: hydrogen column density
nH (3 &plusmn; 2) &times; 1021 cm-2, photon
index 2.03 &plusmn; 0.01, power-law normalization 0.15 &plusmn; 0.01, and
reduced chi-squared of 1.15 for 1313 degrees of freedom. The unabsorbed
fluxes are (4 &plusmn; 0.01) &times; 10-10 and (6.72 &plusmn;
0.01) &times; 10-10 erg cm-2 s-1 in the
0.3-10 and 3-70 keV ranges, respectively. Uncertainties on the spectral
parameters are quoted at the 90% confidence level. Our spectral analysis
does not reveal clear signatures of a soft X-ray thermal component, potentially
associated with an accretion disk or boundary layer, nor the emission line
in the 6-7 keV iron band.

The NuSTAR observation confirms that EP250916a remains in an active state
and is mostly consistent with the properties of an accreting compact object
in a hard state. Detailed analysis is ongoing. We encourage continued multiwavelength
monitoring to constrain the nature of this transient source.

We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request,
and the NuSTAR SOC for promptly carrying out the observation.

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Password Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
https://www.astronomerstelegram.org/?read=17421
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ATEL #17422							     ATEL #17422

Title:	MAXI/GSC observation of an X-ray brightening from the symbiotic
		X-ray binary 4U 1954+319
Author:	M. Nakajima (Nihon U.), T. Mihara (RIKEN), H. Negoro (Nihon
		U.), K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Tamagawa, N.
		Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y.
		Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H.
		Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang,
		T. Nakamoto (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S.
		Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA),
		Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi
		(Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.),
		W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U)
Queries:	nakajima.motoki@nihon-u.ac.jp
Posted:	26 Sep 2025; 13:24 UT
Subjects:X-ray, Binary, Neutron Star, Transient, Pulsar

MAXI/GSC has observed an X-ray brightening from the symbiotic X-ray binary
4U 1954+319 since 2025-08-25 (MJD 60912).  The 1-day averaged flux in the
4–10 keV band began increasing at a rate of ~3 mCrab/day and reached
99 +/- 6 mCrab on 2025-09-22 (MJD 60940).  This flux is comparable to the
highest level recorded by MAXI/GSC in 2010 (ATel #2702).  The current flare
shows a longer brightening phase than the major flare in 2010, which rose
to its peak within ~20 days.  There is a possibility that the source may
develop into the brightest flare ever observed.  We encourage follow-up
observations.

The latest light curve and the image of this source are available at the
following URL:

http://maxi.riken.jp/pubdata/v7.7l/J1955+320/index.html

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Password Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
https://www.astronomerstelegram.org/?read=17422
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ATEL #17423							     ATEL #17423

Title:	NLSy1 1H0323+342: Upper limits from a neutrino search with
		IceCube
Author:	Alicia Mand (University of Wisconsin-Madison), Erik Blaufuss
		(University of Maryland)
Queries:	blaufuss@umd.edu
Posted:	26 Sep 2025; 20:30 UT
Subjects:TeV, Neutrinos, AGN

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed a search [1] for track-like muon neutrino events
arriving from the direction of the flaring radio-loud narrow-line Seyfert
1 1H 0323+342, also known as 4FGL J0324.8+3412 detected by Fermi-LAT (ATel
#17407) and Swift (ATeL #17411). For this search we used a time window
of (2025-09-20 00:00:00.000 to 2025-09-24 23:59:59.000) during which IceCube
was collecting good quality data. We report a p-value of 1.0, consistent
with background expectation.

We accordingly derive a time-integrated muon-neutrino flux upper limit
for this source with an E^-2.0 spectrum, expressed as E^2 dN/dE evaluated
at 1 TeV = 5.3e-02 GeV cm^-2. 90% of events IceCube would detect from a
source at this declination with an E^-2.0 spectrum have energies in the
approximate energy range between 7e+02 and 5e+05 GeV.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector
operating at the geographic South Pole, Antarctica. The IceCube realtime
alert point of contact can be reached at roc@icecube.wisc.edu.

[1] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021)

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Password Certification: Erik Blaufuss (blaufuss@umd.edu)
https://www.astronomerstelegram.org/?read=17423
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