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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17682, 17683
Message-Id: <20260217071730.07F532880789@server.astronomerstelegram.org>
Date: Tue, 17 Feb 2026 02:17:30 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17682							     ATEL #17682

Title:	MAXI/GSC detection of a very hard X-ray burst probably from
		4U 0513-40 (GRB 260206A)
Author:	H. Negoro, M. Nakajima (Nihon U.), J. A. Kennea (PSU), T.
		Mihara (RIKEN)
Queries:	negoro.hitoshi@nihon-u.ac.jp
Posted:	16 Feb 2026; 11:38 UT
Subjects:X-ray, Neutron Star

This is a cross-posting of GCN 43763.

In Swift-XRT followup observations of GRB 260206A detected with the MAXI-GSC
(GCN Circ. 43624), the XRT marginally detected a faint, uncatalogued X-ray
source near the center of the MAXI error region (Lenava et al. GCN Circ.
43630). The XRT also detected a bright known X-ray source, LSXPS J051406.5-400237,
of which a count rate was 7.3 +/- 0.3 cts/s, approximately 70% higher than
the mean count rate (Source 1: Evans et al. GCN Circ. 43625).

We analyzed the GSC energy spectrum of the burst, and found that a power-law
model gave a better fit to the spectrum than a blackbody model. This is
because many hard X-ray photons were detected above 10 keV. We fixed the
equivalent hydrogen column density to 3.84e20 (obtained with the nH Calculator
at NASA/HEASARC for the direction of 4U 0513-40), and obtained a photon
index of 1.7 (+0.7, -0.6 at 90% confidence interval) and a blackbody temperature
of 1.5 (+0.6, - 0.4) keV in each fit.

The burst exhibited a double-peaked-like structure with an e-folding decay
time of 5.8 +/- 1.0 sec at 2-20 keV without any collimator response correction.
The peak X-ray flux is around 1.22e-8 erg/cm2/s at 2-10 keV, which corresponds
to 2.2e38 erg/s if the source is in the globular cluster NGC 1851 hosting
4U 0513-40 (aka X0512-401) (12.2 kpc away). MAXI detected more than ten
X-ray bursts with a variety of time profiles originating from the direction
of NGC 1851. These strongly suggest that this hard energetic burst also
came from 4U 0513-40 (or ABK A, see Homer et al. 2001, ApJ, L155), within
the error circle of LSXPS J051406.5-400237, though 4U 0513-40 is just outside
the MAXI error box if we take account of the systematic uncertainty of
the localization, 0.1 deg (GCN Circ. 43624).

probable XRB from 4U 0513-40: https://maxi.riken.jp/alert/novae/1077288702/1077288702.htm
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Password Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)
https://www.astronomerstelegram.org/?read=17682
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ATEL #17683							     ATEL #17683

Title:	Additional Photometry of a Possible Very Slowly Evolving Nova,
		M31N 2025-12f
Author:	K. Hornoch, H. Kucakova (Astronomical Institute, Ondrejov,
		Czech Republic), A. W. Shafter (SDSU)
Queries:	k.hornoch@centrum.cz
Posted:	16 Feb 2026; 19:35 UT
Subjects:Optical, Nova, Transient

In ATel #17576 we reported independent discovery of a possible very slowly
evolving nova in M31 designated PNV J00432344+4124357 = M31N 2025-12f =
AT 2025ajxy, and as ZTF25acghjar by the ZTF.

After discovery, the object brightened very slowly and reached R = 19.0
mag one month later. We also measured its color and found (R-I) = 0.5 +/-
0.2 mag, excluding the possibility that the transient is a Mira-type variable
star. We predicted that if it is a nova in M31, it will continue to brighten
further like very slowly evolving nova PNV J00423788+4112491 = AT 2022abzg
= M31N 2022-09b (see ATel #15831).

To confirm the prediction we followed the object photometrically using
the 0.65-m telescope at Ondrejov (D65) yielding the following magnitudes:

<span class="inner-pre" style="font-size: 13px"> 
Date [UT]    |   Mag  | Err  | Filter | Telescope

2026-01-10.779 |  18.9  | 0.2  |   R    | D65 
2026-01-11.683 |  19.0  | 0.25 |   R    | D65 
2026-01-17.912 |  18.8  | 0.15 |   R    | D65 
2026-01-18.688 |  18.8  | 0.2  |   R    | D65 
2026-01-19.705 |  18.7  | 0.3  |   R    | D65 
2026-01-20.928 |  18.9  | 0.15 |   R    | D65 
2026-02-10.718 |  18.9  | 0.1  |   R    | D65 
2026-02-13.850 |  17.83 | 0.10 |   R    | D65 
2026-02-15.796 |  17.6  | 0.1  |   R    | D65

</span>

Based on its photometric evolution, we conclude that M31N 2025-12f is likely
a very slowly evolving classical nova in M31. As with other very slow novae,
we wouldn't be surprised if this nova has a complex lightcurve with multiple
re-brightenings, so follow-up photometric and especially spectroscopic
observations of this object are strongly encouraged.

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Password Certification: Allen W. Shafter (ashafter@sdsu.edu)
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