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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel 17815, 17816
Message-Id: <20260527070530.8D7352880998@server.astronomerstelegram.org>
Date: Wed, 27 May 2026 03:05:30 -0400 (EDT)

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ATEL #17815							     ATEL #17815

Title:	MAXI/GSC Observation of an X-ray Outburst from the Be/X-ray
		Binary Pulsar 1A 0535-66 (1A 0538-66)
Author:	M. Nakajima (Nihon U.), T. Mihara (RIKEN), M. Sugizaki (Kanazawa
		U.), H. Negoro, K. Saito, T. Yokoyama, M. Wada (Nihon U.), T. Tamagawa,
		N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita,
		Y. Kawakubo, H. Hiramatsu, A. Yoshida (AGU), Y. Tsuboi, H. Sugai,
		N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto,
		M. Uenishi, T. Usuki, S. Yatsuzuka (Ehime U.), I. Takahashi, Y. Yatsu
		(Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Kurihara (JAXA),
		Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), M. Yamauchi, M. Nishio,
		C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), W. Iwakiri (Chiba
		U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U), S. Ogawa (TUS)
Queries:	nakajima.motoki@nihon-u.ac.jp
Posted:	26 May 2026; 14:00 UT
Subjects:X-ray, Binary, Neutron Star, Transient, Pulsar

The MAXI/GSC nova alert system triggered on an X-ray brightening from the
Be/X-ray binary pulsar 1A 0535-66 (also known as 1A 0538-66) on 2026 May
21 (MJD 61181; see the MAXI alert page at https://maxi.riken.jp/alert/novae/1181387701/1181387701.htm
). This marks the third firm detection of this source by MAXI/GSC. The
previous activities were recorded on 2025 September 30 (MJD 60948; ATel
#17455, #17467) and 2026 February 27 (MJD 61098, ATel #17705). Similar
to the previous events, the current outburst exhibited a fast-rise and
exponential-decay (FRED) profile. To obtain a correct light curve in a
crowded region, we performed a 'image-fit' analysis (Morii et al. 2016)
following the procedure used in our previous report (ATel #17705). We
extracted the light curve while avoiding contamination from the nearby
source LMC X-4. By fitting the obtained a 90-minute binned light curve
in the 4-10 keV band with a FRED model, we determined the parameters of
the outburst (onset time, peak time, and peak flux). The outburst onset
time was 18:01 UT on 2026 May 20 (MJD 61180.751) and the peak time was
13:12 UT on May 21 (MJD 61181.550). Assuming the orbital ephemeris of
Porb = 16.64002 &plusmn; 0.00026 d and T0 = 55673.71
&plusmn; 0.05 (MJD) given by Rajoelimanana et al. (2017) and updated by
Ducci et al. (2022), these onset and peak times correspond to the orbital
phases of 0.952 and 0.000, respectively. The derived peak flux in the
4-10 keV band is 0.126 &plusmn; 0.016 photons s-1 cm-2
(104 &plusmn; 13 mCrab). This corresponds to the X-ray luminosity of &sim;3
&times; 1038 erg s-1, assuming a Crab-like single
power-law spectrum and a distance of 50 kpc. After the peak, the outburst
exponentially decayed with &tau; = 0.57 &plusmn; 0.15 d. The obtained
outburst parameters are similar to those of the outburst observed in 2025
September (ATel #17455, #17467). A comparison of three outburst profiles
is available at the following link.

A comparison of three outburst profiles.: http://maxi.riken.jp/plot/A0535-66_lc.gif
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Password Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
https://www.astronomerstelegram.org/?read=17815
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ATEL #17816							     ATEL #17816

Title:	MAXI/GSC detection of an X-ray outburst from the Be/X-ray binary
		pulsar LS V +44 17 (RX J0440.9+4431)
Author:	M. Nakajima (Nihon U.), T. Mihara (RIKEN), H. Negoro, K. Saito,
		T. Yokoyama, M. Wada (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka
		(RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu,
		A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo
		U.), M. Shidatsu, C. Kang, T. Nakamoto, M. Uenishi, T. Usuki, S. Yatsuzuka
		(Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S.
		Ueno, H. Tomida, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara, S. Kobayashi
		(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), S. Ogawa (TUS)
Queries:	nakajima.motoki@nihon-u.ac.jp
Posted:	27 May 2026; 02:00 UT
Subjects:X-ray, Binary, Neutron Star, Transient, Pulsar

The MAXI/GSC nova alert system triggered on a weak X-ray brightening from
the Be/X-ray binary pulsar LS V +44 17 (RX J0440.9+4431) on 2026 May 22
(MJD 61182). This source had been below the detection limit of MAXI/GSC
for three years since its last major outburst in 2023 (ATel #15835, #15848,
#15868, #15874, #15880, #15892, #15907, #15913). The day-averaged flux
in the 4-10 keV band, derived from the MAXI/GSC public data (https://maxi.riken.jp/v7l3h/J0440+445/index.html),
was 0.025&plusmn;0.006 photons cm-2 s-1 (21&plusmn;5
mCrab) on May 22 (MJD 61182). Because the region with the LS V +44 17
was outside the FOV of MAXI/GSC until May 18 (MJD 61178), the exact onset
time of this brightening is unclear. The source flux remained at a nearly
constant level of &sim;0.019 photons cm-2 s-1 (&sim;16
mCrab) for a week since May 19 (MJD 61179). However, on May 26 (MJD 61186),
the source flux exhibited a further brightening. The flux doubled to 0.044&plusmn;0008
photons cm-2 s-1 (36&plusmn;6 mCrab). Multi-wavelength
follow-up observations are encouraged.

The latest X-ray light-curve can be checked at the following pages;

MAXI ( https://maxi.riken.jp/v7l3h/J0440+445/index.html ),

Swift/BAT ( https://swift.gsfc.nasa.gov/results/transients/weak/LSVp4417/
),

Fermi/GBM ( https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/rxj0440.html
).

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https://www.astronomerstelegram.org/?read=17816
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