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From: atel@astronomerstelegram.org
Subject: ATel 17705
Message-Id: <20260303070321.4E26C28804FC@server.astronomerstelegram.org>
Date: Tue, 3 Mar 2026 02:03:21 -0500 (EST)

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ATEL #17705							     ATEL #17705

Title:	MAXI/GSC observation of a bright outburst from a Be/X-ray binary
		1A 0538-66
Author:	M. Nakajima, H. Negoro (Nihon U.), T. Mihara (RIKEN), M. Sugizaki
		(Kanazawa U.), K. Takagi, H. Nishio (Nihon U.), T. Tamagawa, N. Kawai,
		M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo,
		H. Hiramatsu, Y. Kondo, D. Iijima, 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, S. Ogawa,
		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)
Queries:	nakajima.motoki@nihon-u.ac.jp
Posted:	3 Mar 2026; 02:26 UT
Subjects:X-ray, Binary, Neutron Star, Transient, Pulsar

The MAXI/GSC nova alert system triggered on an X-ray transient source in
the Large Magellanic Cloud (LMC) region on 2026 February 27 (MJD 61098).
Assuming that the source flux was constant over the seven transits between
02:14 UT and 11:32 UT, we derived the source position as (R.A., Dec.) =
(83.624 deg, -66.880 deg) = (05 34 29, -66 52 47) (J2000), with a statistical
90% C.L. elliptical error region with long and short radii of 0.22 deg
and 0.21 deg, respectively.  The roll angle of the major axis from the
north direction is 135.0 deg counterclockwise.  There is an additional
systematic uncertainty of 0.1 deg (90% containment radius).  The derived
position is consistent with that of the Be/X-ray binary pulsar 1A 0538-66.
This is the second firm detection of this source by the MAXI/GSC observation
; the previous X-ray outburst was recorded on 2025 September 30 (MJD 60948;
ATel #17455, #17467).

Since the bright X-ray source LMC X-4 is located in the vicinity of 1A
0538-66, we cannot extract the light curve of 1A 0538-66 using the standard
data analysis (e.g., the MAXI on-demand process; https://maxi.riken.jp/mxondem)
without contamination. Therefore, we applied an 'image-fit' analysis (Morii
et al. 2016), following the procedure adopted in the previous report (ATel
#17467), to derive the source light curve. The derived 90-min binned light
curve in the 4-10 keV band shows that the current outburst started at 02:15
UT on February 27, 2026 (MJD 61098.094) and peaked at 03:50 UT on the same
day (MJD 61098.160). The observed peak flux in 4-10 keV is 0.178 &plusmn;
0.034 photons s-1 cm-2 (147 &plusmn; 28 mCrab). 
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), the outburst onset and
peak times correspond to orbital phases of 0.9842 and 0.9882, respectively.
These outburst parameters - onset phase, peak phase, and peak flux - are
consistent with those of the previous event (ATel #17467).

The remarkable feature of the current outburst is that the flux increased
again at an orbital phase of 0.03 (MJD 61098.868) instead of following
the fast-rise exponential-decay profile observed in the previous outburst
(ATel #17467). The peak flux of this second episode reached approximately
200 mCrab at 09:14 UT on February 28 (MJD 61099.385). No significant spectral
(color) changes were observed around the second peak. The 4-10 keV luminosity
during the second peak exceeded that of the first peak. Assuming a Crab-like
single power-law spectrum, the peak luminosity of the second episode is
estimated to be approximately 8 &times; 1038 erg s-1,
assuming a distance of 50 kpc.

The latest observation shows that the source flux in 4-10 keV band decreased
to 0.067 &plusmn; 0.025 photons s-1 cm-2 (55 &plusmn;
21 mCrab) at 10:01 UT on 2026 March 1 (MJD 61100.418).

1A 0538+66 light curve: https://maxi.riken.jp/figure/A0538/A0538_outburst2026.gif
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Password Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
https://www.astronomerstelegram.org/?read=17705
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