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From: atel@astronomerstelegram.org
Subject: ATel  17772
Message-Id: <20260502070313.0F755288082D@server.astronomerstelegram.org>
Date: Sat,  2 May 2026 03:03:13 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17772							     ATEL #17772

Title:	MAXI/GSC detection of an X-ray burst probably from GS 0836-429
		(MX 0836-42)
Author:	H. Negoro, M. Nakajima (Nihon U.), H. Aoe (Ehime U.), K. Saito,
		T. Yokoyama, M. Wada (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai,
		M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo,
		H. Hiramatsu, A. Yoshida (AGU), Y. Tsuboi, Y. Ishihara, H. Ogaki (Chuo
		U.), M. Shidatsu, C. Kang, 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), and S. Ogawa (TUS)
Queries:	negoro.hitoshi@nihon-u.ac.jp
Posted:	1 May 2026; 15:12 UT
Subjects:X-ray, Neutron Star

The MAXI/GSC nova alert system triggered on a bright X-ray transient source
at 08:47:31 UT on April 30, 2026. Assuming that the source flux was constant
over the transit, we obtain the source position using the GSC_2 counter
at

(R.A., Dec) = (129.480 deg, -43.152 deg) = (08 37 55, -43 09 07) (J2000)

with a statistical 90% C.L. elliptical error region with long and short
radii of 0.79 deg and 0.47 deg, respectively. The roll angle of the long
axis from the north direction is 125.0 deg counterclockwise. There is an
additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 162 +- 52 mCrab (4.0-10.0keV,
1 sigma error).

Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners: 
(R.A., Dec) = (127.826, -43.150) deg = (08 31 18, -43 08 59) (J2000)

(R.A., Dec) = (128.930, -44.429) deg = (08 35 43, -44 25 44) (J2000)

(R.A., Dec) = (132.376, -42.778) deg = (08 49 30, -42 46 40) (J2000)

(R.A., Dec) = (131.231, -41.532) deg = (08 44 55, -41 31 55) (J2000)

There was no significant excess flux in the previous transit at 07:14 UT
and in the next transit at 10:20 UT with an upper limit of 20 mCrab for
each.

We note that this event was detected with GSC_2, GSC_7, and a not-well
calibrated counter GSC_1. GSC_7 yielded a different source position centered
at (R.A., Dec) = (129.892 deg, -43.865 deg). We adopt the GSC_2 result
here due to its superior calibration.

The X-ray burster GS 0836-429 (a.k.a. MX 0836-42 and 4U 0836-429) and the
Be X-ray binary pulsar GS 0834-430 (Makino and the Ginga Team, 1990 IAU
Circ 5139, Aoki et al. 1992, PASJ 44, 641) are both located within the
GSC_2 error region. Since no GSC counters detected significant hard X-rays
in the 10-20 keV band, this event is likely an X-ray burst from GS 0836-429.
If indeed the burst originated from GS 0836-429, this is the first burst
detection from the source by the MAXI/GSC in over 17 years. GS 0836-429
was previously active in 2003-2004, with 61 X-ray bursts detected by the
INTEGRAL/JEM-X (Aranzana et al. 2016, A&A 586, A142, also Chelovekov et
al. 2005, Astron. Letters 31, 681), and a total of 78 bursts recorded between
1996 and 2012 (Galloway et al. 2020, ApJS, 249, 32).  
Since then, the source has remained in a long-term quiescent state. We
then examined recent activity of this source using GSC data, but no significant
activity exceeding 10 mCrab was found.

This burst may signal a transition to a new active state for GS 0836-429
in the near future.

Trigger Information about this burst: http://maxi.riken.jp/alert/novae/1160275146/1160275146.htm
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Password Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)
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