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From: atel@astronomerstelegram.org
Subject: ATel  17842
Message-Id: <20260614070314.220BE2880634@server.astronomerstelegram.org>
Date: Sun, 14 Jun 2026 03:03:14 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17842							     ATEL #17842

Title:	SVOM follow-up of the X-ray transient EP260531a
Author:	F. Cangemi (APC, France), L. Tao (IHEP, China), A. Coleiro
		(APC, F.), L. Zhang (IHEP, C)
Queries:	cangemi@apc.in2p3.fr
Posted:	13 Jun 2026; 08:37 UT
Subjects:X-ray, Transient

We report on SVOM ToO follow-up observations of EP260531a, a new X-ray
transient discovered by the Einstein Probe mission on 2026 May 31 (ATel
#17828). SVOM performed observations at the position RA = 265.71186, Dec
= -31.200138 between 2026 June 7 at 23:49:29 UT and 2026 June 8 at 04:40:42
UT.

The SVOM/MXT spectrum (5747 s of exposure) in the 0.3-10 keV energy range
can be well described (chi2/dof = 1073.74/1000) by an absorbed diskbb model
with a temperature of kT = 0.47 +/- 0.04 keV and a column density of nH
= 2.90 (+0.28 / -0.25) 10^22 cm^-2. The 0.5-10 keV flux is 1.73 (+0.06
/ -0.59) e-10 ergs/cm2/s. The addition of a power-law component does not
significantly improve the fit. All uncertainties are quoted at the 90%
confidence level.

The temperature and the flux derived from the MXT spectrum are consistent
with that reported from Einstein Probe/FXT observations (ATel #17833) on
2026 June 4 suggesting that the source may have reached a plateau. Such
a temperature is compatible with a black hole X-ray binary observed in
the low-hard state.

No significant emission was detected with SVOM/ECLAIRs. Assuming a power-law
spectrum with a photon index of Gamma = 2, we derived a 3-sigma upper limit
on the source flux of 2.21 e-10 ergs/cm2/s in the 4-150 keV energy range.

Optical observations performed with SVOM/VT at the position of the X-ray
source do not show any significant variability in the VT_R band of the
possible optical counterpart reported in ATel #17840. We note, however,
that several optical sources are present within the error circle, preventing
an unambiguous identification of the counterpart based on the current VT
observations alone.

Additional SVOM observations are scheduled for 2026 June 15.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission
led by the Chinese National Space Administration (CNSA, China), National
Center for Space Studies (CNES, France) and the Chinese Academy of Sciences
(CAS, China), which is dedicated to observing gamma-ray bursts and other
transient phenomena in the energetic universe. ECLAIRs was developed jointly
by APC, CEA, CNES and IRAP.

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