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From: atel@astronomerstelegram.org
Subject: ATel  17840
Message-Id: <20260611070316.93FFC28807D2@server.astronomerstelegram.org>
Date: Thu, 11 Jun 2026 03:03:16 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17840							     ATEL #17840

Title:	A candidate NIR counterpart of the new X-ray transient EP260531a
		and indications of a black-hole accretor from archival X-ray data
Author:	Lara Sidoli (INAF-IASF Milano, Italy)
Queries:	sidoli@lambrate.inaf.it
Posted:	10 Jun 2026; 08:57 UT
Subjects:Infra-Red, Optical, X-ray, Transient

A new X-ray transient has been recently discovered by the Einstein Probe
(EP), EP260531a (ATel#17828). Its sky position has been refined with the
Follow-up X-ray Telescope (FXT; ATel#17833).

No known X-ray sources are consistent with the new FXT position. 
Two XMM-Newton observations cover EP260531a sky position (0886011301 and
0934200701), with no detections, nor in EPIC or in the Optical Monitor.

In the longest XMM observation (26.5 ks, ObsID 0886011301), the faintest
detected source (according to the EPIC PPS source list reported in the
XMM Science Archive) has an observed flux of 1.4e-15 erg/cm2/s (0.2-12
keV). Assuming this value as a rough indication of the quiescence, EP260531a
spans more than five orders of magnitudes in X-ray flux during outburst.

The X-ray outburst reported by ATel#17828 and ATel#17833 shows a days-long
rise with an absorbed (3-5x10^22 cm-2) soft spectrum (well described by
a double component continuum model with a steep power-law and a 0.44 keV
blackbody model), more compatible with a neutron star or black-hole transient
in a low mass X-ray binary, than with a high mass X-ray binary. The high
absorbing column density and the Galactic position indicate an X-ray binary
in the Galactic Bulge.

Searching the WFCAM Science Archive, JHK images from the UKIDSS Galactic
Plane Survey (Lawrence et al 2007, Lucas et al., 2008) are investigated
to search for NIR counterparts. Several sources lie within the new error
circle, yet two are markedly brighter than the others:

Star n.1)- UKIDSS_ID 438752313020 (=2MASS 17425134-3112003), a star located
at R.A.(J2000)=265.7139235, dec=-31.2001045, 6.6 arcsec offset from EP-FXT
position, with J=14.5718 mag (+/-0.0036), H=12.6469 mag (+/- 0.0013), K=11.6413
mag (+/-0.0014), and no counterparts in the Gaia catalogue (implying G>20
mag).

Star n.2)- UKIDSS_ID 438752313754 (=2MASS 17425083-3111515), a star located
at R.A.(J2000)=265.7117957, dec=-31.1976875, 9.2 arcsec offset from EP-FXT
position, with J=14.2089 mag (+/-0.0029), H=13.3489 mag (+/-0.0024), K=12.8594
mag (+/-0.0038). This star has a Gaia counterpart (GaiaDR3 4056622694415235584)
with G=17.9 mag, Teff=8600 K, located at a photo-geometric distance (Bailer-Jones
et al. 2023) of 1.7 kpc (+/- 0.4 kpc).

The star n.1 has no Gaia counterpart, unknown distance and it is highly
reddened (J-K=2.93 mag). Assuming it is a late type star, it should be
a giant (likely M-type) star located in the Galactic Bulge, otherwise it
is difficult to reconcile the huge reddening with the stellar brightness
for a dwarf, late star.

The star n.2 (the Gaia-detected one) is an A-type dwarf star which only
needs an extinction Av=6-8 mag to explain its NIR colors and magnitudes
at the Gaia distance. This appears incompatible with the much larger value
of the extinction (Av=10-20 mag) inferred from the X-ray spectral fit (absorbing
column density 5e22 cm-2), more naturally placing the X-ray transient at
much larger distances, in the Galactic Bulge.

In conclusion, the preferred counterpart of the X-ray transient is star
n.1, a highly reddened counterpart, compatible with a X-ray binary hosting
a late (likely M) giant star, located in the Galactic Bulge.

A final important point concerns the quiescent X-ray luminosity, which
is more consistent with a black-hole accretor: for a quiescent flux below
10^-15 erg/cm2/s (as implied by the XMM archival observations), the luminosity
is around 10^31 erg/s (at 10 kpc), reaching 10^32 erg/s (typical value
for a neutron star in quiescence) only at a distance of about 30 kpc.

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https://www.astronomerstelegram.org/?read=17840
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