A candidate NIR counterpart of the new X-ray transient EP260531a and indications of a black-hole accretor from archival X-ray data

ATel #17840; Lara Sidoli (INAF-IASF Milano, Italy)
on 10 Jun 2026; 08:57 UT
Credential Certification: Lara Sidoli (sidoli@lambrate.inaf.it)

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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.