Multi-wavelength outburst activity from EP J174942.2-384834: a very faint X-ray transient discovered by Einstein Probe

TL;DR

This paper reports the discovery and multi-wavelength analysis of a very faint X-ray transient, EP J174942.2-384834, identified as a black hole candidate, showcasing Einstein Probe's capability to detect faint Galactic accreting objects.

Contribution

It presents the first detailed multi-wavelength characterization of a very faint X-ray transient discovered by Einstein Probe, highlighting its properties and classification as a black hole candidate.

Findings

Detected two major outbursts and a rebrightening over seven months.

Spectral modeling indicates a cool, possibly truncated accretion disk.

Optical/UV emission correlates with X-ray flux, suggesting a disk-dominated origin.

Abstract

We report the discovery and multi-wavelength characterization of the Galactic transient EP J174942.2$-$384834, first detected by the Einstein Probe during a faint X-ray outburst in March 2025. Coordinated follow-up observations revealed two major outbursts and a rebrightening over a seven-month period. Broadband X-ray spectral modeling shows that the outburst emission was dominated by thermal Comptonization of very soft seed photons. The absence of a detected thermal disk component, together with the low inferred seed-photon temperature, is consistent with a cool and possibly truncated accretion disk. The X-ray spectrum remained consistently hard throughout the outburst activity, with a power-law photon index of $\Gamma \approx 1$-2, gradually softening as the flux declined. The optical/UV counterpart brightened in tandem with the X-ray emission and exhibited a blue continuum with broad Balmer absorption features. Together with the optical/UV - X-ray luminosity correlation, this supports a disk-dominated origin of the optical/UV outburst emission, with viscous heating likely playing a major role and irradiation possibly contributing, especially in the UV. No radio counterpart was detected, implying at most very faint jet activity. Taken together, the observed properties support the classification of EP J174942.2$-$384834 as a very faint X-ray transient black hole candidate. This study demonstrates the ability of Einstein Probe to uncover and characterize the faintest accreting compact objects in the Galaxy.