On the Nature of Einstein Probe Transient EP250916a: Insights from X-Ray, Optical, and Radio Observations

Jaisawal, GK; Illiano, G; Carotenuto, F; Bouquin, AL; Russell, DM; Leloudas, G; Sanna, A; Akl, D; Fender, R; Motta, S

Journal article

On the Nature of Einstein Probe Transient EP250916a: Insights from X-Ray, Optical, and Radio Observations

Abstract:: Abstract We report multiwavelength studies of the transient EP250916a, detected by the Einstein Probe on 2025 September 16. Located at low Galactic latitude, the source exhibited a rapid X-ray brightening, reaching an unabsorbed 0.5–10 keV flux of (6.4 ± 0.1) × 10 −10 erg cm −2 s −1 , followed by a plateau and a two-stage decay lasting over 40 days. Swift/X-Ray Telescope (XRT) monitoring shows a persistently hard spectrum (Γ ≈ 1.6–2.2) with only modest softening during decay, while a Nuclear Spectroscopic Telescope Array (NuSTAR) observation confirms a hard-state continuum extending up to 70 keV. Timing analysis of XMM-Newton data reveals a weak quasiperiodic oscillation (QPO) at ∼13 Hz. No other coherent pulsations or thermonuclear bursts are detected. Broadband spectral modeling favors a nonthermal power-law continuum with partial-covering absorption and shows no significant thermal disk component. Optical imaging obtained with Nordic Optical Telescope, Las Campanas Observatory, and GaiaDR3 identifies two faint sources within the 2″ Swift/XRT positional uncertainty. A MeerKAT observation at 1.28 GHz yielded no radio counterpart, with a 3 σ upper limit of 60 μ Jy beam −1 . The combination of a long-lasting outburst, a hard nonthermal X-ray spectrum, a weak QPO detection, the absence of coherent timing features, and faint potential optical counterparts disfavors a stellar-flare or extragalactic origin and supports an accreting compact-object scenario. Comparisons with similar faint, hard-state transients place EP250916a within a growing population of low-luminosity, hard-state black hole X-ray binary candidates.

Publication status:: Published

Peer review status:: Peer reviewed

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APA Style

Jaisawal, G. K., Illiano, G., Carotenuto, F., Bouquin, A. L., Russell, D. M., Leloudas, G., Sanna, A., Akl, D., Fender, R., & Motta, S. (2026). On the Nature of Einstein Probe Transient EP250916a: Insights from X-Ray, Optical, and Radio Observations. The Astrophysical Journal, 1005(2), 161–161.

MLA Style

Jaisawal, GK, et al. “On the Nature of Einstein Probe Transient EP250916a: Insights from X-Ray, Optical, and Radio Observations.” The Astrophysical Journal, vol. 1005, no. 2, 2026, pp. 161–61.

Chicago Style

Jaisawal, GK, G Illiano, F Carotenuto, et al. 2026. “On the Nature of Einstein Probe Transient EP250916a: Insights from X-Ray, Optical, and Radio Observations.” The Astrophysical Journal 1005 (2): 161–61.
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