Multi-wavelength Constraints on the Transient EP250905a
Abstract
Fast X-ray transients (FXTs) are a diverse class of high-energy suggested origins, ranging from stellar explosions to compact object mergers. The Einstein Probe (EP) satellite discovers approximately 100 FXTs per year. We aim to constrain the physical origin of EP250905a. We analyze X-ray, optical, near-infrared (NIR), and radio temporal and spectral properties of EP250905a. In addition, we assess the possible role of weak gravitational lensing in shaping its observed characteristics. EP250905a fades rapidly in X-rays, and we detect no NIR or radio emission, but we detect early optical emission that rapidly fades beyond the detection limits. Two nearby galaxies are identified for which we derive spectroscopic redshifts of z=0.374 (G1) and z=2.714 (G2). Our analysis favors G2 as the host of the FXT EP250905a. The angular separation of 2.56\, between the FXT's optical counterpart and the center of the G1 galaxy suggests the emission of the FXT might be moderately magnified by lensing effects (μ≈3.9) given the inferred Einstein radius of G1 (θE≈1.9 arcsec). The data are best explained as an afterglow from a mildly off-axis structured jet at z=2.714, providing a consistent broadband interpretation that also allows for weakly lensed emission of EP250905a.
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