Characterization of a Peculiar Einstein Probe Transient EP240408a: An Exotic Gamma-Ray Burst or an Abnormal Jetted Tidal Disruption Event?

We present the results of our multiwavelength (X-ray to radio) follow-up campaign of the Einstein Probe transient EP240408a. The initial 10 s trigger displayed bright soft X-ray (0.5-4 keV) radiation with peak luminosity L_X ≳ 10⁴⁹ (10⁵⁰) erg s⁻¹ for an assumed redshift z ≳ 0.5 (2.0). The Neil Gehrels Swift Observatory and Neutron star Interior Composition ExploreR discovered a fading X-ray counterpart lasting for ∼5 days (observer frame), which showed a long-lived (∼4 days) plateau-like emission (t^−0.5) before a sharp power-law decline (t⁻⁷). The plateau emission was in excess of L_X ≳ 10⁴⁶ (10⁴⁷) erg s⁻¹ at z ≳ 0.5 (2.0). Deep optical and radio observations resulted in nondetections of the transient. Our observations with Gemini South revealed a faint potential host galaxy (r ≈ 24 AB mag) near the edge of the X-ray localization. The faint candidate host, and lack of other potential hosts (r ≳ 26 AB mag; J ≳ 23 AB mag), imply a higher redshift origin (z ≳ 0.5), which produces extreme X-ray properties that are inconsistent with many known extragalactic transient classes. In particular, the lack of a bright gamma-ray counterpart, with the isotropic-equivalent energy (10-10,000 keV) constrained by GECam and Konus-Wind to E_γ,iso ≲ 4 × 10⁵⁰ (6 × 10⁵¹) erg at z ≈ 0.5 (2.0), conflicts with known gamma-ray bursts of similar X-ray luminosities. We therefore favor a jetted tidal disruption event as the progenitor of EP240408a at z ≳ 1.0, possibly caused by the disruption of a white dwarf by an intermediate-mass black hole. The alternative is that EP240408a may represent a new, previously unknown class of transient.