Characterization of a Peculiar Einstein Probe Transient EP240408a: An Exotic Gamma-Ray Burst or an Abnormal Jetted Tidal Disruption Event?
Buckley, David; Cenko, S. Bradley; Klingler, Noel; Sakamoto, Takanori; Kouveliotou, Chryssa; Hare, Jeremy; Hammerstein, Erica; Andreoni, Igor; Schulze, Steve; Hu, Lei; Rose, Sam; Ryan, Geoffrey; Freeburn, James; Dobie, Dougal; Karambelkar, Viraj; Cabrera, Tomás; Palmese, Antonella; O'Connor, Brendan; Kunnumkai, Keerthi; Ng, Mason; Troja, Eleonora; Leung, James K.; Bruni, Gabriele; Dichiara, Simone; Chakraborty, Joheen; Srinivasaragavan, Gokul P.; Kasliwal, Mansi; Pasham, Dheeraj; Beniamini, Paz; Ricci, Roberto; Monageng, Itumeleng; Eerten, Hendrik van; Lien, Amy
United States, Israel, Italy, Australia, Canada, South Africa, United Kingdom, Japan
Abstract
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 LX ≳ 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 LX ≳ 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 Eγ,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.