2024/10/29 by O'Connor, B., Pasham, D., Andreoni, I. +30 · 8 citations
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2410.21622
We present the results of our multi-wavelength (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_\textrmX \gtrsim 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 d (observer frame), which showed a long-lived (~4 d) plateau-like emission (t-0.5) before a sharp powerlaw decline (t-7). The plateau emission was in excess of L_\textrmX \gtrsim 1046 (1047) erg s-1 at z>0.5 (2.0). Deep optical and radio observations resulted in non-detections 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 \gtrsim 26 AB mag; J \gtrsim 23 AB mag), implies 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,\textrmiso \lesssim 4×1051 (6×1052) erg at z>0.5 (2.0), conflicts with known gamma-ray bursts (GRBs) of similar X-ray luminosities. We therefore favor a jetted tidal disruption event (TDE) 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.