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

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 ≳ 10 ^49 (10 ^50 ) erg s ^−1 for an assumed redshift z ≳ 0.5 (2.0). The Ne...

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Main Authors: Brendan O’Connor, Dheeraj Pasham, Igor Andreoni, Jeremy Hare, Paz Beniamini, Eleonora Troja, Roberto Ricci, Dougal Dobie, Joheen Chakraborty, Mason Ng, Noel Klingler, Viraj Karambelkar, Sam Rose, Steve Schulze, Geoffrey Ryan, Simone Dichiara, Itumeleng Monageng, David Buckley, Lei Hu, Gokul P. Srinivasaragavan, Gabriele Bruni, Tomás Cabrera, S. Bradley Cenko, Hendrik van Eerten, James Freeburn, Erica Hammerstein, Mansi Kasliwal, Chryssa Kouveliotou, Keerthi Kunnumkai, James K. Leung, Amy Lien, Antonella Palmese, Takanori Sakamoto
Format: Article
Language:English
Published: IOP Publishing 2025-01-01
Series:The Astrophysical Journal Letters
Subjects:
X-ray astronomy
X-ray transient sources
Relativistic jets
Gamma-ray bursts
Black holes
Online Access:https://doi.org/10.3847/2041-8213/ada7f5
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Summary: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 ≳ 10 ^49 (10 ^50 ) 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 ≳ 10 ^46 (10 ^47 ) 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 × 10 ^50 (6 × 10 ^51 ) 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.
ISSN:2041-8205

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