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

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 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.

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Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-01-27
DOI
https://doi.org/10.3847/2041-8213/ada7f5
Citations
17
Primary Topic
Gamma-ray bursts and supernovae
Type
article
Field-Weighted Citation Impact
19.63

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article

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

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The Astrophysical Journal Letters
Gamma-ray bursts and supernovae
19.63
article

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

Erica Hammerstein, Joheen Chakraborty, R. Ricci, C. Kouveliotou, G. Bruni, Gokul P. Srinivasaragavan, Hendrik van Eerten, E. Troja, Sam Rose, James K. Leung, James Freeburn, I. M. Monageng, A. Palmese, Brendan O’Connor, A. Y. Lien, D. A. H. Buckley, S. Dichiara, Jeremy Hare, Tomás Cabrera, Lei Hu, T. Sakamoto, Mason Ng, S. B. Cenko, Igor Andreoni, Steve Schulze, Dougal Dobie, Geoffrey Ryan, Dheeraj R. Pasham, Paz Beniamini, Viraj Karambelkar, Keerthi Kunnumkai, M. M. Kasliwal, Noel Klingler
article en
17 citations

Abstract

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 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.

The Astrophysical Journal LettersVol. 979(2)
Science and Technology Facilities Council
Openalex Percentile: Top 1%
Gamma-ray bursts and supernovae
19.63
DOIOpen Access PDFOpenAlex
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