The curious case of EP241021a: Unraveling the mystery of its exceptional rebrightening

Abstract

Fast X-ray Transients (FXTs) are a rare and poorly understood phenomenon with a variety of possible progenitors. The launch of the Einstein Probe (EP) mission has facilitated a rapid increase in the real-time discovery and follow-up of FXTs. We focus on the recent EP discovered transient EP241021a, which shows a peculiar panchromatic behavior. We obtained optical and near-infrared multi-band imaging and spectroscopy with the Fraunhofer Telescope at Wendelstein Observatory, the Hobby-Eberly Telescope, and the Very Large Telescope over the first 100 days of its evolution. EP241021a was discovered by EP as a soft X-ray trigger, but was not detected at gamma-ray frequencies. The observed soft X-ray prompt emission spectrum is consistent with non-thermal radiation, which requires at least a mildly relativistic outflow with bulk Lorentz factor $\Gamma \gtrsim 4$. The optical and near-infrared lightcurve has a two component behavior where an initially fading component $\sim t^{-1}$ turns to a rise steeper than $\sim t^4$ after a few days before peaking at $M_r\approx -21.8$ mag and quickly returning to the initial decay. The peak absolute magnitude is the most luminous optical emission associated to an FXT, superseding EP240414a. Standard supernova models are unable to reproduce either the absolute magnitude or rapid timescale ($<2$ d) of the rebrightening. The X-ray, optical and near-infrared spectral energy distributions display a red color $r-J\approx 0.8$ mag, and point to a non-thermal origin (${\nu }^{-1}$) for the broadband emission. By considering a gamma-ray burst as a plausible scenario, we favor a refreshed shock as the cause of the rebrightening. This is consistent with the inference of an at least mildly relativistic outflow based on the prompt trigger. Our results suggest a link between EP discovered FXTs and gamma-ray bursts.

Accepted in A&A

References in corpus (48)

• Gaia Early Data Release 3: The astrometric solution
• The magnetar model for Type I superluminous supernovae I: Bayesian analysis of the full multicolour light curve sample with MOSFiT
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• Discovery of the nearby long, soft GRB 100316D with an associated supernova
• A time - luminosity correlation for Gamma Ray Bursts in the X - rays
• Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow
• The HETDEX Instrumentation: Hobby-Eberly Telescope Wide Field Upgrade and VIRUS
• Signatures of a jet cocoon in early spectra of a supernova associated with a $\gamma$-ray burst
• Bulk Lorentz factors of Gamma-Ray Bursts
• Afterglow Lightcurves from Misaligned Structured Jets
• Smooth Light Curves from a Bumpy Ride: Relativistic Blast Wave Encounters a Density Jump
• Science objectives of the Einstein Probe mission
• The Radio to GeV Afterglow of GRB 221009A
• Theoretical Models of Optical Transients. I. A Broad Exploration of the Duration-Luminosity Phase Space
• A deep survey of short GRB host galaxies over $z\sim 0-2$: implications for offsets, redshifts, and environments
• An extremely energetic supernova from a very massive star in a dense medium
• The complex light-curve of the afterglow of GRB071010A
• Extragalactic fast X-ray transient candidates discovered by Chandra (2000-2014)
• GRB 221009A Afterglow from a Shallow Angular Structured Jet
• The Hydrogen-Poor Superluminous Supernovae from the Zwicky Transient Facility Phase-I Survey: II. Light Curve Modeling and Characterization of Undulations
• Blasts from the Past: Supernova Shock Breakouts among X-Ray Transients in the XMM-Newton Archive
• A wind environment and Lorentz factors of tens explain gamma-ray bursts X-ray plateau
• RAPTOR observations of delayed explosive activity in the high-redshift gamma-ray burst GRB 060206
• Constraints on the circumburst environments of short gamma-ray bursts
• Luminous Supernovae: Unveiling a Population Between Superluminous and Normal Core-collapse Supernovae
• X-ray flares, plateaus, and chromatic breaks of GRB afterglows from up-scattered forward-shock emission
• Extragalactic FXT Candidates Discovered by Chandra (2014-2022)
• Cosmological Fast Optical Transients with the Zwicky Transient Facility: A Search for Dirty Fireballs
• What can we learn from "internal plateaus"? The peculiar afterglow of GRB 070110
• A long optical plateau in the afterglow of the Extended Emission short GRB 150424A: Evidence for energy injection by a magnetar?
• A Sensitive Search for Supernova Emission Associated with the Extremely Energetic and Nearby GRB 221009A
• Triggering the Untriggered: The First Einstein Probe-Detected Gamma-Ray Burst 240219A and Its Implications
• No visible optical variability from a relativistic blast wave encountering a wind-termination shock
• The early afterglow of GRB 190829A
• Afterglows from precursors in Gamma Ray Bursts. Application to the optical afterglow of GRB 091024
• Challenging GRB models through the broadband dataset of GRB060908
• The nature of the late achromatic bump in GRB 120326A
• Einstein Probe discovery of EP240408a: a peculiar X-ray transient with an intermediate timescale
• EP240414a: Off-axis View of a Jet-Cocoon System from an Expanded Progenitor Star
• Flares in gamma-ray burst X-ray afterglows as prompt emission from slightly misaligned structured jets
• Astro-COLIBRI -- The COincidence LIBrary for Real-time Inquiry for multimessenger astrophysics
• Extremely energetic supernova explosions embedded in a massive circumstellar medium: the case of SN 2016aps
• Gamma-Ray Bursts
• Spectral Hardening Reveals Afterglow Emergence in Long-Duration Fast X-ray Transients: A Case Study of GRB 250404A/EP250404a
• The Signature of Refreshed Shocks in the of Afterglow of GRB030329
• Late X-ray flares from the interaction of a reverse shock with a stratified ejecta in GRB afterglows: simulations on a moving mesh
• Orphan optical flare as SOSS emission afterglow, localization in time
• Revisiting GRB 060218: new insights into low-luminosity gamma-ray bursts from a revised shock breakout model

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• The soft X-ray transient EP241021a: A cosmic explosion with a complex off-axis jet and cocoon from a massive progenitor
• Characterising EP241107a: Multiwavelength Observations of an Einstein Probe-detected Fast X-ray Transient
• Unveiling the nature of the Einstein Probe transient EP 241021a
• Search for the Optical Counterpart of Einstein Probe Discovered Fast X-ray Transients from Lulin Observatory
• TDE 2025abcr: A Tidal Disruption Event in the Outskirts of a Massive Galaxy
• Multi-epoch afterglow rebrightenings in GRB 250129A: Evidence for successive shock interactions
• Fast X-ray Transients produced by Off-axis Jet-Cocoons from Long Gamma-Ray Bursts