TY - JOUR
T1 - The curious case of EP241021a
T2 - Unraveling the mystery of its exceptional rebrightening
AU - Busmann, Malte
AU - O’Connor, Brendan
AU - Sommer, Julian
AU - Gruen, Daniel
AU - Beniamini, Paz
AU - Gill, Ramandeep
AU - Moss, Michael J.
AU - Palmese, Antonella
AU - Riffeser, Arno
AU - Yang, Yu Han
AU - Troja, Eleonora
AU - Dichiara, Simone
AU - Ricci, Roberto
AU - Klingler, Noel
AU - Gössl, Claus
AU - Hu, Lei
AU - Rau, Arne
AU - Ries, Christoph
AU - Ryan, Geoffrey
AU - Schmidt, Michael
AU - Yadav, Muskan
AU - Zeimann, Gregory R.
N1 - Publisher Copyright:
© The Authors 2025.
PY - 2025/9/17
Y1 - 2025/9/17
N2 - Context. 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. Aims. We focus on the recent EP discovered transient EP241021a, which shows a peculiar panchromatic behavior, with the aim of understanding its origin. Methods. We obtained optical and near-infrared multiband imaging and spectroscopy with the Fraunhofer Telescope at Wendelstein Observatory, the Hobby-Eberly Telescope, and the Very Large Telescope of the newly discovered EP transient EP241021a over the first 100 days of its evolution. Results. 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 nonthermal radiation, which requires at least a mildly relativistic outflow with a bulk Lorentz factor Γ ≳ 4. The optical and near-infrared light curve displays a two-component behavior, where an initially fading component, ∼ t −1, transitions to a rise steeper than ∼ t 3 after a few days, before peaking at an absolute magnitude of M r ≈ −21.8 mag and quickly returning to the initial decay. Standard supernova models are unable to reproduce either the absolute magnitude or the rapid timescale (< 2 d) of the rebrightening. The X-ray, optical and near-infrared spectral energy distributions display a red color, r − J ≈ 0.8 mag, and point to a nonthermal origin (∼ ν −1) for the broadband emission. 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. Conclusions. Our results suggest a link between EP-discovered FXTs and gamma-ray bursts, despite the lack of gamma-ray detections for the majority of EP transients.
AB - Context. 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. Aims. We focus on the recent EP discovered transient EP241021a, which shows a peculiar panchromatic behavior, with the aim of understanding its origin. Methods. We obtained optical and near-infrared multiband imaging and spectroscopy with the Fraunhofer Telescope at Wendelstein Observatory, the Hobby-Eberly Telescope, and the Very Large Telescope of the newly discovered EP transient EP241021a over the first 100 days of its evolution. Results. 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 nonthermal radiation, which requires at least a mildly relativistic outflow with a bulk Lorentz factor Γ ≳ 4. The optical and near-infrared light curve displays a two-component behavior, where an initially fading component, ∼ t −1, transitions to a rise steeper than ∼ t 3 after a few days, before peaking at an absolute magnitude of M r ≈ −21.8 mag and quickly returning to the initial decay. Standard supernova models are unable to reproduce either the absolute magnitude or the rapid timescale (< 2 d) of the rebrightening. The X-ray, optical and near-infrared spectral energy distributions display a red color, r − J ≈ 0.8 mag, and point to a nonthermal origin (∼ ν −1) for the broadband emission. 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. Conclusions. Our results suggest a link between EP-discovered FXTs and gamma-ray bursts, despite the lack of gamma-ray detections for the majority of EP transients.
KW - gamma-ray burst: general
KW - radiation mechanisms: non-thermal
KW - shock waves
KW - stars: jets
UR - https://www.scopus.com/pages/publications/105016904088
U2 - 10.1051/0004-6361/202554626
DO - 10.1051/0004-6361/202554626
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AN - SCOPUS:105016904088
SN - 0004-6361
VL - 701
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
M1 - A225
ER -