Letter to the Editor

Ultra-long MeV transient from a relativistic jet: A tidal disruption event candidate

¹ Gran Sasso Science Institute, Viale F. Crispi 7, L’Aquila (AQ) I-67100, Italy
² INFN – Laboratori Nazionali del Gran Sasso, L’Aquila (AQ) I-67100, Italy
³ Cahill Center for Astronomy & Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁴ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
⁵ University of Science and Teichnology of China, Hefei, Anhui, China
⁶ Tianfu Cosmic Ray Research Center 610000 Chengdu, China
⁷ Yerevan State University, 1 Alek Manukyan St, Yerevan 0025, Armenia
⁸ Department of Astronomy, Astrophysics and Space Engineering Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore, 453552 Madhya Pradesh, India
⁹ Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
¹⁰ Institute for Gravitation & the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA

^⋆ Corresponding authors: gor.oganesyan@gssi.it; ekammoun@caltech.edu; annarita.ierardi@gssi.it

Received: 24 July 2025
Accepted: 11 October 2025

Abstract

On July 2, 2025, the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray space telescope detected three short-duration MeV transients with overlapping sky locations. These events, named as GRB 250702D, B, and E (collectively referred to as DBE), triggered the detector with delays of approximately 1 − 2 hours between each burst. Follow-up observations of this unusually long MeV transient (lasting more than 3 hours) by the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array over a period of ∼10 days revealed a steep temporal decline in soft X-rays (∝t^{−1.9 ± 0.1}). The time-averaged spectra during the outbursts are well described by a single power law, dN_γ/dE ∝ E^−1.5, while upper limits above 100 MeV imply a spectral cutoff between 10 MeV and 100 MeV. Using standard γ-ray transparency arguments, we derived a lower limit on the bulk Lorentz factor. Combined with the steep decline in X-rays, these constraints point to a relativistic jet origin. The properties of DBE are inconsistent with established GRB spectral–energy correlations, disfavoring classical long-GRB progenitors. Instead, the basic characteristics of DBE resemble those of previously reported jetted tidal disruption events (TDEs), though alternative progenitor channels cannot be excluded. In the relativistic TDE scenario, DBE is the first to have a MeV γ-ray emission detected. We argue that the observed emission is most likely produced by synchrotron radiation from sub-TeV electrons.