Transient MeV radiation from a relativistic tidal disruption candidate

Abstract: On July 2, 2025, the Gamma-ray Burst Monitor (GBM) onboard 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 >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 ($\propto t^{-1.9 \pm 0.1}$). These characteristics are consistent with the scenario in which DBE arises from a relativistic jet launched after the tidal disruption of a star by a compact object. In the relativistic Tidal Disruption Event (TDE) scenario, DBE is the first one with detected MeV $\gamma$-ray emission. We show that the time-resolved MeV spectra during the outbursts are best described by a single power-law, $dN_{\gamma}/dE \propto E^{-1.5}$, extending from 10 keV to 40 MeV. The upper limits at higher energies ($>$ 100 MeV) indicate that the spectrum breaks at $10-100$ MeV. From standard MeV $\gamma$-ray transparency arguments, we derive a lower limit on the jet bulk Lorentz factor. These constraints place DBE in the same category as four previously reported relativistic TDEs in terms of outburst luminosity and energetics. We discuss the origin of the MeV emission in several scenarios, including synchrotron and inverse Compton cooling of electrons accelerated in the jet.