Ultra-long MeV transient from a relativistic jet: a tidal disruption event 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 ($\propto t^{-1.9 \pm 0.1}$). The time-averaged spectra during the outbursts are well described by a single power law $dN_{\gamma}/dE \propto E^{-1.5}$, while upper limits above 100 MeV imply a spectral cutoff between 10 MeV and 100 MeV. Using standard gamma-ray transparency arguments, we derive 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 one with detected MeV gamma-ray emission. We argue that the observed emission is most likely produced by synchrotron radiation from sub-TeV electrons.