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Descrizione
Gamma-ray bursts (GRBs) are extragalactic MeV transients lasting from fractions of a second to several minutes, typically associated with mergers of compact objects or the collapse of massive stars. On 2 July 2025, the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope detected three emission episodes with overlapping sky localizations, occurring 1-2 hours apart, collectively designated GRB 250702B. Taken together, the emission lasted more than 3 hours, making it the longest MeV transient ever observed. Follow-up observations with the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array revealed a rapidly decaying soft X-ray counterpart over the subsequent ten days.
Our time-resolved spectral analysis of the prompt emission phase shows that the gamma-ray spectra are harder than those of long GRBs. The spectra are well described by a single power law extending from 10 keV to 40 MeV, with evidence for a spectral break above 50 MeV.
The extreme duration, broadband spectral properties, and X-ray evolution suggest GRB 250702B likely originates from a relativistic jet launched during the tidal disruption of a star by a compact object. From $\gamma\gamma$ opacity constraints, we derive a lower limit on the jet bulk Lorentz factor of $\Gamma$ > 10. The inferred luminosity and energetics are consistent with those of previously reported relativistic tidal disruption events. The MeV spectra disfavour inverse Compton scenarios and are naturally explained by synchrotron emission from sub-TeV electrons.
| Sessione | Oggetti variabili e transienti in astrofisica |
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