Are Ultra-Long Gamma-Ray Bursts Caused by Blue Supergiant Collapsars, Newborn Magnetars, or White Dwarf Tidal Disruption Events?

TL;DR

Ultra-long gamma-ray bursts (ulGRBs) with durations around 10,000 seconds may originate from blue supergiant collapsars, magnetars, or white dwarf tidal disruption events, with recent observations challenging some models and suggesting TDEs as a plausible energy source.

Contribution

This paper proposes that white dwarf tidal disruption events can explain observed supernova-like bumps in ulGRBs, expanding the possible origins of these phenomena.

Findings

WD-TDEs can produce SN-like bumps with observed energies and velocities.

X-ray ionization can suppress hydrogen lines in ejecta.

SN 2011kl's light curve is consistent with multiple models, but magnetar explanation is less natural.

Abstract

Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with extreme durations of $\sim 10^{4}$ s. Leading candidates for their origin are blue supergiant collapsars, magnetars, and white dwarf tidal disruption events (WD-TDEs) caused by massive black holes (BHs). Recent observations of supernova-like (SN-like) bumps associated with ulGRBs challenged both the WD-TDE and the blue supergiant models because of the detection of SNe and the absence of hydrogen lines, respectively. We propose that WD-TDEs can accommodate the observed SN-like bumps if the fallback WD matter releases energy into the unbound WD ejecta. The observed ejecta energy, luminosity, and velocity are explained by the gravitational energy, Eddington luminosity, and escape velocity of the formed accretion disk, respectively. We also show that the observed X-rays can ionize the ejecta, eliminating lines. The SN-like light curves (SN 2011kl) for the ulGRB 111209A are consistent with all three models, although a magnetar model is unnatural because the spin-down time required to power the SN-like bump is a hundred times longer than the GRB. Our results imply that TDEs are a possible energy source for SN-like events in general and for ulGRBs in particular.