Short Gamma-Ray Bursts from the Merger of Two Black Holes (1602.05140v2)

Abstract: Short Gamma-Ray Bursts (GRBs) are explosions of cosmic origin believed to be associated with the merger of two compact objects, either two neutron stars, or a neutron star and a black hole. The presence of at least one neutron star has long been thought to be an essential element of the model: its tidal disruption provides the needed baryonic material whose rapid accretion onto the post-merger black hole powers the burst. The recent tentative detection by the Fermi satellite of a short GRB in association with the gravitational wave signal GW150914 produced by the merger of two black holes has challenged this standard paradigm. Here we show that the evolution of two high-mass, low-metallicity stars with main sequence rotational speeds a few tens of percent of the critical speed eventually undergoing a weak supernova explosion {\em can} produce a short gamma-ray burst. The outer layers of the envelope of the last exploding star remain bound and circularize at large radii. With time, the disk cools and becomes neutral, suppressing the magneto-rotational instability, and hence the viscosity. The disk remains 'long-lived dead' until tidal torques and shocks during the pre-merger phase heat it up and re-ignite accretion, rapidly consuming the disk and powering the short gamma-ray burst.

Short Gamma-Ray Bursts from Black Hole-Black Hole Mergers

The paper entitled "Short Gamma-Ray Bursts from the Merger of Two Black Holes" presents a nuanced exploration into a scenario that challenges conventional astrophysical models associated with Short Gamma-Ray Bursts (SGRBs). Historically, SGRBs have been attributed to mergers involving at least one neutron star, necessary for providing the requisite baryonic material that powers the gamma-ray burst through rapid post-merger accretion onto a black hole. This paper, however, examines an alternative framework in which SGRBs can arise from binary black hole (BH-BH) mergers, thereby questioning established paradigms postulated in earlier works.

Overview of Key Findings

The analysis begins by referencing the significant detection of gravitational waves (GW) from the merger of a high-mass BH-BH system, specifically the well-documented GW150914 event, with masses of approximately 36 and 29 solar masses. Notably, this GW event coincides with a tentative observation of a short GRB by the Fermi satellite, prompting a reassessment of the astrophysical processes involved in SGRBs.

The authors propose that under specific conditions involving two high-mass, low-metallicity stars, a BH-BH merger can indeed produce an SGRB. They suggest that the evolution of such stars, which undergo episodes of weak supernova explosions, leaves behind a 'fallback' disk around at least one of the black holes. This disk, while initially long-lived and inactive due to suppressed magneto-rotational instability, can be reactivated during the pre-merger phase by tidal interactions and shocks, resulting in a short but energetic gamma-ray burst.

Theoretical and Practical Implications

The implications of this research are twofold, affecting both our theoretical understanding and practical observational strategies. Theoretically, it expands the scope of SGRB progenitors by illustrating that specific stellar evolution channels can yield BH-BH systems with the requisite conditions for an SGRB. Practically, this model prompts the inclusion of BH-BH systems in searches for GRB counterparts to GW events, necessitating adjustments in observational strategies.

The fallback disk model described, contingent on parameters like metallicity and rotational velocity, also underscores the importance of metallicity in the evolutionary pathways of massive stars, influencing mass loss and angular momentum retention. The findings indicate that BH-BH mergers might not merely be silent gravitational wave events but could exhibit detectable electromagnetic counterparts, provided certain conditions are met.

Numerical Results and Predictions

In their quantitative analysis, the authors highlight conditions under which a disk can remain 'dead' for extended periods, only to be revived shortly before the merger. The disk accretion and subsequent GRB duration, derived from parameters such as the outer disk radius and viscosity values, are estimated to align well with observed SGRB durations.

The paper calculates that a modest disk mass of around $10^{-4} - 10^{-3} M_\odot$ is sufficient to generate a GRB luminosity comparable to that observed in conjunction with GW150914. These calculations reinforce the plausibility of BH-BH mergers contributing to the population of SGRBs if the disks possess certain characteristics regarding mass and angular momentum distribution.

Future Directions in Astrophysical Research

The potential convergence of GW and EM astronomy as suggested by this paper opens avenues for further exploration into the electromagnetic signatures of BH-BH mergers. This may spur targeted future observations and theoretical modeling efforts aimed at refining the understanding of merger dynamics and disk properties in these systems.

Future research may explore more varied environmental and initial star conditions, as well as detailed simulations of disk reactivation processes at various stages of binary evolution. Such endeavors could also provide insights into the broader implications for stellar evolution and the end stages of massive star lifecycles, potentially impacting related fields such as nucleosynthesis and black hole population statistics.

In conclusion, while the association of SGRBs with BH-BH mergers remains a topic of ongoing investigation, the framework offered in this paper represents a significant shift in understanding and serves as a foundation for both theoretical pursuits and observational campaigns in the context of multimessenger astronomy.