Electromagnetic Counterparts to Black Hole Mergers Detected by LIGO
The paper by Abraham Loeb investigates the potential electromagnetic (EM) counterparts to the merger of stellar-mass black holes (BHs) observed by the LIGO collaboration. LIGO's detection of gravitational waves (GWs) from such BH mergers, exemplified by GW150914, traditionally suggests a purely gravitational phenomenon devoid of any EM signals. Interestingly, the Fermi Gamma-ray Burst Monitor (GBM) reported a γ-ray transient occurring 0.4 seconds subsequent to the GW150914 event, overlapping with the event’s sky localization. This unexpected observation drives the exploration of plausible scenarios linking BH mergers with EM emissions.
Loeb presents a model wherein the GW and γ-ray signals share a common genesis from a rapidly rotating, massive-star collapse. According to this hypothesis, during the star’s collapse, the core bifurcates into two clumps positioned in a dumbbell configuration. This bifurcation results in the formation of two black holes, which eventually merge, an event detected by LIGO. Crucially, Loeb suggests that the residual accretion disk formed around the merger remnant could power a GRB through jet emissions, akin to the collapsar model concept for long-duration GRBs.
A key inference from the analysis is the requirement for the progenitor star to exhibit rapid rotation and massive composition, exceeding 100 solar masses (M⊙) before the collapse. Such characteristics point to a formation path involving the merger of two less massive stars, supporting a high-infall rate during the collapse. The resultant mass accretion and angular momentum conditions align with the rotational dynamics necessary for generating a GRB in this model.
Notably, the study suggests that the GRB's luminosity, deduced from the Fermi GBM data, implies an extraordinarily high accretion rate surpassing the Eddington limit. This mechanism could thereby elucidate the presence of a GRB signature post-BH merger, contrary to standard expectations.
Furthermore, Loeb discusses the implications for ongoing GW-BH research, indicating a potential paradigm where future LIGO detections could routinely investigate accompanying GRB signals. This prospect entails a systematic search strategy to identify EM counterparts associated with GW events, providing novel pathways for precision cosmology via GW-EM multi-messenger astronomy.
In conclusion, the paper underscores the necessity for further numerical simulations to refine the understanding of binary BH formation and GRB production amidst massive stellar collapses. While Loeb’s model posits a compelling framework for potential GW-EM correlation, empirical validation through additional detections and enhanced observational strategies would substantiate its feasibility. As LIGO's sensitivity evolves, the detection of similar correlated events could profoundly enrich the understanding of massive star evolution, BH merger dynamics, and the interplay between gravitational and electromagnetic phenomena in the cosmos.
