INTEGRAL Upper Limits on Gamma-Ray Emission from a Gravitational Wave Event
The study conducted by Savchenko et al. presents an analysis of potential gamma-ray emission associated with a gravitational wave event detected by the LIGO/Virgo collaboration, referred to as GW150914. The analysis utilized data from the INTEGRAL satellite, focusing specifically on the ability of the SPI-ACS (Spectrometer on INTEGRAL Anti-Coincidence Shield) to detect or constrain hard X-ray and gamma-ray emissions coincident with the gravitational wave signal.
Overview of Methodology
The central instrument used in this investigation, the SPI-ACS, is known for its omni-directional monitoring capability, which provides it with a substantial field of view and no reliance on triggering mechanisms, allowing continuous data recording. This quality is essential for capturing transient events such as gamma-ray bursts (GRBs) that might accompany gravitational wave signals. The paper emphasizes conducting simulations to understand the instrument's response given its position and the satellite's structure, which could attenuate incoming signals.
The researchers performed a meticulous search for short-duration (0.05 to 10 seconds) gamma-ray signals around the time of the LIGO trigger. The absence of any significant excess above the background noise allowed them to derive upper limits on gamma-ray fluences across the LIGO event's sky localization region.
Key Results
The analysis reports 3-sigma upper limits on the gamma-ray fluence in the range of $F_{\gamma}=2 \times 10{-8}$~erg~cm${-2}$ to $F_{\gamma}=10{-6}$~erg~cm${-2}$ within the energy range of 75~keV~-~2~MeV. These limits vary based on the assumed spectral models of GRBs and their sky positions, acknowledging INTEGRAL's coverage of about 95% of the LIGO/Virgo event locality. These restrictive bounds indicate that any gamma-ray emission associated with the GW150914 event would have to be significantly weaker than the gravitational wave emission, quantified by the ratio E$\gamma/$E$\mathrm{GW}<10{-6}$.
In addition to SPI-ACS, the paper outlines attempts to detect gamma-ray signals using the IBIS instruments within INTEGRAL, which, however, were constrained by their field of view relative to the event's location.
Discussion of Implications
The paper engages in an exploration of the physical implications of these results. Given that the LIGO detection aligns with a binary black hole in-spiral, existing theories suggest minimal to no gamma-ray emission; thus, the non-detection by INTEGRAL aligns with such expectations. The limits imposed on any potential emission provide novel constraints for theoretical models suggesting electromagnetic emission from such events, particularly those involving massive circumbinary environments or unusual stellar remnants like exotic compact objects.
Considerations and Future Directions
This study significantly contributes to the understanding of multi-messenger astronomy, implying that high-energy gamma-ray emissions may not universally accompany gravitational wave signals from black hole mergers. The rigorous constraints on the gamma-ray to gravitational wave energy ratio established by this study call for future collaborative efforts between gravitational wave detectors and electromagnetic observatories to refine these boundaries. Further enhancements in detection sensitivities and multi-band observational campaigns will aid in testing theoretical predictions, particularly those pertaining to exotic astrophysical objects.
In conclusion, Savchenko et al.'s investigation serves as a robust reference point for constraining the gamma-ray emissions in the context of gravitational wave astronomy and highlights the complexities involved in detecting weak electromagnetic counterparts to high-energy cosmic events.
