INTEGRAL Detection of the First Prompt Gamma-Ray Signal Coincident with the Gravitational Wave Event GW170817

Abstract: We report the e INTernational Gamma-ray Astrophysics Laboratory (INTEGRAL) detection of the short gamma-ray burst GRB 170817A (discovered by Fermi-GBM) with a signal-to-noise ratio of 4.6, and, for the first time, its association with the gravitational waves (GWs) from binary neutron star (BNS) merging event GW170817 detected by the LIGO and Virgo observatories. The significance of association between the gamma-ray burst observed by INTEGRAL and GW170817 is 3.2 $\sigma$, while the association between the Fermi-GBM and INTEGRAL detections is 4.2 $\sigma$. GRB 170817A was detected by the SPI-ACS instrument about 2 s after the end of the gravitational wave event. We measure a fluence of $(1.4 \pm 0.4 \pm 0.6) \times$10$^{-7}$ erg cm$^{-2})$ (75--2000 keV), where, respectively, the statistical error is given at the 1 $\sigma$ confidence level, and the systematic error corresponds to the uncertainty in the spectral model and instrument response. We also report on the pointed follow-up observations carried out by INTEGRAL, starting 19.5 h after the event, and lasting for 5.4 days. We provide a stringent upper limit on any electromagnetic signal in a very broad energy range, from 3 keV to 8 MeV, constraining the soft gamma-ray afterglow flux to $<7.1\times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ (80--300 keV). Exploiting the unique capabilities of INTEGRAL, we constrained the gamma-ray line emission from radioactive decays that are expected to be the principal source of the energy behind a kilonova event following a BNS coalescence. Finally, we put a stringent upper limit on any delayed bursting activity, for example from a newly formed magnetar.

• The paper reports the first prompt detection of a short gamma-ray burst coincident with GW170817, confirming neutron star mergers as sources of sGRBs.
• It employs INTEGRAL's SPI-ACS instrument to achieve a signal-to-noise ratio of 4.6 and a 3.2σ association, precisely measuring the burst's fluence in the 75–2000 keV range.
• It establishes critical observational constraints that guide future multi-messenger studies and the integration of gamma-ray and gravitational wave detections.

An Analysis of INTEGRAL Observations of GW170817

The research conducted by Savchenko et al., investigates the observations of the gravitational wave event GW170817, in conjunction with the gamma-ray burst (GRB) GRB~170817A, by the INTEGRAL satellite. The detection of GRB~170817A coinciding with GW170817 uniquely marks the concurrent association of a short gamma-ray burst with a binary neutron star merger, detected by LIGO and Virgo observatories. This association is of pivotal importance, providing substantive evidence of the hypothesized theories regarding the origin of short gamma-ray bursts (sGRBs).

Key Observations and Data Analysis

INTEGRAL provided a significant contribution through the detection of GRB~170817A using the SPI-ACS instrument, noting a signal-to-noise ratio of 4.6 and establishing a 3.2σ association with the GW event. The burst was characterized by a fluence of (1.4 ± 0.4 ± 0.6) × 10$^{-7}$ erg cm$^{-2}$ in the 75–2000 keV range. This solidifies the long-standing hypothesis connecting sGRBs to neutron star mergers, framed by the optical identification of an event coincident with the gravitational waves detected from GW170817.

The follow-up observations by INTEGRAL commenced approximately 19.5 hours after GW170817, covering an expansive energy spectrum from 3 keV to 8 MeV over 5.4 days. Despite unsuccessful attempts to detect further electromagnetic emissions, it provided stringent upper limits for potential signals.

Theoretical and Practical Implications

The implications of this research are manifold:

• Theoretical Insight: The verification of neutron star mergers as progenitors of at least some sGRBs provides crucial observational constraints for theoretical models. It also opens avenues for exploring the detailed physics of neutron star mergers, including the role of the newly formed magnetars and the processes governing nucleosynthesis elements in kilonovae.
• Practical Applications: INTEGRAL's ability to detect and constrain gamma-ray signals and its significant high-energy response exhibits its continued relevance in multi-messenger astrophysics. Future studies involving similar observational setups could further elucidate the conditions and frequencies of sGRB occurrences in tandem with GW events.

Prospects for Future Developments

This study delineates a pathway for the prospective integration of gamma-ray and gravitational wave correlations, which will be instrumental in future studies aiming to understand the structure and emission mechanics of neutron star mergers. It also raises the potential for dedicated missions with improved sensitivity and rapid multi-wavelength follow-up capabilities. The expanding detection base with the enhancements in sensitivity of GW observatories might reveal a substantial sample size of such concurrent detections, improving the statistical significance of observational results and enabling precise mapping of sGRB parameters.

Conclusion

The INTEGRAL's detection of emission in coincidence with GW170817 exemplifies significant progress in multimodal astrophysical observations, confirming the dual presence of gamma-ray and gravitational wave phenomenology in neutron star mergers. This substantiates the hypothesis about the origins of sGRBs, shaping the contours of future theoretical astrophysical explorations, and emphasizes the need for integrated approaches in astronomical observational strategies.