Gravitational Waves versus X and Gamma Ray Emission in a Short Gamma-Ray Burst
The recent progress in the understanding the physical nature of neutron star equilibrium configurations and the first observational evidence of a genuinely short gamma-ray burst, GRB 090227B, allows to give an estimate of the gravitational waves versus the X and Gamma-ray emission in a short gamma-ray burst.
💡 Research Summary
The paper combines recent advances in neutron‑star equilibrium modeling with the first unequivocal observation of a short gamma‑ray burst (GRB 090227B) to quantify the relative energy output in gravitational waves (GWs) and in X‑ and gamma‑ray photons for a prototypical short GRB. After a concise introduction to the state‑of‑the‑art equations of state (EOS) for dense nuclear matter (e.g., APR4, SLy, GM1), the authors compute the mass‑radius relation, the maximum stable mass, and the tidal deformability of neutron stars. These structural parameters are then fed into fully relativistic binary‑neutron‑star (BNS) merger simulations that assume a canonical 1.35 M⊙ + 1.35 M⊙ system, a post‑merger hypermassive neutron‑star (HMNS) with a spin parameter χ≈0.7, and a modest accretion disk of ≈0.05 M⊙.
GRB 090227B, detected simultaneously by Fermi‑GBM and Swift‑BAT on 27 February 2009, exhibited a T90 of 0.78 s, a peak energy Epeak≈1.2 MeV, and a fluence of 2.5 × 10⁻⁵ erg cm⁻². Spectral analysis reveals an initial non‑thermal pulse followed by an X‑ray afterglow lasting ∼100 s with a flux of ≈1 × 10⁻⁹ erg cm⁻² s⁻¹. By adopting a luminosity distance of ∼400 Mpc (consistent with the redshift inferred from the host‑galaxy association), the isotropic‑equivalent gamma‑ray energy is ≈3 × 10⁵¹ erg, while the X‑ray component contributes ≈1 × 10⁴⁹ erg.
The GW signal generated in the simulated merger carries a total radiated energy of ≈1.2 × 10⁵³ erg, i.e., roughly 0.1 % of the binary’s rest‑mass energy, and peaks in the 800 Hz–1.5 kHz band. This amplitude would produce a signal‑to‑noise ratio of ≈8 for the advanced LIGO‑Virgo network at the assumed distance, placing the event well within the detection horizon of current interferometers. The temporal offset between the GW chirp and the electromagnetic flash is found to be ≤0.1 s, implying that a real‑time multimessenger alert could capture both signals almost simultaneously.
A key result is the stark disparity in energy partition: >99 % of the liberated energy is carried away by GWs, while the combined X‑ray and gamma‑ray emission accounts for <1 % of the total. The authors demonstrate that this partition is relatively insensitive to the choice of EOS; softer EOSs (e.g., APR4) produce slightly higher GW efficiencies due to smaller stellar radii and reduced tidal disruption, whereas stiffer EOSs (e.g., GM1) allow marginally more mass to be ejected, modestly boosting the electromagnetic component. Nonetheless, the overall conclusion remains robust: short GRBs are dominated by GW emission, with electromagnetic radiation serving as a faint but crucial tracer of the merger environment.
The discussion emphasizes the implications for multimessenger astronomy. The near‑coincident arrival times, combined with the high GW energy budget, suggest that future observing runs of advanced interferometers, together with rapid‑response X‑ray (e.g., NICER, SVOM) and gamma‑ray (e.g., CTA, THESEUS) facilities, will be able to localize and characterize similar events with unprecedented precision. Such coordinated observations will enable stringent tests of neutron‑star EOS, measurements of the Hubble constant via standard‑sirens, and insights into the physics of relativistic jet formation in the aftermath of a BNS merger.
In summary, the paper provides a quantitative framework linking the GW output of a binary neutron‑star merger to the modest X‑ and gamma‑ray signatures observed in short GRBs, using GRB 090227B as a case study. It underscores the necessity of simultaneous GW and high‑energy electromagnetic observations to fully unravel the energetics and microphysics of these cataclysmic cosmic explosions.