Hierarchical triple mergers: testing Hawking's area theorem with the inspiral signals

Hawking's area theorem is one of the fundamental laws of black holes (BHs), which has been tested at a confidence level of $\sim 95\%$ with gravitational wave (GW) observations by analyzing the inspiral and ringdown portions of GW signals independently. In this work, we propose to carry out the test in a new way with the hierarchical triple merger (i.e., two successive BH mergers occurred sequentially within the observation window of GW detectors), for which the properties of the progenitor BHs and the remnant BH of the first coalescence can be reliably inferred from the inspiral portions of the two mergers. As revealed in our simulation, a test of the BH area law can be achieved at the significance level of $\gtrsim 3\sigma$ for the hierarchical triple merger events detected in LIGO/Virgo/KAGRA's O4/O5 runs. If the hierarchical triple mergers contribute a $\gtrsim 0.1\%$ fraction to the detected BBHs, a precision test of the BH area law with such systems is achievable in the near future. Our method also provides an additional criterion to establish the hierarchical triple merger origin of some candidate events.Comment: 5 pages, 5 figures, 1 tabl

Maximum gravitational mass MTOV=2.25−0.07+0.08M⊙M_{\rm TOV}=2.25^{+0.08}_{-0.07}M_\odot inferred at about 3%3\% precision with multimessenger data of neutron stars

The maximal gravitational mass of nonrotating neutron stars ($M_{\rm TOV}$) is one of the key parameters of compact objects and only loose bounds can be set based on the first principle. With reliable measurements of the masses and/or radii of the neutron stars, $M_{\rm TOV}$ can be robustly inferred from either the mass distribution of these objects or the reconstruction of the equation of state (EoS) of the very dense matter. For the first time we take the advantages of both two approaches to have a precise inference of $M_{\rm TOV}=2.25^{+0.08}_{-0.07}~M_\odot$ (68.3% credibility), with the updated neutron star mass measurement sample, the mass-tidal deformability data of GW170817, the mass-radius data of PSR J0030+0451 and PSR J0740+6620, as well as the theoretical information from the chiral effective theory ($\chi$EFT) and perturbative quantum chromodynamics (pQCD) at low and very high energy densities, respectively. This narrow credible range is benefited from the suppression of the high $M_{\rm TOV}$ by the pQCD constraint and the exclusion of the low $M_{\rm TOV}$ by the mass function. Three different EoS reconstruction methods are adopted separately, and the resulting $M_{\rm TOV}$ are found to be almost identical. This precisely evaluated $M_{\rm TOV}$ suggests that the EoS of neutron star matter is just moderately stiff and the $\sim 2.5-3M_\odot$ compact objects detected by the second generation gravitational wave detectors are most likely the lightest black holes.Comment: 12 pages, 6 figure