An axion-like scalar field environment effect on binary black hole merger

The environment, such as an accretion disk, could modify the signal of the gravitational wave from astrophysical black hole binaries. In this article, we model the matter field around intermediatemass binary black holes by means of an axion-like scalar field and investigate their joint evolution. In detail, we consider equal mass binary black holes surrounded by a shell of axion-like scalar field both in spherically symmetric and non-spherically symmetric cases, and with different strengths of the scalar field. Our result shows that the environmental scalar field could essentially modify the dynamics. Firstly,in the spherically symmetric case, with increase of the scalar field strength, the number of circular orbits for the binary black hole is reduced. This means that the scalar field could significantly accelerate the merger process. Secondly, once the scalar field strength exceeds a certain critical value, the scalar field could collapse into a third black hole with its mass being larger than that of the binary. Consequently,the new black hole that collapses from the environmental scalar field could accrete the binary promptly and the binary collides head-on with each other. In this process, there is almost no quadrupole signal produced, and, consequently, the gravitational wave is greatly suppressed. Thirdly, when the scalar field strength is relatively smaller than the critical value, the black hole orbit could develop eccentricity through accretion of the scalar field. Fourthly, during the initial stage of the inspiral, the gravitational attractive force from the axion-like scalar field could induce a sudden turn in the binary orbits, hence resulting in a transient wiggle in the gravitational waveform. Finally, in the non-spherical case, the scalar field could gravitationally attract the binary moving toward the center of mass for the scalar field and slow down the merger process.

Relation between gravitational mass and baryonic mass for non-rotating and rapidly rotating neutron stars

With a selected sample of neutron star(NS)equations of state(EOSs)that are consistent with the current observations and have a range of maximum masses,we investigate the relations between NS gravitational mass Mg and baryonic mass and the relations between the maximum NS mass supported through uniform rotation(Mmax)and that of nonrotating NSs(Mtov).We find that for an EOS-independent quadratic,universal transformation formula(Mb=Mg+A×M^2/g),the best-fit A value is 0.080 for non-rotating NSs,0.064 for maximally rotating NSs,and 0.073 when NSs with arbitrary rotation are considered.The residual error of the transformation is?0.1M⊙ for non-spin or maximum-spin,but is as large as?0.2M⊙ for all spins.For different EOSs,we find that the parameter A for non-rotating NSs is proportional to R^-1/1.4(where R1.4 is NS radius for 1.4M⊙ in units of km).For a particular EOS,if one adopts the best-fit parameters for different spin periods,the residual error of the transformation is smaller,which is of the order of O.O1M⊙ for the quadratic form and less than O.O1M⊙ for the cubic form(Mb=Mg+A1×M^2/g+A2×M^3/g).We also find a very tight and general correlation between the normalized mass gain due to spin △m≡(Mmax-MTOV)MTOV and the spin period normalized to the Keplerian period P,i.e.,log10 △m=(-2.74±0.05)log10 P+log10(0.20±0.01),which is independent of EOS models.These empirical relations are helpful to study NS-NS mergers with a long-lived NS merger product using multi-messenger data.The application of our results to GW170817 is discussed.