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Register a new userGravitational waves from Population III binary black holes are consistent with LIGO/Virgo O3a data for the chirp mass larger than $sim 20M_{odot}$
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The probability number distribution function of binary black hole mergers observed by LIGO/Virgo O3a has double peaks as a function of chirp mass $M_c$, total mass $M_t$, primary black hole mass $M_1$ and secondary one $M_2$, respectively. The larger chirp mass peak is at $M_c cong 30 M_{odot}$. The distribution of $M_2$ vs. $M_1$ follows the relation of $M_2cong 0.7M_1$. For initial mass functions of Population III stars in the form of $f(M) propto M^{-alpha}$, population synthesis numerical simulations with $0leq alpha leq 1.5$ are consistent with O3a data for $M_c gtrsim 20M_{odot}$. The distribution of $M_2$ vs. $M_1$ for simulation data also agrees with $M_2cong 0.7M_1$ relation of O3a data.
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The recent discovery of the gravitational wave source GW150914 has revealed a coalescing binary black hole (BBH) with masses of $sim 30~M_odot$. Previous proposals for the origin of such a massive binary include Population III (PopIII) stars. PopIII stars are efficient producers of BBHs and of a gravitational wave background (GWB) in the $10-100$ Hz band, and also of ionizing radiation in the early Universe. We quantify the relation between the amplitude of the GWB ($Omega_{rm gw}$) and the electron scattering optical depth ($tau_{rm e}$), produced by PopIII stars, assuming that $f_{rm esc}approx 10%$ of their ionizing radiation escapes into the intergalactic medium. We find that PopIII stars would produce a GWB that is detectable by the future O5 LIGO/Virgo if $tau_{rm e} gtrsim 0.07$, consistent with the recent Planck measurement of $tau_e=0.055 pm 0.09$. Moreover, the spectral index of the background from PopIII BBHs becomes as small as ${rm d}ln Omega_{rm gw}/{rm d}ln flesssim 0.3$ at $f gtrsim 30$ Hz, which is significantly flatter than the value $sim 2/3$ generically produced by lower-redshift and less-massive BBHs. A detection of the unique flattening at such low frequencies by the O5 LIGO/Virgo will indicate the existence of a high-chirp mass, high-redshift BBH population, which is consistent with the PopIII origin. A precise characterization of the spectral shape near $30-50$ Hz by the Einstein Telescope could also constrain the PopIII initial mass function and star formation rate.
In the case of zero-metal (population III or Pop III) stars, we show that the total mass of binary black holes from binary Pop III star evolution can be $sim 150 ,M_{odot}$, which agrees with the mass of the binary black hole GW190521 recently discovered by LIGO/Virgo. The event rate of such binary black hole mergers is estimated as 0.13--0.66$~(rho_{rm SFR}/(6times10^5~M_{odot}/{rm Mpc}^3))~Err_{rm sys}~{rm yr^{-1}~Gpc^{-3}}$, where $rho_{rm SFR}$ and $Err_{rm sys}$ are the cumulative comoving mass density of Pop III stars depending on star formation rate and the systematic errors depending on uncertainties in the Pop III binary parameters, respectively. The event rate in our fiducial model with $rho_{rm SFR}=6times10^5~M_{odot}/{rm Mpc}^3$ and $ Err_{rm sys}=1$ is 0.13--0.66$~{rm yr^{-1}~Gpc^{-3}}$, which is consistent with the observed value of 0.02--0.43$~{rm yr^{-1}~Gpc^{-3}}$.
We present a search for gravitational waves from 116 known millisecond and young pulsars using data from the fifth science run of the LIGO detectors. For this search ephemerides overlapping the run period were obtained for all pulsars using radio and X-ray observations. We demonstrate an updated search method that allows for small uncertainties in the pulsar phase parameters to be included in the search. We report no signal detection from any of the targets and therefore interpret our results as upper limits on the gravitational wave signal strength. The most interesting limits are those for young pulsars. We present updated limits on gravitational radiation from the Crab pulsar, where the measured limit is now a factor of seven below the spin-down limit. This limits the power radiated via gravitational waves to be less than ~2% of the available spin-down power. For the X-ray pulsar J0537-6910 we reach the spin-down limit under the assumption that any gravitational wave signal from it stays phase locked to the X-ray pulses over timing glitches, and for pulsars J1913+1011 and J1952+3252 we are only a factor of a few above the spin-down limit. Of the recycled millisecond pulsars several of the measured upper limits are only about an order of magnitude above their spin-down limits. For these our best (lowest) upper limit on gravitational wave amplitude is 2.3x10^-26 for J1603-7202 and our best (lowest) limit on the inferred pulsar ellipticity is 7.0x10^-8 for J2124-3358.
We report a search for gravitational waves from the inspiral, merger and ringdown of binary black holes (BBH) with total mass between 25 and 100 solar masses, in data taken at the LIGO and Virgo observatories between July 7, 2009 and October 20, 2010. The maximum sensitive distance of the detectors over this period for a (20,20) Msun coalescence was 300 Mpc. No gravitational wave signals were found. We thus report upper limits on the astrophysical coalescence rates of BBH as a function of the component masses for non-spinning components, and also evaluate the dependence of the search sensitivity on component spins aligned with the orbital angular momentum. We find an upper limit at 90% confidence on the coalescence rate of BBH with non-spinning components of mass between 19 and 28 Msun of 3.3 times 10^-7 mergers /Mpc^3 /yr.
With approximately 50 binary black hole events detected by LIGO/Virgo to date and many more expected in the next few years, gravitational-wave astronomy is shifting from individual-event analyses to population studies. We perform a hierarchical Bayesian analysis on the GWTC-2 catalog by combining several astrophysical formation models with a population of primordial black holes. We compute the Bayesian evidence for a primordial population compared to the null hypothesis, and the inferred fraction of primordial black holes in the data. We find that these quantities depend on the set of assumed astrophysical models: the evidence for primordial black holes against an astrophysical-only multichannel model is decisively favored in some scenarios, but it is significantly reduced in the presence of a dominant stable-mass-transfer isolated formation channel. The primordial channel can explain mergers in the upper mass gap such as GW190521, but (depending on the astrophysical channels we consider) a significant fraction of the events could be of primordial origin even if we neglected GW190521. The tantalizing possibility that LIGO/Virgo may have already detected black holes formed after inflation should be verified by reducing uncertainties in astrophysical and primordial formation models, and it may ultimately be confirmed by third-generation interferometers.
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