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Register a new userA Unicorn in Monoceros: the $3M_odot$ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate
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Added by
Tharindu Jayasinghe
Publication date
2021
fields
Physics
and research's language is
English
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We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($dsim460,rm pc$), bright ($Vsimeq8.3$~mag), evolved ($T_{rm eff, giant}simeq4440$~K, and $L_{rm giant}simeq173~L_odot$) red giant in a high mass function, $f(M)=1.72pm 0.01~M_odot$, nearly circular binary ($P=59.9$ d, $esimeq 0$). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey (ASAS), Kilodegree Extremely Little Telescope (KELT), and Transiting Exoplanet Survey Satellite (TESS) light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities and stellar temperature give an inclination of $87.0^circ{}^{+1.7^{circ}}_{-1.4^{circ}} $, a mass ratio of $qsimeq0.33pm0.02$, a companion mass of $M_{rm comp}=3.04pm0.06~M_odot$, a stellar radius of $R_{rm giant}=24.9pm0.7~R_odot$, and a giant mass of $M_{rm giant}=1.00pm0.07~ M_odot$. We identify a likely non-stellar, diffuse veiling component with contributions in the $B$ and $V$-band of ${sim}63%$ and ${sim}24%$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion disks. The X-ray luminosity of the system is $L_{rm X}simeq7.6times10^{29}~rm ergs~s^{-1}$, corresponding to $L/L_{rm edd}{sim}10^{-9}$. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the mass gap.
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Thompson et al. (Reports, 1 November 2019, p. 637, Science) interpreted the unseen companion of the red giant star 2MASS J05215658+4359220 as most likely a black hole. We argue that if the red giant is about one solar mass, its companion can be a close binary consisting of two main-sequence stars. This would explain why no X-ray emission is detected from the system.
Stellar evolution theory predicts a gap in the black hole birth function caused by the pair instability. Presupernova stars that have a core mass below some limiting value, Mlo, after all pulsational activity is finished, collapse to black holes, whereas more massive ones, up to some limiting value, Mhi, explode, promptly and completely, as pair-instability supernovae. Previous work has suggested Mlo is approximately 50 solar masses and Mhi is approximately 130 solar masses. These calculations have been challenged by recent LIGO observations that show many black holes merging with individual masses, Mlo is least some 65 solar masses. Here we explore four factors affecting the theoretical estimates for the boundaries of this mass gap: nuclear reaction rates, evolution in detached binaries, rotation, and hyper-Eddington accretion after black hole birth. Current uncertainties in reaction rates by themselves allow Mlo to rise to 64 solar masses and Mhi as large as 161 solar masses. Rapid rotation could further increase Mlo to about 70 solar masses, depending on the treatment of magnetic torques. Evolution in detached binaries and super-Eddington accretion can, with great uncertainty, increase Mlo still further. Dimensionless Kerr parameters close to unity are allowed for the more massive black holes produced in close binaries, though they are generally smaller.
van den Heuvel & Tauris argue that if the red giant star in the system 2MASS J05215658+4359220 has a mass of 1 solar mass (M$_odot$), then its unseen companion could be a binary composed of two 0.9 M$_odot$ stars, making a triple system. We contend that the existing data are most consistent with a giant of mass $3.2^{+1.0}_{-1.0}$ M$_odot$, implying a black hole companion of $3.3^{+2.8}_{-0.7}$ M$_odot$.
Gravitational lensing allows the detection of binary black holes (BBH) at cosmological distances with chirp masses that appear to be enhanced by $1+z$ in the range $1<z<4$, in good agreement with the reported BBH masses. We propose this effect also accounts for the puzzling mass gap events (MG) newly reported by LIGO/Virgo, as distant, lensed NSBH events with $1<z<4$. The fitted mass of the neutron star member becomes $(1+z)times 1.4M_odot$, and is therefore misclassified as a low mass black hole. In this way, we derive a redshift of $zsimeq 3.5$ and $zsimeq 1.0$ for two newly reported mass asymmetric events GW190412 & GW190814, by interpreting them as lensed NSBH events, comprising a stellar mass black hole and neutron star. Over the past year an additional 31 BBH events and 5 MG events have been reported with high probability ($>95%$), from which we infer a factor $simeq 5$ higher intrinsic rate of NSBH events than BBH events, reflecting a higher proportion of neutron stars formed by early star formation. We predict a distinctive locus for lensed NSBH events in the observed binary mass plane, spanning $1<z<4$ with a narrow mass ratio, $q simeq 0.2$, that can be readily tested when the waveform data are unlocked. All such events may show disrupted NS emission and are worthy of prompt follow-up as the high lensing magnification means EM detections are not prohibitive despite the high redshifts that we predict. Such lensed NSBH events provide an exciting prospect of directly charting the history of coalescing binaries via the cosmological redshift of their waveforms, determined relative to the characteristic mass of the neutron star member.
The Zwicky Transient Facility (ZTF) reported the event ZTF19abanrhr as a candidate electromagnetic (EM) counterpart at a redshift $z=0.438$ to the gravitational wave (GW) emission from the binary black hole merger GW190521. Assuming that ZTF19abanrhr is the {it bona fide} EM counterpart to GW190521, and using the GW luminosity distance estimate from three different waveforms NRSur7dq4, SEOBNRv4PHM, and IMRPhenomPv3HM, we report a measurement of the Hubble constant $H_0= 50.4_{-19.5}^{+28.1}$ km/s/Mpc, $ 62.2_{-19.7}^{+29.5}$ km/s/Mpc, and $ 43.1_{-11.4}^{+24.6}$ km/s/Mpc (median along with $68%$ credible interval) respectively after marginalizing over matter density $Omega_m$ (or dark energy equation of state $w_0$) assuming the flat LCDM (or wCDM) model. Combining our results with the binary neutron star event GW170817 with its redshift measurement alone, as well as with its inclination angle inferred from Very Large Baseline Interferometry (VLBI), we find $H_0= 67.6_{-4.2}^{+4.3}$ km/s/Mpc, $Omega_m= 0.47_{-0.27}^{+0.34}$, and $w_0= -1.17_{-0.57}^{+0.68}$ (median along with $68%$ credible interval) providing the most stringent measurement on $H_0$ and the first estimation on $Omega_m$ and $w_0$ from bright standard siren. In the future, $1.3%$ measurement of $H_0=68$ km/s/Mpc and $28%$ measurement of $w_0=-1$ is possible from about $200$ GW190521-like sources.
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