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Ed P. J. van den Heuvel
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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.
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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$_odot$.
All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M$_{odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$alpha$ emission line require the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.
There are very few confirmed black holes with a mass that could be $sim! 4, M_odot$ and no neutron stars with masses greater than $sim! 2, M_odot$, creating a gap in the observed distribution of compact star masses. Some black holes with masses between 2 and $4, M_odot$ might be hiding among other X-ray sources, whose masses are difficult to measure. We present new high-speed optical photometry of the low-mass X-ray binary V1408 Aql (= 4U 1957+115), which is a persistent X-ray source thought to contain a black hole. The optical light curve of V1408~Aql shows a nearly sinusoidal modulation at the orbital period of the system superimposed on large night-to-night variations in mean intensity. We combined the new photometry with previously-published photometry to derive a more precise orbital period, $P = 0.388893(3)$ d, and to better define the orbital light curve and night-to-night variations. The orbital light curve agrees well with a model in which the modulation is caused entirely by the changing aspect of the heated face of the secondary star. The lack of eclipses rules out orbital inclinations greater than $65^{circ}$. Our best models for the orbital light curve favor inclinations near $13^{circ}$ and black hole masses near $3, M_odot$ with a 90% upper bound of $6.2, M_odot$, and a lower bound of $2.0, M_odot$ imposed solely by the maximum mass of neutron stars. We favor a black hole primary over a neutron star primary based on evidence from the X-ray spectra, the high spin of the compact object, and the fact that a type I X-ray burst has not been observed for this system. Although uncertainties in the data and the models allow higher masses, possibly much higher masses, the compact star in V1408~Aql is a viable candidate for a black hole lying in the mass gap.
At about 70 solar masses, the recently-discovered dark object orbited by a B-type star in the system LB-1 is difficult to understand as the end point of standard stellar evolution, except as a binary black hole (BBH). LB-1 shows a strong, broad H-alpha emission line that is best attributed to a gaseous disk surrounding the dark mass. We use the observed H-alpha line shape, particularly its wing extension, to constrain the inner radius of the disk and thereby the separation of a putative BBH. The hypothesis of a current BBH is effectively ruled out on the grounds that its merger time must be a small fraction of the current age of the B star. The hypothesis of a previous BBH that merged to create the current dark mass is also effectively ruled out by the low orbital eccentricity, due to the combination of mass loss and kick resulted from gravitational wave emission in any past merger. We conclude that the current dark mass is a single black hole produced by the highly mass-conserving, monolithic collapse of a massive star.
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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