arXiv subjects

Youjun Lu

Publications and source records attributed to Youjun Lu.

At least 55 records · Page 3Linked to original sources

arXiv2023

Pulsar timing array detections of supermassive binary black holes: implications from the detected common process signal and beyond

Pulsar timing arrays (PTAs) are anticipated to detect the stochastic gravitational wave background (GWB) from supermassive binary black holes (BBHs) as well as the gravitational waves from individual BBHs. Recently, a common process signal was reported by several PTAs. In this paper, we investigate the constraints on the BBH population model(s) by current PTA observations and further study the detections of both the GWB and individual BBHs by current/future PTAs. We find that the MBH--host galaxy scaling relation, an important ingredient of the BBH population model, is required to either evolve significantly with redshift or have a normalization $\sim0.86-1.1$ dex higher than the empirical ones, if the GWB is the same as the common process signal. For both cases, the estimated detection probability for individual BBHs is too small for a positive detection by current PTAs. By involving either the constrained scaling relations or those empirical ones into the BBH population models, we estimate that the GWB may be detected with a signal-to-noise ratio $\gtrsim3$ by the PTAs based on the Five hundred meter Aperture Spherical radio Telescope (CPTA) and the Square Kilometer Array (SKAPTA) after $\sim2-3$ (or $\sim6-11$) years' observation, if it is the same as (or an order of magnitude lower than) the common process signal. The detection time of individual BBHs by CPTA and SKAPTA is close to that of the GWB detection. We show that the BBH population model can be strongly constrained by the number and property distributions of BBHs to be detected by future PTAs.

astro-ph.HE↗

arXiv2023

The correlations of stellar tidal disruption rates with properties of massive black holes and their host galaxies

Stars can be either disrupted as tidal disruption events (TDEs) or swallowed as a whole by massive black holes (MBHs) at galactic centers when they approach sufficiently close to these MBHs. In this work, we investigate the correlations of such stellar consumption rates with both the MBH mass $M_{\rm bh}$ and the inner slope of the host galaxy mass density distribution $α$. We introduce a simplified analytical power-law model with a power-law stellar mass density distribution surrounding MBHs and separate the contributions of two-body relaxation and stellar orbital precession for the stellar orbital angular momentum evolution in nonspherical galaxy potentials. The stellar consumption rates derived from this simplified model can be well consistent with the numerical results obtained with a more realistic treatment of stellar distributions and dynamics around MBHs, providing an efficient way to estimate TDE rates. The origin of the correlations of stellar consumption rates with $M_{\rm bh}$ and $α$ are explained by the dependence of this analytical model on those MBH/host galaxy properties and by the separation of the stellar angular momentum evolution mechanisms. We propose that the strong positive correlation between the rates of stellar consumption due to two-body relaxation and $α$ provides one interpretation for the overrepresentation of TDEs found in some rare E+A/poststarburst galaxies. We find high TDE rates for giant stars, up to those for solar-type stars. The understanding of the origin of the correlations of the stellar consumption rates will be necessary for obtaining the demographics of MBHs and their host galaxies via TDEs.

astro-ph.HE↗

arXiv2023

Multiband gravitational wave observations of stellar binary black holes at the low to middle and high frequencies

The ground-based gravitational wave (GW) observatories discover a population of merging stellar binary black holes (BBHs), which are promising targets for multiband observations by the low-, middle-, and high-frequency GW detectors. In this paper, we investigate the multiband GW detections of BBHs and demonstrate the advantages of such observations in improving the localization and parameter estimates of the sources. We generate mock samples of BBHs by considering different formation models as well as the merger rate density constrained by the current observations (GWTC-3). We specifically consider the astrodynamical middle-frequency interferometer GW observatory (AMIGO) in the middle-frequency band and estimate that it may detect $21$-$91$ BBHs with signal-to-noise ratio $\varrho\geq8$ in a $4$-yr observation period. The multiband observations by the low-frequency detectors [Laser Interferometer Space Antenna (LISA) and Taiji] and AMIGO may detect $5$-$33$ BBHs with $\varrho_{\rm LT}\geq5$ and $\varrho_{\rm AMI}\geq5$, which can evolve to the high-frequency band within $4$ yr and can be detected by the Cosmic Explorer (CE) and Einstein Telescope (ET). The joint observations of LISA-Taiji-AMIGO-ET-CE may localize the majority of the detectable BBHs in sky areas of $7\times10^{-7}$ to $2\times10^{-3}$ deg$^2$, which is improved by a factor of $\sim120$, $\sim2.4\times10^{5}$, $\sim1.8\times10^{4}$, or $\sim1.2\times10^{4}$, comparing with those by only adopting CE-ET, AMIGO, LISA-Taiji, or LISA-Taiji-AMIGO. These joint observations can also lead to an improvement of the measurement precision of the chirp mass (symmetric mass ratio) by a factor of $\sim5.5\times10^{4}$ ($33$), $\sim16$ ($8$), $\sim120$ ($90$), or $\sim5$ ($5$), comparing with those by CE-ET, AMIGO, LISA-Taiji, or LISA-Taiji-AMIGO.

astro-ph.HE↗

arXiv2023

On networks of space-based gravitational-wave detectors

The space-based laser interferometers, LISA, Taiji and TianQin, are targeting to observe milliHz gravitational waves (GWs) in the 2030s. The joint observations from multiple space-based detectors yield significant advantages. In this work, we recap the studies and investigations for the joint space-based GW detector networks to highlight: 1) the high precision of sky localization for the massive binary black hole (BBH) coalescences and the GW sirens in the cosmological implication, 2) the effectiveness to test the parity violation in the stochastic GW background observations, 3) the efficiency of subtracting galactic foreground, 4) the improvement in stellar-mass BBH observations. We inspect alternative networks by trading off massive BBH observations and stochastic GW background observation.

gr-qc↗

arXiv2023

Inspiraling streams of enriched gas observed around a massive galaxy 11 billion years ago

Stars form in galaxies, from gas that has been accreted from the intergalactic medium. Simulations have shown that recycling of gas-the reaccretion of gas that was previously ejected from a galaxy-could sustain star formation in the early Universe. We observe the gas surrounding a massive galaxy at redshift 2.3 and detect emission lines from neutral hydrogen, helium, and ionized carbon that extend 100 kiloparsecs from the galaxy. The kinematics of this circumgalactic gas is consistent with an inspiraling stream. The carbon abundance indicates that the gas had already been enriched with elements heavier than helium, previously ejected from a galaxy. We interpret the results as evidence of gas recycling during high-redshift galaxy assembly.

astro-ph.GA↗

arXiv2023

The luminosity functions of kilonovae from binary neutron star mergers under different equation of states

Kilonovae produced by mergers of binary neutron stars (BNSs) are important transient events to be detected by time domain surveys with the alerts from the ground-based gravitational wave detectors. The observational properties of these kilonovae depend on the physical processes involved in the merging processes and the equation of state (EOS) of neutron stars (NSs). In this paper, we investigate the dependence of kilonova luminosities on the parameters of BNS mergers, and estimate the distribution functions of kilonova peak luminosities (KLFs) at the u, g, r, i, y, and z bands as well as its dependence on the NS EOS, by adopting a comprehensive semi-analytical model for kilonovae (calibrated by the observations of GW170817), a population synthesis model for the cosmic BNSs, and the ejecta properties of BNS mergers predicted by numerical simulations. We find that the kilonova light curves depend on both the BNS properties and the NS EOS, and the KLFs at the considered bands are bimodal with the bright components mostly contributed by BNS mergers with total mass $\lesssim 3.2M_\odot$/$2.8M_\odot$ and fainter components mostly contributed by BNS mergers with total mass $\gtrsim 3.2M_\odot$/$2.8M_\odot$ by assuming a stiff/soft (DD2/SLy) EOS. The emission of the kilonovae in the KLF bright components is mostly due to the radiation from the wind ejecta by the remnant discs of BNS mergers, while the emission of the kilonovae in the KLF faint components is mostly due to the radiation from the dynamical ejecta by the BNS mergers.

astro-ph.HE↗

arXiv2022

Constraining the origin of stellar binary black hole mergers by detections of their lensed host galaxies and gravitational wave signals

A significant number of stellar binary black hole (sBBH) mergers may be lensed and detected by the third generation gravitational wave (GW) detectors. Their lensed host galaxies may be detectable, which thus helps to accurately localize these sources and provide a new approach to study the origin of sBBHs. In this paper, we investigate the detectability of the lensed host galaxies for the lensed sBBH mergers. We find that the detection fraction of the host galaxies to the lensed GW events can be significantly different for a survey with a given limiting magnitude if sBBHs are produced by different mechanisms, such as the evolution of massive binary stars, the dynamical interactions in dense star clusters, and that assisted by active galactic nuclei or massive black holes. Furthermore, we illustrate that the statistical spatial distribution of those lensed sBBHs in its hosts resulting from different sBBH formation channels can be different from each other. Therefore, with the third generation GW detectors and future large scale galaxy surveys, it is possible to independently constrain the sBBH origin via the detection fraction of those lensed events with identifiable lensing host signatures and/or even constrain the contribution fractions from different sBBH formation mechanisms

astro-ph.HE↗

arXiv2022

On Detecting Nearby Nano-Hertz Gravitational Wave Sources via Pulsar Timing Arrays

Massive binary black holes (MBBHs) in nearby galactic centers, if any, may be nano-Hertz gravitational wave (GW) sources for pulsar timing arrays (PTAs) to detect. Normally the objective GWs for PTA experiments are approximated as plane waves because its sources are presumably located faraway. For nearby GW sources, however, this approximation may be inaccurate due to the curved GW wave front and the GW strength changes along the paths of PTA pulsar pulses. In this paper, we analyze the near-field effect in the PTA detection of nearby sources and find it is important if the source distance is less than a few tens Mpc, and ignoring this effect may lead to a significant signal-to-noise underestimation especially when the source distance is comparable to the pulsar distances. As examples, we assume a nano-Hertz MBBH source located at either the Galactic Center (GC) or the Large Magellanic Cloud (LMC) according to the observational constraints/hints on the MBBH parameter space, and estimate its detectability by current/future PTAs. We find that the GC MBBH may be detectable by the Square Kilometer Array (SKA) PTA. It is challenging for detecting the LMC MBBH; however, if a number ($N\gtrsim10$) of stable millisecond pulsars can be found in the LMC center, the MBBH may be detectable via a PTA formed by these pulsars. We further illustrate the near-field effects on the PTA detection of an isotropic GW background contributed mainly by nearby GW sources, and the resulting angular correlation is similar to the Hellings-Downs curve.

astro-ph.HE↗

arXiv2022

Probing the nature of dark matter via gravitational waves lensed by small dark matter halos

Dark matter (DM) occupies the majority of matter content in the universe and is probably cold (CDM). However, modifications to the standard CDM model may be required by the small-scale observations, and DM may be self-interacting (SIDM) or warm (WDM). Here we show that the diffractive lensing of gravitational waves (GWs) from binary black hole mergers by small halos ($\sim10^3-10^6M_\odot$; mini-halos) may serve as a clean probe to the nature of DM, free from the contamination of baryonic processes in the DM studies based on dwarf/satellite galaxies. The expected lensed GW signals and event rates resulting from CDM, WDM, and SIDM models are significantly different from each other, because of the differences in halo density profiles and abundances predicted by these models. We estimate the detection rates of such lensed GW events for a number of current and future GW detectors, such as the Laser Interferometer Gravitational Observatories (LIGO), the Einstein Telescope (ET), the Cosmic Explorer (CE), Gravitational-wave Lunar Observatory for Cosmology (GLOC), the Deci-Hertz Interferometer Gravitational Wave Observatory (DECIGO), and the Big Bang Observer (BBO). We find that GLOC may detect one such events per year assuming the CDM model, DECIGO (BBO) may detect more than several (hundreds of) such events per year, by assuming the CDM, WDM (with mass $>30$\,keV) or SIDM model, suggesting that the DM nature may be strongly constrained by DECIGO and BBO via the detection of diffractive lensed GW events by mini-halos. Other GW detectors are unlikely to detect a significant number of such events within a limited observational time period. However, if the inner slope of the mini-halo density profile is sufficiently steeper than the Navarro-Frenk-White (NFW) profile, e.g., the pseudo-Jaffe profile, one may be able to detect one to more than hundred such GW events by ET and CE.

astro-ph.CO↗

arXiv2022

On Detecting Stellar Binary Black Holes via the LISA-Taiji Network

The detection of gravitational waves (GWs) by ground-based laser interferometer GW observatories (LIGO/Virgo) reveals a population of stellar binary black holes (sBBHs) with (total) masses up to $\sim 150M_\odot$, which are potential sources for space-based GW detectors, such as LISA and Taiji. In this paper, we investigate in details on the possibility of detecting sBBHs by the LISA-Taiji network in future. We adopt the sBBH merger rate density constrained by LIGO/VIRGO observations to randomly generate mock sBBHs samples. Assuming an observation period of $4$ years, we find that the LISA-Taiji network may detect several tens (or at least several) sBBHs with signal-to-noise ratio (SNR) $>8$ (or $>15$), a factor $2-3$ times larger than that by only using LISA or Taiji observations. Among these sBBHs, no more than a few that can merge during the $4$-year observation period. If extending the observation period to $10$ years, then the LISA-Taiji network may detect about one hundred (or twenty) sBBHs with SNR $>8$ (or $>15$), among them about twenty (or at least several) can merge within the observation period. Our results suggest that the LISA-Taiji network may be able to detect at least a handful to twenty or more sBBHs even if assuming a conservative SNR threshold ($15$) for ``detection'', which enables multi-band GW observations by space and ground-based GW detectors. We also further estimate the uncertainties in the parameter estimations of the sBBH systems ``detected'' by the LISA-Taiji network. We find that the relative errors in the luminosity distance measurements and sky localization are mostly in the range of $0.05-0.2$ and $1-100°^2$, respectively, for these sBBHs.

astro-ph.HE↗

arXiv2022

On Dark Gravitational Wave Standard Sirens as Cosmological Inference and Forecasting the Constraint on Hubble Constant using Binary Black Holes Detected by Deci-hertz Observatory

Gravitational wave (GW) signals from compact binary coalescences can be used as standard sirens to constrain cosmological parameters if their redshift can be measured independently. However, mergers of stellar binary black holes (BBHs) may not have electromagnetic counterparts and thus have no direct redshift measurements. These dark sirens may be still used to statistically constrain cosmological parameters by combining their GW measured luminosity distances and localization with deep redshift surveys of galaxies around it. We investigate this dark siren method in detail by using mock BBH and galaxy samples. We find that the Hubble constant can be constrained well with an accuracy $\lesssim1\%$ with a few tens or more of BBH mergers at redshift up to $1$ if GW observations can provide accurate estimates of their luminosity distance (with relative error of $\lesssim0.01$) and localization ($\lesssim0.1~\rm{deg}^2$), though the constraint may be significantly biased if the luminosity distance and localization errors are larger. We also introduce a simple method to correct this bias and find it is valid when the luminosity distance and localization errors are modestly large. We further generate mock BBH samples, according to current constraints on BBH merger rate and the distributions of BBH properties, and find that the Deci-hertz Observatory (DO) in a half year observation period may detect about one hundred BBHs with signal-to-noise ratio $\varrho\gtrsim30$, relative luminosity distance error $\lesssim0.02$, and localization error $\lesssim0.01\rm{deg}^2$. By applying the dark standard siren method, we find that the Hubble constant can be constrained to the $\sim0.1-1\%$ level using these DO BBHs, an accuracy comparable to the constraints obtained by using electromagnetic observations in the near future, thus it may provide insight into the Hubble tension.

astro-ph.CO↗

arXiv2021

The Gravitational-Wave Physics II: Progress

It has been a half-decade since the first direct detection of gravitational waves, which signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave cosmology. The increasing number of the detected gravitational-wave events has revealed the promising capability of constraining various aspects of cosmology, astronomy, and gravity. Due to the limited space in this review article, we will briefly summarize the recent progress over the past five years, but with a special focus on some of our own work for the Key Project ``Physics associated with the gravitational waves'' supported by the National Natural Science Foundation of China. In particular, (1) we have presented the mechanism of the gravitational-wave production during some physical processes of the early Universe, such as inflation, preheating and phase transition, and the cosmological implications of gravitational-wave measurements; (2) we have put constraints on the neutron star maximum mass according to GW170817 observations; (3) we have developed a numerical relativity algorithm based on the finite element method and a waveform model for the binary black hole coalescence along an eccentric orbit.

gr-qc↗

arXiv2021

Investigating the existence of gravitomagnetic monopole in M87*

We examine the possibility for the existence of gravitomagnetic monopole ($n_*$) in M87* by using the results obtained from its first Event Horizon Telescope image. By numerically deducing the shadow sizes in Kerr-Taub-NUT (KTN) spacetime, we show that the shadow size increases with increasing $|n_*|$ for a fixed Kerr parameter $|a_*|$ in case of the KTN black hole, whereas for a KTN naked singularity it increases with increasing $n_*$ for a fixed $a_* > 0$ if $n_* > -\cot 17^{\circ}$ . In general, the asymmetry of shadow shape increases if the central dark object in M87 is a KTN/Kerr naked singularity instead of a KTN/Kerr black hole. We find that a non-zero gravitomagnetic monopole is still compatible with the current EHT observations, in which case the upper limit of $n_*$ cannot be greater than $1.1$, i.e., $n_* \lesssim 1.1$ for the prograde rotation ($a_* > 0$), and the lower limit of $n_*$ cannot be less than $-1.1$, i.e., $ n_* \gtrsim -1.1$ for the retrograde rotation ($a_* < 0$). Moreover, if the circularity of the shadow can be measured on a precision of $\lesssim 1\%$, the Kerr and KTN naked singularities can be falsified for M87*.

astro-ph.HE↗

arXiv2021

Investigating the co-evolution of massive black holes in dual active galactic nuclei and their host galaxies via galaxy merger simulations

Major galaxy mergers can trigger nuclear activities and are responsible for high-luminosity quasi-stellar objects /active galactic nuclei (QSOs/AGNs). In certain circumstances, such mergers may cause dual active galactic nuclei (dAGN) phenomenon. This study investigates dAGN triggering and evolution of massive black holes (MBHs) during the merging processes using hydrodynamic code GADGET-2 to simulate several gas-rich major mergers at redshift $z=2$ and $3$, respectively. Results reveal that gas-rich major mergers can trigger significant nuclear activities after the second and third pericentric passages and the formation of dAGN with significant time duration ($\sim 10 - 390$ Myr). During the merging processes, galactic bulge evolves with time because of the rapid star formation in each (or both) galactic centers and initial mixing of stars in galactic disks due to violent relaxation. MBHs grow substantially due to accretion and finally merge into a bigger black hole. The growth of galactic bulges and corresponding increases of its velocity dispersions predate the growth of MBHs in the dAGN stages. The MBHs in these stages deviate below the relation between MBH mass and bulge mass (or velocity dispersion), and they revert to the relation after the final mergers due to the significant accretion that occurs mostly at a separation less than a few kpc. Then, the two MBHs merge with each other.

astro-ph.GA↗

arXiv2021

Hydrodynamical Simulations of the Triggering of Nuclear Activities by Minor Mergers of Galaxies

Major mergers of galaxies are considered to be an efficient way to trigger Active Galactic Nuclei and are thought to be responsible for the phenomenon of quasars. This has however recently been challenged by observations of a large number of low luminosity Active Galactic Nuclei at low redshift ($z\lesssim1$) without obvious major merger signatures. Minor mergers are frequently proposed to explain the existence of these Active Galactic Nuclei. In this paper, we perform nine high resolution hydrodynamical simulations of minor galaxy mergers and investigate whether nuclear activities can be efficiently triggered by minor mergers, by setting various properties for the progenitor galaxies of those mergers. We find that minor galaxy merger scan activate the massive black hole in the primary galaxy with an Eddington ratio of $f_{\rm Edd}>0.01$ and $>0.05$ (or a bolometric luminosity $>10^{43}$ and $>10^{44}\mathrm{erg\, s^{-1}}$) with a duration of $2.71$ and $0.49$ Gyr (or $2.69$ and $0.19$ Gyr), respectively. The nuclear activity of primary galaxy strongly depends on the nucleus separation, the nucleus is more active as the two nuclei approach to each other. Dual Active Galactic Nuclei systems can still possibly form by minor mergers of galaxies, the time period for dual Active Galactic Nuclei is only $\sim 0.011$ Gyr and $\sim 0.017$ Gyr with Eddington ratio of $f_{\rm Edd}>0.05$ and bolometric luminosity $>10^{44}\mathrm{erg\, s^{-1}}$. This time period is typically shorter than that of dual Active Galactic Nuclei induced by galaxy major mergers.

astro-ph.GA↗

arXiv2021

Variations of Broad Emission Lines from periodicity QSOs under the interpretation of supermassive binary black holes with misaligned circumbinary broad line regions

Quasars with periodic light curves are considered as candidates of supermassive binary black hole (BBH) systems. One way for further confirmations may be searching for other characteristic signatures, such as those in their broad emission lines (BELs), if any, which require a thorough understanding on the response of BELs to the BBH systems. In Ji et al. (2021), we have investigated the response of circumbinary broad line region (BLR) to the central active secondary black hole under the relativistic Doppler boosting (BBH-DB) and intrinsic variation (BBH-IntDB) dominant mechanisms for continuum variation by assuming the middle plane of the BLR aligned with the BBH orbital plane. In this paper, we explore how the BEL profiles vary when the BLR is misaligned from the BBH orbital plane with different offset angles under both the BBH-DB and BBH-IntDB scenarios. Given a fixed inclination angle of the BBH orbital plane viewed in edge-on and similar continuum light curves produced by the two scenarios, increasing offset angles make the initial opening angle of the circumbinary BLR enlarged due to orbital precession caused by the BBH system, especially for clouds in the inner region, which result in Lorentz-like BEL profiles for the BBH-DB model but still Gaussian-like profiles for the BBH-IntDB model at the vertical BLR case. The amplitude of profile variations decrease with increasing offset angles for the BBH-DB scenario, while keep nearly constant for the BBH-IntDB scenario, since the Doppler boosting effect is motion direction preferred but the intrinsic variation is radiated isotropically. If the circumbinary BLR is composed of a coplanar and a vertical components with their number of clouds following the mass ratio of the BBHs, then the bi-BLR features are more significant for the BBH-IntDB model that require larger mass ratio to generate similar continuum variation than the BBH-DB model.

astro-ph.GA↗

arXiv2021

Formation and Evolution of Binary Neutron Stars: Mergers and Their Host Galaxies

In this paper, we investigate the properties of binary neutron stars (BNSs) and their mergers by combining population synthesis models for binary stellar evolution (BSE) with cosmological galaxy formation and evolution models. We obtain constraints on BSE model parameters by using the observed Galactic BNSs and local BNS merger rate density ($R_0$) inferred from Gravitational Wave (GW) observations, and consequently estimate the host galaxy distributions of BNS mergers. We find that the Galactic BNS observations imply efficient energy depletion in the common envelope (CE) phase, a bimodal kick velocity distribution, and low mass ejection during the secondary supernova explosion. However, the inferred $R_0$ does not necessarily require an extremely high CE ejection efficiency and low kick velocities, different from the previous claims, mainly because the latest inferred $R_0$ is narrowed to a lower value ($320_{-240}^{+490}\,{\rm Gpc^{-3}\,yr^{-1}}$). The BNS merger rate density resulting from the preferred model can be described by $R(z)\sim R_0(1+z)^ζ$ at low redshift ($z\lesssim0.5$), with $R_0\sim316$-$784\,{\rm Gpc^{-3}\,yr^{-1}}$ and $ζ\sim1.34$-$2.03$, respectively. Our results also show that $R_{0}$ and $ζ$ depend on settings of BSE model parameters, and thus accurate estimates of these parameters by future GW detections will put strong constraints on BSE models. We further estimate that the fractions of BNS mergers hosted in spiral and elliptical galaxies at $z\sim0$ are $\sim81$%-$84$% and $\sim16$%-$19$%, respectively. The BNS merger rate per galaxy can be well determined by the host galaxy stellar mass, star formation rate, and metallicity, which provides a guidance in search for most probable candidates of BNS host galaxies.

astro-ph.GA↗

arXiv2021

Wind-powered ultraluminous X-ray sources

Although ultra-luminous X-ray sources (ULX) are important for astrophysics due to their extreme apparent super-Eddington luminosities, their nature is still poorly known. Theoretical and observational studies suggest that ULXs could be a diversified group of objects composed of low-mass X-ray binaries, high-mass X-ray binaries and marginally also systems containing intermediate-mass black holes, which is supported by their presence in a variety of environments. Observational data on the ULX donors could significantly boost our understanding of these systems, but only a few were detected. There are several candidates, mostly red supergiants (RSGs), but surveys are typically biased toward luminous near-infrared objects. Nevertheless, it is worth exploring if RSGs can be members of ULX binaries. In such systems matter accreted onto the compact body would have to be provided by the stellar wind of the companion, since a Roche-lobe overflow could be unstable for relevant mass-ratios. Here we present a comprehensive study of the evolution and population of wind-fed ULXs and provide a theoretical support for the link between RSGs and ULXs. Our estimated upper limit on contribution of wind-fed ULX to the overall ULX population is $\sim75$--$96\%$ for young ($<100$ Myr) star forming environments, $\sim 49$--$87\%$ for prolonged constant star formation (e.g., disk of Milky Way), and $\lesssim1\%$ for environments in which star formation ceased long time ($>2$ Gyr) ago. We show also that some wind-fed ULXs (up to $6\%$) may evolve into merging double compact objects (DCOs), but typical systems are not viable progenitors of such binaries because of their large separations. We demonstrate that, the exclusion of wind-fed ULXs from population studies of ULXs, might have lead to systematical errors in their conclusions.

astro-ph.HE↗