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IC 10 X-1 is one of close X-ray binaries containing a Wolf-Rayet donor, which can provide an evolutionary link between high-mass X-ray binaries and gravitational wave sources. It is still unclear about the precise nature of the accreting compact object in IC 10 X-1, although it looks more like a black hole than a neutron star. In this work, we use a binary population synthesis method to simulate the formation of IC 10 X-1 like binaries by assuming different common-envelope ejection efficiencies. This work represents a big step forward over previous studies since we adopt new criteria of mass-transfer stability. These criteria allow the formation of IC 10 X-1 like systems without experiencing common envelope evolution. Based on our calculations, we propose that the compact object in IC 10 X-1 is a black hole with mass of $\sim 10-30M_\odot$ and the progenitor evolution of this binary probably just experienced stable mass transfer., Comment: 17 pages, 10 figures, accepted by ApJ

Neutron star$-$white dwarf (NSWD) binaries are one of the most abundant sources of gravitational waves (GW) in the Milky Way. These GW sources are the evolutionary products of primordial binaries that experienced many processes of binary interaction. We employ a binary population synthesis method to investigate the properties of Galactic NSWD binaries detectable by the Laser Interferometer Space Antenna (LISA). In this paper, only the NSWD systems with a COWD or ONeWD component are included. We consider various models related to mass transfer efficiencies during primordial binary evolution, supernova explosion mechanisms at NS formation, common envelope ejection efficiencies, and critical WD masses that determining the stability of mass transfer between WDs and NSs. Based on our calculations, we estimate that tens to hundreds of LISA NSWD binaries exist in the Milky Way. We find that the detection of LISA NSWD binaries is able to provide profound insights into mass transfer efficiencies during the evolution of primordial binaries and critical WD masses during mass transfer from a WD to an NS., Comment: 11 pages, 5 figures, 1 table, accepted to MNRAS

V1082 Sgr is a cataclysmic variable with accretion luminosity above $10^{34}$erg s$^{-1}$, indicating a mass transfer rate above $10^{-9}M_{\odot}$yr$^{-1}$. However, its K type companion was suggested to be under-filling its Roche lobe (RL), making the high mass transfer rate a mystery. In this work we propose a possible model to explain this discrepancy. The system is proposed to be an intermediate polar, with its K type companion filling its RL. The mass of the white dwarf star is evaluated to be $0.77\pm0.11M_{\odot}$ from both X-ray continuum fitting and Fe line flux ratio measurements. We make numerical simulations to search for the possible progenitors of the system. The results show that a binary with an initial 1.5--2.5$M_{\odot}$ companion in a 1--2 day orbit (or an initial 1.0--1.4$M_{\odot}$ companion in a 3.2--4.1 day orbit) may naturally evolve to a cataclysmic variable with a $\sim 0.55 \pm 0.11M_{\odot}$, Roche-lobe filling companion in a 0.86 day orbit. The effective temperature of the donor star, the mass transfer rate, and the derived V band magnitude are all consistent with previous observations., Comment: 6 pages, 3 figures, 2 tables, accepted by MNRAS

Previous X-ray observations toward the Nuclear Star Cluster (NSC) at the Galactic center have discovered thousands of point sources, most of which were believed to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting from a low-mass companion. However, the population properties of these CVs remain unclear, which otherwise contain important information about the evolutionary history of the NSC. In this work we utilize ultradeep archival \textit{Chandra} observations to study the spectral properties of the NSC CVs, in close comparison with those in the Solar vicinity. We find that the NSC CVs have strong Fe XXV and Fe XXVI lines (both of which show equivalent widths $\sim200-300$ eV), indicating metal-rich companions. Moreover, their Fe XXVI to Fe XXV line flux ratio is used to diagnose the characteristic white dwarf mass ($M_{\rm WD}$) of NSC CVs. The results show that the CVs with $L_{\rm 2-10 keV}>6\times10^{31}$ erg s$^{-1}$ have a mean $M_{\rm WD}$ of $\sim0.6/1.0\,M_{\odot}$ if they are magnetic/non-magnetic CVs; while those with $L_{\rm 2-10 keV}$ between $1-6\times10^{31}$ erg s$^{-1}$ have a mean $M_{\rm WD}$ of $\sim0.8/1.2\,M_{\odot}$ if they are magnetic/non-magnetic CVs. All these \textit{Chandra}-detected CVs collectively contribute $\sim$30-50\% of the unresolved 20-40 keV X-ray emission from the NSC. The CV population with massive (i.e., $M_{\rm WD}\sim1.2M_{\odot}$) WDs have not been observed in the Solar vicinity or the Galactic bulge, and they might have been formed via dynamical encounters in the NSC., Comment: 16 pages, 1 Table, 4 Figures, accepted by the Astrophysical Journal

The flux ratio of Fe XXVI--Ly$\alpha$ to Fe XXV--He$\alpha$ lines ($I_{\rm 7.0}/I_{\rm 6.7}$) is a sensitive indicator of the maximum temperature ($T_{\rm max}$), and therefore the mass of white dwarf stars ($M_{\rm WD}$) in cataclysmic variables (CVs). To examine and calibrate the theoretical $I_{\rm 7.0}/I_{\rm 6.7}$--$T_{\rm max}$--$M_{\rm WD}$ relations, reliable measurements of $T_{\rm max}$ and $I_{\rm 7.0}/I_{\rm 6.7}$ are necessary. In this work, we conduct a thorough investigation on 3--50 keV X-ray spectra of 25 solar neighborhood magnetic and non-magnetic CVs based on archival \textit{NuSTAR} and \textit{Suzaku} observations. The measured $T_{\rm max}$ are compared to the $I_{\rm 7.0}/I_{\rm 6.7}$ and $M_{\rm WD}$. The results show the sampled CVs closely follow the theoretical $I_{\rm 7.0}/I_{\rm 6.7}$--$T_{\rm max}$ relation. Moreover, all the $M_{\rm WD}$ estimated from $I_{\rm 7.0}/I_{\rm 6.7}$ are consistent with the dynamically measured ones. We conclude that $I_{\rm 7.0}/I_{\rm 6.7}$ can be used as a good diagnostic for $T_{\rm max}$ and $M_{\rm WD}$ in both magnetic and non-magnetic CVs., Comment: 15 pages, 4 figures, Accepted for publication in ApJ

The X-ray emissivity (i.e., luminosity per unit stellar mass) of globular clusters are an important indicator of their dynamical evolution history. Based on deep archival \textit{Chandra} observations, we report a stacking analysis of 44 globular clusters (GCs) with 0.5-8 keV luminosities $L_{\rm X} \lesssim 10^{35} {\rm~erg~s^{-1}}$ in the M\,31 bulge, which are supposed to be dominated by cataclysmic variables (CVs) and coronally active binaries (ABs). We obtain a significant detection at $5\sigma$ level in 0.5-8 keV band. The average X-ray luminosity per GC and the average X-ray emissivity are determined to be $5.3 \pm 1.6\times10^{33}\rm~erg~s^{-1}$ and $13.2\pm4.3\times10^{27}\rm~erg~{\rm s^{-1}~M^{-1}_{\odot}}$, respectively. Both of these values are consistent with those of MW GCs. What's more, the measured emissivity of M31 GCs is also consistent with that of the MW field stars. Massive GCs have X-ray luminosities which are marginally higher with less massive ones. Massive GCs also show a lower emissivity ($4.5\pm 2.4\times10^{27}{\rm~erg~s^{-1}~M^{-1}_{\odot}}$) than less massive ones ($15.0\pm 7.8\times10^{27}{\rm~erg~s^{-1}~M^{-1}_{\odot}}$), which is consistent with the scenario that the (progenitors of) CVs and ABs were more efficiently destructed via stellar encounters in the more massive GCs. No dependence of the X-ray emissivity on GC color or on the projected galactocentric distance of GCs were found., Comment: 15 pages, 4 figures, accepted in ApJ

The mean white dwarf (WD) mass in the Galactic bulge cataclysmic variables (CVs) was measured by applying the the shock temperature-WD mass correlation of magnetic cataclysmic variables (mCVs) to the Galactic Bulge X-ray Emission (GBXE) spectra. However, the resulting mean WD mass is lower than that of the local CVs. This discrepancy could be explained by the dominating sources in the GBXE are non-mCVs instead of mCVs. In this work, we conduct an thorough investigation on the X-ray spectra of local DNe from \suzaku\ archives, and derive semi-empirical correlations between the shock temperature $T_{\rm max}$, the flux ratio of \fec\ to \feb\ lines, and WD mass for quiescent, non-magnetic CVs. By applying these correlations to the GBXE, we derive the average WD mass of CVs in the Galactic bulge to be $0.81\pm 0.07M_\odot$. This value is consistent with previous optical measurements of WD mass in local CVs., Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical Journal

The properties of the Galactic Ridge X-ray Emission (GRXE) observed in the 2-10 keV band place fundamental constraints on various types of X-ray sources in the Milky Way. Although the primarily discrete origin of the emission is now well established, the responsible populations of these sources remain uncertain, especially at relatively low fluxes. To provide insights into this issue, we systematically characterize the Fe emission line properties of the candidate types of the sources in the solar neighborhood and compare them with those measured for the GRXE. Our source sample includes 6 symbiotic stars (SSs), 16 intermediate polars (IPs), 3 polars, 16 quiescent dwarf novae (DNe) and 4 active binaries (ABs). We find that the mean equivalent width ($EW_{6.7}$) of the 6.7-keV line and the mean 7.0/6.7-keV line ratio are $107\pm16.0$ eV and $0.71\pm 0.04$ for intermediate polars and $221\pm 135$ eV and $0.44\pm 0.14$ for polars, respectively, which are all substantially different from those ($490\pm15 $~eV and $0.2\pm 0.08$) for the GRXE. Instead, the GRXE values are better agreed by the $EW_{6.7}$ ($438\pm 84.6$~eV) and the ratio ($0.27\pm 0.06$) observed for the DNe. We further find that the $EW_{6.7}$ is strongly correlated with the 2-10-keV luminosity of the DNe, which can be characterized by the relation $EW_{6.7}=(438\pm95 {\rm~eV}) (L/10^{31}~{\rm~ergs~s^{-1} })^{(-0.31\pm0.15)}$. Accounting for this correlation, the agreement can be improved further, especially when the contributions from other classes sources to the GRXE are considered, which all have low $EW_{6.7}$ values. We conclude that the GRXE mostly consists of typically faint, but numerous DNe, plus ABs, while magnetic CVs are probably mainly the high-flux representatives of the responsible populations and dominate the GRXE only in harder energy bands., Comment: 35 pages, 8 figures, accepted by ApJ

The ultra-luminous X-ray source (ULX) M82 X-1 is one of the most promising intermediate mass black hole candidates in the local universe based on its high X-ray luminosities ($10^{40}-10^{41} {\rm erg s^{-1}}$) and quasi-periodic oscillations, and is possibly associated with a radio flare source. In this work, applying the sub-pixel technique to the 120 ks Chandra observation (ID: 10543) of M82 X-1, we split M82 X-1 into two sources separated by 1.1$\arcsec$. The secondary source is not detected in other M82 observations. The radio flare source is found to associate not with M82 X-1, but with the nearby transient source S1 with an outburst luminosity of $\sim 10^{39} {\rm erg s^{-1}}$. With X-ray outburst and radio flare activities analogous to the recently discovered micro-quasar in M31, S1 is likely to be a micro-quasar hidden in the shadow of M82 X-1., Comment: 5 pages,2 figures and 2 tables, accepted for publication in ApJL

In the single degenerate scenario for the progenitors of type Ia supernovae (SNe Ia), a white dwarf rapidly accretes hydrogen- or helium-rich material from its companion star, and appears as a supersoft X-ray source. This picture has been challenged by the properties of the supersoft X-ray sources with very low-mass companions and the observations of several nearby SNe Ia. It has been pointed out that, the X-ray radiation or the wind from the accreting white dwarf can excite wind or strip mass from the companion star, thus significantly influence the mass transfer processes. In this paper we perform detailed calculations of the wind-driven evolution of white dwarf binaries. We present the parameter space for the possible SN Ia progenitors, and for the surviving companions after the SNe. The results show that the ex-companion stars of SNe Ia have characteristics more compatible with the observations, compared to those in the traditional single degenerate scenario., Comment: 20 pages, 7 figures, accepted for publication in ApJ

The binding energy parameter $\lambda$ plays an important role in common envelope (CE) evolution. Previous works have already pointed out that $\lambda$ varies throughout the stellar evolution, though it has been adopted as a constant in most of the population synthesis calculations. We have systematically calculated the binding energy parameter $\lambda$ for both Population I and Population II stars of masses $1-20 M_{\sun}$, taking into account the contribution from the internal energy of stellar matter. We present fitting formulae for $\lambda$ that can be incorporated into future population synthesis investigations. We also briefly discuss the possible applications of the results in binary evolutions., Comment: 23 pages, 3 figures, accepted for publication in ApJ.

Thermal timescale mass transfer generally occurs in close binaries where the donor star is more massive than the accreting star. The mass transfer rates are usually estimated in terms of the Kelvin-Helmholtz timescale of the donor star. But recent investigations indicate that this method may overestimate the real mass transfer rates in accreting white dwarf or neutron star binary systems. We have systematically investigated the thermal-timescale mass transfer processes in intermediate-mass X-ray binaries, by calculating binary evolution sequences with various initial donor masses and orbital periods. From the calculated results we find that on average the mass transfer rates are lower than traditional estimates by a factor of $\sim 4$., Comment: 13 pages, 4 figures, and 2 tables, accepted for publication in A&A