arXiv subjects

Xiao-jie Xu

Publications and source records attributed to Xiao-jie Xu.

10 recordsLinked to original sources

arXiv2026

Mechanisms for magnetic braking boost and disruption: the role of irradiation-driven winds and convective turnover time spike in cataclysmic variables

The saturated, boosted, and disrupted magnetic braking (SBD MB) model is an empirical prescription that has recently gained support from close-binary observations. Different boosting ($K$) and disruption ($η$) parameters appear necessary for different systems, but their physical origins remain uncertain. We aim to identify the mechanisms that boost magnetic braking (MB) and cause its disruption at the fully convective boundary in cataclysmic variables (CVs). We modelled CV evolution with MESA and compared the results with observed CV properties. We computed the convective turnover time ($τ_c$) directly from the donor's structure rather than adopting empirical relations. We also included irradiation from the accreting white dwarf, which heats the donor's outer layers and can drive additional winds that enhance MB. The structure-based $τ_c$ calculation reveals a pronounced spike as the donor approaches full convection, which drives the disruption parameter $η$ and initiates the period gap in CVs. The outcome of irradiation is sensitive to the accretion, irradiation, and wind efficiencies, all of which are poorly constrained from observations. Despite these uncertainties, plausible parameter choices allow irradiation-driven winds to provide the required boost $K$ during accreting phases. We refer to the combined prescription as the i$τ$SBD MB model and find that it yields evolutionary tracks broadly consistent with the main CV properties. Our i$τ$SBD MB framework offers a physically motivated interpretation of the empirical boost and disruption factors in SBD MB for CV evolution. We suggest that the convective turnover time spike at the fully convective boundary may drive MB disruption for fast-rotating stars in the saturated regime, while irradiation-driven winds may be the dominant mechanism boosting MB in accreting binaries and other strongly irradiated close systems.

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arXiv2026

X-ray luminous late-type giants: an overlooked population contributing to the Galactic ridge iron line emission

The origin of the highly ionized iron emission (Fe XXV at $6.7\,\mathrm{keV}$) characterizing the Galactic ridge X-ray emission (GRXE) remains a fundamental puzzle in high-energy astrophysics. Although the GRXE continuum is largely resolved into discrete populations of cataclysmic variables and coronally active stars, these sources exhibit Fe XXV equivalent widths significantly lower than that of the total GRXE, leaving the intense iron line emission unexplained. In this work, we cross-correlated the XMM-Newton survey of the inner Galactic disk with Gaia DR3 astrometry to identify and characterize hard X-ray sources ($>2\,\mathrm{keV}$) with reliable stellar counterparts. We selected 107 X-ray sources located within the red giant branch of the color-magnitude diagram, many of which are verified long-period variables. These sources exhibit high X-ray luminosities ($L_{\mathrm{X}} \approx 10^{31}$--$10^{33}\,\mathrm{erg~s^{-1}}$), significantly exceeding the typical coronal saturation levels of single giants. Their X-ray spectra are notably harder than those of quiescent stellar coronae, with plasma temperatures reaching up to $kT \approx 6\,\mathrm{keV}$ and a prominent emission feature at $\sim 6.7\,\mathrm{keV}$. The combination of high $L_{\mathrm{X}}$, hard spectra, and intense Fe XXV emission identifies this population as accretion-powered binaries associated with late-type giants. Our analysis demonstrates that this population contributes $\sim 20\%$ of the total GRXE continuum and $\sim 40\%$ of its iron line emission, providing a key component to resolving the Galactic X-ray background puzzle.

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arXiv2025

Unveiling the soft X-ray source population towards the inner Galactic disk with XMM-Newton

Across the Galactic disk lies a diverse population of X-ray sources, with the fainter end remaining poorly understood due to past survey sensitivity limits. We aim to classify and characterize faint X-ray sources detected in the eROSITA All-Sky Survey (eRASS1) towards the inner Galactic disk ($350^\circ < l < 360^\circ$, $-1^\circ < b < 1^\circ$) using deeper XMM-Newton observations (typical exposure of $\sim 20\,\text{ks}$). We analyzed 189 eRASS1 sources, combining X-ray spectral fitting ($0.2$--$10\,\text{keV}$) with Gaia astrometric and photometric data for robust classification. Our results show that the eRASS1 catalog towards the Galactic disk is overwhelmingly dominated by coronal sources ($\sim 74\%$), primarily active stars and binaries, with $\sim 8\%$ being wind-powered massive stars and $\sim 18\%$ being accreting compact objects. We propose an empirical hardness-ratio cut ($\text{HR} > -0.2$) to efficiently isolate these non-coronal sources. By stacking the classified population and comparing with the Galactic Ridge X-ray Emission (GRXE), we estimate that $\sim 6\%$ of the GRXE flux in the $0.5$--$2.0\,\text{keV}$ band is resolved into point sources above the eRASS1 flux limit ($\sim 5\times 10^{-14}\,\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$). This resolved soft-band emission is dominated by active stars, while hard-band flux originates primarily from X-ray binaries. We conclude that the eRASS1 catalog retains a non-negligible population of compact objects that can be effectively distinguished using X-ray color selection.

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arXiv2019

V1082-Sgr: A magnetic cataclysmic variable with a lobe-filling companion star

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.

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arXiv2019

Massive White Dwarfs in the Galactic Center: A Chandra X-ray Spectroscopy of Cataclysmic Variables

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.

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arXiv2019

The Fe Line Flux Ratio as a diagnostic of the maximum temperature and the white dwarf mass of Cataclysmic Variables

The flux ratio of Fe XXVI--Ly$α$ to Fe XXV--He$α$ 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.

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arXiv2018

An Empirical Correlation of $T_{\rm max}$ - $M_{\rm WD}$ of Dwarf Novae and The Average White Dwarf Mass in Cataclysmic Variables in the Galactic Bulge

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.

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arXiv2018

A Chandra Study of The Stellar X-ray Emissivity of Globular Clusters in M\,31 Bulge

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 $5σ$ 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.

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arXiv2016

Fe Line Diagnostics of Cataclysmic Variables and Galactic Ridge X-ray Emission

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.

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arXiv2015

A likely Micro-quasar in the Shadow of M82 X-1

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.

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