arXiv subjects

Zhi-Xiang Zhang

Publications and source records attributed to Zhi-Xiang Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of Repeating Transitions in 16 Changing-look Active Galactic Nuclei

The repeating changing-look active galactic nuclei (RCL AGNs) exhibit multiple appearances and disappearances of broad emission lines (BELs), whose underlying mechanism remains a puzzle. Expanding the sample of RCL AGNs is valuable for constraining the transition timescale and probing the accretion physics driving CL behaviors. This study aims to identify RCL AGNs using the multi-epoch spectroscopic data of confirmed CL AGNs from the Sloan Digital Sky Survey, Large Sky Area Multi-Object Fiber Spectroscopic Telescope, and Dark Energy Spectroscopic Instrument, supplemented with mid-infrared (MIR) light curves. Through selection criteria and visual inspection, we identify 22 RCL AGNs among 299 CL AGNs, corresponding to an occurrence rate of about 7\%, indicating that repeated transitions are not extremely rare in CL AGNs. Among the 22 RCL AGNs, 16 are newly identified, which significantly expands the known RCL AGN sample. Based on the spectra and densely sampled MIR light curves, we derive MIR variability timescales for 18 RCL AGNs, and find no significant correlation between the timescale and the black hole mass.

astro-ph.GA↗

arXiv2026

Classical Be Stars and Classical Be Star Binaries from LAMOST DR12

Classical Be (CBe) stars are rapidly rotating B-type stars with Balmer emission lines that originated from the decretion disks surrounding them in their spectra. Accounting for $\sim$20% of all B-type stars, most CBe stars are thought to form through mass and angular momentum transfer from their companions. It follows that in most close CBe star binaries, the companions are expected to be post-main-sequence stars rather than main-sequence (MS) stars. Hitherto, $\sim$100 CBe star binaries have been identified, the majority of which are Be/X-ray binaries. As expected, none of the others have indeed been confirmed as CBe+MS binary stars. To further study and verify the origin of CBe stars, identifying additional CBe star binaries is indispensable. In this study, we report 504 CBe stars identified using data from Data Release 12 of the Large sky Area Multi-Object fiber Spectroscopic Telescope. Among these, 141 are newly identified and 14 exhibiting radial velocity variations are identified as CBe star binaries. Besides, 60 CBe stars with high normalized unit weight error (RUWE) but not confirmed by dynamics are proposed as potential CBe star binaries. We also find that 34 CBe stars are potential cluster members. By calculating peculiar velocities, 37 runaway stars are identified with peculiar velocities ranging from $\sim$40 km s$^{-1}$ to $\sim$101 km s$^{-1}$.

astro-ph.SR↗

arXiv2026

Searching for Contact Binaries with LAMOST and TESS

Contact binaries (CBs) serve as fundamental laboratories for studying complex stellar interactions, including mass transfer, tidal effects, and angular momentum loss. In this work, we search for CB with high-precision light curves from the Transiting Exoplanet Survey Satellite (TESS) and large radial-velocity variation from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). We derive a sample of 1,281 CB candidates, among which 266 are newly reported. Our sample with both high-precision photometry and medium-resolution spectra may provide new constraints on the physical scales, luminosity calibration, and population distribution of CBs, offering valuable insights into their evolutionary role within the stellar population.

astro-ph.SR↗

arXiv2026

Weighing Hidden Companions of Compact Object Candidates via Rotational Broadening

The determination of unseen companion masses ($M_1$) is essential for identifying compact objects in binary systems, yet obtaining reliable orbital inclinations remains one of the most difficult challenges. In this study, we focus on ten targets selected from a sample of 89 compact object candidates characterized by large mass functions, rapid rotation, and high-quality Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) spectra. We measure their projected rotational velocities ($v \sin i$) from the LAMOST medium-resolution spectra and, combined with stellar radii, derive orbital inclinations and the corresponding companion masses. Our results show that five sources exhibit mass ratios $M_1 / M_2 > 2/3$, with no detectable spectral signatures of the unseen companions, providing strong evidence for their compact nature. Two particularly notable cases, J0341 and J0359, host companions with inferred masses of $1.39^{+0.09}_{-0.10}$ $M_\odot$ and $1.34^{+0.08}_{-0.09}$ $M_\odot$, respectively. These masses suggest that the invisible objects are either neutron stars or massive white dwarfs with masses close to the Chandrasekhar limit. If they are white dwarfs, these two targets are highly likely to be Type Ia supernova progenitors. This study highlights the potential of $v \sin i$ measurements as a systematic approach to unveiling compact objects in binaries.

astro-ph.SR↗

arXiv2026

X-ray Quasi-Periodic Oscillations in Active Galactic Nuclei and Their Implications for the Changing Look Phenomenon

X-ray timing of active galactic nuclei (AGN) provides a unique probe of gas accretion onto supermassive black holes (SMBHs). Quasi-periodic oscillations (QPOs), which trace gas dynamics in the strongly curved spacetime around SMBHs, are rare in AGN. These signals often are analogs of high-frequency QPOs occasionally seen in some black-hole X-ray binaries, and their scarcity in AGN can partly be attributed to the low frequencies expected for typical SMBH masses. Intriguingly, robust X-ray QPO detections in SMBH systems have so far been reported only in narrow-line Seyfert 1 galaxies (NLS1s) and tidal disruption events (TDEs). Here we report the discovery of a QPO candidate during the 2018 outburst of the changing-look AGN (CL-AGN) NGC 1566. Numerical simulations indicate that the disk epicyclic oscillations responsible for high-frequency QPOs are damped by magnetohydrodynamic turbulence unless the accretion flow is misaligned and/or eccentric. In TDEs, the stellar debris stream is naturally misaligned with the SMBH spin, while NLS1s may host misaligned disks due to their youth. Motivated by the QPO candidate in NGC 1566, we propose that CL-AGN accretion is also misaligned -- potentially fueled by captured, free-falling broad-line region clouds. This model naturally explains why CL-AGN transition timescales are much shorter than the standard disk viscous timescale. This picture can be tested by searching for QPOs or quasi-periodic eruptions in other CL-AGN.

astro-ph.HE↗

arXiv2026

Light curve modeling of the nearest neutron star candidate LAMOST J235456.73+335625.9

The discovery of heavy radioactive elements (e.g., $^{60}\mathrm{Fe}$) on Earth suggests that supernova explosions may have occurred near our planet within the past million years, potentially having a significant impact on the ecological environment. This finding has motivated the search for nearby neutron stars in the Solar neighborhood. In a recent study, a candidate for one of the closest neutron stars to Earth, LAMOST J235456.73+335625.9 (hereafter J2354), was reported. Based on dynamical mass measurements under different inclination angle assumptions, the inferred mass range for the unseen compact companion in the system is $1.4$--$1.6$ $M_{\odot}$. Hence, the unseen companion in J2354 is either a massive cold white dwarf or a neutron star. Here we model the flux variations of J2354 as a combination of ellipsoidal modulation and surface spots. We test both cold spot and hot spot models, setting the number of spots to two in each case, and constrain the spot properties through light curve fitting. In the cold spot scenario, the spots are mostly visible at phases $0.5$--$0.75$, whereas in the hot spot scenario, the spots appear predominantly at phases $0.25$--$0.5$. The hot spot model shows better agreement with the observed H$α$ phase variation than the cold spot model. Furthermore, the thermal radiation of a massive but cold white dwarf cannot produce the level of localized heating required to explain the hot spot unless additional heating mechanisms are involved; in contrast, a neutron star can naturally provide such heating through energetic winds. Our results are consistent with the neutron star interpretation of the compact object in J2354.

astro-ph.SR↗

arXiv2025

Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. XV. Reverberation Mapping of Mg II Emission Lines

As the 15th paper in a series reporting on a large reverberation mapping (RM) campaign of super-Eddington accreting massive black holes (SEAMBHs) in active galactic nuclei (AGNs), we present the results of measurements of the Mg II lines in 18 SEAMBHs monitored spectroscopically from 2017 to 2024. Among these, the time lags of Mg II have been successfully determined for 8 of the 18 objects, thereby expanding the current Mg II RM sample, particularly at higher accretion rates. By incorporating measurements of the line widths, we determine the masses of their central supermassive black holes. Based on these new measurements, we update the relation between the Mg II radius and the monochromatic luminosity at 3000 $\mathring{\mathrm{A}}$ ($R_{\rm MgII}-L_{3000}$ relation), yielding a slope of $0.24 \pm 0.03$, which is slightly shallower than, yet still consistent with, previously reported values. Similar to the H$β$ lines, the Mg II time lags in SEAMBHs are shorter than those of AGNs with normal accretion rates at comparable luminosities. The deviation of AGNs from the best-fit $R_{\rm MgII}-L_{3000}$ relation shows a strong correlation with the accretion rate, while no significant correlation is found between the deviation and the flux ratio of UV iron to Mg II.

astro-ph.GA↗

arXiv2025

Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. IV. Broad Emission Line Evolution Sequence Among Hα, Mg II, and Hβ

From a parent catalog of 561 changing-look active galactic nuclei (CL-AGNs) identified by Guo et al. (2025), we investigate the evolutionary sequence of broad emission lines using a redshift-selected subset (0.35 < z < 0.45) of 54 CL-AGNs whose Dark Energy Spectroscopic Instrument (DESI) spectra simultaneously cover the Hα, H\b{eta}, and Mg II emission lines. To provide a baseline for comparison, we construct a control sample of 19,897 normal Type 1 AGNs within the same redshift range from the DESI Year 1 data. Through stacked spectral analysis and line-continuum luminosity correlations, we identify a clear evolutionary sequence in all AGN where broad H\b{eta} fades first, followed by Mg II, and then Hα, as the AGN luminosity declines - consistent with expectations from reverberation mapping. This trend reflects a radially stratified broad line region (BLR), where each line's responsivity depends on its ionization potential and radial distance from the central engine. In addition, we find that more massive supermassive black holes (SMBHs) require lower Eddington ratios to fully suppress broad emission lines, suggesting that the critical accretion threshold for the CL phenomenon is mass-dependent. Our results present the first statistical confirmation of a stratified broad line fading sequence in AGNs, reinforcing the central role of accretion state in shaping BLR structure and visibility.

astro-ph.GA↗

arXiv2025

A Study of Cataclysmic Variables from the eFEDS Survey

We present 17 cataclysmic variables (CVs) obtained from the crossmatch between the Sloan Digital Sky Survey (SDSS) and eROSITA Final Equatorial Depth Survey (eFEDS), including 8 known CVs before eFEDS and 9 identified from eFEDS. The photometric periods of four CVs are derived from the Zwicky Transient Facility (ZTF) and Catalina Real-Time Transient Survey (CRTS). We focus on two CVs, SDSS J084309.3$-$014858 and SDSS J093555.0+042916, and confirm that their photometric periods correspond to the orbital periods by fitting the radial velocity curves. Furthermore, by the combination of the Gaia distance, the spectral energy distribution, and the variations of $\mathrm{H}\mathrmα$ emission lines, the masses of the white dwarf and the visible star can be well constrained.

astro-ph.SR↗

arXiv2025

A Systematic Search for Gaseous Debris Disks in DESI Early Data Release White Dwarfs

Detecting gaseous debris disks around white dwarfs offers a unique window into the ultimate fate of planetary systems and the composition of accreted planetary material. Here we present a systematic search for such disks through the Ca II infrared triplet using the Dark Energy Spectroscopic Instrument (DESI) Early Data Release. From a parent sample of 2706 spectroscopically confirmed white dwarfs, we identify 22 candidate systems showing tentative emission-line features, which corresponds to a raw occurrence rate of 0.81%, more than ten times higher than previous estimates. The detected emission lines are predominantly weak and require confirmation by follow-up observations. Three of these candidates also exhibit infrared excess in WISE photometry, suggesting a possible coexistence of gas and dust. However, the high candidate rate indicates that most are likely false positives due to telluric residuals or unresolved binaries. This work demonstrates the potential of DESI spectra for blind searches of rare circumstellar phenomena. The recently released DESI DR1, with its substantially larger spectroscopic sample, will enable searches for more gaseous disks and provide better insights into their occurrence and nature.

astro-ph.SR↗

arXiv2025

An H$α$ Cloud in the HI Tail: Recent Star Formation in the Outskirts of NGC 4258 Revealed by Nanshan 1-m Telescope

We present first-light deep H$α$ imaging taken with the Nanshan 1-meter wide-field telescope on the local galaxy NGC 4258, alongside archival data from Hubble Space telescope (HST), Westerbork Synthesis Radio Telescope, and The Dark Energy Camera Legacy Survey. The H$α$ image shows ongoing star formation not only inside the galaxy but also in an HI cloud in the eastern HI tail, which is roughly 16 kpc away from the main galaxy. The HST images reveal several ultra-blue compact objects ($\rm F555W - F814W <-0.5 mag,\, FWHM\sim 0.2''$) in the H$α$ bright region, aligned with the HI tail, suggesting the presence of young open cluster candidate in the HI tail. Our results suggest that wide field H$α$ imaging is a valuable tool for investigating recent star formation in the extended regions of NGC 4258. Furthermore, the star formation in diffuse HI tails could highlight an potential aspect of galaxy stellar halo formation, warranting further investigation of the impact of star formation in halos on galaxy evolution.

astro-ph.GA↗

arXiv2025

A Be star-black hole binary with a wide orbit from LAMOST time-domain survey

Binary systems consisting of an early type star and a black hole (BH) are crucial for understanding various astrophysical phenomena, particularly the origins of detected gravitational wave sources. Be binary systems are expected to represent a key evolutionary stage in hosting BHs. However, while hundreds of Be X-ray binaries are known, the only confirmed BH candidate in a Be binary remains highly controversial. We report the discovery of ALS 8814, a Be star-BH binary with a moderately eccentric ($e = 0.23$) and wide orbit ($P = 176.6$ days), revealed by the radial velocity (RV) measurement of the visible Be star. Our analysis, combining flux-calibrated spectra in the Balmer discontinuity region and spectral template matching, yields a mass of $11.2^{+1.4}_{-1.2}$ $M_\odot$ for the Be star. The minimum mass of the unseen companion, assuming an edge-on inclination ($i = 90^{\circ}$), is $9.8\pm 0.7\,M_\odot$. We rule out the presence of non-degenerate companions in ALS 8814, indicating that it can only be a BH. This discovery represents a robust case of a Be-BH binary, identified purely through precise RV measurements from a single set of lines. The extremely low peculiar velocity of ALS 8814 suggests that the BH is formed via a direct core-collapse with a negligible natal kick, implying an almost perfect alignment between the Be star's spin and the orbital plane. In this context, the binary's inclination angle is estimated to be 22$^{\circ}$-49$^{\circ}$ by analyzing the shallow double-peaked profile of the H$α$ emission line. This inclination range corresponds to a BH mass estimate between $15\,M_\odot$ and $58\,M_\odot$. As the only unambiguous Be-BH binary system known to date, ALS 8814 provides valuable constraints on the BH formation in a binary system with a high-mass companion.

astro-ph.SR↗

arXiv2025

A Sandwich Model for Changing-Look AGNs

The spectral variability of changing-look active galactic nuclei (CL-AGNs) occurred on timescales of years to tens of years, posing a significant challenge to the standard thin disk model. In this work, we propose a sandwich model, including an optically thick disk in the mid-plane (Disk 1) and two disks of low effective optical depth on both sides (Disk 2). These two types of disks are coupled with magnetic fields, which allow viscous torque interaction between them. As a consequence, the radial velocity of Disk 1 can increase by up to three orders of magnitude compared to the standard thin disk, leading to an equivalent decrease in the accretion timescale. Therefore, such a sandwich model can account for the rapid variability in CL-AGNs. In addition, we also discuss the influence of the magnetic pressure on Disk 2. When Disk 2 is dominated by the magnetic pressure, it resembles a "warm corona", which is responsible for the soft X-ray excess.

astro-ph.GA↗

arXiv2025

Newly Discovered Changing-look Active Galactic Nuclei from SDSS and LAMOST Surveys

Changing-look Active Galactic Nuclei (CL AGNs) exhibit drastic variations in broad emission lines (BELs), the mechanism of which remains unclear. Expanding the sample of CL AGNs is helpful to reveal the mechanism. This study aims to identify more CL AGNs by cross-matching spectroscopic data from the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Our approach to identify CL AGNs is based on the automatic spectral fitting, followed by detailed visual inspections. We identify a sample of $51$ CL AGNs through this method, in which $40$ CL AGNs are newly discovered. Within this sample, $41$ CL AGNs primarily show the variability of the H$β$ line, nine exhibit obvious changes in both the H$β$ and H$α$ lines, and one source mainly displays variations in the H$α$ line. Our findings reveal that the sequence of appearance and disappearance of the BELs aligns with the known CL sequence. In addition, we identify 31 candidates exhibiting drastic variations in BELs without accompanying significant photometric variability. We estimate the black hole mass and Eddington ratio for all sources, which range from $2.5\times 10^6$ to $8.0\times 10^8 M_\odot$ and from $0.001$ to $0.13$, respectively. Similar to other studies, we also find that the Eddington ratios of CL AGNs and candidates are lower than those of typical AGNs. Our results support the hypothesis that the CL behavior is driven by the state transitions of the accretion disk.

astro-ph.GA↗

arXiv2025

A Sample of Extreme Eclipsing Binaries with Accretion Disks from LAMOST and ZTF

Extreme eclipsing binaries may harbor peculiar physical properties. In this work, we aim to identify a sample of such systems by selecting binaries with pronounced eclipsing light curves, characterized of large variability ($Δ\mathrm{mag} > 0.3$ in ZTF $g$ band) and significant differences between primary and secondary eclipses (eclipse depth ratio $>$ 20 in ZTF $g$ band). We identified 23 candidates by combining the photometric data and the LAMOST spectroscopic survey. Spectroscopic analysis revealed that all of these systems are dominated by A-type stars in the optical band. Further investigation confirmed that all 23 candidates are Algol-type binaries, with 22 of them being newly discovered. Their orbital periods range from 2.57 to 19.21 days. These systems consist of low-luminosity, highly stripped subgiant donors and accreting A-type stars. The donor stars, with radii of $2.5-8.9~R_\odot$ and effective temperatures around 4000 K, have typical masses of $M_2 \sim 0.3~M_\odot$, indicating substantial mass loss through Roche-lobe overflow. The presence of ellipsoidal variability and H$α$ emission provides strong evidence for ongoing mass transfer. By fitting the spectral energy distributions, spectra, and light curves, we found that most of the accretors have luminosities lower than expected from the mass-luminosity relation, aligning with the predicted faint phase for mass-gaining stars. Three objects of our sample exhibit pulsations with periods from 18 minutes to 8 hours, providing opportunities for asteroseismic studies. The low mass transfer rates and stability make the sample excellent systems for studying mass accretion, advancing our understanding of the Algol-type binary evolution.

astro-ph.SR↗

arXiv2024

Geometrical Distances of Extragalactic Binaries through Spectroastrometry

The growing ``Hubble tension'' has prompted the need for precise measurements of cosmological distances. This paper demonstrates a purely geometric approach for determining the distance to extragalactic binaries through a joint analysis of spectroastrometry (SA), radial velocity (RV), and light curve (LC) observations. A parameterized model for the binary system is outlined, and simulated SA, RV, and LC data are computed to infer the probability distribution of model parameters based on the mock data. The impact of data quality and binary parameters on distance uncertainties is comprehensively analyzed, showcasing the method's potential for high-precision distance measurements. For a typical eclipsing binary in the Large Magellanic Cloud (LMC), the distance uncertainty is approximately 6% under reasonable observational conditions. Within a specific range of data quality and input parameters, the distance measurement precision of individual binary star systems is generally better than 10%. As a geometric method based on the simplest dynamics, it is independent of empirical calibration and the systematics caused by model selections can be tested using nearby binaries with known distances. By measuring multiple binary star systems or monitoring one binary system repeatedly, geometric distance measurements of nearby galaxies can be achieved, providing valuable insights into the Hubble tension and advancing our understanding of the universe's structure and evolution.

astro-ph.CO↗

arXiv2024

A Short-lived Rejuvenation during the Decades-long Changing-look Transition in the Nucleus of Mrk 1018

Changing-look active galactic nuclei (CL-AGNs), characterized by emerging or disappearing of broad lines accompanied with extreme continuum flux variability, have drawn much attention for their potential of revealing physical processes underlying AGN evolution. We perform seven-season spectroscopic monitoring on Mrk~1018, one of the earliest identified CL-AGN. Around 2020, we detect a full-cycle changing-look transition of Mrk~1018 within one year, associated with a nucleus outburst, which likely arise from the disk instability in the transition region between the outer standard rotation-dominated disk and inner advection-dominated accretion flow. Over the past forty-five years, the accretion rate of Mrk~1018 changed 1000 times and the maximum Eddington ratio reached 0.02. By investigating the relation between broad-line properties and Eddington ratio ($L_{\rm bol}/L_{\rm Edd}$), we find strong evidence that the full-cycle type transition is regulated by accretion. There exists a turnover point in the Balmer decrement, which is observed for the first time. The broad Balmer lines change from a single peak in Type 1.0-1.2 to double peaks in Type 1.5-1.8 and the double-peak separation decreases with increasing accretion rate. We also find that the full width at half maximum (FWHM) of the broad Balmer lines obeys FWHM$\propto (L_{\rm bol}/L_{\rm Edd})^{-0.27}$, as expected for a virialized BLR. The velocity dispersion $σ_{\rm line}$ follows a similar trend in Type 1.5-1.8, but displays a sharp increases in Type 1.0-1.2, resulting in a dramatic drop of FWHM/$σ_{\rm line}$. These findings suggest that a virialized BLR together with accretion-dependent turbulent motions might be responsible for the diversity of BLR phenomena across AGN population.

astro-ph.GA↗

arXiv2024

Gleeok's Fire-breathing: Triple Flares of AT 2021aeuk within Five Years from the Active Galaxy SDSS J161259.83+421940.3

We present a noteworthy transient AT 2021aeuk exhibiting three distinct optical flares between 2018 and 2023. It is hosted in a radio-loud narrow-line Seyfert 1 (NLSy1) galaxy, with an optical image showing a minor tidal morphology and a red mid-infrared color W1-W2=1.1. Flares II and III exhibit rapid rises, and long-term decays (around 1000 days) with recurring after-peak bumps. The g-r color after subtracting the reference magnitude exhibited a rapid drop and recovery during Flare II, followed by the minor after-peak evolution in blue colors. We applied a canonical tidal disruption event (TDE) fitting on the light curves which gives a decay index p of $-2.99_{-0.14}^{+0.13}$ for Flare II and $-1.61_{-0.65}^{+0.34}$ for Flare III. The blackbody fitting shows lower temperatures ($\sim 10^{3.8}$K) with minor after-peak evolution. The blackbody radius ($\gtrsim 10^{16}\ \rm cm$) and luminosity ($\sim 10^{45}\rm erg\ s^{-1}$) are larger than the typical TDE sample's. The time lag (in rest frame) between ZTF g- and r-band ($\rm τ_{g,r}=3.4^{+1.0}_{-0.9}$ days) significantly exceeds the prediction from the standard accretion disk. Pre-burst spectra reveal prominent Bowen fluorescence lines, indicating a vigorous or potentially long-lasting process that enriches the local metallicity. Additionally, we derived black hole masses of $\log\it{M}_{\bullet}\rm=7.09^{+0.18}_{-0.31}\ \it{M}_{\odot}$ and $\log\it{M}_{\bullet}\rm=7.52^{+0.08}_{-0.10}\ \it{M}_{\odot}$ using $\rm Hβ$ and $\rm Hα$ emission lines. The variation and recurring features of AT 2021aeuk are not likely induced by the radio-beaming effect or Type-II superluminous supernova (SLSN-II), however, we cannot rule out the possibility of TDE or enhanced active galactic nuclei (AGN) accretion process. The unusually high occurrence of three flares within five years may also induced by the complex local environment.

astro-ph.HE↗