Search Results (55)

Strong magnetic fields and anisotropic stresses can substantially modify the structure and observable properties of compact stars. In this review, we present a unified treatment of magnetically induced anisotropy across neutron stars, hybrid stars, and white dwarfs, connecting the microphysical equation of state effects to macroscopic structure and multimessenger observables. We demonstrate that magnetic-field geometry plays a decisive role: toroidally oriented (transverse) fields enhance the maximum mass by providing additional perpendicular pressure support, whereas radially oriented fields primarily increase central compression with comparatively small mass gain. In neutron stars, anisotropy and magnetic stresses can shift phase-transition thresholds in hybrid models and enable configurations in the lower mass gap with significantly smaller magnetic energy compared to the gravitational binding energy. We further show that continuous gravitational wave emission from magnetically deformed neutron stars provides a complementary probe of internal field geometry through ellipticity-driven strain evolution. In magnetized white dwarfs, super-Chandrasekhar masses arise from the spatial redistribution of magnetic stresses rather than from globally strong magnetic energy. Taken together, these results highlight that magnetic-field geometry and matter anisotropy are as important as field strength in determining mass–radius relations, tidal deformability, gravitational wave detectability, and the emergence of extreme compact-star configurations. Full article

We investigate nucleosynthesis during very strong, non-dynamical recurrent He-flashes that are expected to occur in close binary systems hosting a carbon–oxygen white dwarf and a type-B subdwarf companion. In these systems, due to gravitational wave emissions, the subdwarf star is expected to fill its Roche lobe on a short timescale, resulting in mass transfer onto the companion. As accreted matter also deposits angular momentum, the external layers of the accretor begin to rotate very fast. So, dynamical He burning is avoided, and the WD instead experiences recurrent strong He flashes, which secularly reduce its mass. We consider the PTF J2238+743015.1 system as representative of the whole class of similar objects and compute its evolution by coupling our evolutionary code with a full nuclear network, including isotopes with a lifetime longer than 0.8 s. We find that during He-flash episodes, the delivered neutron flux is typical for the i-process nucleosynthesis, even if it is available for a very short time (1–10 h). As a consequence, only weak s-process nucleosynthesis takes place. The nucleosynthetic path in the ejected matter is quite similar to that of supernovae descending from massive stars. However, due to the rarity of these systems, as well as to the small amount of matter ejected during the He-flashes phase, their contribution to the evolution of the interstellar medium is negligible. Full article

The construction of Dyson spheres, megastructures designed to capture the total radiative output of stars, can be one of the most compelling techno-signature scenarios for advanced extraterrestrial civilizations. By considering equilibrium temperatures, we investigate the luminosities and fluxes of Dyson spheres built around two promising classes of host stars: white dwarfs and red M-dwarfs. Using radiative balance arguments and representative stellar parameters, we compute the temperature–radius relationship for full energy interception and place these hypothetical structures on the Hertzsprung–Russell (H–R) diagram to assess their observational signatures. Our results show that Dyson spheres around white dwarfs produce cooler and fainter blackbody emissions, peaking in the near- to mid-infrared, while those around M-dwarfs radiate more strongly but at longer wavelengths. In both cases, the equilibrium temperature decreases as $R_{\mathrm{D}}^{-1/2}$, while the total luminosity and observed bolometric flux remain fixed by the stellar output. These findings highlight the astrophysical suitability of low-luminosity stars as Dyson sphere hosts and provide practical constraints for future techno-signature searches using infrared surveys. Full article

We present a comprehensive spectral and kinematic analysis of 27 polluted white dwarfs selected from a published catalog of polluted white dwarf candidates. Using LAMOST DR9 and Gaia DR3 data, we derive the effective temperature ($T_{\mathrm{eff}}$), surface gravity ($\log g$), and radial velocity ($RV$), and we measure the Ca II K line parameters, including equivalent width ($E{W}_{\mathrm{Ca}\mathrm{II}\mathrm{K}}$) and radial velocity ($R{V}_{\mathrm{Ca}\mathrm{II}\mathrm{K}}$). In addition, we estimate cooling ages and determine the three-dimensional Galactic kinematics and orbital parameters. Our results show that the majority of the targets lie above the pure-ISM expectation for the Ca II K line, suggesting that the line primarily originates from circumstellar material (CSM) rather than the interstellar medium (ISM). For DA-type white dwarfs in our sample, the Ca II K absorption is more prominent at lower effective temperatures and becomes significantly weaker toward higher temperatures, consistent with previous studies of metal-polluted white dwarfs. Additionally, DA stars show prominent $E{W}_{\mathrm{Ca}\mathrm{II}\mathrm{K}}$ values primarily in the cooling-age bin of $0.9$–$1.4\mathrm{Gyr}$, whereas DB stars are concentrated in the ${\tau }_{\mathrm{cool}}\lesssim 0.5\mathrm{Gyr}$ range, with a similar trend of first increasing and then decreasing $E{W}_{\mathrm{Ca}\mathrm{II}\mathrm{K}}$ with cooling age. Kinematic analysis reveals no significant differences between the Galactic populations of DA and DB white dwarfs. These findings indicate that metal pollution is common across different disk components of the Galaxy, with evidence for ongoing or recurrent evolution of white dwarf planetary systems within various Galactic structures. Full article

The orbital dynamics of compact-object binaries composed of neutron stars (NSs) and white dwarfs (WDs) can be influenced by the gravitational interaction with the gas of dark matter (DM) particles, generating dynamical friction. We discuss the orbital dynamics of detached binaries, quantifying the effect of dynamical friction from DM relative to that driven solely by gravitational-wave emission in vacuum. We focus on fermionic DM within the Ruffini–Arguelles–Rueda (RAR) model, for a fermion of rest-mass in the range 56–300 keV. We find that, for NS-NS, NS-WD, and WD-WD with parameters similar to those of J0737-3039, J0348+0432, and J0651+2844, the DM dynamical friction becomes detectable by space-based GW interferometers such as LISA and TianQin for binaries within a few milliparsec from the Galactic center, and could even dominate the orbital dynamics. Full article

Located in southern China, the five-hundred-meter aperture spherical radio telescope (FAST) is the world’s most sensitive radio telescope, especially for pulsar observation. Since its commissioning in 2016 and full operation in 2020, it has detected over 1100 new pulsars—boosting the globally known various pulsars to over 4000. In this concise overview, we highlight how harnessing FAST’s unique advantages—exceptional precision and ultra-high sensitivity—is set to fuel future discoveries of specialized pulsar types and exotic astrophysical objects. Notable targets include double millisecond pulsar binaries (MSP-MSPs), pulsar/millisecond pulsar–black hole systems (PSR-BHs or MSP-BHs), sub-millisecond pulsars, ultra-long-period pulsars, white dwarf pulsars, and short-orbit double neutron star systems (DNSs)—with orbital periods under one hour. As anticipated, in the 2040s, the combined capabilities of the FAST, the Square Kilometre Array (SKA), and other cutting-edge astronomical instruments will enable over 10,000 pulsar samples, which will usher in a golden era for pulsar research: such breakthroughs will not only significantly broaden and deepen our understanding of the “pulsar paradise” but also drive substantial progress in the field of multi-messenger astronomy. Beyond pulsar-focused research, FAST is poised to play a pivotal role in testing general relativity, detecting nanohertz gravitational waves, studying fast radio bursts (FRBs), and investigating the micro-structure of pulsar emissions. These investigations will not only strengthen our understanding of fundamental physics but also unlock deeper insights into the universe’s profound mysteries. Full article

In the optical band, very few pulsars can be directly detected, but some of the pulsar binary companions can be observed. This study leverages high-precision astrometric data from Gaia Data Release 3 (DR3) to identify pulsar companions in binary systems. Cross-matching the Australia Telescope National Facility (ATNF) Pulsar Catalogue with Gaia DR3 yielded 58 astrometric pairs, including 9 newly confirmed companions—primarily in the southern hemisphere—expanding the known pulsar distribution there. Among newly confirmed companions, eight are redback pulsars, offering insights into millisecond pulsar evolution and companion composition. All 58 companions are classified as main-sequence stars, neutron stars, white dwarfs, or ultra-light companion stars, with ∼40% being spider pulsars. Gaia’s exceptional astrometric precision advances pulsar studies, enabling gravitational wave detection via Pulsar Timing Arrays (PTAs) and improved reference frame link. Future multi-wavelength research will benefit from Gaia DR4, International Pulsar Timing Array (IPTA) collaborations (including Five-hundred-meter Aperture Spherical radio Telescope (FAST)), and Very Long Baseline Interferometry (VLBI) networks like the Chinese VLBI Network (CVN). Full article

We compare the observational properties of rotation-powered binary millisecond pulsars (BMSPs) in the Galactic Field with various companion types. First, BMSPs with diverse companion types exhibit different properties in the relation of binary orbital period versus companion mass, and in the spin period distribution of neutron stars (NSs), etc., implying multiple origins of BMSPs. Second, BMSPs with companions of CO/ONeMg white dwarfs (CO-BMSPs) show fewer sources than those with companions of Helium white dwarfs (He-BMSPs), which may result from the different evolutionary histories or accretion efficiencies in their progenitors. Third, BMSPs with main-sequence companions (MS-BMSPs) and ultra-light companions or planets (UL-BMSPs) are mostly eclipsing sources that are detected in both radio and $\gamma$-ray bands (i.e., radio+$\gamma$ sources), implying that they may be younger systems and share a faster average spin period and higher average accretion rate than CO-BMSPs/He-BMSPs. We propose that the predecessors of MS-BMSPs may share a short binary orbital distance with low-mass companion stars of $M_{\mathrm{c}}\sim 0.5$–$0.8{\mathrm{M}}_{\odot }$, which induces an efficient binary accretion process, and ultimately leaves a BMSP with a main-sequence companion due to the low efficiency of its hydrogen burning. Lastly, radio+$\gamma$ He-BMSPs share a faster average spin period of NSs than radio-only He-BMSPs. Meanwhile, these two groups of sources share similar companion mass distributions, implying the $\gamma$-ray evaporation effect may not obviously strip the companion mass of He-BMSPs during ∼0.3 Gyr, which may be due to the strong gravitational potential energy of the white dwarf companions. Full article

The phenomenon of runaway mass transfers between components of binary systems on a dynamical timescale has been theoretically predicted. However, this phenomenon has been observed for the first time in the history of astronomy just now in a symbiotic system V694 Mon. We employed medium- and high-dispersion spectroscopy, along with multicolor photometry, to study this event in detail. Over 6 years, beginning in 2018, we observed the cessation of disk accretion, the filling of the accretor’s Roche lobe, and the subsequent formation of an A-type star within it. The pulsating envelope of the M giant donor was transferred to the accretor down to its base. Thus, the products from the hydrogen-burning layer appeared on the donor’s surface, and a flash of an emission-line spectrum enriched with s-process elements was detected. We discuss discrepancies between theoretical predictions and observations, as well as other phenomena potentially related to dynamical mass transfer. Full article

Here, we briefly describe the current state of the Zeiss-1000 telescope of the Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS). Principal attention is given to research programs from recent years. The observations made according to allocated requests both by researchers from the observatory and scientists from other institutions and organizations are planned within a half-year schedule. The instrumental facilities provide a wide range of methods for studying objects of interest. They include standard photometry, moderate- and high-resolution spectroscopy, and polarimetry, along with unique “guest” methods (e.g., emission line imaging). The research programs cover different fields of astrophysics. The topic of “hot stars” and other important developments have become possible due to the introduction of new research methods at the telescope. Blazars, gamma-ray burst optical transients, massive supernovae, cataclysmic variables, magnetic stars, white dwarfs, luminous blue variables, red dwarfs, and many others are among its targets. Full article

This work provides the first comparison of the symmetron and dilaton fields in white dwarfs. We show how these screening mechanisms behave inside such stars and their impact on stellar properties. Employing a custom-developed shooting method, we solve the scalar–tensor equilibrium equations in the Newtonian approximation. We consider a Chandrasekhar equation of state and examine a range of potential mass scales and coupling strengths for both fields. Both fields enhance the pressure drop in low-density white dwarfs, leading to smaller stellar masses, radii, and luminosities. Unlike chameleon models, their effects are suppressed in more massive stars, with symmetron fields fully decoupling and dilaton fields weakening but not vanishing. Consequently, no mass–radius curve for screened white dwarfs exceeds the Newtonian prediction in any of these three mechanisms. The mass–radius deviations are generally more pronounced at lower densities, depending on model parameters. Due to their common runaway potential, we confirm that dilaton and chameleon fields display similar field and gradient profiles. In contrast, due to their environment-dependent coupling, the dilaton and symmetron mechanisms exhibit stronger density-dependent screening effects. These findings highlight both phenomenological differences and theoretical similarities among these mechanisms, motivating asteroseismology studies to constrain the symmetron and dilaton parameter spaces. Full article

Symbiotic stars, interacting binaries composed of a cool giant and a hot compact companion, exhibit complex variability across the electromagnetic spectrum. Over the past decades, large-scale photometric and spectroscopic surveys from ground- and space-based observatories have significantly advanced their discovery and characterization. These datasets have transformed the search for new symbiotic candidates, providing extensive time-domain information crucial for their classification and analysis. This review highlights recent observational results that have expanded the known population of symbiotic stars, refined classification criteria, and enhanced our understanding of their variability. Despite these advances, fundamental questions remain regarding their long-term evolution, mass transfer and accretion processes, or their potential role as progenitors of Type Ia supernovae. With ongoing and upcoming surveys, the coming years promise new discoveries and a more comprehensive picture of these intriguing interacting systems. Full article

Fast radio bursts (FRBs) are luminous radio transients with millisecond duration. For some active repeaters, such as FRBs 20121102A and 20201124A, more than a thousand bursts have been detected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The waiting time (WT) distributions of both repeaters, defined as the time intervals between adjacent (detected) bursts, exhibit a bimodal structure well-fitted by two log-normal functions. Notably, the time scales of the long-duration WT peaks for both repeaters show a decreasing trend over time. These similar burst features suggest that there may be a common physical mechanism for FRBs 20121102A and 20201124A. In this paper, we revisit the neutron star (NS)–white dwarf (WD) binary model with an eccentric orbit to account for the observed changes in the long-duration WT peaks. According to our model, the shortening of the WT peaks corresponds to the orbital period decay of the NS-WD binary. We consider two mass transfer modes, namely, stable and unstable mass transfer, to examine how the orbital period evolves. Our findings reveal distinct evolutionary pathways for the two repeaters: for FRB 20121102A, the NS-WD binary likely undergoes a combination of common envelope (CE) ejection and Roche lobe overflow, whereas for FRB 20201124A the system may experience multiple CE ejections. These findings warrant further validation through follow-up observations. Full article

Accreting white dwarf binaries (AWDs) comprise cataclysmic variables (CVs), symbiotics, AM CVns, and other related systems that host a primary white dwarf (WD) accreting from a main sequence or evolved companion star. AWDs are a product of close binary evolution; thus, they are important for understanding the evolution and population of X-ray binaries in the Milky Way and other galaxies. AWDs are essential for studying astrophysical plasmas under different conditions along with accretion physics and processes, transient events, matter ejection and outflows, compact binary evolution, mergers, angular momentum loss mechanisms, and nuclear processes leading to explosions. AWDs are also closely related to other objects in the late stages of stellar evolution, with other accreting objects in compact binaries, and even share common phenomena with young stellar objects, active galactic nuclei, quasars, and supernova remnants. As X-ray astronomy came to a climax with the start of the Chandra and XMM-Newton missions owing to their unprecedented instrumentation, new excellent imaging capabilities, good time resolution, and X-ray grating technologies allowed immense advancement in many aspects of astronomy and astrophysics. In this review, we lay out a panorama of developments on the study of AWDs that have been accomplished and have been made possible by these two observatories; we summarize the key observational achievements and the challenges ahead. Full article

Data on spectral line widths and shifts broadened by interactions with charged particles, for 44 lines in the spectrum of ionized tin, for collisions with electrons and H II and HeII ions, are presented as online available tables. We obtained them by employing the semiclassical perturbation theory for temperatures, T, within the 5000–100,000 K range, and for a grid of perturber densities from 10¹⁴ cm⁻³ to 10²⁰ cm⁻³. The presented Stark broadening data are of interest for the analysis and synthesis of ionized tin lines in the spectra of hot and dense stars, such as, for example, for white dwarfs and hot subwarfs, and for the modelling of their atmospheres. They are also useful for the diagnostics of laser-induced plasmas for high-order harmonics generation in ablated materials. Full article