The SiTian project, designed to utilize 60 telescopes distributed across multiple sites in China, is a next-generation time-domain survey initiative. As a pathfinder for the SiTian project, the Mini-SiTian (MST) has been proposed and implemented to test the SiTian’s brain and data pipeline, and to evaluate the feasibility of its technology and science cases. Mounted at the Xinglong Observatory, the MST project comprises three 30 cm telescopes and has been operated since 2022 November. Each telescope of the MST possesses a large field of view, covering 2 . ° 29 × 1 . ° 53 FOV, and is equipped with g ′ , r ′ and i ′ filters, respectively. Acting as the pioneer of the forthcoming SiTian project, the MST is dedicated to the discovery of variable stars, transients, and outburst events, and has already obtained some interesting scientific results. In this paper, we will summarize the first-two-year operation of the MST project.

Theories and simulations predict that intense space-time curvature near black holes bends the trajectories of light and matter, driving disk and jet precession under relativistic torques. However, direct observational evidence of disk-jet coprecession remains elusive. Here, we report the most compelling case to date: a tidal disruption event (TDE) exhibiting unprecedented 19.6-day quasi-periodic variations in both x-rays and radio, with x-ray amplitudes exceeding an order of magnitude. The nearly synchronized x-ray and radio variations suggest a shared mechanism regulating the emission regions. We demonstrate that a disk-jet Lense-Thirring precession model successfully reproduces these variations while requiring a low-spin black hole. This study uncovers previously uncharted short-term radio variability in TDEs, highlights the transformative potential of high-cadence radio monitoring, and offers profound insights into disk-jet physics.

We report the detection and characterization of ultrafast outflows (UFOs) in the X-ray spectra of the tidal disruption event (TDE) AT2020afhd, based on observations from NICER, Swift, and XMM-Newton. Prominent blueshifted absorption features were detected exclusively during the intermediate phase of the event, occurring between days 172 and 212 within the first 300 days postdiscovery. During this period, the UFO appeared no earlier than day 74, strengthened between days 172 and 194, and disappeared after day 215. This marks the first time that the full evolutionary sequence of X-ray outflows has been observed in a TDE. Moreover, the outflows exhibited a dramatic deceleration from ∼0.19c to ∼0.0097c over a span of approximately 10 days. Photoionization spectral analysis reveals an inverse correlation between outflow velocity and ionization parameter, in contradiction to the predictions from radiation-pressure-driven wind. Finally, we propose that the delayed onset of the outflows may result from an increase in the wind opening angle and/or metal enrichment, particularly iron and oxygen, during the disk formation phase.

It is generally accepted that the limit on the stable rotation of neutron stars is set by gravitational-radiation reaction (GRR) driven instabilities, which cause the stars to emit gravitational waves that carry angular momentum away from them. The instability modes are moderated by the shear viscosity and the bulk viscosity of neutron star matter. Among the GRR instabilities, the f-mode instability plays a historically predominant role. In this work, we determine the instability periods of this mode for three different relativistic models for the nuclear equation of state (EoS) named DD2, ACB4, and GM1L. The ACB4 model for the EoS accounts for a strong first-order phase transition that predicts a new branch of compact objects known as mass-twin stars. DD2 and GM1L are relativistic mean field (RMF) models that describe the meson-baryon coupling constants to be dependent on the local baryon number density. Our results show that the f-mode instability associated with $m=2$ sets the limit of stable rotation for cold neutron stars ($T\lesssim {10}^{10}$ K) with masses between $1M_{\odot }$ and $2M_{\odot }$. This mode is excited at rotation periods between 1 and 1.4 ms (∼20% to ∼40% higher than the Kepler periods of these stars). For cold hypothetical mass-twin compact stars with masses between $1.96M_{\odot }$ and $2.10M_{\odot }$, the $m=2$ instability sets in at rotational stellar periods between 0.8 and 1 millisecond (i.e., ∼25% to ∼30% above the Kepler period).

We present the results of our study of the luminous (L _X ≈ 10³⁹ erg s⁻¹) X-ray binary CXOU J121538.2+361921 in NGC 4214, the high-mass X-ray binary with the shortest known orbital period. Using Chandra data, we confirm the ≈13,000 s (3.6 hr) eclipse period, and an eclipse duration of ≈2000 s. From this, we estimate a mass ratio M ₂/M ₁ ≳ 3 and a stellar density ρ ≈ 6 g cm⁻³, which implies that the donor must be a Wolf–Rayet or a stripped helium star. The eclipse egress is consistently much slower than the ingress. This can be explained by denser gas located either in front of the compact object (as expected for a bow shock) or trailing the donor star (as expected for a shadow wind, launched from the shaded side of the donor). There is no change in X-ray spectral shape with changing flux during the egress, which suggests either variable partial covering of the X-ray source by opaque clumps or, more likely, a gray opacity dominated by electron scattering in a highly ionized medium. We identify the optical counterpart from Hubble images. Photometry blueward of ∼5500 Å indicates a bright (M _B ≈ −3.6 ± 0.3 mag, for a range of plausible extinctions), hot (T ≈ 90,000 ± 30,000 K) emitter, consistent with the Wolf–Rayet scenario. There is also a bright (M _I ≈ −5.2 mag), cool (T ≈ 2700 ± 300 K) component consistent with an irradiated circumbinary disk or with a chance projection of an unrelated asymptotic giant branch star along the same line of sight.