The minimum variation timescale (MVT) of soft gamma-ray repeaters can be an important probe to estimate the emission region in pulsar-like models, as well as the Lorentz factor and radius of the possible relativistic jet in gamma-ray burst (GRB)–like models, thus revealing their progenitors and physical mechanisms. In this work, we systematically study the MVTs of hundreds of X-ray bursts (XRBs) from SGR J1935+2154 observed by Insight-HXMT, GECAM, and Fermi/Gamma-ray Burst Monitor from 2014 July to 2022 January through the Bayesian block algorithm. We find that the MVTs peak at ∼2 ms, corresponding to a light-travel time size of about 600 km, which supports the magnetospheric origin in pulsar-like models. The shock radius and the Lorentz factor of the jet are also constrained in GRB-like models. Interestingly, the MVT of the XRB associated with FRB 200428 is ∼70 ms, which is longer than that of most bursts and implies its special radiation mechanism. In addition, the median of MVTs is 7 ms, shorter than the median MVTs of 40 ms and 480 ms for short GRBs or long GRBs, respectively. However, the MVT is independent of duration, similar to GRBs. Finally, we investigate the energy dependence of MVT and suggest that there is a marginal evidence for a power-law relationship like GRBs, but the rate of variation is at least about an order of magnitude smaller. These features may provide an approach to identify bursts with a magnetar origin.

Starburst galaxies are huge reservoirs of cosmic rays (CRs) and these CRs convert a significant fraction of their energy into gamma-rays by colliding with the interstellar medium (ISM). The produced GeV gamma-ray emission is temporally stable, and the flux is found to correlate well with indicators of star formation rates, such as the total infrared (IR) luminosity L _IR and monochromatic radio continuum luminosity at 1.4 GHz , i.e., following L _γ − L _IR and relations. Recently, gamma-ray excesses have been reported to be spatially coincident with two starburst galaxies, NGC 3424 and UGC 11041, in the fourth Fermi Large Area Telescope (LAT) source catalog (4FGL). Different from other starburst galaxies detected by Fermi-LAT, we find that the gamma-ray emission associated with NGC 3424 and UGC 11041 shows significant flux variability. With relatively weak infrared and radio emission, NGC 3424 and UGC 11041 appear as outliers of the L _γ − L _IR and relations of starburst galaxies. These results suggest that NGC 3424 and UGC 11041 may harbor obscured active galactic nuclei (AGNs) and the AGN activities provide the dominant contribution to the gamma-ray emission as compared to that provided by the starburst activities.

We report the discovery of a new 5.78 ms period millisecond pulsar (MSP), PSR J1740−5340B (NGC 6397B), in an eclipsing binary system discovered with the Parkes radio telescope (now also known as Murriyang) in Australia and confirmed with the MeerKAT radio telescope in South Africa. The measured orbital period, 1.97 days, is the longest among all eclipsing binaries in globular clusters (GCs) and consistent with that of the coincident X-ray source U18, previously suggested to be a “hidden MSP.” Our XMM-Newton observations during NGC 6397B’s radio-quiescent epochs detected no X-ray flares. NGC 6397B is either a transitional MSP or an eclipsing binary in its initial stage of mass transfer after the companion star left the main sequence. The discovery of NGC 6397B potentially reveals a subgroup of extremely faint and heavily obscured binary pulsars, thus providing a plausible explanation for the apparent dearth of binary neutron stars in core-collapsed GCs as well as a critical constraint on the evolution of GCs.

In the eighth data release (DR8) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, more than 318,740 low-resolution stellar spectra with types from B to early F and signal-to-noise ratios >50 were released. With this large volume of the early-type stars, we tried machine-learning algorithms to search for class-one and class-two chemical peculiars (CP1 and CP2), and to detect spectral features to distinguish the two classes in low-resolution spectra. We selected the XGBoost algorithm after comparing the classification efficiency of three machine-learning ensemble algorithms. Using XGBoost followed by the visual investigation, we presented a catalog of 20,694 sources, including 17,986 CP1 and 2708 CP2, in which 6917 CP1 and 1652 CP2 are newly discovered. We also list the spectral features to separate CP1 from CP2 discovered through XGBoost. The stellar parameters (including effective temperature (T _eff), surface gravity (log g), metallicity [Fe/H]), the spatial distribution in Galactic coordinates, and the color magnitude were provided for all of the entries of the catalog. The T _eff for CP1 distributes from ∼6000 to ∼8500 K, while for CP2 it distributes from ∼7000 to ∼13,700 K. The log g of CP1 ranges from 2.8 to 4.8 dex, peaking at 4.5 dex, and of CP2 it ranges from 2.0 to 5.0 dex, peaking at 3.6 dex, respectively. The [Fe/H] of CP1 and CP2 are from −1.4 to 0.4 dex, and the [Fe/H] of CP1 are on average higher than that of CP2. Almost all of the targets in our sample locate around the Galactic plane.

HS 2325+8205 is a long-period eclipsing dwarf nova with an orbital period above the period gap (P_orb > 3 h) and is reported to be a Z Cam-type dwarf nova. We study the light variation and the quasi-periodic oscillations (QPOs) of HS 2325+8205 based on the photometry of theTransiting Exoplanet Survey Satellite (TESS). Using a continuous wavelet transform, a Lomb–Scargle periodogram (LSP), and sine fitting methods, we find for the first time that there is a QPO of ∼2160 s in the long outburst top light curves of HS 2325+8205. Moreover, we find that the oscillation intensity of the QPOs of HS 2325+8205 is related to the orbital phase, and the intensity in orbital phases 0.5–0.9 is stronger than in orbital phases 0.1–0.5. Therefore, the relationship between the oscillation intensity of QPOs and the orbital phase may become a topic of research into the origin of QPOs. In addition, we use the LSP to correct the orbital period of HS 2325+8205 to 0.19433475(6) d.

Spectral lag of the low-energy photons with respect to the high-energy ones is a common astrophysical phenomenon (such as Gamma-ray bursts and the Crab pulsar) and may serve as a key probe to the underlying radiation mechanism. However, spectral lag in keV range of the magnetar bursts has not been systematically studied yet. In this work, we perform a detailed spectral lag analysis with the Li-CCF method for SGR J1935+2154 bursts observed by Insight-HXMT, GECAM and Fermi/GBM from July 2014 to Jan 2022. We discover that the spectral lags of about 61% (non-zero significance >1σ) bursts from SGR J1935+2154 are linearly dependent on the photon energy (E) with t_lag(E) = α(E/keV) + C, which may be explained by a linear change of the temperature of the blackbody-emitting plasma with time. The distribution of the slope (α) approximately follows a Gaussian function with mean and standard deviation of 0.02 ms/keV (i.e. high-energy photons arrive earlier) and 0.02 ms/keV, respectively. We also find that the distribution can be well fitted with three Gaussians with mean values of ∼ -0.009, 0.013 and 0.039 ms/keV, which may correspond to different origins of the bursts. These spectral lag features may have important implications on the magnetar bursts.