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Detection of free precession is very important for analysing the internal structure of neutron stars. So far, because of the interference of the accretion process, no feasible method has been proposed to detect the precession of accreting neutron stars in both theory and observations. Based on the analysis of archive data from Chandra and the Neutron Star Interior Composition ExploreR (NICER) of the ultra-compact X-ray binary system 4U 1820–30, we find that the energy spectra have a stable sinusoidal bi-directional oscillation period near 1000 s, no matter which state the X-ray binary system is in. After we fit the energy spectra and carry out Fourier decomposition of the fitting parameters, we find that this period is more steadily emitted from the neutron star. We discuss the possible origin of this period and conclude that the period is most likely the free precession period of the neutron star in 4U 1820–30. Its nutation can result in the seesaw-like periodical bi-directional oscillation we find in this paper. [ABSTRACT FROM AUTHOR]

Fast transitions between different types of power density spectra (PDS) happening over time-scales of several tens of seconds are rare phenomena in black hole X-ray binaries. In this paper, we report a broad-band spectral-timing analysis of the fast transitions observed in the 2021 outburst of GX 339−4 using NICER and Insight -HXMT observations. We observe transitions between band-limited noise-dominated PDS and type-B quasi-periodic oscillations (QPOs), and their rapid appearance or disappearance. We also make a detailed comparison between the fast transitions in GX 339−4 with those seen in MAXI J1820+070 and MAXI J1348−630. By comparing the spectra of the periods with and without type-B QPOs, we find that the spectral ratios above 10 keV are nearly constant or slightly decreasing, and the values are different between sources. Below 10 keV, the flux change of the Comptonization component is inversely proportional to the flux change of the thermal component, suggesting that the appearance of type-B QPOs is associated with a redistribution of the accretion power between the disc and the Comptonizing emission region. The spectral ratios between the periods with type-B QPO and those with broad-band noise are significantly different from that with type-B QPO and without type-B QPO, where the ratios (type-B QPO/broad-band noise) show a maximum at around 4 keV and then decrease gradually towards high energies. Finally, we discuss the possible change of the geometry of the inner accretion flow and/or jet during the transitions. [ABSTRACT FROM AUTHOR]

Globular clusters (GCs) are the oldest stellar systems in the Galaxy, in which millisecond pulsars are widely believed to be the only steady γ-ray emitters. So far, nine γ-ray GCs have been identified and a few candidates, such as 2MS-GC01 and IC 1257, have been suggested. In this work, after analysing the publicly available Fermi Large Area Telescope data, we confirm the significant γ-ray emission from 2MS-GC01 and IC 1257 and report the discovery of γ-ray emission from NGC 5904 and 6656 within their tidal radii. Also, strong evidence of significant γ-ray emission is found from FSR 1735. From the observed γ-ray luminosities, we estimate the numbers of millisecond pulsars that are expected to be present in these GCs. [ABSTRACT FROM AUTHOR]

After 2010, three outbursts of H1743−322 were detected by RXTE. We have carried out timing and spectral analysis of the data, emphasizing on the two with relatively complete evolution history presented in the RXTE/Proportional Counter Array observations. We then constitute an enlarged outburst sample for H1743−322 which allows us to investigate the spectral transitions in more details. We find that the spectral transitions to high-soft state constrain a region for four outbursts in hardness–intensity diagram. An extension of the region locates in the vicinity of the failed outburst in 2008, and excludes it from a successful group. We therefore suggest that the failed outburst in 2008 may present the first almost successful outburst sample, which is important for modelling the outburst, especially upon the conditions required for transition to high-soft state. [ABSTRACT FROM AUTHOR]

ABSTRACT The relation between the spin period ( Ps) and the orbital period ( Po) in high-mass X-ray binaries is investigated. Be/X-ray binaries and SG/X-ray binaries occupy different areas in the Corbet diagram. The supergiant fast X-ray transients, IGR J18483-0311 and IGR J11215-5952, however, fall into the area occupied by Be/X-ray binaries. After excluding the other evolution scenarios, we find that the positions of the two supergiant fast X-ray transients in the diagram can be interpreted by being the descendants of O-emission X-ray binaries, i.e. the supergiant stars in these systems formed from O-type emission line stars instead of normal main-sequence stars. Therefore, we suggest that there are two types of SG/X-ray binaries: the first one is directly evolved from OB-type main-sequence star and neutron star systems without accretion history, and the other evolved from Be/X-ray binaries with accretion history. [ABSTRACT FROM AUTHOR]