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In the beginning of 2023, the Be transient X-ray pulsar RX J0440.9+4431 underwent a first-ever giant outburst observed from the source peaking in the beginning of February and reaching peak luminosity of ≈4.3 × 1037 erg s−1. Here, we present the results of a detailed spectral and temporal study of the source based on NuSTAR, Swift, INTEGRAL , and NICER observations performed during this period and covering wide range of energies and luminosities. We find that both the pulse profile shape and spectral hardness change abruptly around ≈2.8 × 1037 erg s−1, which we associate with a transition to supercritical accretion regime and erection of the accretion column. The observed pulsed fraction decreases gradually with energy up to 20 keV (with a local minimum around fluorescence iron line), which is unusual for an X-ray pulsar, and then rises rapidly at higher energies with the pulsations significantly detected up to ≈120 keV. The broad-band energy spectra of RX J0440.9+4431 at different luminosity states can be approximated with a two-hump model with peaks at energies of about 10–20 and 50–70 keV previously suggested for other pulsars without additional features. In particular, an absorption feature around 30 keV previously reported and interpreted as a cyclotron line in the literature appears to be absent when using this model, so the question regarding the magnetic field strength of the neutron star remains open. Instead, we attempted to estimate field using several indirect methods and conclude that all of them point to a relatively strong field of around B ∼ 1013 G. [ABSTRACT FROM AUTHOR]

We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January–February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar's geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind. [ABSTRACT FROM AUTHOR]

We report results of the first detailed spectral and temporal studies of the recently discovered Be/X-ray binary eRASSU J050810.4−660653 in Large Magellanic Cloud based on the data from the SRG /ART-XC, NuSTAR and Swift /XRT instruments obtained in 2021 December–2022 May in a wide energy range of 0.5–79 keV. Pulsations with the period of 40.5781 ± 0.0004 s were found in the source light curve with the pulsed fraction monotonically increasing with the energy. An estimate of the orbital period of ∼38 d was obtained based on the long-term monitoring of the system. The source spectrum can be well approximated with a power-law model modified by an exponential cutoff at high energies. The pulse phase-resolved spectroscopy shows a strong variation of spectral parameters depending on the phase of a neutron star rotation. We have not found any features connected with the cyclotron absorption line both in the phase-averaged and phase-resolved spectra of eRASSU J050810.4−660653. However, the neutron star magnetic field was estimated around several 1013 G using different indirect methods. Discovered variations of the hardness ratio over the pulse phase is discussed in terms of physical and geometrical properties of the emitting region. [ABSTRACT FROM AUTHOR]

In this work, the temporal and spectral properties of the poorly studied X-ray pulsar Swift J1808.4−1754 were investigated in the 0.8–79 keV energy range based on the data from the NuSTAR and Swift observatories collected during the 2014 outburst. Strong pulsations with a period of 909.73 ± 0.03 s were detected in the source light curve, with the pulsed fraction demonstrating a non-monotonic dependence on the energy with a local minimum around 17–22 keV. Phase lags in one of the pulse profile components, reaching the maximal value approximately at the same energy, were discovered. The pulse phase-averaged spectrum of the source has a power-law shape with an exponential cutoff at high energies, which is typical of X-ray pulsars. Pulse phase-resolved spectroscopy revealed the presence of a pulse-phase-transient cyclotron absorption line at ∼21 keV, allowing us to estimate the neutron star magnetic field of 2.4 × 1012 G. This makes Swift J1808.4−1754 a member of very small family of X-ray pulsars with a pulse-phase-transient cyclotron line in a narrow phase range. The data from the Nordic Optical Telescope allowed us to study the properties of the infrared companion in the system and to conclude that most probably it is a Be-type star located at a distance of 5–8 kpc. [ABSTRACT FROM AUTHOR]

We report on NuSTAR observations of the well-known wind-accreting X-ray pulsar GX 301–2 during a strong spin-up episode that took place in January–March 2019. A measurement of high luminosity of the source in the most recent observation allowed us to detect a positive correlation of the cyclotron line energy with luminosity. Beyond that, only minor differences in spectral and temporal properties of the source during the spin-up, presumably associated with the formation of a transient accretion disk, and the normal wind-fed state could be detected. Finally, we discuss conditions for the formation of the disk and possible reasons for lack of any appreciable variations in most of the observed source properties induced by the change of the accretion mechanism, and conclude that the bulk of the observed X-ray emission is still likely powered by direct accretion from the wind. [ABSTRACT FROM AUTHOR]

We report the results of a spectral and timing analysis of the poorly studied transient X-ray pulsar 2S 1553-542 using data collected with the NuSTAR and Chandra observatories and the Fermi/GBMinstrument during an outburst in 2015. The properties of the source at high energies (>30 keV) are studied for the first time and the sky position has been essentially improved. The source broad-band spectrum has a quite complicated shape and can be reasonably described by a composite model with two continuum components - a blackbody emission with the temperature about 1 keV at low energies and a power law with an exponential cut-off at high energies. Additionally, an absorption feature at ∼23.5 keV is discovered both in phaseaveraged and phase-resolved spectra and interpreted as the cyclotron resonance scattering feature corresponding to the magnetic field strength of the neutron star B ∼ 3 × 1012 G. Based on the Fermi/GBM data, the orbital parameters of the system were substantially improved, which allowed us to determine the spin period of the neutron star P = 9.27880(3) s and a local spin-up P ≃ -7.5 × 10-10 s s-1 due to the mass accretion during the NuSTAR observations. Assuming accretion from the disc and using standard torque models, we estimated the distance to the system as d = 20 ± 4 kpc. [ABSTRACT FROM AUTHOR]

We report results of our search for fast oscillations in light curve of one of the brightest accretion powered pulsars on the sky V0332+53 with the help of data of the PCA spectrometer of the RXTE (Rossi X-ray Timing Explorer) observatory. In course of this search we have carefully explored complications appearing if one uses only sub-bands of the total bandpass of the PCA spectrometer. We show that light curves collected in the soft sub-band of the PCA spectrometer contains an additional instrumental noise, light curves of harder sub-bands lack some fraction of the anticipated Poisson noise. We show that this noise is caused by a cross-talk of energy bands, which lasts up to ~200 µs. One hypothesis is that these effects are caused by temporarily drop of the PCA detector gain after any occurred event due to slowly moving ions in the detector volume. In order to avoid this effect we searched for fast oscillations in flux of V0332+53 only in the total bandpass of the PCA spectrometer 2-60 keV.We have not detected any quasi-periodic oscillations in light curve of the source with an upper limit at the level of 0.5 per cent in the Fourier frequency range 200-1500 Hz. [ABSTRACT FROM AUTHOR]