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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 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]

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, continues its successful work in observing the sky at energies E > 20 keV. The legacy of the mission already includes a large number of discovered or previously poorly studied hard X-ray sources. The growing INTEGRAL archive allows one to conduct an all-sky survey including a number of deep extragalactic fields and the deepest ever hard X-ray survey of the Galaxy. Taking advantage of the data gathered over 17 yr with the IBIS coded-mask telescope of INTEGRAL , we conducted survey of hard X-ray sources, providing flux information from 17 to 290 keV. The catalogue includes 929 objects, 890 of which exceed a detection threshold of 4.5σ and the rest are detected at 4.0σ–4.5σ and belong to known catalogued hard X-ray sources. Among the identified sources of known or suspected nature, 376 are associated with the Galaxy and Magellanic clouds, including 145 low-mass and 115 high-mass X-ray binaries, 79 cataclysmic variables, and 37 of other types; and 440 are extragalactic, including 429 active galactic nuclei (AGNs), 2 ultra-luminous sources, 1 supernova (AT2018cow), and 8 galaxy clusters. 113 sources remain unclassified. 46 objects are detected in the hard X-ray band for the first time. The Log N -Log S distribution of 356 non-blazar AGNs is measured down to a flux of 2 × 10−12 erg s−1 cm−2 and can be described by a power law with a slope of 1.44 ± 0.09 and normalization 8 × 10−3 deg−2 at 10−11 erg s−1 cm−2. The Log N -Log S distribution of unclassified sources indicates that the majority of them are of extragalactic origin. [ABSTRACT FROM AUTHOR]

The molecular cloud Sgr B2 is a natural Compton mirror in the Central Molecular Zone. It is believed that the observed fading of the Sgr B2 X-ray emission in continuum and the Fe Kα 6.4 keV line indicates past X-ray flare activity of the supermassive black hole Sgr A⋆. Sgr B2 was investigated by the INTEGRAL observatory in the hard X-ray in 2003–2009, showing clear decay of its hard X-ray emission. In this work, we present a long-term time evolution of the Sgr B2 hard X-ray continuum after 2009, associated with the hard X-ray source IGR J17475−2822 as observed by INTEGRAL. The 30–80 keV sky maps, obtained in 2009–2019, demonstrate a significant excess spatially consistent with IGR J17475−2822. The observed 2003–2019 light curve of IGR J17475−2822 is characterized by a linear decrease by a factor of ∼2 until 2011, after which it reaches a constant level of ∼1 mCrab. The source spectrum above 17 keV is consistent with a power-law model with Γ = 1.4 and a high-energy cut-off at ∼43 keV. The Sgr B2 residual emission after ∼2011 shows a good correspondence with models of X-ray emission due to the irradiation of the molecular gas by hard X-rays and low-energy cosmic ray ions. We discuss the possible origin of the residual Sgr B2 emission after 2011 within these models, including theoretically predicted multiply scattered emission. [ABSTRACT FROM AUTHOR]

We report results of the spectral and timing analysis of the Be/X-ray pulsar SXP 4.78 using the data obtained during its recent outburst with NuSTAR, Swift, Chandra , and NICER observatories. Using an overall evolution of the system luminosity, spectral analysis, and variability power spectrum we obtain constraints on the neutron star magnetic field strength. We found a rapid evolution of the variability power spectrum during the rise of the outburst, and absence of the significant changes during the flux decay. Several low frequency quasi-periodic oscillation features are found to emerge on the different stages of the outburst, but no clear clues on their origin were found in the energy spectrum and overall flux behaviour. We use several indirect methods to estimate the magnetic field strength on the neutron star surface and found that most of them suggest magnetic field B ≲ 2 × 1012 G. The strictest upper limit comes from the absence of the cyclotron absorption features in the energy spectra and suggests relatively weak magnetic field B < 6 × 1011 G.) [ABSTRACT FROM AUTHOR]

In this work, we present the first detailed analysis of the supernova remnant RX J1713.7–3946 in the hard X-ray energy range with the Imager on Board the INTEGRAL Satellite (IBIS) coded-mask telescope onboard the INTEGRAL observatory. The shell-type morphology of the entire remnant is mapped in hard X-rays for the first time and significantly detected up to 50 keV. The IBIS sky image of RX J1713.7–3946, accumulated over 14 yr of operations, demonstrates two extended hard X-ray sources. These sources are spatially consistent with north-west and south-west rims of RX J1713.7–3946 and are also clearly visible at energies below 10 keV with XMM–Newton. This points to a single emission mechanism operating in soft and hard X-rays. The INTEGRAL 17–120 keV spectrum of RX J1713.7–3946 is characterized by a power-law continuum with the photon index of Γ ≈ 3 that is significantly softer than Γ ≈ 2 determined by XMM–Newton in the 1–10 keV energy range, suggesting a progressive steepening of the spectrum with the energy. [ABSTRACT FROM AUTHOR]

Deep NuSTAR observation of X-ray pulsar A 0535+262, performed at a very low luminosity of ∼7 × 1034 erg s−1, revealed the presence of two spectral components. We argue that the high-energy component is associated with cyclotron emission from recombination of electrons collisionally excited to the upper Landau levels. The cyclotron line energy of E cyc = 47.7 ± 0.8 keV was measured at the luminosity of almost an order of magnitude lower than what was achieved before. The data firmly exclude a positive correlation of the cyclotron energy with the mass accretion rate in this source. [ABSTRACT FROM AUTHOR]

We report on analysis of properties of the X-ray binary pulsar GRO J1750–27 based on X-ray (Chandra, Swift, and Fermi /GBM), and near-infrared (VVV and UKIDSS surveys) observations. An accurate position of the source is determined for the first time and used to identify its infrared counterpart. Based on the VVV data we investigate the spectral energy distribution (SED) of the companion, taking into account a non-standard absorption law in the source direction. A comparison of this SED with those of known Be/X-ray binaries and early-type stars has allowed us to estimate a lower distance limit to the source at >12 kpc. An analysis of the observed spin-up torque during a giant outburst in 2015 provides an independent distance estimate of 14–22 kpc, and also allows to estimate the magnetic field on the surface of the neutron star at B ≃ (3.5–4.5) × 1012 G. The latter value is in agreement with the possible transition to the propeller regime, a strong hint for which was revealed by Swift /XRT and Chandra. We conclude, that GRO J1750–27 is located far behind the Galactic Centre, which makes it one of the furthest Galactic X-ray binaries known. [ABSTRACT FROM AUTHOR]

Recent NuSTAR and XMM–Newton observations of the molecular cloud around the Arches stellar cluster demonstrate a dramatic change both in morphology and intensity of its non-thermal X-ray emission, similar to that observed in many molecular clouds of the Central Molecular Zone at the Galactic Center. These variations trace the propagation of illuminating fronts, presumably induced by past flaring activities of Sgr A⋆. In this paper, we present results of a long NuSTAR observation of the Arches complex in 2016, taken a year after the previous XMM + NuSTAR observations which revealed a strong decline in the cloud emission. The 2016 NuSTAR observation shows that both the non-thermal continuum emission and the Fe K α 6.4 keV line flux are consistent with the level measured in 2015. No significant variation has been detected in both spectral shape and Fe K α equivalent width EW 6.4 keV, which may be interpreted as the intensity of the Arches non-thermal emission reaching its stationary level. At the same time, the measured 2016 non-thermal flux is not formally in disagreement with the declining trend observed in 2007–2015. Thus, we cannot assess whether the non-thermal emission has reached a stationary level in 2016, and new observations, separated by a longer time period, are needed to draw stringent conclusions. Detailed spectral analysis of three bright clumps of the Arches molecular cloud performed for the first time showed different EW 6.4 keV and absorption. This is a strong hint that the X-ray emission from the molecular cloud is a mix of two components with different origins. [ABSTRACT FROM AUTHOR]

We report on the discovery of a dramatic change in the energy spectrum of the X-ray pulsar GX 304−1 appearing at low luminosity. Particularly, we found that the cut-off power-law spectrum typical for accreting pulsars, including GX 304−1 at higher luminosities of L X ∼ 1036–1037 erg s−1, transformed at lower luminosity of L X ∼ 1034 erg s−1 to a two-component spectrum peaking around 5 and 40 keV. We suggest that the observed transition corresponds to a change of the dominant mechanism responsible for the deceleration of the accretion flow. We argue that the accretion flow energy at low accretion rates is released in the atmosphere of the neutron star, and the low-energy component in the source spectrum corresponds to the thermal emission of the optically thick, heated atmospheric layers. The most plausible explanations for the high-energy component are either the cyclotron emission reprocessed by the magnetic Compton scattering or the thermal radiation of deep atmospheric layers partly Comptonized in the overheated upper layers. Alternative scenarios are also discussed. [ABSTRACT FROM AUTHOR]

We report a discovery of low-frequency quasi-periodic oscillation at 0.3–0.7 Hz in the power spectra of the accreting black hole GRS 1739–278 in the hard-intermediate state during its 2014 outburst based on the NuSTAR and Swift /XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase lags are generally positive (hard) in the 0.1–3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be R in < 7.3 R g. In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with M BH ≈ 100 M⊙ is required. [ABSTRACT FROM AUTHOR]

We report on the monitoring of the final stage of the outburst from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124, which reached ∼40 Eddington luminosities. The main aim of the monitoring program with the Swift /XRT telescope was to measure the magnetic field of the neutron star using the luminosity of transition to the 'propeller' state. The visibility constraints, unfortunately, did not permit us to observe the source down to the fluxes low enough to detect such a transition. The tight upper limit on the propeller luminosity L prop < 6.8 × 1035 erg s−1 implies the dipole component of the magnetic field B < 1013 G. On the other hand, the observed evolution of the pulse profile and of the pulsed fraction with flux points to a change of the emission region geometry at the critical luminosity L crit ∼ 3 × 1038 erg s−1 both in the rising and declining parts of the outburst. We associate the observed change with the onset of the accretion column, which allows us to get an independent estimate of the magnetic field strength close to the neutron stars surface of B > 1013 G. Given the existing uncertainty in the effective magnetosphere size, we conclude that both estimates are marginally compatible with each other. [ABSTRACT FROM AUTHOR]

The luminosity of accreting magnetized neutron stars can largely exceed the Eddington value due to appearance of accretion columns. The height of the columns can be comparable to the neutron star radius. The columns produce the X-rays detected by the observer directly and illuminate the stellar surface, which reprocesses the X-rays and causes additional component of the observed flux. The geometry of the column and the illuminated part of the surface determine the radiation beaming. Curved space–time affects the angular flux distribution. We construct a simple model of the beam patterns formed by direct and reflected flux from the column. We take into account the possibility of appearance of accretion columns, whose height is comparable to the neutron star radius. We argue that depending on the compactness of the star, the flux from the column can be either strongly amplified due to gravitational lensing, or significantly reduced due to column eclipse by the star. The eclipses of high accretion columns result in specific features in pulse profiles. Their detection can put constraints on the neutron star radius. We speculate that column eclipses are observed in X-ray pulsar V 0332+53, leading us to the conclusion of large neutron star radius in this system (∼15 km if M ∼ 1.4 M⊙). We point out that the beam pattern can be strongly affected by scattering in the accretion channel at high luminosity, which has to be taken into account in the models reproducing the pulse profiles. [ABSTRACT FROM AUTHOR]

The ultracompact low-mass X-ray binary 4U1820-30 situated in the globular cluster NGC6624 has an orbital period of only ≈11.4min, which likely implies a white dwarf companion. The observed X-ray bursts demonstrate a photospheric radius expansion phase and therefore are believed to reach the Eddington luminosity, allowing us to estimate the mass and the radius of the neutron star (NS) in this binary. Here, we re-analyse all Rossi X-ray Timing Explorer observations of the system and confirm that almost all the bursts took place during the hard persistent state of the system. This allows us to use the recently developed direct cooling tail method to estimate the NS mass and radius. However, because of the very short, about a second, duration of the cooling tail phases that can be described by the theoretical atmosphere models, the obtained constraints on the NS radius are not very strict. Assuming a pure helium NS atmosphere, we found that the NS radius is in the range 10-12 km, if the NS mass is below 1.7M☉, and in a wider range of 8-12 km for a higher 1.7-2.0M☉ NS mass. The method also constrains the distance to the system to be 6.5 ± 0.5 kpc, which is consistent with the distance to the cluster. For the solar composition atmosphere, the NS parameters are in strong contradiction with the generally accepted range of possible NS masses and radii. [ABSTRACT FROM AUTHOR]

Observations of accreting neutron stars (NSs) with strong magnetic fields can be used not only for studying the accretion flow interaction with the NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating NS (magnetosphere) with the infalling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray pulsars with Be optical companions during their quiescent states, based on data from the Chandra, XMM–Newton and Swift observatories. The whole sample of sources can be roughly divided into two distinct groups: (i) relatively bright objects with a luminosity around ~1034 erg s-1 and (hard) power-law spectra, and (ii) fainter ones showing thermal spectra. X-ray pulsations were detected from five objects in group (i) with quite a large pulse fraction of 50-70 per cent. The obtained results are discussed within the framework of the models describing the interaction of the infalling matter with the NS magnetic field and those describing heating and cooling in accreting NSs. [ABSTRACT FROM AUTHOR]

We report the results of a comprehensive analysis of X-ray (Chandra and Swift observatories), optical (Southern African Large Telescope, SALT) and near-infrared (the VVV survey) observations of the Be/X-ray binary pulsar 2S 1553−542. Accurate coordinates for the X-ray source are determined and are used to identify the faint optical/infrared counterpart for the first time. Using VVV and SALTICAM photometry, we have constructed the spectral energy distribution (SED) for this star and found a moderate NIR excess that is typical for Be stars and arises due to the presence of circumstellar material (disc). A comparison of the SED with those of known Be/X-ray binaries has allowed us to estimate the spectral type of the companion star as B1-2V and the distance to the system as >15 kpc. This distance estimation is supported by the X-ray data and makes 2S 1553−542 one of the most distant X-ray binaries within the Milky Way, residing on the far side in the Scutum-Centaurus arm or even further. [ABSTRACT FROM AUTHOR]

We present results of a deep survey of three extragalactic fields, M81 (exposure of 9.7 Ms), Large Magellanic Cloud (6.8 Ms) and 3C 273/Coma (9.3 Ms), in the hard X-ray (17-60 keV) energy band with the IBIS telescope onboard the INTEGRAL observatory, based on 12 years of observations (2003-2015). The combined survey reaches a 4σ peak sensitivity of 0.18 mCrab (2.6 ? 10-12 erg s-1 cm-2) and sensitivity better than 0.25 and 0.87 mCrab over 10 per cent and 90 per cent of its full area of 4900 deg², respectively. We have detected in total 147 sources at S/N > 4σ, including 37 sources observed in hard X-rays for the first time. The survey is dominated by extragalactic sources, mostly active galactic nuclei (AGN). The sample of identified sources contains 98 AGN (including 64 Seyfert galaxies, seven low-ionization nuclear emission-line region galaxies, three X-ray bright optically normal galaxies, 16 blazars and eight AGN of unclear optical class), two galaxy clusters (Coma and Abell 3266), 17 objects located in the Large and Small Magellanic Clouds (13 high- and two low-mass X-ray binaries and two X-ray pulsars), three Galactic cataclysmic variables, one ultraluminous X-ray source (M82 X-1) and one blended source (SWIFT J1105.7+5854). The nature of 25 sources remains unknown, so that the survey's identification is currently complete at 83 per cent. We have constructed AGN number-flux relations (log N-log S) and calculated AGN number densities in the local Universe for the entire survey and for each of the three extragalactic fields. [ABSTRACT FROM AUTHOR]

We report the results of the deepest Galactic plane (|b| < 17. . 5) survey in the 67.9 and 78.4 keV nuclear de-excitation lines of titanium-44 (44Ti) performed using the data acquired with the IBIS/ISGRI instrument onboard the INTEGRAL satellite during 12 yr of operation. The peak sensitivity of our survey reached an unprecedented level of 4.8 ? 10-6 ph cm-2 s-1 (3σ) that improves the sensitivity of the survey done by Compton Gamma-Ray Observatory/COMPTEL by a factor of ~5. As a result, constraining upper limits for all sources from the catalogue of Galactic supernova remnants (SNRs; Green 2014) are derived. These upper limits can be used to estimate the exposure needed to detect 44Ti emission from any known SNR using existing and prospective X- and gamma-ray telescopes. Among the youngest Galactic SNRs, only Cas A shows significant 44Ti emission flux in good agreement with the NuSTAR measurements. We did not detect any other sources of titanium emission in the Galactic plane at significance level higher than 5σ confirming previous claims of the rarity of such 44Ti-producing SNRs. [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]