SMC X-3: the closest ultraluminous X-ray source powered by a neutron star with non-dipole magnetic field

Aims. The magnetic field of accreting neutron stars determines their overall behavior including the maximum possible luminosity. Some models require an above-average magnetic field strength (greater than or similar to 10(13) G) in order to explain super-Eddington mass accretion rate in the recently discovered class of pulsating ultraluminous X-ray sources (ULX). The peak luminosity of SMCX-3 during its major outburst in 2016-2017 reached similar to 2.5x10(39) erg s(-1) comparable to that in ULXs thus making this source the nearest ULX-pulsar. Determination of the magnetic field of SMCX-3 is the main goal of this paper.Methods. SMCX-3 belongs to the class of transient X-ray pulsars with Be optical companions, and exhibited a giant outburst in July 2016-March 2017. The source has been observed over the entire outburst with the Swift/XRT and Fermi/GBM telescopes, as well as the NuSTAR observatory. Collected data allowed us to estimate the magnetic field strength of the neutron star in SMCX-3 using several independent methods.Results. Spin evolution of the source during and between the outbursts, and the luminosity of the transition to the so-called propeller regime in the range of (0.3-7) x 10(35) erg s(-1) imply a relatively weak dipole field of (1-5) x 10(12) G. On the other hand, there is also evidence for a much stronger field in the immediate vicinity of the neutron star surface. In particular, transition from super-to sub-critical accretion regime associated with the cease of the accretion column and very high peak luminosity favor a field that is an order of magnitude stronger. This discrepancy makes SMCX-3 a good candidate for possessing significant non-dipolar components of the field, and an intermediate source between classical X-ray pulsars and accreting magnetars which may constitute an appreciable fraction of ULX population

Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262

Deep NuSTAR observation of X-ray pulsar A 0535+262, performed at a very low luminosity of similar to 7 x 10(34) 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

GROJ1750-27: A neutron star far behind the Galactic Center switching into the propeller regime

We report on analysis of properties of the X-ray binary pulsar GROJ1750-27 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 similar or equal to (3.5-4.5) x 10(12) 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 GROJ1750-27 is located far behind the Galactic Centre, which makes it one of the furthest Galactic X-ray binaries known

The X-ray properties of Be/X-ray pulsars in quiescence

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 similar to 10(34) 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

Losing a minute every two years: SRG X-ray view of the rapidly accelerating X-ray pulsar SXP 1323

The source SXP 1323 is a peculiar high-mass X-ray binary located in the Small Magellanic Cloud. It is renowned for its rapid spin-up. We investigate for the first time broadband X-ray properties of SXP 1323 as observed by the Mikhail Pavlinsky ART-XC and eROSITA telescopes on board the Spectrum-Roentgen-Gamma observatory. Using ART-XC and eROSITA data, we produced a first broadband 1-20 keV X-ray spectrum and estimated the pulsed fraction box 8 keV. With the addition of archival XMM-Newton observations, we traced the evolution of the spin period of SXP 1323 over the last five years and found that after 2016, the source switched to a linear spin-up with a rate of -29.9 s yr-1. The broadband X-ray spectrum is typical for accreting X-ray pulsars. It has a steep power-law index (Γ = -0.15) and an exponential cutoff energy of 5.1 keV. No significant difference between spectra obtained in states with and without pulsations were found.</p

NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line

We report on the first focused hard X-ray view of the absorbed supergiant system IGR J18027-2016 performed with the Nuclear Spectroscopic Telescope Array observatory. The pulsations are clearly detected with a period of P-spin = 139.866(1)s and a pulse fraction of about 50-60 per cent at energies from 3 to 80 keV. The source demonstrates an approximately constant X-ray luminosity on a time-scale of more than dozen years with an average spin-down rate of P similar or equal to 6 x 10(-10) s s(-1). This behaviour of the pulsar can be explained in terms of the wind accretion model in the settling regime. The detailed spectral analysis at energies above 10 keV was performed for the first time and revealed a possible cyclotron absorption feature at energy similar to 23 keV. This energy corresponds to the magnetic field B similar or equal to 3 x 10(12) G at the surface of the neutron star, which is typical for X-ray pulsars

The unusual behavior of the young X-ray pulsar SXP 1062 during the 2019 outburst

We present the results of the first dedicated observation of the young X-ray pulsar SXP 1062 in the broad X-ray energy band obtained during its 2019 outburst with the NuSTAR and XMM-Newton observatories. The analysis of the pulse-phase averaged and phase-resolved spectra in the energy band from 0.5 to 70 keV did not reveal any evidence for the presence of a cyclotron line. The spin period of the pulsar was found to have decreased to 979.48 +/- 0.06 s implying a similar to 10% reduction compared to the last measured period during the monitoring campaign conducted about five years ago, and is puzzling considering that the system apparently has not shown major outbursts ever since. The switch of the pulsar to the spin-up regime supports the common assumption that torques acting on the accreting neutron star are nearly balanced and thus SXP 1062 likely also spins with a period close to the equilibrium value for this system. The current monitoring of the source also revealed a sharp drop in its soft X-ray flux right after the outburst, which is in drastic contrast to the behavior during the previous outburst when the pulsar remained observable for years with only a minor flux decrease after the end of the outburst. This unexpected off state of the source lasted for at most 20 days after which SXP 1062 returned to the level observed during previous campaigns. We discuss this and other findings in context of the modern models of accretion onto strongly magnetized neutron stars

Basic parameters of the helium-accreting X-ray bursting neutron star in 4U 1820-30

The ultracompact low-mass X-ray binary 4U 1820-30 situated in the globular cluster NGC 6624 has an orbital period of only approximate to 11.4 min, 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 1.7 M-circle dot, and in a wider range of 8-12 km for a higher 1.7-2.0 M-circle dot 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

An apparently normal gamma-ray burst with an unusually low luminosity

Much of progress in gamma-ray bursts has come from the studies of distant events (redshift z~1). The brightest GRBs are the most collimated events and seen across the Universe due to their brilliance. It has long been suspected that nearest (and most common) events have been missed because they are not so collimated or under-energetic or both. Here we report soft gamma-ray observations of GRB 031203, the nearest event to date (z=0.106). This event with a duration of 40 s and peak energy of >190 keV appears to be a typical long duration GRB. However, the isotropic gamma-ray energy <~10^50 erg, about three orders of magnitude smaller than the cosmological population. This event as well as the other nearby but somewhat controversial event GRB 980425 are clear outliers for the much discussed isotropic-energy peak-energy relation and luminosity spectral-lag relations. Radio calorimetry shows that both these events are under-energetic explosions. We conclude that there does indeed exist a large population of under-energetic events.Comment: 11 pages, 3 figure

SRG/ART-XC discovery of SRGA J204318.2+443815: Towards the complete population of faint X-ray pulsars

We report the discovery of the new long-period X-ray pulsar SRGA J204318.2+443815/SRGe J204319.0+443820 in a Be binary system. The source was found in the second all-sky survey by the Mikhail Pavlinsky ART-XC telescope on board the SRG mission. The followup observations with XMM-Newton, NICER, and NuSTAR allowed us to discover a strong coherent signal in the source light curve with a period of ~742 s. The pulsed fraction was found to depend on an increase in energy from ~20% in soft X-rays to >50% at high energies, as is typical for X-ray pulsars. The source has a quite hard spectrum with an exponential cutoff at high energies and a bolometric luminosity of Lx ≃ 4 x 1035 erg s-1. The X-ray position of the source is found to be consistent with the optical transient ZTF18abjpmzf, located at a distance of ~8.0 kpc. Dedicated optical and infrared observations with the RTT-150, NOT, Keck, and Palomar telescopes revealed a number of emission lines (Hα, He I, and the Paschen and Braket series) with a strongly absorbed continuum. According to the SRG scans and archival XMM-Newton data, the source flux is moderately variable (by a factor of 4-10) on timescales of several months and years. All this suggests that SRGA J204318.2+443815/SRGe J204319.0+443820 is a new quasipersistent low-luminosity X-ray pulsar in a distant binary system with a Be-star of the B0-B2e class. Thus the SRG observatory allowed us to unveil a hidden population of faint objects, including a population of slowly rotating X-ray pulsars in Be systems.</p