A new model for the X-ray continuum of the magnetized accreting pulsars

Accreting highly magnetized pulsars in binary systems are among the brightest X-ray emitters in our Galaxy. Although a number of high statistical quality broad-band (0.1-100 keV) X-ray observations are available, the spectral energy distribution of these sources is usually investigated by adopting pure phenomenological models, rather than models linked to the physics of accretion. In this paper, a detailed spectral study of the X-ray emission recorded from the high-mass X-ray binary pulsars Cen X-3, 4U 0115+63, and Her X-1 is carried out by using BeppoSAX and joined Suzaku+NuStar data, together with an advanced version of the compmag model. The latter provides a physical description of the high energy emission from accreting pulsars, including the thermal and bulk Comptonization of cyclotron and bremsstrahlung seed photons along the neutron star accretion column. The compmag model is based on an iterative method for solving second-order partial differential equations, whose convergence algorithm has been improved and consolidated during the preparation of this paper. Our analysis shows that the broad-band X-ray continuum of all considered sources can be self-consistently described by the compmag model. The cyclotron absorption features, not included in the model, can be accounted for by using Gaussian components. From the fits of the compmag model to the data we inferred the physical properties of the accretion columns in all sources, finding values reasonably close to those theoretically expected according to our current understanding of accretion in highly magnetized neutron stars. The updated version of the compmag model has been tailored to the physical processes that are known to occur in the columns of highly magnetized accreting neutron stars and it can thus provide a better understanding of the high energy radiation from these sources.Comment: 19 pages, 10 figures, accepted for publication in A&

Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808

We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of $\sim339.97$ Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of $44.3$ minutes and a projected semi-major axis of $\sim17.6$ lt-ms. Based on the mass function we estimate a minimum companion mass of 0.024 M$_{\odot}$, which assumes a neutron star mass of 1.4 M$_{\odot}$ and a maximum inclination angle of $75^{\circ}$ (derived from the lack of eclipses and dips in the light-curve of the source). We find that the companion star's Roche-Lobe could either be filled by a hot ($5\times 10^{6}$ K) pure helium white dwarf with a 0.028 M$_{\odot}$ mass (implying $i\simeq58^{\circ}$) or an old (>5 Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065$-$0.085 M$_{\odot}$ (16 < $i$ < 21). During the outburst the broad-band energy spectra are well described by a superposition of a weak black-body component (kT$\sim$ 0.5 keV) and a hard cutoff power-law with photon index $\Gamma \sim$ 1.7 and cut-off at a temperature kT$_e\sim$ 130 keV. Up to the latest Swift-XRT observation performed on 2016 July 19 the source has been observed in outburst for almost 150 days, which makes MAXI J0911-655 the second accreting millisecond X-ray pulsar with outburst duration longer than 100 days.Comment: 7 pages, 5 figures, accepted for publication in A&

Discovery of a soft X-ray 8 mHz QPO from the accreting millisecond pulsar IGR J00291+5934

In this paper, we report on the analysis of the peculiar X-ray variability displayed by the accreting millisecond X-ray pulsar IGR J00291+5934 in a 80 ks-long joint NuSTAR and XMM-Newton observation performed during the source outburst in 2015. The light curve of the source was characterized by a flaring-like behavior, with typical rise and decay time scales of ~120 s. The flares are accompanied by a remarkable spectral variability, with the X-ray emission being generally softer at the peak of the flares. A strong quasi periodic oscillation (QPO) is detected at ~8 mHz in the power spectrum of the source and clearly associated with the flaring-like behavior. This feature has the strongest power at soft X-rays (<3 keV). We carried out a dedicated hardness-ratio resolved spectral analysis and a QPO phase-resolved spectral analysis, together with an in-depth study of the source timing properties, to investigate the origin of this behavior. We suggest that the unusual variability of IGR J00291+5934 observed by XMM-Newton and NuSTAR could be produced by an heartbeat-like mechanism, similar to that operating in black-hole X-ray binaries. The possibility that this variability, and the associated QPO, are triggered by phases of quasi-stable nuclear burning, as suggested in the literature for a number of other neutron star binaries displaying a similar behavior, cannot be solidly tested in the case of IGR J00291+5934 due to the paucity of type-I X-ray bursts observed from this source.Comment: Submitted to MNRAS on 23 Sept 2016. Modified according to the referee's suggestions. Comments are welcomed. One reference updated in this versio

A possible cyclotron resonance scattering feature near 0.7 keV in X1822-371

We analyse all available X-ray observations of X1822-371 made with XMM-Newton, Chandra, Suzaku and INTEGRAL satellites. The observations were not simultaneous. The Suzaku and INTEGRAL broad band energy coverage allows us to constrain the spectral shape of the continuum emission well. We use the model already proposed for this source, consisting of a Comptonised component absorbed by interstellar matter and partially absorbed by local neutral matter, and we added a Gaussian feature in absorption at $\sim 0.7$ keV. This addition significantly improves the fit and flattens the residuals between 0.6 and 0.8 keV. We interpret the Gaussian feature in absorption as a cyclotron resonant scattering feature (CRSF) produced close to the neutron star surface and derive the magnetic field strength at the surface of the neutron star, $(8.8 \pm 0.3) \times 10^{10}$ G for a radius of 10 km. We derive the pulse period in the EPIC-pn data to be 0.5928850(6) s and estimate that the spin period derivative of X1822-371 is $(-2.55 \pm 0.03) \times 10^{-12}$ s/s using all available pulse period measurements. Assuming that the intrinsic luminosity of X1822-371is at the Eddington limit and using the values of spin period and spin period derivative of the source, we constrain the neutron star and companion star masses. We find the neutron star and the companion star masses to be $1.69 \pm 0.13$ M$_{\odot}$ and $0.46 \pm 0.02$ M$_{\odot}$, respectively, for a neutron star radius of 10 km.In a self-consistent scenario in which X1822-371 is spinning-up and accretes at the Eddington limit, we estimate that the magnetic field of the neutron star is $(8.8 \pm 0.3) \times 10^{10}$ G for a neutron star radius of 10 km. If our interpretation is correct, the Gaussian absorption feature near 0.7 keV is the very first detection of a CRSF below 1 keV in a LMXB. (abridged)Comment: 14 pages, 12 figures, accepted for publication in A&

Spectral and timing properties of IGR J00291+5934 during its 2015 outburst

We report on the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015 outburst. The source is in a hard state dominated at high energies by a comptonization of soft photons ($\sim0.9$ keV) by an electron population with kT$_e\sim30$ keV, and at lower energies by a blackbody component with kT$\sim0.5$ keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a $3\sigma$ confidence level interval $-6.6\times 10^{-13}$ s/s $< \dot{P}_{orb} < 6.5 \times 10^{-13}$ s/s on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse-profile, indicating that this component resides at the neutron star surface (hot-spot).Comment: 9 pages, 7 figures. Accepted for publication in MNRA

COMPARAÇÃO ANATÔMICA DA ENERVAÇÃO DA ARTICULAÇÃO COXO-FEMORAL EM HUMANOS E CÃES

The arrangement of the nerve fibers responsible by the sensitive route of dogs and human beings were compared, since surgical techniques that have been developed for animal patients are based in similar ones used for pain control in arthrosis cases in human beings.  In this review it was outstanding that in human beings the sensitive innervation of the acetabular area is composed of branches from the superior gluteal, ischiatic, femoral and obturator nerves, while in dogs the responsible nerves are the cranial gluteal, ischiatic and femoral nerves. The obturator nerve is not related to this function in most animals, hence it can be considered an anatomic variation. The innervation, both in human beings and animals, presents bilateral symmetry and, apart from that, the difference between the human bipedal support and the canine quadrupedal support generates biomechanical forces in distinct points, leading to different nerve fiber concentrations between the two species. While the first presents a bigger nerve fiber density in the anteromedial area, for which the obturator nerve is responsible, the second presents a bigger density in the craniolateral and dorsal areas, for which the cranial gluteal and the ischiatic nerves are responsible.Foi comparada a disposição das fibras nervosas responsáveis pela cadeia sensitiva de cães e seres humanos, uma vez que técnicas cirúrgicas vêm sendo desenvolvidas para os pacientes veterinários, baseadas em similares utilizadas no controle da dor de quadros de artrose de seres humanos. Destaca-se nesta revisão que em seres humanos a enervação sensitiva da região acetabular é composta por ramos oriundos dos nervos glúteo superior, isquiático, femoral e obturador, enquanto em cães são responsáveis os nervos glúteo cranial, isquiático e femoral, estando o nervo obturador não relacionado a esta função na maioria dos animais, podendo ser assim considerado variação anatômica. A enervação tanto em seres humanos como em animais apresenta simetria bilateral e, além disso, que a diferença entre o apoio bipedal humano e quadrupedal canino gera forças biomecânicas em pontos distintos, promovendo diferente concentração de fibras nervosas em relação às duas espécies. Enquanto a primeira apresenta maior densidade de fibras nervosas na região anteromedial, de responsabilidade do nervo obturador, a segunda apresenta maior densidade na região crânio-lateral e dorsal, de responsabilidade dos nervos glúteo cranial e isquiático