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&

The accretion environment of Supergiant Fast X-ray Transients probed with XMM-Newton

Supergiant fast X-ray transients (SFXTs) are characterized by a remarkable variability in the X-ray domain, widely ascribed to the accretion from a clumpy stellar wind. In this paper we performed a systematic and homogeneous analysis of sufficiently bright X-ray flares from the SFXTs observed with XMM-Newton to probe spectral variations on timescales as short as a few hundred of seconds. Our ultimate goal is to investigate if SFXT flares and outbursts are triggered by the presence of clumps and eventually reveal whether strongly or mildly dense clumps are required. For all sources, we employ a technique developed by our group, making use of an adaptive rebinned hardness ratio to optimally select the time intervals for the spectral extraction. A total of twelve observations performed in the direction of five SFXTs are reported. We show that both strongly and mildly dense clumps can trigger these events. In the former case, the local absorption column density may increase by a factor of >>3, while in the latter case, the increase is only by a factor of 2-3 (or lower). Overall, there seems to be no obvious correlation between the dynamic ranges in the X-ray fluxes and absorption column densities in SFXTs, with an indication that lower densities are recorded at the highest fluxes. This can be explained by the presence of accretion inhibition mechanism(s). We propose a classification of the flares/outbursts from these sources to drive future observational investigations. We suggest that the difference between the classes of flares/outbursts is related to the fact that the mechanism(s) inhibiting accretion can be overcome more easily in some sources compared to others. We also investigate the possibility that different stellar wind structures, rather than clumps, could provide the means to temporarily overcome the inhibition of accretion in SFXTs.Comment: Accepted for publication on A&

RX J0440.9+4431: a persistent Be/X-ray binary in outburst

The persistent Be/X-ray binary RX J0440.9+4431 flared in 2010 and 2011 and has been followed by various X-ray facilities Swift, RXTE, XMM-Newton, and INTEGRAL. We studied the source timing and spectral properties as a function of its X-ray luminosity to investigate the transition from normal to flaring activity and the dynamical properties of the system. We have determined the orbital period from the long-term Swift/BAT light curve, but our determinations of the spin period are not precise enough to constrain any orbital solution. The source spectrum can always be described by a bulk-motion Comptonization model of black body seed photons attenuated by a moderate photoelectric absorption. At the highest luminosity, we measured a curvature of the spectrum, which we attribute to a significant contribution of the radiation pressure in the accretion process. This allows us to estimate that the transition from a bulk-motion-dominated flow to a radiatively dominated one happens at a luminosity of ~2e36 erg/s. The luminosity dependency of the size of the black body emission region is found to be $r_{BB} \propto L_X^{0.39\pm0.02}$. This suggests that either matter accreting onto the neutron star hosted in RX J0440.9+4431 penetrates through closed magnetic field lines at the border of the compact object magnetosphere or that the structure of the neutron star magnetic field is more complicated than a simple dipole close to the surfaceComment: Accepted for publication by A&

Properties and observability of glitches and anti-glitches in accreting pulsars

Several glitches have been observed in young, isolated radio pulsars, while a clear detection in accretion-powered X-ray pulsars is still lacking. We use the Pizzochero snowplow model for pulsar glitches as well as starquake models to determine for the first time the expected properties of glitches in accreting pulsars and their observability. Since some accreting pulsars show accretion-induced long-term spin-up, we also investigate the possibility that anti-glitches occur in these stars. We find that glitches caused by quakes in a slow accreting neutron star are very rare and their detection extremely unlikely. On the contrary, glitches and anti-glitches caused by a transfer of angular momentum between the superfluid neutron vortices and the non-superfluid component may take place in accreting pulsars more often. We calculate the maximum jump in angular velocity of an anti-glitch and we find that it is expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting pulsars usually have rotational angular velocities lower than those of isolated glitching pulsars, both glitches and anti-glitches are expected to have long rise and recovery timescales compared to isolated glitching pulsars, with glitches and anti-glitches appearing as a simple step in angular velocity. Among accreting pulsars, we find that GX 1+4 is the best candidate for the detection of glitches with currently operating X-ray instruments and future missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor changes to match the final A&A versio

Swift J1734.5-3027: a new long type-I X-ray bursting source

Swift J1734.5-3027 is a hard X-ray transient discovered by Swift while undergoing an outburst in September 2013. Archival observations showed that this source underwent a previous episode of enhanced X-ray activity in May-June 2013. In this paper we report on the analysis of all X-ray data collected during the outburst in September 2013, the first that could be intensively followed-up by several X-ray facilities. Our data-set includes INTEGRAL, Swift, and XMM-Newton observations. From the timing and spectral analysis of these observations, we show that a long type-I X-ray burst took place during the source outburst, making Swift J1734.5-3027 a new member of the class of bursting neutron star low-mass X-ray binaries. The burst lasted for about 1.9 ks and reached a peak flux of (6.0$\pm$1.8)$\times$10$^{-8}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5-100 keV energy range. The estimated burst fluence in the same energy range is (1.10$\pm$0.10)$\times$10$^{-5}$ erg cm$^{-2}$. By assuming that a photospheric radius expansion took place during the first $\sim$200 s of the burst and that the accreted material was predominantly composed by He, we derived a distance to the source of 7.2$\pm$1.5 kpc.Comment: Accepted for publication on A&

Glancing through the accretion column of EXO 2030+375

We took advantage of the large collecting area and good timing capabilities of the EPIC cameras on-board XMM-Newton to investigate the accretion geometry onto the magnetized neutron star hosted in the high mass X-ray binary EXO 2030+375 during the rise of a source Type-I outburst in 2014. We carried out a timing and spectral analysis of the XMM-Newton observation as function of the neutron star spin phase. We used a phenomenological spectral continuum model comprising the required fluorescence emission lines. Two neutral absorption components are present: one covering fully the source and one only partially. The same analysis was also carried out on two Suzaku observations of the source performed during outbursts in 2007 and 2012, to search for possible spectral variations at different luminosities. The XMM-Newton data caught the source at an X-ray luminosity of $2\times10^{36}$ erg s$^{-1}$ and revealed the presence of a narrow dip-like feature in its pulse profile that was never reported before. The width of this feature corresponds to about one hundredth of the neutron star spin period. From the results of the phase-resolved spectral analysis we suggest that this feature can be ascribed to the self-obscuration of the accretion stream passing in front of the observer line of sight. We inferred from the Suzaku observation carried out in 2007 that the self-obscuration of the accretion stream might produce a significantly wider feature in the neutron star pulsed profile at higher luminosities ($\gtrsim$$2\times10^{37}$ erg s$^{-1}$).Comment: Accepted for publication on A&

IGRJ17361-4441: a possible new accreting X-ray binary in NGC6388

IGRJ17361-4441 is a newly discovered INTEGRAL hard X-ray transient, located in the globular cluster NGC6388. We report here the results of the X-ray and radio observations performed with Swift, INTEGRAL, RXTE, and the Australia Telescope Compact Array (ATCA) after the discovery of the source on 2011 August 11. In the X-ray domain, IGRJ17361-4441 showed virtually constant flux and spectral parameters up to 18 days from the onset of the outburst. The broad-band (0.5-100 keV) spectrum of the source could be reasonably well described by using an absorbed power-law component with a high energy cut-off (N_H\simeq0.8x10^(22) cm^(-2), {\Gamma}\simeq0.7-1.0, and E_cut\simeq25 keV) and displayed some evidence of a soft component below \sim2 keV. No coherent timing features were found in the RXTE data. The ATCA observation did not detect significant radio emission from IGRJ17361-4441, and provided the most stringent upper limit (rms 14.1 {\mu}Jy at 5.5 GHz) to date on the presence of any radio source close to the NGC6388 center of gravity. The improved position of IGRJ17361-4441 in outburst determined from a recent target of opportunity observation with Chandra, together with the X-ray flux and radio upper limits measured in the direction of the source, argue against its association with the putative intermediate-mass black hole residing in the globular cluster and with the general hypothesis that the INTEGRAL source is a black hole candidate. IGRJ17361-4441 might be more likely a new X-ray binary hosting an accreting neutron star. The ATCA radio non-detection also permits us to derive an upper limit to the mass of the suspected intermediate massive black hole in NGC6388 of <600 M\odot. This is a factor of 2.5 lower than the limit reported previously.Comment: Accepted for publication on A&A lette