Monitoring clumpy wind accretion in supergiant fast X-ray transients with XMM-Newto

Supergiant fast X-ray transients (SFXTs) are a sub-class of supergiant high mass X-ray binaries hosting a neutron star accreting from the stellar wind of a massive OB companion. Compared to the classical systems, SFXTs display a pronounced variability in X-rays that has long been (at least partly) ascribed to the presence of clumps in the stellar wind. We report here on the first set of results of an on-going XMM-Newton observational program aimed at searching for spectroscopic variability during the X-ray flares and outbursts of the SFXTs. The goal of the paper is to present the observational program and show that the obtained results are according to expectations, with a number of flares (between one and four) generally observed per source and per observation (20~ks-long, on average). We base our work on a systematic and uniform analysis method optimized to consistently search for spectral signatures of a variable absorption column density, as well as other parameters of the spectral continuum. Our preliminary results show that the program is successful and the outcomes of the analysis support previous findings that most of the X-ray flares seem associated to the presence of a massive structure approaching and getting accreted by the compact object. However, we cannot rule out that other mechanisms are at work together with clumps to enhance the X-ray variability of SFXTs. This is expected according to current theoretical models. The success of these observations shows that our observational program can be a powerful instrument to deepen our understanding of the X-ray variability in SFXTs. Further observations will help us in achieving a statistically robust sample. This is required to conduct, in the future, a systematic analysis on the whole SFXT class with the ultimate goal of disentangling the role of different mechanisms giving rise to these events.Comment: Accepted for publication on A&

Disk precession to explain the super-orbital modulation of LMC X-4: results from the Swift monitoring campaign

We studied the spectral changes of the high-mass X-ray binary system LMC X-4 to understand the origin and mechanisms beyond its super-orbital modulation (30.4 days). To this aim, we obtained a monitoring campaign with Swift/XRT (0.3-10 keV) and complemented these data with the years-long Swift/BAT survey data (15-60 keV). We found a self-consistent, physically motivated, description of the broadband X-ray spectrum using a Swift/XRT and a NuSTAR observation at the epoch of maximum flux. We decomposed the spectrum into the sum of a bulk+thermal Comptonization, a disk-reflection component and a soft contribution from a standard Shakura-Sunyaev accretion disk. We applied this model to 20 phase-selected Swift spectra along the super-orbital period. We found a phase-dependent flux ratio of the different components, whereas the absorption column does not significantly vary. The disk emission is decoupled with respect to the hard flux. We interpret this as a geometrical effect in which the inner parts of the disk are tilted with respect to the obscuring outer regions.Comment: 14 pages, 15 figures, Accepted for publication in MNRA

TNFα expressed on the surface of microparticles modulates endothelial cell fate in rheumatoid arthritis

Background: Rheumatoid arthritis (RA) is associated with a high prevalence of atherosclerosis. Recently increased levels of microparticles (MPs) have been reported in patients with RA. MPs could represent a link between autoimmunity and endothelial dysfunction by expressing tumor necrosis factor alpha (TNFα), a key cytokine involved in the pathogenesis of RA, altering endothelial apoptosis and autophagy. The aim of this study was to investigate TNFα expression on MPs and its relationship with endothelial cell fate. Methods: MPs were purified from peripheral blood from 20 healthy controls (HC) and from 20 patients with RA, before (time (T)0) and after (T4) 4-month treatment with etanercept (ETA). Surface expression of TNFα was performed by flow cytometry analysis. EA.hy926 cells, an immortalized endothelial cell line, were treated with RA-MPs purified at T0 and at T4 and also, with RA-MPs in vitro treated with ETA. Apoptosis and autophagy were then evaluated. Results: RA-MPs purified at T0 expressed TNFα on their surface and this expression significantly decreased at T4. Moreover, at T0 RA-MPs, significantly increased both apoptosis and autophagy levels on endothelial cells, in a dose-dependent manner. RA-MPs did not significantly change these parameters after 4 months of in vivo treatment with ETA. Conclusions: Our data demonstrate that MPs isolated from patients with RA exert a pathological effect on endothelial cells by TNFα expressed on their surface. In vivo and in vitro treatment with ETA modulates this effect, suggesting anti-TNF therapy protects against endothelial damage in patients with RA

Highly Structured Wind in Vela X-1

We present an in-depth analysis of the spectral and temporal behavior of a long almost uninterrupted INTEGRAL observation of Vela X-1 in Nov/Dec 2003. In addition to an already high activity level, Vela X-1 exhibited several very intense flares with a maximum intensity of more than 5 Crab in the 20 40 keV band. Furthermore Vela X-1 exhibited several off states where the source became undetectable with ISGRI. We interpret flares and off states as being due to the strongly structured wind of the optical companion: when Vela X-1 encounters a cavity in the wind with strongly reduced density, the flux will drop, thus potentially triggering the onset of the propeller effect which inhibits further accretion, thus giving rise to the off states. The required drop in density to trigger the propeller effect in Vela X-1 is of the same order as predicted by theoretical papers for the densities in the OB star winds. The same structured wind can give rise to the giant flares when Vela X-1 encounters a dense blob in the wind. Further temporal analysis revealed that a short lived QPO with a period of 6800 sec is present. The part of the light curve during which the QPO is present is very close to the off states and just following a high intensity state, thus showing that all these phenomena are related