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X-ray Binary Systems

Group members: Lara Sidoli, Ada Paizis, Sandro Mereghetti, Nicola La Palombara, Paolo Esposito

Supergiant Fast X-ray Transients (SFXTs) are one of the most spectacular discoveries obtained by the INTEGRAL satellite (see ESA Pages). They are a subclass of High Mass X-ray Binaries (HMXBs) displaying extreme transient behavior, with a huge dynamic range from 2 to 5 orders of magnitude. They are believed to be accretors from the winds of the blue supergiant companions, but the actual physical mechanism responsible for the unusually short duration of the outbursts (with respect to Be/X-ray Binaries) is controversial. The study of archival INTEGRAL data of many SFXTs has allowed us to discover for the first time a periodicity in the outburst recurrence from a SFXT, IGRJ11215-5952. The properties of the X-ray light-curve suggest an alternative outburst mechanism, linked to the presence of a preferential plane for the outflowing supergiant wind, inclined with respect to the orbital plane of the system. The outburst is produced when the neutron star crosses this second wind component. The presence of large scale structures in supergiant winds seems to be a viable explanation also for the flaring activity observed in XTEJ1739-302, the prototype of the SFXTs class. From 2007 to 2009 we performed a monitoring of a sample of SFXTs with Swift/XRT which, for the first time, allowed us to unveil that these transients spend a large fraction of their life in an intermediate level of X-ray emission. The kind of X-ray variability we observed with XMM-Newton in the SFXT IGRJ16418-4532, its dynamic range and time-scale, together with the sporadic presence of quasi-periodic flaring, are all suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. This hypothesis could explain the behaviour of IGRJ16418-4532 and, possibly, other SFXTs with similarly short orbital periods. A sensitive and long observation of most of the orbital period of a SFXT (IGRJ16479-4514) with Suzaku allowed us to perform for the first time a study of the wind and X-ray variability properties along the orbit in a SFXT.

In the field of Low Mass X-ray Binaries (LMXBs), we investigate a unified scenario for the high energy emission processes, with particular focus on weakly magnetised neutron star LMXBs. We have also developed a new Comptonisation model (CompTB) and its application to private and archival high energy data is being carried on.

Multi-wavelength campaigns are in place in order to unveil the nature of newly discovered INTEGRAL sources, mainly using Chandra (proprietary data), but also other available X-ray, radio, optical and infrared facilities.

The high sensitivity of current X-ray observatories has allowed us to reveal a new class of X-ray binaries in which the mass donor is a hot subdwarf star. Hot subdwarfs are evolved low-mass stars that lost most of their hydrogen envelopes and are now in the stage of helium core burning. A possible mechanism responsible for the loss of the massive H envelopes necessary to form hot subdwarfs is mass transfer in a binary. Evolutionary models predict that many hot subdwarfs should have neutron star or white dwarf companions. X-rays can be used to discover such systems, which are difficult to identify with optical spectroscopic campaigns. In the course of this project we obtained interesting results for two luminous subdwarfs of O spectral type: HD 49798 and BD+37 442. We showed that the companion of HD 49798 is the fastest spinning white dwarf (P=13.2 s) and one of the most massive with a dynamical mass measurement (see ESA Press Release). BD+37 442 is a luminous He-rich sdO that was believed to be single, until our discovery of an X-ray periodicity of 19 s. Searches for other members of the class with Chandra and Swift are ongoing, while XMM-Newton and ground based optical observations are used to study in more detail the two already known systems. The long term goal of this line of research is to shed light on the poorly known processes taking place during common envelope evolutionary phases and on the properties of wind mass loss from hot subdwarfs.

The analysis of XMM-Newton data of persistent BeXRBs has allowed us to investigate their spectral and timing properties. In this way we found that the pulse period evolution is unrelated to the luminosity evolution and that the spectra of these sources display a clear data excess over the main power-law component. We also found that this excess is clearly pulsed and varies with the phase; in all cases it can be modeled with a rather hot (kT>1 keV) black-body component of small area (R < 0.5 km), therefore this component can be due to emission from the neutron star polar caps. Finally, we have checked that the same type of feature has been observed also in other low-luminosity and long-period HMXBs, thus suggesting that it is a common property of this type of sources.