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Please use this identifier to cite or link to this item: http://hdl.handle.net/10277/296

Authors: Ducci, Lorenzo
Internal Tutor: TREVES, ALDO
Title: Exploring accretion theory with a new subclass of high mass x-ray binaries: interpretation of integral observations of the supergiant fast x-ray transients.
Abstract: In the last eight years the INTEGRAL satellite has allowed to discover a new class of transient X-ray binaries associated with OB supergiants called Supergiant Fast X-ray Transients (SFXTs). They display a high dynamic range, spanning 3-5 orders of magnitude from a quiescent luminosity of 1032 erg s-1 up to a peak luminosity of  1036 - 1037 erg s-1, and they spend most of the time in an intermediate aring level of emission at around 1033 - 1034 erg s-1. The enigmatic properties emerging from this class of sources raise interesting problems to the standard theories for the accretion of matter onto compact objects, and to the properties of OB supergiants in SFXTs. The aim of this thesis is to gain more information about the peculiar transient behaviour of SFXTs and to explore the accretion mechanisms involved in these enigmatic X-ray sources. I present a new clumpy wind model for OB supergiants, with both spherical and non-spherical geometry, that I have developed to investigate the e_ects of accretion from a clumpy wind on the X-ray variability of SFXTs. I assume that the clumps are con_ned by ram pressure of the ambient gas, and I assume that a fraction of the stellar wind is in form of clumps with power law mass and radius distributions. Then, I compute the expected X-ray lightcurves in the framework of the Bondi-Hoyle accretion theory, modi_ed to take into account the presence of clumps, and I apply this model to reproduce the X-ray lightcurves of _ve representative HMXBs: two persistent supergiant systems (Vela X-1 and 4U 1700-377), and the SFXTs IGR J16479-4514, IGR J11215-5952 and IGR J18483-0311. The model can reproduce the observed lightcurves well, but requiring in all cases an overall mass loss from the supergiants about a factor 3-10 smaller than the values inferred from UV lines studies that assume homogeneous wind, but in agreement with recent studies that assume clumpy winds. Then, I report the systematic analysis of all INTEGRAL observations from 2003 to 2009 of 14 SFXTs (con_rmed and candidates), implying a net exposure time of about 30 Ms. This analysis led to discover several new outbursts from SFXTs. I discuss the e_ects of X-ray photoionization on the accretion in close binary systems, and I show that, because of X-ray photoionization, there is a high probability of formation of transient accretion discs from the capture of angular momentum in IGR J16479-4514. This result suggests that the formation of transient accretion discs could be partly responsible for the aring activity in SFXTs with narrow orbits or in SFXTs with higher orbital periods and high eccentricities, when the neutron star is close to periastron. I also propose an alternative way to explain the origin of ares with peculiar shapes observed with INTEGRAL applying the intermittent accretion model of Lamb et al. (1977). Then, I report the study of the candidate SFXT IGR J16418-4532, for which I obtain a re_ned estimate of the orbital period from Swift/BAT data, and a re_ned estimate of the spin period of the neutron star from INTEGRAL data. I con_rm the presence of a region of the orbital lightcurve with a low ux, probably due to an eclipse, or due to the onset of the centrifugal inhibition of accretion. The uncertainties of the results from infrared observations do not allow an assessment of the spectral class of the counterpart of IGR J16418-4532, which could be a main sequence, giant, or O8.5 supergiant. From considerations involving the expected X-ray luminosities, the duration of the likely eclipse, and the onset of the centrifugal inhibition of accretion, I _nd that in all these cases it is possible to exclude the presence of a O8.5 V star, and the presence of a supergiant is favoured.
Keywords: supergiant fast x-ray transients (SFXT)
Issue Date: 2010
Doctoral course: Astronomia e Astrofisica
Academic cycle: 23
Publisher: Università degli Studi dell'Insubria
Citation: Ducci, L.Exploring accretion theory with a new subclass of high mass x-ray binaries: interpretation of integral observations of the supergiant fast x-ray transients. (Doctoral Thesis, Università degli Studi dell'Insubria, 2010).

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