This Thesis deals with a particular class of X-ray binary pulsars, namely the class of pulsars in which accretion takes place directly from the intense stellar wind coming from the optical companion, usually an early type star. Among the wind-fed X-ray binary pulsars observed by the Medium Energy experiment on board the X-ray satellite EXOSAT, we have chosen the pulsars 4U 1538-52 and GX 301-2. We analysed the four EXOSAT observations of 4U 1538-52 and ten GX 301-2 observations, deriving correlations among the observed quantities. We further elaborated a phenomenological model of inhomogeneous wind accretion, developing our theory in two closely related directions: from a statistical point of view our model describes the observed properties of the GX 301-2 power frequency spectra; from the physical point of view we derive the physical properties of the discrete blobs falling onto the neutron star surface, properties which are in agreement with the observations, and we explain the observed correlations between the root mean square variability (rms) associated to the power spectrum and the pulse period derivative, and between the rms and the X-ray luminosity. The X-ray binary pulsar 4U 1538-52 was observed on four occasions during 1984 March and August. The observed pulsation periods of 529.97 ± 0.16 sec (March 17) and 530.14 ± 0.03 sec (August 10) indicate that the neutron star in the 4U 1538-52 continued to spin down with PP = + 3.9 · 10-9 sec/sec (Fp/ Pp = + 2.3 · 10-4 yr-1 ) in the period 1976-1988. From the observed orbital parameter of the system 4U 1538-52/QV Nor, we were not able to discriminate whether this pulsar is a wind-fed or a disk-fed system, but we assumed the wind accretion hypothesis from the pulse period behaviour, which is in agreement with that expected from the class of wind-fed binaries. In the first observation an absorbing episode has been observed, which may be attributed to an inhomogeneity in the stellar wind of QV Nor; the scale height of this blob is 2 · 109-2 · 1010 cm, assuming a blob velocity of 100-1000 Km/ sec, of the same kind of inhomogeneities observed in the Vela X-1 system (Nagase et al., 1986). The eclipse transition time of,..,_, 0.06 days gives a scale height of,..,_, 1.5·1011 cm for the atmosphere of the optical counterpart. The frequency spectra for the two observations out of eclipse are well fitted by a power law plus a constant, with slope a= 1.63±0.09 and a= l.73±0.07for the March 17 and August 10 observations, respectively. The rms's are in agreement with those observed in other high mass X-ray pulsars (Belloni and Hasinger, 1990): (26±5)3 for the March observation and (36±6)3 for the August observation. The phaseaverage X-ray energy spectrum, consistent with a power-law of photon spectral index,..,_, 1.5 (March 17) and ,..,_, 1.4 (August 10), shows an Iron line with an E.W. of,..,_, 100 eV. We performed a fit to the observed pulse profiles in order to extract some information on the emission geometry of the neutron star in 4U 1538-52. We utilized the angular and energy dependence of the X-ray emission given by Meszaros and Nagel (1985) and its analytical fit given by Leahy (1990). We found that the EXOSAT observations are in agreement with the hypothesis that the X-ray emission is emitted mainly in a direction parallel to the magnetic field lines ("pencil" beam emission pattern). The timing analysis performed on ten ME EXOSAT observations of the wind-fed X-ray binary pulsar GX 301-2 showed that in the 1984 observations the source continued to spin-down while in the 1985 observations GX 301-2 was spinning up, with average pulse period derivatives PP = + 2.2 · 10-8 sec/ sec and PP = - 8.4 · 10-8 sec/ sec, respectively. We found that the pulse period history of GX 301-2 alternates periods of general spin-up to periods of spin-down trend, in a sort of "saw-tooth" behaviour. This same type of recurrence has been observed in another transient X-ray binary pulsar, A0535+25 (Ziolkowski, 1985). The observed "saw-tooth" time scale is of the order of 100-1000 years. The pulse profiles show two peaks of different heights, 7r separated in phase. The secondary maximum height seems to be correlated with the X-ray intensity, in the sense that lower the X-ray intensity, higher the secondary peak. We interpret this behaviour in terms of change of beaming pattern. As for 4U 1538-52, we repeated the Leahy's analysis (Leahy, 1990) on the EXOSAT pulse profiles of GX 301-2. We found that the most probable beaming pattern for this source is a pencil beam emission. The power spectra of all the GX 301-2 observations show a change of slope in the frequency range 0.01 < /0 < 0.1 Hz (it is quite interesting that a "knee" in the same frequency range has been observed in the power spectra of the black hole candidate Cyg X-1 (Nolan et al., 1981)). The rms's measured in the power spectra, of the order of 20-303, correlate with the X-ray intensity and the pulse period derivative, showing a different behaviour according to the spinning state of the source. From a statistical point of view, we were able to describe the presence of an inflexion point in the GX 301-2 power spectra in terms of a shot noise process with a special response function, dependent on two free parameters. We derived a relation between these two parameters in order to obtain the inflexion point in the observed frequency range. Then we linked the statistical description with a physical scenario of inhomogeneous wind accretion onto the neutron star, in which we have assumed that the inhomogeneities are produced by the magnetohydrodynamical instabilities that occur at the magnetospheric limit. It is important to stress that, in our model, the discretization in the accreting mass flux occurs at the level of plasma penetration and not at the level of the incoming stellar wind. By assuming the theory of plasma penetration derived in a series of papers by Arons and Lea (Arons and Lea, 1976b; Arons and Lea, 1976a; Arons and Lea, 1980; Burnard et al., 1983), we derived the physical properties of these instability blobs and verified that they are in agreement with the observations. In the frame of this accretion scenario we explain the observed correlation between pulse period derivative and rms 's. Furthermore we found that the physical properties of the instability blobs at the neutron star surface are the same of those derived in the frame of disk accretion (Morfill et al., 1984). We looked for a correlation between pulse period derivative and rms in the disk-fed X-ray transient binary pulsar EXO 2030+375 (Parmar et al., 1989; Angelini et al., 1989): a correlation seems to be present but with a worse significance than that found in the case of GX 301-2. Finally we gave a physical meaning to the two free parameters of the response function: we relate a time scale to the duration of a shot, while the other to the inverse Compton cooling time scale of the blobs.
|Titolo:||Time Properties of Two Wind Fed X-Ray Binary Pulsars Observed with EXOSAT: 4U 1538-52 and GX 301-2|
|Relatore/i esterni:||Frontera, Filippo|
|Data di pubblicazione:||5-dic-1990|
|Appare nelle tipologie:||8.1 PhD thesis|