Since the discovery of the very first X-ray point source SMC X-1, the study of X-ray binaries has received an enormous interest due to the observations over a wide band (from eV to MeV) of electromagnetic spectrum. Among the X-ray sources in the sky, the binary systems in our Galaxy are considered as the brightest sources. An X-ray binary system contains a compact object which is either a neutron star, a black hole or a white dwarf, accreting matter from the binary companion. In such systems, the compact object and the binary companion rotate around the common center of mass. The primary factors that determine the emission properties of an accreting compact object are (1) the nature of the compact object, (2) the strength and geometry of the magnetic field if the compact object is a neutron star, (3) the geometry of the accretion flow from the companion, and (4) the mass accretion rate and the mass of the compact object. These factors determine the emission region of the compact object such as whether it is the small magnetic polar cap of a neutron star, a hot accretion disk surrounding a black hole, a shock heated region in a spherical inflow, or the boundary layer between the surface of the neutron star and the accretion disk. The overall luminosity, spectral shape and the time variability of the X-ray emitted from the binary system is largely influenced by the rate of mass accretion from the companion to the compact object.
X-ray Pulsars:Most of the bright X-ray sources in our Galaxy are ``X-ray binaries'', in which a normal star and a compact object rotate around a common center of mass. The compact object in the binary binary system can be a black hole, neutron star or a white dwarf. Many of these binary systems are X-ray pulsars, in which the magnetic axis of a spinning neutron star sweeps through the earth's line-of-sight. The "pulses" of high-energy radiation we see are due to the misalignment of the rotation axis and magnetic axis of the neutron star. The neutron star in the binary system accretes matter from the binary companion: this material follows the strong magnetic field lines and falls onto the magnetic poles of neutron star. The poles of the neutron star get extremely hot since all energy of the falling material is converted into heat. This hot surface then emits X-rays which are seen as pulses due to the rotation of the neutron star.
The pulse period and orbital period of these pulsars are being measured with various instruments onboard many different satellites since last three decades. The observed change in the pulse and orbital period helps in understanding the orbital evolution of the binary system. The orbital parameters of binary stellar systems are best measured if one or both of the stars are rotating neutron stars. In case of low mass X-ray binaries, the evolution is mainly due to mass transfer from the companion to the compact object and in case of high mass X-ray binaries, it is due to mass loss from the system. Orbital evolution of binary stellar systems are measured very accurately in several accreting X-ray binary pulsars from pulse timing measurements spanning about one/two decades. This is possible due to the strong tidal interaction between the two stellar components and/or huge mass loss through stellar wind that causes rapid orbital evolution in such systems.
Pulse-averaged and pulse-resolved X-ray spectroscopy of accretion powered X-ray pulsars can be used to understand the distribution of matter around the neutron star. The energy spectra of these pulsars can be represented by a power-law with a high-energy cutoff. Since most of the binary pulsars are located in the Galactic plane, their spectra are usually subjected to strong soft X-ray absorption. However, there are some binary X-ray pulsars outside the Galaxy which show the presence of a soft excess over the extended hard power-law component. The soft component is detectable only in pulsars which do not suffer from absorption by material along the line of sight. Pulsations in the soft spectral component with a certain phase difference with respect to the hard component are also seen in a few X-ray pulsars. Apart from the hard and soft spectral components, iron emission line features are also seen in many of the X-ray pulsars. Iron K shell emission lines in X-ray pulsars are believed to be produced by illumination of neutral or partially ionized material in accretion disk, stellar wind of the high mass companion, material in the form of circumstellar shell, material in the line of sight, or in the accretion column. Pulse-phase-averaged and pulse phase-resolved spectroscopy, therefore, provide important information in understanding these systems in more detail.
The pulse phase-resolved spectroscopy analysis technique provides a unique opportunity to have a clear understanding of the distribution of matter around the neutron star in the X-ray binary pulsars. The shape of the continuum and the line profiles vary with pulse phase. High resolution spectroscopy with Suzaku, Chandra and XMM-Newton provides a capability to distinguish the ions of different elements by detecting various emission/absorption lines at low energies. The study of the dependence of the spectral parameters of different emission/absorption lines with different phases of the pulse period of X-ray pulsars can provide information regarding the temperature and density of ion distribution and the state of ionization of different elements in surrounding medium of the X-ray source.
Study of cyclotron resonance features in X-ray pulsars helps in estimating and understanding the nature of the magnetic field of the neutron star. Cyclotron lines in accreting X-ray pulsar spectra result from the resonant scattering of X-rays by electrons in Landau orbits in the intense (~10^12 G) magnetic fields near the poles of the neutron star. For this reason they are known as cyclotron resonance scattering features (CRSFs). Because Landau transition energies are proportional to field strength (E_cyc = 12 keV approximately corresponds to B = 10^12 G), CRSF energies give us our most direct measures of neutron star magnetic fields. Though the cyclotron line features are detected in some X-ray pulsars, the pulse-phase dependence of the line parameters which can give the distribution of the magnetic field lines around the neutron star along with other properties are unknown. This can be studied from the phase averaged spectroscopy of data from observatories such as Suzaku, BeppoSAX, Astrosat etc.
Phase-resolved spectroscopy of cyclotron features in X-ray pulsars helps in understanding the distribution of the magnetic lines of forces around the neutron star. Pulse phase dependent cyclotron line properties, such as line energy, depths, widths, and presence of multiple harmonics, are strongly dependent on the details of the geometry and environment at the base of the accretion column and the pulsar magnetic field. Observations of these lines directly measure extremely intense magnetic fields, and their properties have implications for magnetic field decay in neutron stars and X-ray emission processes and the behavior of matter in strong magnetic fields. The intensity of the cyclotron lines appear to depend on different phases of the pulse period. The study of the cyclotron line features, therefore, is a most important tool to obtain the information on the structure of the emitting plasma and the effects of the strong magnetic fields on the plasma. Studying the cyclotron features, parameters like accretion rate, intensity of the magnetic field can also be estimated.