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Register a new userEffects of variability of X-ray binaries on the X-ray luminosity functions of Milky Way
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The X-ray luminosity functions of galaxies have become a useful tool for population studies of X-ray binaries in them. The availability of long term light-curves of X-ray binaries with the All Sky X-ray Monitors opens up the possibility of constructing X-ray luminosity functions, by also including the intensity variation effects of the galactic X-ray binaries. We have constructed multiple realizations of the X-ray luminosity functions (XLFs) of Milky Way, using the long term light-curves of sources obtained in the 2-10 keV energy band with the RXTE-ASM. The observed spread seen in the value of slope of both HMXB and LMXB XLFs are due to inclusion of variable luminosities of X-ray binaries in construction of these XLFs as well as finite sample effects. XLFs constructed for galactic HMXBs in the luminosity range 10^{36} - 10^{39} erg/sec is described by a power-law model with a mean power-law index of -0.48 and a spread due to variability of HMXBs as 0.19. XLFs constructed for galactic LMXBs in the luminosity range 10^{36} - 10^{39} erg/sec has a shape of cut-off power-law with mean power-law index of -0.31 and a spread due to variability of LMXBs as 0.07.
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Strongly magnetized, accreting neutron stars show periodic and aperiodic variability over a wide range of time scales. By obtaining spectral and timing information on these different time scales, we can have a closer look into the physics of accretion close to the neutron star and the properties of the accreted material. One of the most prominent time scales is the strong pulsation, i.e., the rotation period of the neutron star itself. Over one rotation, our view of the accretion column and the X-ray producing region changes significantly. This allows us to sample different physical conditions within the column but at the same time requires that we have viewing-angle-resolved models to properly describe them. In wind-fed high-mass X-ray binaries, the main source of aperiodic variability is the clumpy stellar wind, which leads to changes in the accretion rate (i.e., luminosity) as well as absorption column. This variability allows us to study the behavior of the accretion column as a function of luminosity, as well as to investigate the structure and physical properties of the wind, which we can compare to winds in isolated stars.
We have obtained three epochs of Chandra ACIS-I observations (totaling $sim$184 ks) of the nearby spiral galaxy NGC~300 to study the logN-logS distributions of its X-ray point source population down to $sim$2$times$10$^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the 0.35-8 keV band (equivalent to $sim$10$^{36}$ erg s$^{-1}$). The individual epoch logN-logS distributions are best described as the sum of a background AGN component, a simple power law, and a broken power law, with the shape of the logN-logS distributions sometimes varying between observations. The simple power law and AGN components produce a good fit for persistent sources (i.e., with fluxes that remain constant within a factor of $sim$2). The differential power law index of $sim$1.2 and high fluxes suggest that the persistent sources intrinsic to NGC~300 are dominated by Roche lobe overflowing low mass X-ray binaries. The variable X-ray sources are described by a broken power law, with a faint-end power law index of $sim$1.7, a bright-end index of $sim$2.8-4.9, and a break flux of $sim$8$times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ ($sim$4$times10^{36}$ erg s$^{-1}$), suggesting they are mostly outbursting, wind-fed high mass X-ray binaries, although the logN-logS distribution of variable sources likely also contains low-mass X-ray binaries. We generate model logN-logS distributions for synthetic X-ray binaries and constrain the distribution of maximum X-ray fluxes attained during outburst. Our observations suggest that the majority of outbursting X-ray binaries occur at sub-Eddington luminosities, where mass transfer likely occurs through direct wind accretion at $sim$1-3% of the Eddington rate.
We present the first direct measurement of the spatial cross-correlation function of high-mass X-ray binaries (HMXBs) and active OB star-forming complexes in the Milky Way. This result relied on a sample containing 79 hard X-ray selected HMXBs and 458 OB associations. Clustering between the two populations is detected with a significance above 7-sigmas for distances < 1 kpc. Thus, HMXBs closely trace the underlying distribution of the massive star-forming regions that are expected to produce the progenitor stars of HMXBs. The average offset of 0.4+-0.2 kpc between HMXBs and OB associations is consistent with being due to natal kicks at velocities of the order of 100+-50 km/s. The characteristic scale of the correlation function suggests an average kinematical age (since the supernova phase) of ~4 Myr for the HMXB population. Despite being derived from a global view of our Galaxy, these signatures of HMXB evolution are consistent with theoretical expectations as well as observations of individual objects.
We have developed a clumpy stellar wind model for OB supergiants in order to compare predictions of this model with the X-ray behaviour of both classes of persistent and transient High Mass X-ray Binaries (HMXBs).
Galactic black-hole X-ray binaries (BHBs) emit a compact, optically thick, mildly relativistic radio jet when they are in the hard and hard-intermediate states. In these states, BHBs exhibit a correlation between the time lag of hard photons with respect to softer ones and the photon index of the power law component that characterizes the X-ray spectral continuum above $sim$ 10 keV. The correlation, however, shows large scatter. Our objective is to investigate the role that the inclination of the system plays on the correlation between the time lag and the photon index. We find that the correlation between the time lag and the photon index is tight in low-inclination systems and becomes weaker in high-inclination systems. The amplitude of the lags is also larger at low and intermediate inclination angles than at high inclination. Our jet model that reproduces the process of Comptonization in an extended jet can account for the observations remarkably well.
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