Keigo Fukumura

MAGNETOHYDRODYNAMIC ACCRETION DISK WINDS AS X-RAY ABSORBERS IN ACTIVE GALACTIC NUCLEI

We present the two-dimensional (2D) ionization structure of self-similar magnetohydrodynamic (MHD) winds off accretion disks around irradiated by a central X-ray point source. Based on earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r) ∝ 1/r (r is the spherical radial coordinate). We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. We focus our attention on the distribution of the column density of the various ions as a function of the ionization parameter ξ (or equivalently r) and the angle θ. Particular attention is paid to the absorption measure distribution (AMD), namely their Hydrogen-equivalent column per logarithmic ξ interval, dNH/d log ξ, which provides a measure of the winds’ radial density profiles. For the chosen density profile n(r) ∝ 1/r the AMD is found to be independent of ξ, in good agreement with its behavior inferred from the X-ray spectra of several active galactic nuclei (AGNs). For the specific wind structure and X-ray spectrum we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v ∼ 100−300 km s (at log ξ ∼ 2−3) for Fexvii and v ∼ 1, 000 − 4, 000 km s (at log ξ ∼ 4 − 5) for Fexxv, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate ṁ and LOS angle θ. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing ṁ and increases steeply with the LOS inclination angle θ. As such, we concur with previous authors that these Email: Keigo.Fukumura@nasa.gov University of Maryland, Baltimore County (UMBC/CRESST), Baltimore, MD 21250 Astrophysics Science Division, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 Research Center for Astronomy, Academy of Athens, Athens 11527, Greece Department of Physics, Technion, Haifa 32000, Israel Senior NPP Fellow

QPOs in the time domain: an autocorrelation analysis

Context. Motivated by the recent proposal that one can obtain quasi-periodic oscillations (QPOs) by photon echoes manifesting as non-trivial features in the autocorrelation function (ACF), we study the ACFs of the light curves of three accreting black hole candidates and a neutron star already known to exhibit QPOs namely, GRS 1915+105, XTE J1550-564, XTE J1859+226 and Cygnus X-2. Aims. We present a comparative study of the timing properties of these systems in the frequency and time domain in search for similarities/differences that may provide clues to the physics underlying the QPO phenomenon. Methods. We compute and focus on the form of the ACFs in search of systematics or specific temporal properties at the time scales associated with the known QPO frequencies in comparison with the corresponding PDS. Results. Even within our small object sample we find both similarities as well as significant and subtle differences in the form of the ACFs both amongst black holes and between black holes and neutron stars to warrant a closer look at the QPO phenomenon in the time domain: the QPO features manifest as an oscillatory behavior of the ACF at lags near zero; the oscillation damps exponentially on time scales equal to the inverse QPO width to a level of a percent or so. In black holes this oscillatory behavior is preserved and easily discerned at much longer lags while this is not the case for the neutron star system Cyg X-2. The ACF of GRS 1915+105 provides an exception to this general behavior in that its decay is linear in time indicating an undamped oscillation of coherent phase. We present simple ad hoc models that reproduce these diverse time domain behaviors and we speculate that their origin is the phase coherence of the underlying oscillation. Conclusions. It appears plausible that time domain analyses, complementary to the more common frequency domain ones, could impose tighter constraints and provide clues for the driving mechanisms behind the QPO phenomenon.

Magnetically advected winds

Observations of X-ray absorption lines in magnetically driven disc winds around black hole binaries and active galactic nuclei yield a universal radial density profile ρ ∝ r - 1.2 in the wind. This is in disagreement with the standard Blandford & Payne profile ρ BP ∝ r - 1.5 expected when the magnetic field is neither advected nor diffusing through the accretion disc. In order to account for this discrepancy, we establish a new paradigm for magnetically driven astrophysical winds according to which the large scale ordered magnetic field that threads the disc is continuously generated by the Cosmic Battery around the inner edge of the disc and continuously diffuses outward. We obtain self-similar solutions of such magnetically advected winds (MAW) and discuss their observational ramifications.