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Becker P.A.,George Mason University | Klochkov D.,University of Tubingen | Schonherr G.,Leibniz Institute for Astrophysics Potsdam | Nishimura O.,Nagano National College of Technology | And 6 more authors.
Astronomy and Astrophysics

Context. Accretion-powered X-ray pulsars exhibit significant variability of the cyclotron resonance scattering feature (CRSF) centroid energy on pulse-to-pulse timescales, and also on much longer timescales. Two types of spectral variability are observed. For sources in group 1, the CRSF energy is negatively correlated with the variable source luminosity, and for sources in group 2, the opposite behavior is observed. The physical basis for this bimodal behavior is currently not well understood. Aims. We explore the hypothesis that the accretion dynamics in the group 1 sources is dominated by radiation pressure near the stellar surface, and that Coulomb interactions decelerate the gas to rest in the group 2 sources. Methods. We derive a new expression for the critical luminosity, L crit, such that radiation pressure decelerates the matter to rest in sources with X-ray luminosity L X > L crit. The formula for L crit is based on a simple physical model for the structure of the accretion column in luminous X-ray pulsars that takes into account radiative deceleration, the energy dependence of the cyclotron cross section, the thermodynamics of the accreting gas, the dipole structure of the pulsar magnetosphere, and the diffusive escape of radiation through the column walls. We show that for typical neutron star parameters, L crit = 1.5 × 10 37B 12 16/15 erg, s -1, where B 12 is the surface magnetic field strength in units of 10 12 G. Results. The formula for the critical luminosity is evaluated for five sources, using the maximum value of the CRSF centroid energy to estimate the surface magnetic field strength B 12. The results confirm that the group 1 sources are supercritical (L X > L crit) and the group 2 sources are subcritical (L X < L crit), although the situation is less clear for those highly variable sources that cross over the line L X = L crit. We also explain the variation of the CRSF energy with luminosity as a consequence of the variation of the characteristic emission height. The sign of this dependence is opposite in the supercritical and subcritical cases, hence creating the observed bimodal behavior. Conclusions. We have developed a new model for the critical luminosity in accretion-powered X-ray pulsars that explains the bimodal dependence of the CRSF centroid energy on the X-ray luminosity L X. Our model provides a physical basis for the observed variation of the CRSF energy as a function of L X for both the group 1 (supercritical) and the group 2 (subcritical) sources as a result of the variation of the emission height in the column. © 2012 ESO. Source

Vasudevan R.V.,Institute of Astronomy | Fabian A.C.,Institute of Astronomy | Reynolds C.S.,Institute of Astronomy | Reynolds C.S.,University of Maryland University College | And 3 more authors.
Monthly Notices of the Royal Astronomical Society

The cosmic X-ray background (CXB) is the total emission from past accretion activity on to supermassive black holes in active galactic nuclei (AGN) and peaks in the hard X-ray band (30 keV). In this paper, we identify a significant selection effect operating on the CXB and flux-limited AGN surveys, and outline how they must depend heavily on the spin distribution of black holes. We show that, due to the higher radiative efficiency of rapidly spinning black holes, they will be over-represented in the X-ray background, and therefore could be a dominant contributor to the CXB. Using a simple bimodal spin distribution, we demonstrate that only 15 per cent maximally spinning AGN can produce 50 per cent of the CXB. We also illustrate that invoking a small population of maximally spinning black holes in CXB synthesis models can reproduce the CXB peak without requiring large numbers of Compton-thick AGN. The spin bias is even more pronounced for flux-limited surveys: 7 per cent of sources with maximally spinning black holes can produce half of the source counts. The detectability for maximum spin black holes can be further boosted in hard (>10 keV) X-rays by up to ~60 per cent due to pronounced ionized reflection, reducing the percentage of maximally spinning black holes required to produce half of the CXB or survey number counts further. A host of observations are consistent with an over-representation of high-spin black holes. Future NuSTAR and ASTRO-H hard X-ray surveys will provide the best constraints on the role of spin within the AGN population. © 2016 The Authors. Source

Keck M.L.,Boston University | Brenneman L.W.,Harvard - Smithsonian Center for Astrophysics | Ballantyne D.R.,Georgia Institute of Technology | Bauer F.,University of Santiago de Chile | And 21 more authors.
Astrophysical Journal

We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin a > 0.9 accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. We discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features. © 2015. The American Astronomical Society. All rights reserved. Source

Fabian A.C.,Institute of Astronomy | Parker M.L.,Institute of Astronomy | Wilkins D.R.,Institute of Astronomy | Miller J.M.,University of Michigan | And 3 more authors.
Monthly Notices of the Royal Astronomical Society

We discuss the application of simple relativistically blurred X-ray reflection models to the determination of the spin and the inner radius of the disc in accreting black holes. Observationally, the nature of the corona is uncertain a priori, but a robust determination of the inner disc radius can be made when the disc emissivity index is tightly constrained. When the inner disc is well illuminated, the black hole spin can also be determined. Using reflection modelling derived from ray tracing, we show that robust determination of disc truncation requires that the location of the coronal source is quasi-static and at a height and radius less than the truncation radius of the disc. Robust spin measurements require that at least part of the corona lies less than about 10 gravitational radii above the black hole in order that the innermost regions, including the innermost stable circular orbit, are well illuminated. The width of the blurring kernel (e.g., the iron line) has a strong dependence on coronal height. These limitations may be particularly applicable at low Eddington fractions (e.g. the low/hard state and low-luminosity AGN) where the height of the corona may be relatively large, or outflowing, and tied to jet production. © 2014 The Authors. Source

Garcia J.A.,Harvard - Smithsonian Center for Astrophysics | Steiner J.F.,Harvard - Smithsonian Center for Astrophysics | McClintock J.E.,Harvard - Smithsonian Center for Astrophysics | Remillard R.A.,Massachusetts Institute of Technology | And 2 more authors.
Astrophysical Journal

We analyze simultaneously six composite RXTE spectra of GX 339-4 in the hard state comprising 77 million counts collected over 196 ks. The source spectra are ordered by luminosity and span the range 1.6%-17% of the Eddington luminosity. Crucially, using our new tool pcacorr, we re-calibrate the data to a precision of 0.1%, an order of magnitude improvement over all earlier work. Using our advanced reflection model relxill, we target the strong features in the component of emission reflected from the disk, namely, the relativistically broadened Fe K emission line, the Fe K edge, and the Compton hump. We report results for two joint fits to the six spectra: For the first fit, we fix the spin parameter to its maximal value (a∗ = 0.998) and allow the inner disk radius Rin to vary. Results include (i) precise measurements of Rin, with evidence that the disk becomes slightly truncated at a few percent of Eddington and (ii) an order-of-magnitude swing with luminosity in the high energy cutoff, which reaches >890 keV at our lowest luminosity. For the second fit, we make the standard assumption in estimating spin that the inner edge of the accretion disk is located at the innermost stable circular orbit (Rin = RISCO) and find a∗ = 0.95+0.003 -0.05(90% confidence, statistical). For both fits, and at the same level of statistical confidence, we estimate that the disk inclination is i = 48° ± 1° and that the Fe abundance is super-solar, AFe = 5 ± 1. © 2015. The American Astronomical Society. All rights reserved. Source

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