2,164 research outputs found

    Normal Branch Quasi-Periodic Oscillations in Sco X-1: Viscous Oscillations of a Spherical Shell Near the Neutron Star

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    We present a comprehensive classification of all observed QPOs within the framework of the transition layer model using a large set of Rossi X-ray Timing Explorer (RXTE) data for Sco X-1. The model assumes an optically thin material along the observer's line of sight in the horizontal branch and an increasingly optically thick material while in the other two branches that is consistent with X-ray and radio observations and the disk transition layer model of QPOs. We identify the ~ 6 Hz frequencies in the normal branch as acoustic oscillations of a spherical shell around the neutron star (NS) that is formed after radiation pressure near the Eddington accretion rate destroys the disk. The size of the shell is on the order of one NS radii from the NS. We also estimate the upper limit of Sco X-1's magnetic field to be 0.7 x 10^6 G at about one NS radii above the NS surface while in the horizontal X-ray branch.Comment: 12 pages and 2 figures, to be published in the Astrophysical Journal Letter

    The X-ray evolving universe: (ionized) absorption and dust, from nearby Seyfert galaxies to high-redshift quasars

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    (Abridged) Cold and warm absorbers have beeen detected in all types of active galaxies (AGN) from low to high redshift. This gas, located in the black hole region of AGN, is thought to play an important role in AGN unification scenarios, in explaining the X-ray background, in black hole growth and AGN evolution. We provide a review of the observations of dusty and dust-free warm and cold absorbers at low and high redshift, including most recent results and exciting questions still open. Emphasis is on the science issues that we will be able to address with XEUS for the first time, particularly at high redshift, including: (i) determination of metal abundances of X-ray (cold) absorbers by detection of metal absorption edges, (ii) analysis of the composition of dust mixed with cold and ionized gas (K-edges of metals in cold dust and cold gas will be resolvable from each other for the first time), (iii) measurement of the velocity field of the gas, (iv) utilization of these results to investigate the evolution of gas and dust in AGN from high to low redshift: the evolution of abundances, dust content, ionization state, amount and velocity of gas, and its role in feeding the black hole. We emphasize the importance of iron absorption measurements with XEUS at high redshift for two key issues of cosmology: the early star formation history of the universe, and the measurement of cosmological parameters. As an example, we discuss recent XMM-Newton observations of the high-redshift BAL quasar APM 08279+5255.Comment: 8 pages, to appear in the proc. of the workshop "XEUS - studying the evolution of the universe", G. Hasinger et al. (eds), MPE Report, in press. Related papers on the high-z BAL quasar APM 08279+5255 (and MPG press release), and on ionized absorbers are available at http://www.xray.mpe.mpg.de/~skomossa

    Deep Extragalactic X-ray Surveys

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    Deep surveys of the cosmic X-ray background are reviewed in the context of observational progress enabled by the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission-Newton. The sources found by deep surveys are described along with their redshift and luminosity distributions, and the effectiveness of such surveys at selecting active galactic nuclei (AGN) is assessed. Some key results from deep surveys are highlighted including (1) measurements of AGN evolution and the growth of supermassive black holes, (2) constraints on the demography and physics of high-redshift AGN, (3) the X-ray AGN content of infrared and submillimeter galaxies, and (4) X-ray emission from distant starburst and normal galaxies. We also describe some outstanding problems and future prospects for deep extragalactic X-ray surveys.Comment: 32 pages; Annu. Rev. Astron. Astrophys., Volume 43 (2005); updated to match accepted versio

    The X-ray Background and AGNs

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    Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is largely due to the accretion onto supermassive black holes, integrated over the cosmic time. These surveys have resolved more than 80% of the 0.1-10 keV X-ray background into discrete sources. Optical spectroscopic identifications show that the sources producing the bulk of the X-ray background are a mixture of unobscured (type-1) and obscured (type-2) AGNs, as predicted by the XRB population synthesis models. A class of highly luminous type-2 AGN, so called QSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. The new Chandra AGN redshift distribution peaks at much lower redshifts (z~0.7) than that based on ROSAT data, and the new X-ray luminosity function indicates that the space density of Seyfert galaxies peaks at significantly lower redshifts than that of QSOs. It is shown here, that the low redshift peak applies both to absorbed and unabsorbed AGN and is also seen in the 0.5-2 keV band alone. Previous findings of a strong dependence of the fraction of type-2 AGN on luminosity are confirmed with better statistics here. Preliminary results from an 800 ksec XMM-Newton observation of the Lockman Hole are discussed.Comment: Proceedings of the conference: "The restless high energy universe", held in Amsterdam, May 2003. To be published in: Nucl. Physics B. Suppl. Ser., E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers (eds.). 10 pages, 5 figure

    A Possible Explanation for the "Parallel Tracks" Phenomenon in Low-Mass X-Ray Binaries

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    An explanation is proposed for the fact that in LMXBs the correlation between most observable X-ray spectral and timing parameters (such as kHz QPO frequency) on the one hand, and Lx on the other, while generally good in a given source on a time scale of hours, is absent both on longer time scales and between sources. This leads to parallel tracks in plots of such parameters vs. Lx. Where previous explanations require at least two time-variable independent parameters, e.g. Mdot through the disk and through a radial inflow, one is in fact sufficient if the systemic response to time variations in this variable has both a prompt and a time-averaged component. I explore a scenario in which most observable spectral and timing parameters to first order depend on disk accretion rate normalized by its own long-term average rather than on any individual Mdot; Lx just depends on total Mdot. Thus, parameters can be uncorrelated to Mdot, yet vary in response to Mdot variations. Numerical simulations of the model describing the dependence of kHz QPO frequency on Lx, which observationally is characterized by a striking pattern of parallel tracks both in individual sources and between sources, reproduce the observations remarkably well. A physical interpretation involving a radial inflow with a rate that derives through a time averaging process from the disk accretion rate, and an inner disk radius that depends on the balance between the accretion through the disk and the total luminosity seems particularly promising. The consequences of this idea for our understanding of states and tracks in LMXBs are discussed, and the applicability of the idea to black-hole candidates, where the observational situation is more complex, is briefly addressed.Comment: 17 pages 3 figures - version accepted for publication in the ApJ; tentatively scheduled for the v561 n2 p1 ApJ November 10, 2001 issue. Some corrections and clarifications w/r to details of the argumen
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