arXiv subjects

Doron Chelouche

Publications and source records attributed to Doron Chelouche.

45 records · Page 3Linked to original sources

arXiv2006

QSO Absorption Lines from QSOs

We present the results of a search for metal absorption lines in the spectra of background QSOs whose sightlines pass close to foreground QSOs. We detect MgII(2796,2803) absorption in Sloan Digital Sky Survey (SDSS) spectra of four z>1.5 QSOs whose lines of sight pass within 26-98 kpc of lower redshift (z~0.5-1.5) QSOs. The 100% [4/4 pairs] detection of MgII in the background QSOs is clearly at odds with the incidence of associated (z_abs ~ z_em) systems -- absorbers which exist towards only a few percent of QSOs. Although the quality of our foreground QSO spectra is not as high as the SDSS data, absorption seen towards one of the background QSOs clearly does not show up at the same strength in the spectrum of the corresponding foreground QSO. This implies that the absorbing gas is distributed inhomogeneously around the QSO, presumably as a direct consequence of the anisotropic emission from the central AGN. We discuss possible origins for the MgII lines, including: absorption by gas from the foreground QSO host galaxy; companion galaxies fuelling the QSO through gravitational interactions; and tidal debris left by galaxy mergers or interactions which initiated the QSO activity. No single explanation is entirely satisfactory, and we may well be seeing a mixture of phenomena.

astro-ph↗

arXiv2005

XMM-Newton Spectroscopy of the Starburst Dominated Ultra Luminous Infrared Galaxy NGC 6240

We present new XMM-Newton observation of the Ultra Luminous Infrared Galaxy (ULIRG) NGC 6240. We analyze the reflecting grating spectrometer (RGS) data, and data from the other instruments, and find a starburst dominated 0.5-3 keV spectrum with global properties resembling those observed in M82 but with a much higher luminosity. We show that the starburst region can be divided into an outer zone, beyond a radius of about 2.1 kpc, with a gas temperature of about 10^7 K and a central region with temperatures in the range (2-6) x 10^7 K. The gas in the outer region emits most of the observed Oviii Lyman-alpha line and the gas in the inner region the emission lines of higher ionization ions, including a strong Fexxv line. We also identify a small inner part, very close to the active nuclei, with typical Seyfert 2 properties including a large amount of photoionized gas producing a strong Fe K-alpha 6.4 keV line. The combined abundance, temperature and emission measure analysis indicates super solar Ne/O, Mg/O, Si/O, S/O and possibly also Fe/O. The analysis suggests densities in the range of (0.07-0.28) x epsilon^(-1/2) cm^(-3) and a total thermal gas mass of about 4 x 10^8 x epsilon^(1/2) solar masses, where epsilon is the volume filling factor. We used a simple model to argue that a massive starburst with an age of about 2 x 10^7 years can explain most of the observed properties of the source. NGC 6240 is perhaps the clearest case of an X-ray bright luminous AGN, in a merger, whose soft X-ray spectrum is dominated by a powerful starburst.

astro-ph↗

arXiv2005

Gravitational Microlensing and the Structure of Quasar Outflows

We show that invaluable information on the structure of quasar outflows can be obtained by considering microlensing (ML) induced variability of absorption line troughs in lensed quasars. Depending on the structure and geometry of the outflowing gas, such extrinsic line variability can be manifested as changes to the equivalent width of the line as well as line profile distortions. Here we consider several physically distinct outflow models, having very similar spectral predictions, and show how ML induced absorption line variability can be used to distinguish between them. The merits of future systematic studies of these effects are exemplified.

astro-ph↗

arXiv2005

Dynamical and Spectral Modeling of the Ionized Gas and Nuclear Environment in NGC 3783

We present a new approach for calculating the physical properties of highly ionized X-ray flows in active galactic nuclei (AGN). Our method relies on a detailed treatment of the structure, dynamics and spectrum of the gas. A quantitative comparison of our model predictions with the 900 ksec Chandra/HETG X-ray spectrum of NGC 3783 shows that: (1) The highly ionized outflow is driven by thermal pressure gradients and radiation pressure force is less important. (2) A full featured dynamical model that provides a very good fit to the high resolution X-ray spectrum requires a multi-phased flow with a density power spectrum reminiscent of the interstellar medium. (3) Adiabatic cooling is an important factor and so is an additional heating source that may be related to the apparent multi-phase and turbulent nature of the flow. (4) The base of the flow is ~1pc from the central object, in agreement with some, but not all previous estimates. (5) The mass loss rate is in the range 0.01-0.1 solar masses per year which is smaller than previous estimates and of the same order of the mass accretion rate in this object.

astro-ph↗

arXiv2004

The Properties and the Evolution of the Highly Ionized Gas in MR2251-178

(abridged) We present the first XMM-Newton observations of the radio-quiet quasar MR2251-178 obtained in 2000 and 2002. The EPIC-pn spectra show a power-law continuum with a slope of 1.6 at high energies absorbed by at least two warm absorbers (WAs) intrinsic to the source. The underlying continuum in the earlier observation shows a soft excess at low X-ray energies which can be modeled as an additional power-law with a slope of 2.9. The high-resolution grating spectrum obtained in 2002 shows emission lines from N VI, O VII, O VIII, Ne IX, and Ne X, as well as absorption lines from the low-ionization ions of O III, O IV, and O V, and other confirmed and suspected weaker absorption lines. We suggest a model for the high-resolution spectrum which consist of two or three WA components. The two-components model has a high-ionization WA with a column density of 10^21.5 - 10^21.8 cm^-2 and a low-ionization absorber with a column density of 10^20.3 cm^-2. In the three-components model we add a lower ionization component that produce the observed iron M-shell absorption lines. We investigate the spectral variations in MR2251-178 over a period of 8.5 years using data from ASCA, BeppoSAX, and XMM-Newton. All X-ray observations can be fitted with the above EPIC-pn model. The 8.5 year history of the source suggests a changing X-ray absorber due to material that enters and disappears from the line-of-sight on timescales of several months. We also present the entire FUSE spectrum of MR2251-178. We detect emission from N III, C III, and O VI and at least 4 absorption systems in C III, H I, and O VI, one at -580 km/s and at least 3 others which are blended together and form a wide trough covering the velocity range of 0 to -500 km/s. The general characteristics of the UV and X-ray absorbers are consistent with an origin in the same gas.

astro-ph↗

arXiv2003

Non-Sobolev modelling of radiation pressure driven flows in Active Galactic Nuclei

We present a new general scheme for calculating the structure and dynamics of radiation pressure driven, photoionized flows. The new method goes one step beyond the Sobolev approximation. It involves a numerical solution of the radiative transfer in absorption lines including the effects of differential expansion and line interactions such as line locking and blanketing. We also present a new scheme for calculating the radiation pressure due to trapped line photons in finite, differentially expanding flows. We compare our results for the radiation pressure force with those obtained using the Sobolev approximation and show the limitations of the latter. In particular, we demonstrate that the Sobolev method gives a poor approximation near discontinuity surfaces and its neglect of line blanketing can lead to erroneous results in high velocity flows. We combine the newly calculated radiation pressure force with self-consistent photoionization and thermal calculations to study the dynamics and spectral features of broad absorption line flows and highly ionized gas flows in AGN. A comparison with Sobolev-type calculations shows that the latter over estimates the flow's terminal velocity and, conversely, under estimates its opacity. We also show that line locking on broad emission lines can have a significant effect on the dynamics and spectral features of AGN flows.

astro-ph↗

arXiv2003

Strong Gravitational Lensing and the Structure of Quasar Outflows

We show that by analyzing the spectra of lensed broad absorption line quasars (BALQSOs), it is possible to reveal key properties of the outflowing gas in the inner regions of these objects. This results from the fact that each image of the quasar corresponds to a different line of sight through the outflow. This combined with dynamical estimates for the location of the flow, adds new information concerning the lateral, non line of sight structure of the absorbing gas. Here we consider a sample of lensed BALQSOs and note that the similarity of BAL profiles of different images of the same quasar implies that the flow is relatively isotropic on small scales. We show that its geometry is inconsistent with the ballistically accelerated spherical cloud model, and that wind models provide a better description of the flow structure. Furthermore, observations seem to disagree with naive interpretations of recent time-dependent wind simulations. This may hint on several important physical processes that govern the structure and dynamics of such flows. Future prospects for the study of quasar outflows with the effect of strong gravitational lensing are discussed.

astro-ph↗

arXiv2003

Continuum Shielding and Flow Dynamics in Active Galactic Nuclei

We study the ionization, thermal structure, and dynamics of AGN flows which are partially shielded from the central continuum. We utilize detailed non-LTE photoionization and radiative transfer code using exact (non-Sobolev) calculations. We find that shielding has a pronounced effect on the ionization, thermal structure, and the dynamics of such flows. Moderate shielding is especially efficient in accelerating flows to high velocities since it suppresses the ionization level of the gas. The ionization structure of shielded gas tends to be distributed uniformly over a wide range of ionization levels. In such gas, radiation pressure due to trapped line photons can dominate over the thermal gas pressure and have a significant effect on the thermal stability of the flow. Heavily shielded flows are driven mainly by line radiation pressure, and so line locking has a large effect on the flow dynamics. We show that the observed ``L_αghost'' is a natural outcome in highly ionized flows that are shielded beyond the Lyman limit. We suggest that high velocity AGN flows occupy only a small fraction of the volume and that their density depends only weakly on the velocity field.

astro-ph↗

arXiv2001

Radiation Pressure Acceleration by X-rays in Active Galactic Nuclei

We present calculations of the dynamics of highly ionized gas clouds that are confined by external pressure, and are ionized by AGN continuum. We focus on the gas that is seen in absorption in the X-ray spectrum of many AGN and show that such gas can reach hydrostatic equilibrium under various conditions. The principal conclusion is that the clouds can be accelerated to high velocities by the central X-ray source. The dynamical problem can be reduced to the calculation of a single parameter, the average force multiplier, . The typical value of is ~10 suggesting that radiation pressure acceleration by X-rays is efficient for L/L_Edd>0.1. The terminal velocity scales with the escape velocity at the base of the flow and can exceed it by a large factor. The typical velocity for a HIG flow that originates at R=1e17 cm in a source with L_x=1e44 erg/s is ~1000 km/s, i.e. similar to the velocities observed in several X-ray and UV absorption systems. Highly ionized AGN clouds are driven mainly by bound-free absorption and bound-bound processes are less important unless the lines are significantly broadened or the column density is very small. Pressure laws that result in constant or outward decreasing ionization parameters are most effective in accelerating the flow.

astro-ph↗