arXiv subjects

Amri Wandel

Publications and source records attributed to Amri Wandel.

33 records · Page 2Linked to original sources

arXiv2001

Supermassive Black Holes in Active Galactic Nuclei. I. The Consistency of Black Hole Masses in Quiescent and Active Galaxies

We report the first results of a program to measure accurate stellar velocity dispersions in the bulges of the host galaxies of active galactic nuclei (AGNs) for which accurate black hole (BH) masses have been determined via reverberation mapping. We find good agreement between BH masses obtained from reverberation mapping, and from the M(BH) - sigma relation as defined by quiescent galaxies, indicating a common relationship between active and quiescent black holes and their large-scale environments.

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arXiv2000

The Black Hole-Host Galaxy Relation in Active Galactic Nuclei

The masses of the central black holes of Active Galactic Nuclei estimated by the reverberation virial technique seem to be significantly smaller than predicted from the black hole mass-bulge luminosity relation for massive black holes detected by kinematic methods in less active and normal galaxies. We reexamine the relation for updated data and suggest a few reasons that may cause this discrepancy.

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arXiv2000

Evidence for Supermassive Black Holes in Active Galactic Nuclei from Emission-Line Reverberation

Emission-line variability data for Seyfert 1 galaxies provide strong evidence for the existence of supermassive black holes in the nuclei of these galaxies, and that the line-emitting gas is moving in the gravitational potential of that black hole. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, which is then combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. In the case of the best-studied galaxy, NGC 5548, various emission lines spanning an order of magnitude in distance from the central source show the expected velocity proportional to inverse square root of the distance correlation between distance and line width, and are thus consistent with a single value for the mass. Two other Seyfert galaxies, NGC 7469 and 3C 390.3, show a similar relationship. We compute the ratio of luminosity to mass for these three objects and the narrow-line Seyfert 1 galaxy NGC 4051 and find that that the gravitational force on the line-emitting gas is much stronger than radiation pressure. These results strongly support the paradigm of gravitationally bound broad emission-line region clouds.

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arXiv1999

On the Baldwin Effect in Active Galactic Nuclei: II. Intrinsic Spectral Evolution

We suggest that the growth of the central black hole in active galactic nuclei (AGN) due to the matter accreted over the AGN lifetime causes an evolution of the luminosity and spectrum. In a previous paper we have shown that the effective temperature of the UV continuum spectrum and the emission-line equivalent width are anti-correlated with the black-hole mass. Here we estimate the change in the equivalent width of the emission lines due to the growth of the black hole mass caused by the accreted material and show that for several plausible accretion scenarios and effective-temperature models the evolving equivalent width is anti-correlated with continuum luminosity, implying that intrinsic evolution could contribute to the Baldwin effect.

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arXiv1999

On the Baldwin Effect in Active Galactic Nuclei: I. The Continuum-Spectrum - Mass Relationship

We suggest that the Baldwin Effect is a result of the spectral dependence of the line-driving ionizing continuum on the black hole mass. We derive a relationship between the mass of the central black hole and the broad emission line luminosity in active galactic nuclei (AGN). Assuming the UV spectrum of AGN is emitted from an optically thick medium we find an expression for the characteristic energy of the ``UV bump'' in terms of the observable luminosity and emission-line width. We show empirically and analytically that the bump energy is anti-correlated with the black-hole mass and with the continuum luminosity. Our model reproduces the observed inverse correlation between equivalent width and continuum luminosity, yielding an explanation of the Baldwin effect from first principles. The model gives a good fit to the Baldwin Effect of the CIV line for a mean quasar EUV spectrum (Zheng et al. 1997) and for several model spectra. The model also predicts a correlation between the strength of the Baldwin Effect (the slope of the equivalent width as a function of luminosity) and the ionization potential, consistent with recent data.

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arXiv1999

Keplerian Motion of Broad-Line Region Gas as Evidence for Supermassive Black Holes in Active Galactic Nuclei

Emission-line variability data on NGC 5548 argue strongly for the existence of a mass of order 7 x 10^7 solar masses within the inner few light days of the nucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, and these determinations are combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. The data for several different emission lines spanning an order of magnitude in distance from the central source show the expected V proportional to r^{-1/2} correlation and are consistent with a single value for the mass.

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arXiv1999

The Black Hole to Bulge Mass Relation in Active Galactic Nuclei

The masses of the central black holes in Active Galactic Nuclei (AGNs) can be estimated using the broad emission-lines as a probe of the virial mass. Using reverberation mapping to determine the size of the Broad Line Region (BLR) and the width of the variable component of the line profile H$β$ line it is possible to find quite accurate virial mass estimates for AGNs with adequate data. Compiling a sample of AGNs with reliable central masses and bulge magnitudes we find an average black-hole-to-bulge mass ratio of 0.0003, a factor of 20 less than the value found for normal galaxies and for bright quasars. This lower ratio is more consistent with the back hole mass density predicted from quasar light, and is similar to the central black hole/bulge mass ratio in our Galaxy. We argue that the black hole/bulge mass ratio actually has a significantly larger range than indicated by mssive black holes detected in normal galaxies (using stellar dynamics) and in bright quasars, which may be biased towards large black holes. We derive a scenario of black hole growth that explains the observed distribution.

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arXiv1998

Evolutionary Baldwin Effect in AGN

Assuming Active Galactic Nuclei are powered by accretion onto a massive black holes we suggest that the growth of the central black hole due to the matter accreted over the AGN lifetime causes an evolution of the luminosity and spectrum. We show that the effective temperature of the UV continuum spectrum is likely to be anti-correlated with the black-hole mass and with the accretion-powered luminosity. We estimate the change in the equivalent width of the emission lines due to the growth of the black hole and show that for plausible evolutionary tracks and effective-temperature models the equivalent width is anti-correlated with continuum luminosity thus implying an evolutionary origin to the Baldwin effect.

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arXiv1998

Spectral Dependence of the Broad Emission-Line Region in AGN

We derive a theoretical relation between $R_{BLR}$, the size of the broad-emission-line region of active galactic nuclei, and the observed soft X-ray luminosity and spectrum. We show that in addition to the well known $R_{BLR}\sim L^{1/2}$ scaling, $R_{BLR}$ should depend also on the spectral shape and derive the expected relation between $R_{BLR}$ and the X-ray luminosity and spectral index. Applying this relation to calculate a predicted BLR radius for ten AGN with reverberational data, we show that including the dependence on the spectrum improves the agreement between the calculated BLR radius and the radius independently determined from reverberation mapping. Similarly, we evaluate an expression for the line width, and show that including the dependence on the spectrum significantly improves the agreement between the calculated BLR velocity dispersion and the observed FWHM of the H$β$ line.

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arXiv1998

The Mass-Luminosity Relation in AGN

Probably the most fundamental characteristic of the quasar-AGN power house, the mass of the central black hole, is the least well known. I review the three main classes of mass estimation methods---broad emission-line kinematics, X-ray variability and accretion-disk modeling, and the masses they give in terms of the Eddington ratio, $L/L_{\rm Edd}$. The broad emission lines are probably the best probe of the central mass. They provide mass estimates that suggest a narrow spread for the Eddington ratio - 1--2 orders of magnitude over more than six orders of magnitude in continuum luminosity, possibly indicating a universal $M/L$ ratio for AGN. However, other methods give a larger spread and possibly a luminosity- dependent Eddington ratio. Potential sources of errors and bias are discussed.

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arXiv1997

BLR sizes and the X-ray spectrum in AGN

Recent ROSAT studies of narrow-line Seyfert 1 galaxies revealed these objects (with FWHM $H_β \le 2000 km s^{-1}$) generally show steeper soft X-ray spectra than broad line Seyfert 1's and that there are no AGN with broad lines and steep X-ray spectra. We derive a simple theoretical model which explains the observed correlation between the line width and the spectral index for Seyfert 1 galaxies by assuming the line width is due to gravitational velocity dispersion and showing that steeper soft X-ray spectra yield larger BLR radii. We test the model over a hetrogeneous (normal and narrow-line) sample of some 50 AGN finding a good agreement with the data. We give empirical relations for the dependence of the line width and Eddington ratio on the X-ray luminosity and apply variability analyses for direct estimation of the mass part in the line width.

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arXiv1997

Spectral Dependence of the Broad Emission-Line Region in AGN

We derive a theoretical relation between $R_{BLR}$, the size of the broad-emission-line region of active galactic nuclei, and the observed soft X-ray luminosity and spectrum. We show that in addition to the well known $R_{BLR}\sim L^{1/2}$ scaling, $R_{BLR}$ should depend also on the soft X-ray spectral slope and derive the expected relation between $R_{BLR}$ and the X-ray luminosity and spectral index. Applying this relation to calculate a predicted BLR radius for ten AGN with reverberation data, we show that including the dependence on the spectrum improves the agreement between the calculated BLR radius and the radius independently determined from reverberation mapping. Similarly, we evaluate an expression for the line width, and show that including the dependence on the spectrum significantly improves the agreement between the calculated BLR velocity dispersion and the observed FWHM of the H$β$ line.

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arXiv1997

Secondary Accceleration of Cosmic Rays by Supernova Shocks

In the common model supernova shock-acceleration of cosmic rays there are two open questions: 1. where does the high energy cosmic rays below the knee (10$^4-10^6$ Gev) come from, and 2. are cosmic ray accelerated only at their origin or contineuosly during their residence in the Galaxy. We show that $10^15$ eV light nuclei are probably accelerted by associations of supernovae. The ratio of the spectra of secondary to primary cosmic rays would be affected by repeated acceleration (also called reacceleration or secondary acceleration) in the ISM during their propagation in the galaxy. The observed secondary and primary CR spectra are used to constrain the amount of such reacceleration by supernova remnants (SNR). Two cases are considered: weak shocks ($1 3$) of relatively young remnants. It is shown that weak shocks produce more reacceleration than what is permitted in the framework of the standard leaky box (SLB) model, making it inconsistent with dispersed acceleration that should be produced by SNR. If the SLB is modified to allow a moderate amount of RA by week shocks, the RA produced by old SNRs agrees with the rate required to fit the secondary-to primaray cosmic-ray data, making a self consistent picture. Significant reacceleration by strong shocks of young SNRs should lead to flattening of the secondary-to primaray ratio at high energies, near 1TeV/nucleon.

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arXiv1997

X-ray variability and spectral scaling: a measure of BLR sizes in AGN

We apply a new method of determination of the size of the broad emission-line region (BLR) in active galactic nuclei. This method relates the radius of the broad-line region of AGN to the soft X-ray luminosity and spectral index. Comparing the BLR distances calculated from our photoionization scaling model to the BLR distances determined by reverberation mapping shows that the scaling law agrees with the $R\sim L^{1/2}$ empirical relation. We investigate a complimentary method of estimating the BLR distance - based on the Keplerian broadening of the emission lines and the central mass estimated from X-ray variability.

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arXiv1995

Turbulent Convection in Thin Accretion Disks

A self-consistent solution for a thin accretion disk with turbulent convection is presented. The disk viscosity and the convective flux are derived from a physical model for turbulence, and expressed in terms of the local physical conditions of the disk which, in turn, are controlled by the former two. In the gas pressure region, two distinct solutions are obtained. In one, the convective flux is much larger than the radiative flux and the blackbody region extends over the entire gas pressure region and could also extend down to the inner boundary of the disk. In this solution the temperature profile is close to adiabatic. In the other solution, the convective flux is about a third of the total flux, and there exist the gas pressure blackbody and electron scattering regions as well as the radiation pressure region. In the radiation pressure region, the temperature profile is very close to adiabatic, and the disk is geometrically thin and optically thick even for super Eddington accretion rates. The fraction of the convective flux, out of the total flux, increases with the accretion rate, and for accretion rates comparable to the Eddington limit is close to 1. This variation stabilizes the, radiation pressure region, so that the disk regions are secularily stable.

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