arXiv subjects

Francesco Haardt

Publications and source records attributed to Francesco Haardt.

96 records · Page 6Linked to original sources

arXiv1996

The instrinsic UV/soft x-ray spectrum of quasars

The detection of a HeII absorption trough in the spectra of three high-redshift quasars provides new constraints on the spectral shape of the UV extragalactic background. The relative strengths of the observed flux decrements at the rest-frame wavelengths of 1216 and 304 A require a relatively soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic flux is dominated by the integrated light from QSOs, and the HeII reionization of the universe was completed well before z~3, then the UV/soft X-ray spectrum of individual quasars at these epochs must, once the cosmological ``filtering'' through material along the line of sight is taken into account, satisfy similar constraints on the average. We model the propagation of AGN-like ionizing radiation through the intergalactic medium using CUBA, a numerical code developed in our prior work. We show that, in order to explain the reported HeII absorption, any thermal component responsible for the ``soft X-ray excess'' observed in the spectra of AGNs at z<0.3 must, at z~3, have a typical temperature >80 eV, and a luminosity not exceeding 20% of that of the ``UV bump''.

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arXiv1996

X-ray variability and correlations in the two phase disk-corona model for Seyfert galaxies

We discuss in detail the broad band X-ray variability expected in Seyfert galaxies in the framework of a Compton cooled corona model. Variations in the optical depth tau of the corona cause spectral changes in the Comptonized emission in the sense that the spectrum steepens and the temperature decreases with increasing tau. If the corona is pair dominated, tau is determined by the compactness (proportional to luminosity for fixed size). This yields a definite relation between spectral shape and intensity implying a variation in intensity of a factor 10 for a change of 0.2 in the 2-10 keV spectral index. For low pair density coronae, no definite prediction is possible. In the absence of substantial changes in the structure of the corona, the soft thermal component correlates with the medium-hard Comptonized component. Examples are computed using the ROSAT response matrix, showing that current observations of NGC 5548 and Mrk 766 showing different behaviours can be reconciled with the model. The spectral behaviour of Mrk 766 strongly suggests that the coronal plasma in this source is not pair dominated.

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arXiv1995

Radiative Transfer in a Clumpy Universe: II. The Utraviolet Extragalactic Background

We present a detailed calculation of the propagation of AGN-like ionizing radiation through the intergalactic space. We model the ionization state of absorbing clouds, and show that the universe will be more opaque above $4\,$Ryd than previously estimated. Singly ionized helium in \Lya forest clouds and Lyman-limit systems is found to be very efficient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a significant fraction of the absorbed primary photons (emitted, e.g., by quasar sources) will be reradiated by the photoionized gas through \Lya line emission, two-photon continuum, and recombination continuum radiation. In the light of new data and recent studies, we also reassess the contribution of the QSOs observed in optical surveys to the UV extragalactic background, and find that the stochastic reprocessing of quasar Lyman continuum radiation by hydrogen and helium along the line of sight will significantly affect the amplitude, spectral shape, and fluctuations properties of the metagalactic flux. In a scenario in which QSOs are the primary source of ionizing photons in the universe, the integrated \HI \Lya emission at $z=0$ from photoionized \Lya clouds and Lyman-limit systems is found to be at a level of less than 5\% of current observational limits on the far-UV extragalactic radiation flux. We show that $J_{912}$ increases from $\approx 10^{-23} \uvunits$ at the present epoch to $\approx 5\times 10^{-22}\uvunits$ at $z=2.5$. The attenuated direct quasar emission plus recombination radiation from intergalactic gas appears to provide enough hydrogen-ionizing photons to satisfy the proximity effect at large redshift. The \HeII/\HI ratio in the diffuse intergalactic medium and the \Lya clouds increases from $\approx25$ at

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arXiv1994

A Model for the X-Ray and UV Emission from Seyfert Galaxies and Galactic Black Holes

We propose that the X--ray emission from radio quiet AGN and galactic black holes is due to Comptonization of soft thermal photons emitted by the underlying accretion disk in localized structures (blobs). The power per unit area produced by the blobs, impinging on the disk, can easily dominate the radiation internally produced by the disk. In this case the electron temperature and the high energy spectrum can be determined in a similar way as in the previously studied homogeneous model (Haardt \& Maraschi 1991). However in the present model: a) the emitted spectrum is largely independent of the {\it fraction} of gravitational power dissipated in the blobs; b) the X--ray spectrum can be harder depending on a form factor of the blobs; c) the UV (or soft X--ray for galactic objects) luminosity that is not intercepted by the blobs can be larger than the X--ray luminosity. In the framework of a simplified accretion disk $α-Ω$ dynamo model, we make order of magnitude estimates of the number of active blobs, their size, luminosity and hence their compactness, finding values in agreement with what is observed. The expected UV to X--ray spectra and correlations of X--ray and UV light curves are discussed.

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arXiv1994

On Thermal Comptonization in e+- Pair Plasmas

We study $e^{\pm}$ pair plasmas in pair equilibrium, which emit high energy radiation by thermal Comptonization of soft photons. We find that the maximum luminosity to size ratio of the source (i.e. the compactness) depends not only on the hot plasma temperature, but also on the spectral index of the resulting Comptonized spectrum. In the observationally interesting range, sources of same compactness can be hotter if their spectrum is steeper. Instruments observing in the 50--500 keV energy range, such as OSSE on board CGRO, and especially the future SAX satellite, can be more successful in detecting sources moderately steep, the flattest sources being characterized by an high energy cut--off at too low frequencies. For any given pair of values of spectral index and temperature, Comptonization theory alone fixes the $ratio$ of the compactnesses in hard and soft photons. However, if the source is pair dominated, the absolute values of the two compactnesses are fixed. Therefore there is a a one--to--one correspondence between the physical parameters of the source (the compactnesses in soft and hard photons) and the observable quantities (spectral index and temperature). This correspondence can be extremely useful in interpreting the physical behaviour of the sources, especially during variations, and can help discriminating between different models for the high energy emission of compact sources.

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arXiv1994

The Contribution of the Obscuring Torus to the X--Ray Spectrum of Seyfert Galaxies: A Test for the Unification Model

The presence of an obscuring torus around the nucleus of Seyfert galaxies, as supposed in the popular unification scheme, can strongly modify the X--ray spectrum for both type 1 and 2 Seyfert galaxies. We find that if the torus is Compton thick, it can scatter at small angles a significant fraction of the nuclear radiation, and contribute to the continuum of Seyfert 1 galaxies above $\sim$10 keV, and to the fluorescence iron line at 6.4 keV. At large inclination angles and for large torus column densities, the spectrum is attenuated by photoabsorption and Compton scattering, while the iron fluorescence line produced by the torus can have large equivalent widths. Even after dilution by the continuum scattered by the warm material outside the torus, this iron line could be strong enough to explain the ``cold" component in the spectrum of NGC 1068. We stress that the complex pattern of the predicted variability can be a powerful tool for constraining the parameters of the model, such as the column density of the torus, its inclination and the amount of warm scattering material.

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