arXiv subjects

Julien Malzac

Publications and source records attributed to Julien Malzac.

46 records · Page 3Linked to original sources

arXiv2004

Jet-Disc coupling in the accreting black hole XTEJ1118+480

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arXiv2004

Full radiative coupling in two-phase models for accreting black holes

The emission from galactic black holes and Seyfert galaxies is generally understood in term of two-phase models (Haardt and Maraschi 1991, 1993). Such models postulate that a hot plasma coexists with relatively colder material in the inner part of the accretion flow. We present the first simulated broad-band spectra produced by such a system and accounting simultaneously for energy balance and Comptonisation in the hot phase, together with reflection, reprocessing, ionization and thermal balance in the cold phase. This was made possible by coupling three radiative transfer codes: a non-linear Monte-Carlo code (NLMC), a photo-ionization code TITAN and a linear Monte-Carlo code NOAR. The equilibrium comptonisation spectrum appears to be sensitive to the shape of the reprocessed spectrum that, in turn, depends on the ionization parameter, but also on the structure of the irradiated cold material. This is illustrated by a comparison of simulations assuming constant density or a constant pressure in the cold phase. We also compare our results with simplified models where reprocessing is approximated by a blackbody spectrum. Our detailed treatment leads to noticeably different spectral energy distributions (SEDs) characterised by harder X-ray spectra. Even at low ionization parameters the commonly used blackbody approximation is poor, leading to X-ray spectra that are too soft. The effect, however, seems not to be strong enough to reconcile the slab corona model with the hardest observed spectra, unless the reflector has a constant density and the ionization parameter is large.

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arXiv2004

Jet-disc coupling through a common energy reservoir in the black hole XTE J1118+480

We interpret the rapid correlated UV/optical/ X-ray variability of XTE J1118+480 as a signature of the coupling between the X-ray corona and a jet emitting synchrotron radiation in the optical band. We propose a scenario in which the jet and the X-ray corona are fed by the same energy reservoir where large amounts of accretion power are stored before being channelled into either the jet or the high energy radiation. This time dependent model reproduces the main features of the rapid multi-wavelength variability of XTE J1118+480. Assuming that the energy is stored in the form of magnetic field, we find that the required values of the model parameters are compatible with both a patchy corona atop a cold accretion disc and a hot thick inner disc geometry. The range of variability timescales for the X-ray emitting plasma are consistent with the dynamical times of an accretion flow between 10 and 100 Schwarzschild radii. On the other hand, the derived range of timescales associated with the dissipation in the jet extends to timescales more than 10 times larger, confirming the suggestion that the generation of a powerful outflow requires large scale coherent poloidal field structures. A strong requirement of the model is that the total jet power should be at least a few times larger than the observed X-ray luminosity. This would be consistent with the overall low radiative efficiency of the source. We present independent arguments showing that the jet probably dominates the energetic output of all accreting black holes in the low-hard state.

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arXiv2003

Cyclotron-Synchrotron: harmonic fitting functions in the non-relativistic and trans-relativistic regimes

The present work investigates the calculation of absorption and emission cyclotron line profiles in the non-relativistic and trans-relativistic regimes. We provide fits for the ten first harmonics with synthetic functions down to 10^(-4) of the maximum flux with an accuracy of 20 per cent at worst. The lines at a given particle energy are calculated from the integration of the Schott formula over the photon and the particle solid angles relative to the magnetic field direction. The method can easily be extended to a larger number of harmonics. We also derive spectral fits of thermal emission line plasmas at non-relativistic and trans-relativistic temperatures extending previous parameterisations.

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arXiv2002

Reflection at large distance from the central engine in Seyfert 1s

We consider the possibility that most of the reflection component, observed in the hard X-ray spectra of Seyfert galaxies, could be formed on an extended medium, at large distance from the central source of primary radiation (e.g. on a torus). Then, the reflector cannot respond to the rapid fluctuations of the primary source. The observed reflected flux is controlled by the time-averaged primary spectrum rather than the instantaneous (observed) one. We show that this effect strongly influences the spectral fit parameters derived under the assumption of a reflection component consistent with the primary radiation. We find that a pivoting primary power-law spectrum with a nearly constant Comptonised luminosity may account for the reported correlation between the reflection amplitude R and the spectral index Gamma, and simultaneously produces an iron line EW that is nearly independent of Gamma. We emphasize the effects of the modelling of the primary component on the determination of the reflection amplitude, and show that in NGC 5548, when these effects are taken into account, the RXTE data are consistent with the reflection features being produced mainly at large distance from the central source.

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arXiv2002

X-ray spectra produced by a hot plasma containing cold clouds

We compute the hard X-ray spectra from a hot plasma pervaded by small cold dense clouds. The main cooling mechanism of the plasma is Compton cooling by the soft thermal emission from the clouds. We compute numerically the equilibrium temperature of the plasma together with the escaping spectrum. The spectrum depends mainly on the amount of cold clouds filling the hot phase. The clouds covering factor is constrained to be low in order to produce spectra similar to those observed in Seyfert galaxies and X-ray binaries, implying that an external reflector is required in order to reproduce the full range of observed reflection amplitudes. We also derive analytical estimates for the X-ray spectral slope and reflection amplitude using an escape probability formalism.

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arXiv2001

A quasi-spherical inner accretion flow in Seyfert galaxies ?

We study a phenomenological model for the continuum emission of Seyfert galaxies. In this quasi-spherical accretion scenario, the central X-ray source is constituted by a hot spherical plasma region surrounded by spherically distributed cold dense clouds. The cold material is radiatively coupled with the hot thermal plasma. Assuming energy balance, we compute the hard X-ray spectral slope Gamma and reflection amplitude R. This simple model enables to reproduce both the range of observed hard X-ray spectral slopes, and reflection amplitude R. It also predicts a correlation between R and Gamma which is very close to what is observed. Most of the observed spectral variations from source to source, would be due to differences in the cloud covering fraction. If some internal dissipation process is active in the cold clouds, darkening effects may provide a simple explanation for the observed distributions of reflection amplitudes, spectral slopes, and UV to X-ray flux ratios.

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arXiv2001

X-ray spectra of accretion discs with dynamic coronae

We compute the X-ray spectra produced by non-static coronae atop accretion discs around black holes and neutron stars. The hot corona is radiatively coupled to the underlying disc (the reflector) and generates an X-ray spectrum which is sensitive to the bulk velocity of the coronal plasma, beta=v/c. We show that an outflowing corona reproduces the hard-state spectrum of Cyg X-1 and similar objects. The dynamic model predicts a correlation between the observed amplitude of reflection R and the X-ray spectrum slope Gamma since both strongly depend on beta. A similar correlation was observed and its shape is well fitted by the dynamic model. The scattering of soft radiation in an outflowing corona can also account for the observed optical-UV polarization pattern in active galactic nuclei.

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arXiv2000

Temporal properties of flares in accretion disk coronae

Using a non-linear Monte-Carlo code we investigate the radiative response of an accretion disk corona system to static homogeneous flares. We model a flare by a rapid (comparable to the light crossing time) energy dissipation in the corona or the disk. If the flares originate from the disk, the coronal response to the soft photon shots produces a strongly non-linear Comptonised radiation output, with complex correlation/anti-correlations between energy bands. This behavior strongly differs from those found with usual linear calculations. Thus any model for the rapid aperiodic variability of X-ray binaries invoking a varying soft photon input as a source for the hard X-ray variability has to take into account the coronal temperature response. On the other hand, if the flare is due to a violent heating of the corona, when the perturbation time scale is of the order of a few corona light crossing times, the shot spectrum evolves from hard to soft. This general trend is independent of the shot profile and geometry. We show that for short dissipation time, the time averaged spectra are generally harder than in steady state situation. In addition, annihilation line and high energy tails can be produced without need for non-thermal processes.

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arXiv1999

IC emission above a disk

We developed a non-linear Monte Carlo code based on the LP method proposed by Stern et al (1995). We use it to simulate an extended non-thermal X/gamma ray source coupled with a cold accretion disk. The source radiates by inverse Compton process. We show that very low optical depths are required for the 2nd Compton order to be negligible. Such optical depths can be reached only if the source is very close to the disk (corona-like geometries). And even in this configuration, 2nd Compton order is negligible only for the hardest observed spectral slopes. We also account for Compton reflection on the disk. This kind of coupling leads to a strongly angle dependent spectrum with a reflection component much stronger than in standard isotropic models, leading to unobserved reflection dominated spectra at low inclination.

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