Physical constraints for the active regions in Seyfert galaxies

Abstract

We discuss several physical constraints on the nature of the Active Regions (AR) in Seyfert 1 Galaxies, and show that a plausible model consistent with these constraints is one in which the ARs are magnetically confined and "fed". The unique X-ray index of these sources points to a large compactness parameter (l ≫ 1). This, together with the conditions required to account for the observed optical depth being close to unity, suggests that the magnetic energy density in the AR should be comparable to the equipartition value in the accretion disk, and that it should be released in a flare-like event above the surface of the cold accretion disk. We consider the various issues pertaining to magnetic flares and attempt to construct a coherent picture, including a reason for the optical depth in the AR being ∼ 1, and an understanding of the characteristics of the X-ray reflection component and the power density spectra associated with this high-energy emission.

Time Domain Studies of X‐Ray Shot Noise in Cygnus X‐1

“…The notion that shot parameters such as timescale and peak rate are distributed -i.e., that the shots are not all the same -is well supported. Models for generation of the shots include inhomogeneity in the orbiting matter (Bao & Ostgaard 1995), magnetic flares above the disk, with energy released either through reconnection, as in solar flares, or through Comptonization of soft radiation in the disk by the plasma trapped in the flare (Nayakshin & Melia 1997;Poutanen & Fabian 1999). Reconnection events in the disk might also give rise to short-term variability and hard emission.…”

Magnetic flares and the observed τ[sub T]∼1 in Seyfert Galaxies

“…This value does not depend on luminosity, but it does of course depend on the geometry and v a . Nayakshin & Melia (1997a) show that the Alfvén velocity is close to c for typical conditions in an accretion disk if its luminosity is a sizable fraction of the Eddington luminosity and the magnetic field strength in the AR is comparable to the disk thermal energy density. Suppose now that the geometry is not perfectly spherically symmetric, and that instead the Alfvén waves can enter the X-ray source through an area A a , but the radiation leaves through an area A r ∼ > A a , which is plausibly just the total area of the AR.…”

“…To understand the scale represented by the bracketed quantities in this equation, let us consider the physical conditions that are likely to be attained during a short-lived and very energetic magnetic flare above the standard α-disk. The magnetic field energy density is a fraction of the underlying disk energy density and the typical size ∆R a of the flare is expected to be of the order of the disk scale height (Galeev et al 1979;Haardt et al 1994;Nayakshin & Melia 1997a). Now, the confinement of the plasma inside the flare, and the observed condition l ≫ 1, require that B 2 /8π ≫ P r ≫ P g .…”

“…The local compactness parameter l is defined as l ≡ Lσ T /Rm e c 2 , where L is the source luminosity, R is the source size and σ T is the Thomson cross section. In principle, it is possible to obtain still larger values for the compactness parameter (l ∼ few hundred; see Nayakshin & Melia 1997a), thus creating enough pairs to account for the observed τ T ≃ 1.0 +0.4 −0.2 (Zdziarski et al 1996). This explanation for the observed value of τ T ∼ 1 based on the pair equilibrium condition, however, relies on the assumption that the particles are confined to a rigid box, so that no pressure constraints need to be imposed.…”

Big Blue Bump and transient active regions in Seyfert Galaxies

“…In these studies, the incident X-ray intensity is always assumed to be stationary in time. However, the most recent work on the physics of the high-energy sources suggests that a likely origin for the illuminating X-rays are magnetic flares above the surface of the cold accretion disk (e.g., Haardt et al 1994;Nayakshin & Melia 1997). The lifetime of these flares is probably much smaller than the disk's hydrostatic time scale, for which the assumption of timeindependent reflection is in that case not warranted.…”