Constraints on Jet‐driven Disk Accretion in Sagittarius A*

Abstract

We revisit theoretical and observational constraints on geometrically-thin disk accretion in Sagittarius A ⋆ (Sgr A ⋆ ). We show that the combined effects of mass outflows and electron energization in the hot part of the accretion flow can deflate the inflowing gas from a geometrically-thick structure. This allows the gas to cool and even thermalize on an inflow timescale. As a result, a compact, relatively cool disk may form at small radii. We show that magnetic coupling between the relativistic disk and a steady-state jet results in a disk that is less luminous than a standard relativistic disk accreting at the same rate. This relaxes the observational constraints on thin-disk accretion in Sgr A ⋆ (and by implication, other Low-Luminosity Active Galactic Nulcei, LLAGN). We find typical cold gas accretion rates of a few × 10 −9 M ⊙ yr −1 . We also find that the predicted modified disk emission is compatible with existing near-infrared (NIR) observations of Sgr A ⋆ in its quiescent state provided that the disk inclination angle is > ∼ 87 • and that the jet extracts more than 75% of the accretion power.

Accretion discs in blazars

“…In this section, we present an explicit model for the magnetic coupling between an accretion disc and jet, resulting in a modified disc spectrum (Jolley & Kuncic 2008a). The standard accretion disc theory (Shakura & Sunyaev 1973) predicts the following radiative flux from an accretion disc: where G is the gravitational constant, is the gravitational radius, is the dimensionless radius and x i is the dimensionless last marginally stable orbit.…”

“…The standard accretion disc theory (Shakura & Sunyaev 1973) predicts the following radiative flux from an accretion disc: where G is the gravitational constant, is the gravitational radius, is the dimensionless radius and x i is the dimensionless last marginally stable orbit. The disc flux can be generalized by including a relativistic correction term f NT (Novikov & Thorne 1973; Page & Thorne 1974) and a torque at the innermost radial boundary f r (Agol & Krolik 2000), as well as a torque across the disc surface f z (Kuncic & Bicknell 2004; see Jolley & Kuncic 2008a for further details). Thus, the generalized radiative disc flux for a black hole with mass M and accretion rate can be expressed as …”

“…The jet is divided into cylindrical sections, and the contribution from each component is integrated to give the total emission spectrum. The observed specific luminosity due to the net contribution from each jet component is (see also Freeland et al 2006; Jolley & Kuncic 2008a) where S syn ν is the synchrotron source function and is the synchrotron optical depth along a path length Δ s ≈ 2 r /cos θ j , where r ≈ z φ, through each cylindrical section.…”

“…Once the parameter ε j /ε a has been set, we use the above equation to normalize the electron number density N e,0 at the base of the jet. Further details of the model are described in Jolley & Kuncic (2008a).…”

“…Because blazars exhibit such powerful relativistic jets, the accretion disc emission should be strongly modified, making these sources ideal candidates for detecting jet‐modified disc accretion. The black hole mass M and mass accretion rates of these sources could be substantially higher than that inferred from the standard accretion disc theory if the jets are accretion‐powered (Jolley & Kuncic 2008a).…”