Previous abstract Next abstract

Session 15 -- AGNs II: Models

Oral presentation, Monday, June 12, 1995, 2:00pm - 3:30pm

[15.03] The Synchrotron Self-absorbed Spectrum of a Spherically \\Accreting Nuclear Radio Source

I consider a scenario in which stellar winds within the dense stellar nucleus are intercepted by the dormant massive black hole (BH) sitting at the galaxian center, and quasi-sperical accretion of the wind onto the BH occurs resulting in acceleration of relativistic electrons to produce the spectrum $N(\gamma)\propto \gamma^{-p}$. Under assumption on equipartition between the local energy density of the magnetic field and that of relativistic electrons, the spectrum of synchrotron radio emission is derived to be $\alpha=(13+2p)/(22+5p)$. It ranges between $\alpha =0.51$ to 0.56 for the range of $p$=3 to 1. The radius of the source is predicted to vary with frequency as $r\propto \nu^{m}$, where $m=-4(4+p)/(22+5p)\approx -0.7$; the higher the frequency, the more inner parts of the source are seen. Since at radii $r \nu_0$. Inverse Compton emission offers additional tests of this model in the X-ray energy range. Two different states of accretion, a ``low" and a ``high", depending on the accretion rate $\dot M$, are discussed to predict the variability patterns.

This model is applicable to nuclear radio sources in the nuclei of nearby spiral galaxies such as M 81, M 104, etc., which have revealed the presence of a central compact radio source whose prototype in the Milky Way galaxy is Sgr A$^\star$. Similarly, in many early-type galaxies, parsec-scale radio cores have been found. The most important common feature of these sources is the inverted radio spectrum $S\propto \nu^\alpha$ with $\alpha =+0.3$ to $+0.5$, which is consistent with the above model. The observed radio spectra enable us to evaluate some important physical parameters of the accreting sources.

Monday program listing