Probing the Jet Turnover Frequency Dependence on Black Hole Mass and Mass Accretion Rate
Hammerstein, Erica; ; King, Ashley
The Astrophysical Journal, Volume 875, Issue 2, article id. 82, 8 pp. (2019).
We have examined a sample of 13 sub-Eddington supermassive black holes hosted by galaxies spanning a variety of morphological classifications to further understand the empirical fundamental plane of black hole activity. This plane describes black holes from stellar-mass to supermassive and relates the mass of an accreting black hole and its radio and X-ray luminosities. A key factor in studying the fundamental plane is the turnover frequency, i.e., the frequency at which the radio continuum emission becomes optically thin. We measured this turnover frequency using new Very Large Array observations combined, when necessary, with archival Chandra observations. Radio observations are in the range of 5-40 GHz across four frequency bands in B configuration, giving high spatial resolution to focus on the core emission. We use Markov chain Monte Carlo methods to fit the continuum emission in order to find the turnover frequency. After testing for correlations, the turnover frequency does not display a significant dependence on either the mass or mass accretion rate, indicating that more complicated physics than simple scaling and optical depth effects are at play, as has been suggested by recent theoretical work.
A paper written by former UM undergraduate Erica Hammerstein (now a PhD student at the University of Maryland, College Park). AGN that are very sub-Eddington should be jet dominated. Where the jet is optically thick, the jet’s spectrum should look flat in F_nu vs nu space; and have a negative slope at higher frequencies where the jet is optically thin. The frequency at which this transition happens is called the turnover frequency. If jets are simple, we would expect the turnover frequency to show strong correlations with black hole mass and mass accretion rate. We used VLA to observe two samples of low-luminosity AGN that either had a very similar black hole mass or a very similar mass accretion rate. We measured the turnover frequency (or put limits on it) for all of the sources. Then we tested how the turnover frequency scales with black hole mass and mass accretion rate, and found that there was no strong correlation, indicating that jets are not so simple.