Reading the Olympics: the discobolus

The lecture looked at the changing meaning of the figure of the discobolus from the first Olympic games to today

AGN Flickering and Chaotic Accretion

Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic timescale $\sim 10^5$ yr. We show that this is the timescale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion disc feeding event.Comment: 3 pages. Accepted for publication in MNRAS Letter

Do jets precess... or even move at all?

Observations of accreting black holes often provoke suggestions that their jets precess. The precession is usually supposed to result from a combination of the Lense-Thirring effect and accretion disc viscosity. We show that this is unlikely for any type of black hole system, as the disc generally has too little angular momentum compared with a spinning hole to cause any significant movement of the jet direction across the sky on short timescales. Uncorrelated accretion events, as in the chaotic accretion picture of active galactic nuclei, change AGN jet directions only on timescales \gtrsim 10^7 yr. In this picture AGN jet directions are stable on shorter timescales, but uncorrelated with any structure of the host galaxy, as observed. We argue that observations of black-hole jets precessing on timescales short compared to the accretion time would be a strong indication that the accretion disc, and not the standard Blandford-Znajek mechanism, is responsible for driving the jet. This would be particularly convincing in a tidal disruption event. We suggest that additional disc physics is needed to explain any jet precession on timescales short compared with the accretion time. Possibilities include the radiation warping instability, or disc tearing.Comment: 4 pages. Accepted for publication in ApJ Letter

Abstracting Builtins for Groundness Analysis

This note clarifies how to handle solution gathering meta-calls, asserts and retracts in the groundness analysis of Prolog

HLX-1 may be an SS433 system

We show that the hyperluminous source HLX--1 may be a stellar--mass binary system like SS433, but seen along its X--ray beams. The precession of these beams gives the $\sim 1$~yr characteristic timescale of the light curve, while the significant X--ray duty cycle means that the precession angle must be comparable with the beam opening angle, which is of order $1.6^{\circ}$. The X--ray light curve appears to result from geometric collimation and scattering as the beam moves through the line of sight. Encouragingly, the distance $\sim 95$~Mpc suggested for HLX--1 is only a few times larger than the minimum distance at which we can expect to view such a highly--beamed system along its axis. This picture allows a simple interpretation of HLX--1 as the most extreme known member of the ULX population.Comment: accepted for publication in MNRA