Peter Boorman and Chris Fragile

Date: 

Tuesday, October 16, 2018, 1:30pm to 2:30pm

Location: 

BHI Conference Room 20, 20 Garden Street, Cambridge (Lunch will be served at 1pm).

Peter Boorman, University of South Hampton UK

Title: Constructing a Census of Growing Supermassive Black Holes - First Results from the NuLANDS Survey

Abstract: Very little is known about how black holes at the centres of galaxies grow to become “super" massive. Sensitive X-ray observations have enabled studies of the closest regions to growing supermassive black holes (aka Active Galactic Nuclei - AGN) and have revealed the majority are obscured by gas. Unfortunately, though, the most obscured AGN are typically too faint to be detected in wide-field X-ray surveys. This has led to estimates of the fraction of heavily obscured AGN relative to the total population varying drastically between ~20-70%, yet a determination of this fraction is essential to understand the evolution and demographics of all supermassive black holes. The NuSTAR Local AGN N(H) Distribution Survey (NuLANDS) is one of the largest legacy surveys of AGN currently underway with the X-ray space telescopes NuSTAR, XMM-Newton and the Neil Gehrels Swift Observatory (> 4 million seconds of total exposure) aimed at resolving this issue. Via multiwavelength techniques, NuLANDS is targeting AGN representatively of the underlying population, and has thus already led to the discovery and classification of multiple heavily obscured AGN previously hidden at X-ray wavelengths. The initial results from the survey indicate that > 30% of all AGN in the local Universe are heavily obscured, and that high-energy X-ray selection alone preferentially selects less-obscured AGN. As such, NuLANDS marks a major step in completing the local census of supermassive black hole growth and will pave the way in quantifying how many AGN, on average, are heavily obscured in the local Universe.

 

Chris Fragile, College of Charleston

Title: Lense-Thirring Precession in Astrophysical Black Holes
Abstract: The gravitomagnetic torque from a rotating black hole is known to cause test particles on orbits not co-aligned with the equatorial plane to precess around the rotation axis of the black hole. In the weak field limit, this precession is known by the names of the scientists who first calculated its frequency — Josef Lense & Hans Thirring. While we do not have test particles that we can observe orbiting black holes in nature, the same gravitomagnetic torque can cause precession and other interesting phenomena in black hole accretion disks, which we can observe. Thus, Lense-Thirring precession may manifest itself in a number of ways, including (quasi-)periodic changes in the X-ray light curve, spectrum, and polarization, as well as a corkscrew pattern in the outflows (jets and winds) from the system.