MATs (Monday Afternoon Talks), 09/18/2023 — Speakers: Jack Neustadt & Jon Zink

Monday September 18, 2023 3:00 pm

MATs (Monday Afternoon Talks)

3:00pm – 3:30pm, Jack Neustadt

Looking beyond the lamppost: a new method of understanding AGN continuum variability

The variability of Active Galactic Nuclei (AGNs) has been studied for decades, with the UV/optical continuum observed to stochastically fluctuate at the 10% level over timescales of weeks to months. Fundamentally, this variability should be driven by temperature fluctuations in the accretion disk surrounding the central black hole. Where multiband lightcurves are available, the variability is similar in all bands, but the bluer wavelengths vary earlier than the redder wavelengths with delays typical of the light travel time across the disk. These observations have led to the commonly used “lamppost” model, where central luminosity fluctuations – typically linked with X-ray emission – irradiate the disk to drive the UV/optical variability. However, it seems unlikely that this is the only source of variability in the disk. We introduce a new approach to understanding disk variability where we invert the multiband UV/optical lightcurves of AGNs, including those of AGN STORM targets NGC 5548 and Mrk 817, into “maps” of the disk resolved in time and in radius under the assumption of axisymmetry. In addition to a lamppost “signal”, we see strong evidence for small amplitude, slow-moving temperature fluctuations in the disk that exist over timescales longer than those related to the lamppost. These could be indicative of viscosity fluctuations propagating through the disk or opacity-driven convection waves, both of which have been proposed in the literature and examined in simulations, though we are unsure of the exact physical mechanisms that are driving observed fluctuations. Regardless, these slow-moving temperature fluctuations could provide an answer to many open questions in AGN variability studies.

3:00pm – 3:30pm, Jon Zink

Exoplanets in the Galaxy: Insights from Kepler and K2

The quest to understand the vast expanse of our galaxy and the multitude of planetary systems it hosts has led to groundbreaking discoveries in the field of exoplanetary science. The Scaling K2 collaboration embarked on a journey that has produced the only homogeneous sample of exoplanets observed by the K2 mission, complemented by corresponding measures of sample completeness and reliability. This comprehensive catalog has not only expanded the inventory of known exoplanets, but also broadened our perspective on galactic latitudes, stellar ages, masses, and metallicities. I will provide an overview of the recent findings from this project, including the discovery that small planets are less common around stars with high galactic midplane oscillation amplitudes. The origin of this trend remains unclear, but it highlights the interplay between the galactic environment and planetary systems. We have also identified an interwoven connection between these sub-Saturns and sub-Neptunes, suggesting that the gaseous envelopes of these planets have undergone intriguing inflationary processes. Moreover, our exploration of the dataset led to the intriguing revelation that the prevalence of hot Jupiters is not strongly correlated with the mass of their host stars. This insight challenges conventional wisdom and hints at a deeper and more complex interplay between the formation of giant and terrestrial planets. Our findings challenge existing paradigms and propel us closer to unraveling the complex tapestry of planetary systems across the galaxy.


Hosts: Minghao Yue, Daniele Michilli


  • Jack Neustadt, Ohio State University
  • Jon Zink, CalTech