arXiv subjects

Zachary Steyn

Publications and source records attributed to Zachary Steyn.

2 recordsLinked to original sources

arXiv2026

Continuum Reverberation in Bright Quasars Using NASA/ATLAS

Variable continuum emission from AGN can be used to probe the structure of their accretion disks via reverberation mapping. Assuming a variable, hot inner light source irradiates the surrounding accretion disk, time delays between different continuum band light curves reveal light-travel times between their respective emission regions. Inter-band delays measured in several low-luminosity AGN are ubiquitously $\sim 3$ times longer than expected from standard disk theory, with evidence this size discrepancy may decrease in more luminous AGN. We have analysed high-cadence light curves of 9,498 of the brightest quasars between redshift 0.3-2.5 in the largest continuum reverberation study to date. Given the large sample size, we construct bins and fit delays jointly to combine inference across the parameter space and improve lag detections. We find that the size discrepancy persists in our high-luminosity sample, and that the previously seen anti-correlation with luminosity is likely driven by wavelength effects. The complex, non-monotonic wavelength dependence of delay amplitudes strongly suggests that contamination of inter-band delays by variable diffuse emission is widespread in the AGN population. We test delay behaviour against a variety of quasar properties finding longer lags in quasars with: higher Eddington ratios, redder colours, stronger optical FeII equivalent widths, higher iron ratios (both UV FeII/MgII and optical FeII/H$\beta$), CIV broad absorption troughs, and lower CIV blueshift.

astro-ph.GA↗

arXiv2023

Probing Quasar Viewing Angle with the Variability Structure Function

Given the anisotropic emission from quasar accretion discs, their viewing angle affects estimates of the quasar luminosity, black-hole mass and Eddington ratio. Discs appear overluminous when viewed pole-on and underluminous when viewed at high inclination. In radio-quiet quasars, the viewing angle is usually unknown, although spectroscopic indicators have been proposed. Here, we use a recently discovered universality in the variability structure function (SF) of quasar light curves (LCs), where all quasars show the same SF when clocks run in units of orbital timescale. As an offset from the mean relation can be caused by incorrect orbital timescales and thus incorrect luminosities, we correlate these offsets with suggested inclination indicators. We derive SFs from NASA/ATLAS LCs spanning $\sim 6$ years of observation, using a sample of 183 luminous quasars with measured H$β$ lines as well as 753 quasars with CIV and MgII lines. Starting from the proposed orientation indicators, we expect quasars with narrower H$β$ lines and with more blueshifted CIV lines to be viewed more pole-on and thus appear overluminous. In contrast, our SF analysis finds that presumed pole-on discs appear underluminous, consistently for both line indicators. We discuss possible explanations for the behaviour of quasars with highly blueshifted CIV lines irrespective of inclination angle, including dusty outflows that might render the accretion disc underluminous and flatter disc temperature profiles with longer orbital timescales than in thin-disc models but reach no satisfying conclusion.

astro-ph.GA↗