Temperature fluctuations in quasar accretion discs from spectroscopic monitoring data

Abstract

ABSTRACT NK22 proposed a new method to reconstruct the temperature perturbation map (as functions of time and disc radius) of active galactic nuclei (AGN) accretion discs using multiwavelength photometric light curves. We apply their technique to 100 quasars at z = 0.5–2 from the Sloan Digital Sky Survey Reverberation Mapping project, using multi-epoch spectroscopy that covers rest-frame UV-optical continuum emission from the quasar and probes days to months time-scales. Consistent with NK22 for low-redshift AGNs, we find that the dominant pattern of disc temperature perturbations is either slow inward/outward moving waves with typical amplitudes $\delta T/T_0\sim 10~{{\ \rm per \, cent}}$ traveling at ∼0.01–0.1c, with a typical radial frequency of ∼ 0.5 dex in log R, or incoherent perturbations. In nearly none of the cases do we find clear evidence for coherent, fast outgoing temperature perturbations at the speed of light, reminiscent of the lamppost model; but such lamppost signals may be present in some quasars for limited periods of the monitoring data. Using simulated data, we demonstrate that high-fidelity temperature perturbation maps can be recovered with high-quality monitoring spectroscopy, with limited impact from seasonal gaps in the data. On the other hand, reasonable temperature perturbation maps can be reconstructed with high-cadence photometric light curves from the Vera C Rubin Observatory Legacy Survey of Space and Time. Our findings, together with NK22, suggest that internal disc processes are the main driver for temperature fluctuations in AGN accretion discs over days to months time-scales.