Temperature fluctuations in quasar accretion discs from spectroscopic monitoring data

Abstract

Neustadt & Kochanek (2022, hereafter NK22) proposed a new method to reconstruct the temperature perturbation map (as functions of time and disc radius) of AGN accretion discs using multi-wavelength 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 timescales. 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 $\delta T/T_0\sim 10~{{\%}}$ 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 timescales.

AGN STORM 2. VI. Mapping Temperature Fluctuations in the Accretion Disk of Mrk 817

“…While the apparent motions are roughly linear on the maps, the radial scale is logarithmic, so the apparent velocity is increasing with radius, roughly as v ∝ R, implying that the timescale associated with the fluctuations does not strongly depend on radius. This is also seen in the δT maps in NK22 and Stone & Shen (2023). Both of the dashed lines, inward and outward, correspond to physical velocities at R R log 2 in = of v ∼ 1500 km s −1 , which is roughly 13% of the orbital velocity at this radius and 0.005c.…”

“…The recovered lag t lag and scale s lag parameters are almost identical to those of the inserted signal. the other AGNs analyzed in NK22 and Stone & Shen (2023). We find that the timescales associated with the temperature fluctuations do not strongly depend on radius-e.g., the inner radii probed by our model are not significantly more variable than the outer radii-and we estimate this timescale to be of order 100 days.…”

“…Indeed, these accretion fluctuations have been invoked in previous studies to explain the UV/ optical and X-ray variability on timescales longer than reverberation (e.g., Arévalo et al 2008Arévalo et al , 2009Breedt et al 2009). These viscosity-driven fluctuations are thought to move only inward along the disk, whereas the maps from NK22 and Stone & Shen (2023) show fluctuations moving both outward and inward. Another explanation is that these fluctuations are driven by opacity conditions, particularly the iron opacity bump, that can produce strong variations in temperature and luminosity at the disk radii producing the observed flux and on timescales of hundreds of days (Jiang et al 2019;Jiang & Blaes 2020).…”

“…Stone & Shen (2023) used the NK22 model on a sample of SDSS (Ahumada et al 2020) quasars that had been spectroscopically monitored for years as part of the SDSS Reverberation Mapping campaign (SDSS-RM; Shen et al 2015Shen et al , 2019. Despite the dramatically different data set (Stone & Shen 2023 used time series of spectra, while NK22 used multiband lightcurves), different cadences, and different AGN properties (the SDSS-RM quasars are much more massive, more luminous, and higher redshift than the NK22 sample), Stone & Shen (2023) found similar results; the majority of the temperature maps are dominated by coherent, slow-moving, radial temperature fluctuations. They also found little to no evidence for reverberation signals in their maps, although this is probably due to the slow observing cadence of the SDSS-RM data relative to the light travel time of the disk.…”