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Register a new userQuasar Microlensing Variability Studies Favor Shallow Accretion Disk Temperature Profiles
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Added by
Matthew Cornachione
Publication date
2020
fields
Physics
and research's language is
English
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We compare the microlensing-based continuum emission region size measurements in a sample of 15 gravitationally lensed quasars with estimates of luminosity-based thin disk sizes to constrain the temperature profile of the quasar continuum accretion region. If we adopt the standard thin disk model, we find a significant discrepancy between sizes estimated using the luminosity and those measured by microlensing of $log(r_{L}/r_{mu})=-0.57pm0.08,text{dex}$. If quasar continuum sources are simple, optically thick accretion disks with a generalized temperature profile $T(r) propto r^{-beta}$, the discrepancy between the microlensing measurements and the luminosity-based size estimates can be resolved by a temperature profile slope $0.37 < beta < 0.56$ at $1,sigma$ confidence. This is shallower than the standard thin disk model ($beta=0.75$) at $3,sigma$ significance. We consider alternate accretion disk models that could produce such a temperature profile and reproduce the empirical continuum size scaling with black hole mass, including disk winds or disks with non-blackbody atmospheres.
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We use thirteen seasons of R-band photometry from the 1.2m Leonard Euler Swiss Telescope at La Silla to examine microlensing variability in the quadruply-imaged lensed quasar WFI 2026-4536. The lightcurves exhibit ${sim},0.2,text{mag}$ of uncorrelated variability across all epochs and a prominent single feature of ${sim},0.1,text{mag}$ within a single season. We analyze this variability to constrain the size of the quasars accretion disk. Adopting a nominal inclination of 60$^text{o}$, we find an accretion disk scale radius of $log(r_s/text{cm}) = 15.74^{+0.34}_{-0.29}$ at a rest-frame wavelength of $2043,unicode{xC5}$, and we estimate a black hole mass of $log(M_{text{BH}}/M_{odot}) = 9.18^{+0.39}_{-0.34}$, based on the CIV line in VLT spectra. This size measurement is fully consistent with the Quasar Accretion Disk Size - Black Hole Mass relation, providing another system in which the accretion disk is larger than predicted by thin disk theory.
Microlensing perturbations to the flux ratios of gravitationally lensed quasar images can vary with wavelength because of the chromatic dependence of the accretion disks apparent size. Multiwavelength observations of microlensed quasars can thus constrain the temperature profiles of their accretion disks, a fundamental test of an important astrophysical process which is not currently possible using any other method. We present single-epoch broadband flux ratios for 12 quadruply lensed quasars in eight bands ranging from 0.36 to 2.2 microns, as well as Chandra 0.5--8 keV flux ratios for five of them. We combine the optical/IR and X-ray ratios, together with X-ray ratios from the literature, using a Bayesian approach to constrain the half-light radii of the quasars in each filter. Comparing the overall disk sizes and wavelength slopes to those predicted by the standard thin accretion disk model, we find that on average the disks are larger than predicted by nearly an order of magnitude, with sizes that grow with wavelength with an average slope of ~0.2 rather than the slope of 4/3 predicted by the standard thin disk theory. Though the error bars on the slope are large for individual quasars, the large sample size lends weight to the overall result. Our results present severe difficulties for a standard thin accretion disk as the main source of UV/optical radiation from quasars.
The rest-frame UV/optical variability of the quasars in the Sloan Digital Sky Survey (SDSS) Stripe 82 is used to test the Corona-Heated Accretion-disk Reprocessing (CHAR) model of Sun et al. 2020. We adopt our CHAR model and the observed black-hole masses ($M_{mathrm{BH}}$) and luminosities ($L$) to generate mock light curves that share the same measurement noise and sampling as the real observations. Without any fine-tuning, our CHAR model can satisfactorily reproduce the observed ensemble structure functions for different $M_{mathrm{BH}}$, $L$, and rest-frame wavelengths. Our analyses reveal that a luminosity-dependent bolometric correction is disfavored over the constant bolometric correction for UV/optical luminosities. Our work demonstrates the possibility of extracting quasar properties (e.g., the bolometric correction or the dimensionless viscosity parameter) by comparing the physical CHAR model with quasar light curves.
Microlensing observations indicate that quasar accretion discs have half-light radii larger than expected from standard theoretical predictions based on quasar fluxes or black hole masses. Blackburne and colleagues have also found a very weak wavelength dependence of these half-light radii. We consider disc temperature profile models that might match these observations. Nixon and colleagues have suggested that misaligned accretion discs around spinning black holes will be disrupted at radii small enough for the Lense-Thirring torque to overcome the discs viscous torque. Gas in precessing annuli torn off a disc will spread radially and intersect with the remaining disc, heating the disc at potentially large radii. However, if the intersection occurs at an angle of more than a degree or so, highly supersonic collisions will shock-heat the gas to a Compton temperature of T~10^7 K, and the spectral energy distributions (SEDs) of discs with such shock-heated regions are poor fits to observations of quasar SEDs. Torn discs where heating occurs in intermittent weak shocks that occur whenever the intersection angle reaches a tenth of a degree pose less of a conflict with observations, but do not have significantly larger half-light radii than standard discs. We also study two phenomenological disc temperature profile models. We find that discs with a temperature spike at relatively large radii and lowered temperatures at radii inside the spike yield improved and acceptable fits to microlensing sizes in most cases. Such temperature profiles could in principle occur in sub-Keplerian discs partially supported by magnetic pressure. However, such discs overpredict the fluxes from quasars studied with microlensing except in the limit of negligible continuum emission from radii inside the temperature spike.
Gravitational microlensing by the stellar population of lensing galaxies provides an important opportunity to spatially resolve the accretion disk structure in strongly lensed quasars. Some of the objects (like Einsteins cross) are reasonably consistent with the predictions of the standard accretion disk model. In other cases, the size of the emitting region is larger than predicted by the standard thin disk theory and practically independent on wavelength. This may be interpreted as an observational manifestation of an optically-thick scattering envelope possibly related to super-Eddington accretion with outflows.
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