Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 = the Einstein Cross. II. Energy profile of the accretion disk

TL;DR

This study uses microlensing and Bayesian analysis to determine the energy profile of a quasar's accretion disk, revealing a power-law relation with a well-constrained exponent, marking a significant advance in understanding quasar structure.

Contribution

It provides the first accurate, model-independent measurement of the energy profile of a quasar accretion disk on small scales using microlensing data.

Findings

The caustic crossing timescale is approximately 4 months.

The energy profile follows a power-law with an exponent of about 1.2.

This is the first small-scale, model-independent measurement of quasar accretion disk energy profile.

Abstract

We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237+0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0+/-1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law R lambda^{zeta} with zeta=1.2+/-0.3, where R is the source size responsible for the emission at wavelength lambda. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.