Spitzer observations of a gravitationally lensed quasar, QSO 2237+0305

TL;DR

This study uses Spitzer observations to analyze the infrared spectral energy distribution and microlensing effects in the gravitationally lensed quasar QSO 2237+0305, testing quasar models and the structure of the emission regions.

Contribution

First infrared spectral analysis of QSO 2237+0305 with Spitzer, confirming the dusty torus model and microlensing effects on flux ratios across wavelengths.

Findings

IR SED similar to other bright radio-quiet quasars

Dusty torus model fits IR SED shape but with wavelength offset

Microlensing affects near-IR flux ratios, especially at shorter wavelengths

Abstract

The four-image gravitationally lensed quasar QSO 2237+0305 is microlensed by stars in the lens galaxy. The amplitude of microlensing variability can be used to infer the relative size of the quasar as a function of wavelength; this provides a test of quasar models. Toward this end, we present Spitzer Space Telescope Infrared Spectrograph and Infrared Array Camera (IRAC) observations of QSO 2237+0305, finding the following. (1) The infrared (IR) spectral energy distribution (SED) is similar to that of other bright radio-quiet quasars, contrary to an earlier claim. (2) A dusty torus model with a small opening angle fits the overall shape of the IR SED well, but the quantitative agreement is poor due to an offset in wavelength of the silicate feature. (3) The flux ratios of the four lensed images can be derived from the IRAC data despite being unresolved. We find that the near-IR fluxes are increasingly affected by microlensing toward shorter wavelengths. (4) The wavelength dependence of the IRAC flux ratios is consistent with the standard quasar model in which an accretion disk and a dusty torus both contribute near 1 micron in the rest frame. This is also consistent with recent IR spectropolarimetry of nearby quasars.