30 Years of Optical Monitoring Reveals Detailed Structure of Gravitationally Lensed Quasar
Astronomers have compiled roughly 9,000 days of optical light curves for the quadruply-imaged gravitationally lensed quasar QSO 2237+0305, known as the Einstein Cross, using a new photometric technique that minimizes contamination from the foreground lensing galaxy. The study combines observations from multiple observatories across both hemispheres in the V, R, and I bands, capturing multi-band microlensing variability in unprecedented detail. The findings provide direct observational evidence that the quasar's emission region grows with wavelength, placing tight empirical constraints on models of quasar accretion disk structure.
A new study accepted by Astronomy & Astrophysics presents the most extensive optical monitoring campaign to date of QSO 2237+0305, the Einstein Cross, spanning approximately 9,000 days across the VRI optical bands. The research team developed a photometric technique that determines sky background levels from regions well away from the lens system and combines analytical and numerical modeling of the lensing galaxy's structure to minimize contamination. The resulting light curves for all four quasar images capture microlensing variability across nearly the full optical spectrum with a level of detail not previously achieved. A preliminary microlensing analysis, assuming a mean microlens mass of 0.3 solar masses and concentric Gaussian source profiles, finds that the half-light radius of the g-band emission region is 9.6 ± 2.7 light-days. Crucially, the source size scales with wavelength following a power-law index of α = 0.94 ± 0.05, consistent with an almost linear relationship, providing direct evidence for the wavelength-stratified structure of the quasar's optical emission region. The authors conclude that these long-baseline light curves set stringent empirical constraints on both quasar emission models and the physics of gravitational microlensing.
What's missing
The analysis is described as 'preliminary' regarding microlensing constraints, and the results depend on assumed priors for microlens mass (0.3 solar masses) and source velocity distributions taken from a prior study; different assumptions could shift the inferred source sizes and power-law index. The paper does not yet present a full Bayesian microlensing inversion, and the authors acknowledge that the velocity distribution peaks rather than full distributions were used, which may introduce systematic uncertainties.
What different sources said
- arXiv astro-phCenter
Joining forces: 30 years of optical monitoring of the Einstein Cross
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