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The lensed quasar Q2237+0305, better known as the Einstein Cross, has been observed with the X‑Shooter spectrograph for the first time over its full 3000–20000 Å range, giving the most complete spectrum of this system to date. By measuring the flux ratios between the quasar’s continuum and the cores of its broad emission lines, researchers established a baseline that isolates microlensing effects that change with wavelength.
Using a thin‑disk model with a Gaussian brightness profile (I(R)∝exp(−R²/2r_s²)) and a size‑wavelength relation r_s∝λᵖ, the team generated magnification maps for each of the four images (A–D) with the inverse‑polygon‑mapping algorithm. The maps employed the macromodel parameters (κ,γ) from Kochanek (2004): κ = 0.394, 0.375, 0.743, 0.635 and γ = 0.395, 0.390, 0.733, 0.623 for images A, B, C, and D respectively. Visual 6 shows these values.
| Q2237+0305 | A | B | C | D |
| κ | 0.394 | 0.375 | 0.743 | 0.635 |
| γ | 0.395 | 0.390 | 0.733 | 0.623 |
A joint likelihood analysis of three independent chromatic microlensing measurements yielded a logarithmic accretion‑disk size ln r_s = 1.59⁺⁰.⁸³₋⁰.⁹6 lt‑day and a power‑law index p = 0.77⁺⁰.⁴⁷₋⁰.²⁸. These numbers indicate a disk that is smaller and has a shallower temperature profile than the standard thin‑disk prediction of p = 4/3. Visual 8 displays the probability distribution that led to these values.
The study also found that extinction differences between image pairs are minimal, with no significant dust reddening seen in the spectrum of image C relative to A. However, the broad‑line cores of Ly α and C IV show only marginal evidence for microlensing in their wings, limited by low signal‑to‑noise and telluric contamination. Thus, while the continuum behaviour strongly supports a chromatic microlensing signal, firm conclusions about the broad‑line region remain tentative.
These results refine our picture of the inner workings of a quasar behind a lens galaxy, showing that the emitting region is smaller and displays a different temperature gradient than classical models. Future higher‑resolution observations will be needed to confirm the subtle microlensing of the line wings and to tighten constraints on the accretion‑disk structure.