There are four images of this quasar, three of them can be fitted with dust
extinction in the intervening galaxy, all with unrealistically low values of
R_V_ and an E(B-V) between 0.5 and 1.2, depending upon which image is studied
and the parameterisation that is used. The extremely low preferred values of
R_V_ for image A and C, are difficult to justify physically due to the strange
appearance of the extinction laws for low R_V_ (e.g. see the CCM
parameterisation with R_V_=0.5 in Fig. 1). Image D has a larger R_V_ value, with
a higher chi-square probability. Image B, which has one deviating band, could
not be fitted by dust extinction.
The extinction properties of this system have been measured by Falco et al.
(1999), using the differential method. They obtained a high value of R_V_ of
5.29 (4.47-6.11), which is inconsistent with our result. Eliasdottir et al.
(2006) used the same technique and derived values of R_V_ between 2.9 and 3.1,
with errors of the order of 1.5. In our analysis, values below 1.3, were
We note that the colours of the object have been shown to vary with time
(Moreau et al. 2005) and therefore there must be an effect, in addition to or
instead of dust extinction, affecting the colours, for example microlensing.
This would explain why we derive best-fit values for R_V_ that are
unrealistically low, and why completely different values have been obtained
using other data sets.
Q2237+0305.-We use HST astrometry from CASTLES. For the broadband optical
fluxes, we use the microlensing light curves from Wozniak et al. (2000). We
correct for differential extinction using the reddening deduced by Falco et al.
(1999). For the radio fluxes, we use the data from Falco et al. (1996). In
addition, Agol et al. (2000) report mid-infrared flux ratios measured at 8.9
and 11.7 microns.