NAM logo, by David Le Conte

B0218+357: time delays and new MERLIN/VLA 5 GHz maps of the Einstein ring

Andy Biggs (Jodrell)

The ``golden lens'' B0218+357 is now fully realising its potential as a system for determining the Hubble constant. An accurate time delay has been measured (10.5 +/- 0.4 days at 95% confidence) from VLA monitoring of total flux density and polarisation at two frequencies, 8.4 and 15 GHz. Recent analysis of 5 GHz MERLIN monitoring data show them to be consistent with the VLA value, but do not allow us to improve the time delay determination.

Modelling of this system is still at an early stage, but an initial Singular Isothermal Ellipsoid model based on the VLBI substructure of the two compact images and the flux density ratio found from the VLA monitoring give an H₀ of 69 +13 -19 km/s/Mpc (95% confidence).

To date, no models have exploited the Einstein ring for constraints on the lensing due to a lack of resolution and sensitivity in existing radio maps. A new combined MERLIN/VLA 5 GHz image of the system shows for the first time substructure in the Einstein ring. With this we hope to substantially improve the value of H₀ determined from this system.

Maintained by Ian Howarth