Abell 370 revisited: refurbished Hubble imaging of the first strong lensing cluster

We present a strong lensing analysis of the galaxy cluster Abell 370 (z= 0.375) based on the recent multicolour images by Advanced Camera for Surveys obtained as part of the Early Release Observation (ERO) that followed the Hubble Service Mission #4. Back in 1987, the giant gravitational arc (z= 0.725) in Abell 370 was one of the first pieces of evidence that massive clusters are dense enough to act as strong gravitational lenses. The new observations reveal in detail its disclike morphology, and we show that it can be interpreted as a complex five-image configuration, with a total magnification factor of 32 ± 4. Moreover, the high-resolution multicolour information allowed us to identify 10 multiply imaged background galaxies. We derive a mean Einstein radius of θE= 39 ± 2 arcsec for a source redshift at z= 2, corresponding to a mass of M(