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G. Chartas (PSU), E.E. Falco (CFA), G. Garmire (PSU), V. Gupta (PSU), C. Jones (CFA), I.I. Shapiro (CFA)
We report the detection of the cluster lens of the gravitational lens (GL) system Q0957+561 from a deep Chandra ACIS observation. Extended X-ray emission is found to be centered NE of image B. Its spectrum can be modeled well with a thermal plasma model consistent with the emission originating from a cluster at a redshift of 0.36. Our best fit estimates of the cluster temperature of ~ 3~keV and mass distribution of the cluster are used to derive the convergence parameter \kappa, the ratio of the cluster surface mass density to the critical density required for lensing. The observed cluster center, mass distribution and convergence parameter \kappa provide additional constraints to lens models for this system. Our new results break a sheet-mass degeneracy in GL models of this system and provide better constraints on the Hubble constant.