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We used a set of moderately-deep and high-resolution optical observationsobtained with the Hubble Space Telescope to investigate the properties of thestellar population in the heavily obscured bulge globular cluster NGC 6256. Theanalysis of the color-magnitude diagram revealed a stellar population with anextended blue horizontal branch and severely affected by differentialreddening, which was corrected taking into account color excess variations upto \delta E(B-V) ~ 0.51. We implemented a Monte Carlo Markov Chain technique toperform the isochrone fitting of the observed color-magnitude diagram in orderto derive the stellar age, the cluster distance and the average color excess inthe cluster direction. Using different set of isochrones we found that NGC 6256is characterized by a very old stellar age around 13.0 Gyr, with a typicaluncertainty of ~ 0.5 Gyr. We also found an average color excess E(B-V) = 1.19and a distance from the Sun of 6.8 kpc. We then derived the clustergravitational center and measured its absolute proper motion using the Gaia-DR2catalog. All this was used to back-integrate the cluster orbit in a Galaxy-likepotential and measure its integrals of motion. It turned out that NGC 6256 iscurrently in a low-eccentricity orbit entirely confined within the bulge andits integrals of motion are fully compatible with a cluster purely belonging tothe Galaxy native globular cluster population. All these pieces of evidencesuggest that NGC 6256 is an extremely old relic of the past history of theGalaxy, formed during the very first stages of its assembly.

Digging for relics of the past: the ancient and obscured bulge globular cluster NGC6256