The radial distribution of blue stragglers in Galactic globular cluster NGC 6656-clues to the dynamical status

We present the dynamical status of the Galactic globular cluster NGC 6656 using the spatial distribution of blue straggler stars (BSSs). A combination of multi-wavelength high-resolution space- and ground-based data are used to cover a large cluster region. We determine the centre of gravity (C-grav) and construct the projected density profile of the cluster using the probable cluster members selected from the Hubble Space Telescope and Gaia Data Release 2 proper-motion data sets. The projected density profile in the investigated region is nicely reproduced by a single-mass King model, with core (r(c)) and tidal (r(t)) radii of 75 ''.2 +/- 3 ''.1 and 35 ''.6 +/- 1 ''.1 respectively. In total, 90 BSSs are identified on the basis of proper-motion data in the region of radius 625 ''. The average mass of the BSSs is determined as 1.06 +/- 0.09 M-circle dot and with an age range of 0.5 to 7 Gyr. The BSS radial distribution shows a bimodal trend, with a peak in the centre, a minimum at r similar to r(c) and a rising tendency in the outer region. The BSS radial distribution shows a flat behaviour in the outermost region of the cluster. We also estimate the A(rh)(+) parameter as an alternative indicator of the dynamical status of the cluster; this is found to be 0.038 +/- 0.016. Based on the radial distribution and A(rh)(+) parameter, we conclude that NGC 6656 is an intermediate dynamical age cluster.