In the past years the experimental detection of the Hawking radiation in Bose-Einstein condensates (BECs) has taken a remarkable leap forward (J. Steinhauer, Nature Physics 12, 2016 and J.R.M. De Nova et al., Nature 569, 2019). Exploiting the particular features of hard core bosons in 1D, i.e. the Tonks-Girardeau gas (S. Giovanazzi, Phys. Rev. Lett. 94, 2005), we are able to obtain the exact solution of a BEC flowing against an obstacle and we examine it in the framework of sonic black holes; in this limit we recover Hawking result without making use of the gravitational analogy and we find that a precise correspondence between the emission of phonons in the upstream region and the Hawking-like mechanism requires additional conditions to be met (A. Parola, M. Tettamanti and S. L. Cacciatori EPL 119, 2017). Moreover, we investigate the correlations between the Hawking quanta and the in-falling partner and we describe the model also from a semiclassical perspective (M. Tettamanti, S. L. Cacciatori and A. Parola, Phys. Rev. D 99, 2019).