Prospects of a statistical detection of the 21-cm forest and its potential to constrain the thermal state of the neutral IGM during reionization

The 21-cm forest signal is a promising probe of the Epoch of Reionization complementary to other 21-cm line observables and Lyα forest signal. Prospects of detecting it have significantly improved in the last decade thanks to the discovery of more than 30 radio-loud quasars at these redshifts, upgrades to telescope facilities, and the notion that neutral hydrogen islands persist down to z ≲ 5.5. We forward-model the 21-cm forest signal using semi-numerical simulations and incorporate various instrumental features to explore the potential of detecting the 21-cm forest at z = 6, both directly and statistically, with the currently available (uGMRT) and forthcoming (SKA1-low) observatories. We show that it is possible to detect the 1D power spectrum of the 21-cm forest spectrum, especially at large scales of $k\lesssim 8.5\, \rm MHz^{-1}$ with the $500\, \rm hr$ of the uGMRT time and $k\lesssim 32.4\, \rm MHz^{-1}$ with the SKA1-low over $50\, \rm hr$ if the intergalactic medium (IGM) is 25% neutral and these neutral hydrogen regions have a spin temperature of $\lesssim 30\, \rm K$. On the other hand, we infer that a null-detection of the signal with such observations of 10 radio-loud sources at z ≈ 6 can be translated into constraints on the thermal and ionization state of the IGM which are tighter than the currently available measurements. Moreover, a null-detection of the 1D 21-cm forest power spectrum with only $50\, \rm hr$ of the uGMRT observations of 10 radio-loud sources can already be competitive with the Lyα forest and 21-cm tomographic observations in disfavouring models of significantly neutral and cold IGM at z = 6.