Connecting X-ray absorption and 21cm neutral hydrogen absorption in
  obscured radio AGN
  Many radio galaxies show the presence of dense and dusty gas near the active
nucleus. This can be traced by both 21cm HI absorption and soft X-ray
absorption, offering new insight into the physical nature of the circumnuclear
medium of these distant galaxies. To better understand this relationship, we
investigate soft X-ray absorption as an indicator for the detection of
associated HI absorption, as part of preparation for the First Large Absorption
Survey in HI (FLASH) to be undertaken with the Australian Square Kilometre
Array Pathfinder (ASKAP). We present the results of our pilot study using the
Boolardy Engineering Test Array, a precursor to ASKAP, to search for new
absorption detections in radio sources brighter than 1 Jy that also feature
soft X-ray absorption. Based on this pilot survey, we detected HI absorption
towards the radio source PKS 1657-298 at a redshift of z = 0.42. This source
also features the highest X-ray absorption ratio of our pilot sample by a
factor of 3, which is consistent with our general findings that X-ray
absorption predicates the presence of dense neutral gas. By comparing the X-ray
properties of AGN with and without detection of HI absorption at radio
wavelengths, we find that X-ray hardness ratio and HI absorption optical depth
are correlated at a statistical significance of 4.71{\sigma}. We conclude by
considering the impact of these findings on future radio and X-ray absorption
studies.
