One sightline, many systems: a FLASH discovery of HI towards scintillating quasar PKS 0405-385

TL;DR

This paper reports the discovery of an intervening HI 21cm absorption line towards quasar PKS 0405-385, combining radio and optical observations to study intervening gas and quasar variability.

Contribution

It presents the first detection of HI absorption towards this quasar and revises its size estimate, integrating new spectroscopy and scintillation theory insights.

Findings

Detected HI absorption at z=0.882 towards PKS 0405-385.

Identified multiple metal-line systems at different redshifts.

Revised the quasar's linear size to over 0.3 parsecs, with no new intraday variability evidence.

Abstract

We report the discovery of an intervening 21\,cm absorption line at z = 0.882 towards the z = 1.284 quasar PKS 0405-385, identified in the First Large Absorption Survey in HI (FLASH). This quasar once displayed the most rapid known intraday variability at radio frequencies, from which it earned the title of `the smallest radio quasar'. Although its size was revised upwards soon after based on updated scattering theory, PKS 0405-385 remains an important probe of Galactic plasma, and now also of intervening gas discovered through HI absorption. We present new long-slit spectroscopy spanning both PKS 0405-385 and the candidate host of the intervening HI gas. We identify MgII and FeII absorption lines in this spectrum consistent with the redshift of the intervening HI, as well as two additional, independent metal-line systems at z = 0.907 and z = 0.966, but we cannot accurately pinpoint the host(s) of this intervening gas in current data. We revisit the radio variability of PKS 0405-385 in light of advances in scintillation theory, as well as extended monitoring with the Australia Telescope Compact Array and the Australian SKA Pathfinder, and find a revised linear size >0.3 pc, but no new evidence of repeating intraday variability.