X-ray obscurers: bridging the gap between warm absorbers and ultra-fast outflows

Outflows from active galactic nuclei are found over a broad range of distances from the supermassive black hole and with a large range of velocities. The so-called "warm absorbers" cover a broad range of ionization parameters but at modest velocities of a few hundred to thousand km/s and relatively modest column densities of typically less than 1% of the Thomson depth. On the other extreme, ultra-fast outflows with speeds of order 0.1c have column densities up to a Thomson depth but are almost fully ionized.In this contribution, we discuss a newly discovered component, the X-ray obscurers which occupy the velocity gap and likely also the distance gap between warm absorbers and ultra-fast outflows. These obscurers have been studied in detail using deep joint monitoring observations with XMM-Newton, HST, Swift and NuSTAR in for example NGC 5548 and NGC 3783. The gas is generally lowly ionized, has high column densities up to 10% of the Thomson depth and is outflowing at speeds of several thousands of km/s. They block a significant fraction of the ionizing radiation from the central regions, are likely due to a strong accretion disk wind, and occur more frequently than previously anticipated. We discuss the impact of these obscurers on their environment, their frequency and the observational challenges to study them.