The interconnection between an accreting black hole and its environment

In active galactic nuclei (AGN) the radiation from the accretion disk is primarily emitted in the

far or extreme ultraviolet, in the energy range ~10-100 eV. This UV radiation is completely

absorbed by hydrogen in our own Galaxy and hence unobservable. However, this central

radiation ionizes the gas and dust present in the immediate vicinity of the black hole (0.01 – 100

parsecs). This reprocessing produces characteristic signatures in the spectra of AGN – the strong

broad and narrow emission lines observed in the optical and the ultraviolet (UV), and the

absorption lines common in the UV and X-rays. Understanding the reprocessing of the UV

continuum can (a) give crucial insight into the intimate surroundings of the black hole, and (b)

provide direct observable signatures of the accretion disk environment, from which the physics

of accretion onto super massive black holes may be understood.

For smaller, stellar mass black holes in binaries (BHBs), the radiation from the accretion disk is

directly observable in X-rays (~ 1 keV). As X-rays do not suffer from Galactic extinction, this

gives us direct observations to connect the central radiation to the properties of the gas. In case of

the BHBs, the signature of matter-light reprocessing is found as absorption lines in the X-ray

spectra, at > 1 keV. Hence only with the launch of the high resolution X-ray spectrometers on

XMM-Newton and Chandra, at the turn of the century, this has become an addressable topic.

To study various aspects of the interaction between the black hole and its nearby environment, I

have applied various photoionization tools. In my Talk I shall present the status of our

understanding of the light-gas interaction in astrophysical black holes and elaborate on how they

are being studied using the absorption and emission lines.