Description |
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.
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