Working Group Members:

• Prajval Shastri, IIA, Bangalore

• Ashok Pati, IIA, Bangalore

• Mousumi Das, RRI, Bangalore

• P. Shreekumar, ISAC, Bangalore

• Ranjeev Misra, IUCAA, Pune

• Kiran Baliyan, PRL, Ahmedabad

• U. C. Joshi, PRL, Ahmedabad

A number of research areas in active galactic nuclei science have been identified that will be studied with priority using the TAUVEX observations.

Active Galactic Nuclei science areas:

A small fraction of all galaxies were historically called active galaxies, because they were seen to produce very powerful emission from their nuclei that could not be explained by stellar processes alone. This powerful emission was found to occur at all observed wavelengths from radio to gamma-rays, with the peak of the emission often occurring in the rest-frame ultraviolet. Active Galactic Nuclei (AGNs) are grouped into several types, where the main classes are: quasars, quasi-stellar objects (QSOs), Seyfert galaxies, BL Lacertae objects (BL Lacs), and radio galaxies. They all are unified by the non-thermal emmision from their nucleus, manifesting itself in many ways, including a flat UV-to-optical continuum spectrum and variability at all wavelengths, from radio to the gamma-ray regime. The standard picture for the origin of this nuclear activity is that a supermassive black hole (of order 10⁸ solar masses), situated in the centre of the host galaxy, accretes gas from the host. In this process, gravitational binding energy is released, part of which is transformed into radiation.

AGN generate energy that emerges both as kinetic energy of a bipolar outflow as well as radiative energy of the accretion and the synchrotron-emitting plasma outflow, and create an axisymmetric system. The anisotropy of the system is further enhanced by the relativistic speeds of the plasma outflow and anisotropically distributed obscuring dust, perhaps in the form of a torus. Consequently, orientation plays a dominant role in the appearance of the galaxies as observed from earth. A corollary is that AGNs that might appear very different might actually be intrinsically similar (the Unification hypothesis).

Why the Ultraviolet with TAUVEX?

The AGN phenomenon manifests itself very strongly in the Ultraviolet band and TAUVEX can be used to determine their spatial, broadband spectral and temporal structure (i.e., variability on scales hours to months). Ultraviolet imaging of galaxies can be used to trace their star-formation history and thus are important for insights into the synergy between evolution of the central black hole and the galaxy.

What can be measured with TAUVEX?

• UV Continuum Properties

• UV Variability of well-known from previous surveys AGNs and its correlation with X-ray variability.

-This could be a unique feature of TAUVEX, as, for example, GALEX is not engaged in it.

• Obscuration

• Population properties:

--- The local population of AGNs
-- Redshift evolution of AGNs

Blazars and BL Lacs science areas:

Intimately related to the study of active galactic nuclei is the subject of blazars. Though being members of an AGN group, blazars themselves are not a homogeneous group and can be divided into two: highly variable quasars, sometimes called Optically Violently Variable (OVV) quasars (these are a small subset of all quasars) and BL Lacertae objects ("BL Lacs"). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both OVV quasars and BL Lac objecs. The name "blazar" was originally coined in 1978 by astronomer Ed Spiegel to denote the combination of these two classes.
Blazars are AGN with a relativistic jet that is pointing in the general direction of the Earth. We observe "down" the jet, or nearly so, and this accounts for the rapid variability and compact features of both types of blazars. Many blazars have apparent superluminal features within the first few parsecs of their jets, probably due to relativistic shock fronts.Some examples for blazars include 3C273, BL Lacertae, PKS 2155-304, Markarian 421, Markarian 501. The latter two are also called TeV Blazars for their high energy (Tera electron volt range) X-ray emission.

Why Observe Blazars?

BL Lacs are point sources in most small to medium sized telescope with wild and extreme variability. Observations are greately needed by the scientific community. Careful study of BL Lac and other blazars can help support or refute the widely accepted unified theory of AGN, which says that all AGN are basically the same - the differences are caused only in their orientation to our point of view. Many multi-wavelength BL Lac observing compaigns have been organized (such as the Whole Earth Blazar Telescope consisting of amateurs and professionals alike) and many more are certainly on the horizon.

• We can learn about jets and the processes that produce jets.

-For a blazar, the jet swamps out everything else.

• With enough data we can begin to understand jets and the central engine.

Blazars are among the most highly variables known and their variability is totally irregular. This variability may be accounted for the fact that we are looking straight ``down'' the jet.

• By observing blazar jets we get to see the most direct effects of the central supermassive black hole.

-When we observe blazars, we are seeing as close as humans may ever see to the central engine of an AGN, we are looking right at a naked supermassive black hole (?down the throat of the dragon?).

Why Observing with TAUVEX?

• Variability of BL Lacs is greately enhanced in the UV because the cool star component is supressed.

• TAUVEX unique total sky coverage

There is an incompleteness in the total blazars surveys---there are no sources in the south celestial hemisphere. At high northern latitudes few sources with magnitudes greater than 18 are found, but nothing higher than 80°. TAUVEX with its extremely long exposure time at extreme north and south declinations presents a great opportunity to find new sources and to study known ones with better exposure. Below is the list of high declination sources brighter than 16 mag that TAUVEX is very suitable to observe.

Possible Observing Programs

• Deep exposures of well-observed AGN fields, for example, HDF, CDFS, CDFN, HELLAS2XMM, Goods. Objectives here may be:

--- Observations in several UV colours at all (?) redshifts
--- UV Variability and its correlation with X-ray variability
--- Obscuration (Type I, Type II in the UV)
--- Correlation of UV observations with mid-IR from Spitzer

• TAUVEX study of shallow, large area AGN surveys, for example SDSS, 2df surveys. Objectives here may be:

--- Measuriing UV variability on various time scales (1 day - 3 years). Advantages for TAUVEX are that there is a large number of sources in these fields with full spectroscopic follow-up and that there is no UV and variability information currently avaiable on them.

Return to top