The astrophysics of quasars and BL Lac objects



Our research team is responsible for the preliminary work before launch and for the exploitation of the satellite data.

Planck satellite will scan the whole sky twice at high radio frequencies. Several types of active galaxies will be found in these maps, giving us lots of new information at many new radio frequencies. It is possible that we will find new types of active galaxies, too!

Planck will help us study, among others, the following topics:

Physical models of active galactic nuclei

Planck data will help studying which models best describe active galactic nuclei (AGNs). For example, the appearance of an AGN depends on the angle that we look at it. Are we looking along its powerful radio jet, or towards the centre of the galaxy? Sometimes there is a dusty torus between us and the centre so we can't see it. All this is why, when we are looking at an active galaxy from different angles, we see a variety of properties and events which lead to the division of active galaxies into several classes. There may be a way of unifying all the different classes of AGNs -by studying their viewing angles, brightness, structure, and spectra. We can also use the information we get of the bright knots -or shocks- moving along the powerful radio jets for developing and testing shock models.
Figure: C.M. Urry & P. Padovani
Model of a quasar, with a central black hole,
                    accretion disk, and radio jets

BL Lac objects

BL Lac objects (BLOs) are active galaxies with no emission lines in their spectra (or the lines are very weak). There are different types of BLOs, depending on the frequency they are brightest at (radio or X-ray domain). Currently we are not sure whether this is due to different viewing angles or if the sources are intrinsically different. A class of BLOs intermediate to the two main classes has been recently discovered. With the extensive data from the Planck satellite we can study all these classes of BL Lac objects, and try to find out how and why they are different.

Fainter flat-spectrum AGNs

So far all full-sky surveys have been performed at low frequencies -at 8.4 GHz or lower. This is because it is usually presumed, by looking at their spectra which are flat at lower frequencies but get much steeper at higher frequencies, that quasars are very faint at higher frequencies. Our group has recently discovered that some sources are unexpectedly bright at high radio frequencies. This is possibly due to a different (flatter) shape of their spectra or to the variability of their brightness over time. The Planck satellite survey will provide us with a catalog of fainter sources like this, and we can study their spectra, as well as the potential differences between the bright and the faint flat-spectrum radio sources.


Flux density curve of quasar 3C 454.3, N.B. variability of
            brightness over time Planck will produce observations at a variety of timescales, from a few hours (position of the receiver horns, changes in the satellite's orbit) to six months (whole sky scanned twice in one year). These variability data can be used in several ways. With the longer timescale data we can study the structure of the radio jets of quasars, the variability of individual shocks in the jets, and possibly the variability of the core.
Click figure to see a larger version

With the shorter timescale data, especially the data taken at an interval of a few hours, we can study intraday variability (IDV). It is usually believed to be generated by interstellar scintillation. However, the high frequency observations provided by Planck can help to distinguish whether IDV is indeed produced in this manner, or if it is actually generated within the sources themselves (which would require rewriting the whole synchrotron model of AGNs!).

Follow-up satellite and ground based observations

Several other satellites, at other frequencies, will be operational about the same time as Planck. This will enable us to observe the interesting sources found by Planck at several frequencies. This is very important, since all these observations tell us about the sources different, but equally important facts! The satellites include, for example, GLAST, Integral, and AGILE at gamma-ray frequencies, XMM and Chandra at the X-ray domain, and IRIS/ASTRO-F and Herschel at infrared frequencies. GLAST satellite

Follow-up observations at radio and optical frequencies are of course possible from the ground. Our group in Metsähovi and Tuorla will use our own facilities for this purpose. Very high-energy gamma-rays can be observed from the ground as well (VERITAS, HESS, and Magic -instruments).

Quick Detection System

The brightest active galaxies have been monitored in Metsähovi already for many years, so we know quite well how they behave. However, Planck will observe a wealth of much less known objects. We are also interested in studying unexpectedly bright or otherwise peculiar sources. The best way to do this is to observe them at a lot of different frequencies and techniques as soon after the original Planck measurement as possible. For this purpose we are developing, in cooperation with the Planck LFI Data Processing Centre (DPC), a Quick Detection System software package. It will alert scientists quickly, in a week or two from the initial Planck detection, to unexpectedly bright sources. A dedicated Work Group (WG 6.1) was set up for the QDS development, coordinated by our Metsähovi and Tuorla team researchers. Juha Aatrokoski from Metsähovi is responsible for the design and coding of the system.

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