Extragalactic astronomy

The discovery of the first quasi-stellar object (QSO) in 1963 was a landmark in extragalactic astronomy. In a few years it became clear that there were a class of galaxies with very active nuclei whose electromagnetic emission could not be explained as the result of the contributions from the individual stars in them. Strongly relativistic processes in these galaxies lead to non-thermal emission from radio to gamma-rays.  

In the 1970s, the standard picture of Active Galactic Nuclei (AGNs) emerged: a central, supermassive and compact object (presumably a black hole) accretes matter from the host galaxy forming an accretion disk around it. This disk emits optical, UV and X-ray photons and anchors a strong magnetic field. Electromagnetic processes in the black hole magnetosphere lead to the formation and acceleration of jets of relativistic particles. These jets open their way through the galaxy reaching the intergalactic medium where they finally form hot spots of synchrotron radiation. 

By the end of the 1980s and the early 1990s, two discoveries led to question some aspects of the standard model of AGNs. First, intraday variability at radio wavelengths was found in several blazars. The explanation of this variability imposed severe constraints on the size of the emitting region in the blazar and required the existence of extreme bulk motion of the radiating plasma. On the other hand, after the launching of the Compton Gamma-Ray Observatory in 1991, it was found that at least some AGNs radiate most of their power at gamma-ray energies. 

These two discoveries triggered a lot of research on AGNs around the world and the members of our group were not alien to these problems. In particular Gustavo E. Romero carried out an extensive monitoring program searching for intraday radio variability in southern blazars during the early 90s. This search led to the first statistical approach to intraday variability in southern samples and to the discovery, in 1993, of extreme intraday variations. These variations were observed by first time in the blazar PKS 0537-441, implying, under the incoherent synchrotron assumption, apparent brightness temperatures of about 10²¹ K. Relativistic interpretations require unrealistic bulk motions with Doppler factors of about 1000 in order to get the brightness temperatures below the inverse Compton limit. The origin of this extraordinary behaviour, since then observed in other sources, remains a mystery.

In recent years Romero and co-workers extended the variability studies of southern blazars at other wavelengths. The CASLEO 2.15-m telescope was successfully used in a series of microvariability campaigns which yielded outstanding results, including the detection of extreme microvariability in the blazars AO 0235+164 and PKS 0537-441. At the same time that these observational studies were performed, theoretical research on the origin of the variability was conducted. Models for radio, optical and gamma-ray variability were developed, and the consequences of superluminal gravitational microlensing of blazars were explored. 

Extreme intranight variability observed in the BL Lac object AO 0235 + 164 with the 2.15-m CASLEO telescope. From G.E. Romero, S.A. Cellone, J.A. Combi, Astronomy &. Astrophysics Letters 360, L47-L50 (2000).

Currently, the group is engaged in both observational and theoretical projects related with AGN variability. These projects include international cooperation in multifrequency observational campaigns and satellite-borne observations. Special attention is given to microvariability studies of the optical polarization in blazars, because they may give important clues on the behaviour of the magnetic fields in the innermost regions of the active nuclei. A careful evaluation of methodology errors, both in photometric and polarimetric observations, allowed our group to establish observational and data-reduction procedures which have been adopted as standards in AGN microvariability studies.

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