The Physics of QSOs

This type of variable sources differs in many ways from the ones discussed previously. First, QSOs aren’t stars but are associated with the centers of active galaxies. Second, they show no periodic behavior but stochastic. Third, their astronomical application is not distance estimation, but establishing an astrometric reference frame.

QSOs are, like their higher-level type AGN, composed of supermassive black holes (SMBH) in the order of 105 to 109 Mand surrounding accretions disks. The gas in the disk heats up during

accretion, resulting into the production of emission in the optical and ultraviolet range. Some QSOs also show radio or X-ray emission. They luminosities can be as large as 1047 ergs s−1 in tiny volumes (≈ 2 × 1014_{cm, Edelson et al. 1996).}

QSOs were discovered during the first radio surveys in the late 1950s. Due to their star-like appearance as point sources, but showing properties inconsistent with stars, they were named “quasi-stellar objects”. The exact cause for their enormous total luminosities – of up to 104 _times

the luminosity of a typical galaxy – within a small volume (as implied by Spitzer and Saslaw 1966) were unclear until the physics of accretion disks were understood and imaging and spectroscopic observations were available. Such observations can give evidence for the existence of massive compact objects at the centers of galaxies (e.g. Kormendy et al. 1996a,b; Magorrian et al. 1998; van der Marel et al. 1997). Observed line broadening (e.g. Peterson 1997) indicates the presence of gas moving in a relativistic potential wall.

Light Curve Properties

Unlike variable stars, whose variability is often periodic or a least dominated by periodic com- ponents, AGN (and thus QSOs) show mostly no periodic variability. In consequence, QSO light

curves are described as a stochastic process, e.g. a Gaussian process (Rybicki and Press 1992), whose parameters can be determined by using a structure function (Rybicki and Press 1992). QSOs vary in every waveband. Continuum variability in the optical was established even before the emission-line redshifts were understood.

Variability of QSOs occur on many different time scales. They range from weeks for changes on the thermal time scales in the accretion disk, over months for superpositions of stochastic processes up to several years for changes in the large-scale structure of accretion disks or lens crossing times.

Most, but not all, AGN continuum spectra have a spectacularly different appearance from normal galaxy spectra. Whereas in the UV, large fluctuations are common and occur on time scales from weeks to years, in the optical, the fluctuations are rather small.

A particularly well observed example, NGC 4151, is shown in the top panel of Fig. 2.16 in its UV, as well as one of NGC 5548 in the optical.

Figure 2.16(Top) A long-term UV (1455 ˚A) light curve for NGC 4151; (bottom) a shorter optical

(5100 ˚A) light curve for NGC 5548. In the UV, fluctuations of several are common and can occur on timescales ranging from weeks to years. In the optical band, the fluctuations tend to be rather smaller. Taken from Krolik (1999).

The Importance of QSOs

Originally, active galactic nuclei were considered to be a rare phenomenon. However, studies on the Palomar Survey (L. C. Ho and A. V. Filippenko and W. L. Sargent 1995) point a different picture. Out of the Palomar Survey sample, 86% of the galaxies show emission lines, among them, ∼40% of the galaxies show H ii emission (an indicator for star formation), and ∼50% belong to the active galaxies, in detail, ∼30% are low-ionization nuclear emission-line regions (LINER), ∼13% are Seyfert I and II, and ∼10% of all galaxies have a broad Hα component.

It is assumed that almost all galaxies undergo active phases during their evolution, so AGN evolution is assumed to be closely related to galaxy formation and evolution in the Universe. Their large luminosities make them to be traced even at high redshifts and thus large distances and early stages of the Universe. As AGN are very good tracers of distributions of both visible and dark matter (Ferrarese et al. 2001), they enable a view on the large-scale structures in the early Universe. The evolution of supermassive black holes (SMBH) residing in the centers of AGN can be probe the intergalactic medium (IGM). “Feedback” from AGN affects the host galaxies and IGM (Silk and Rees 1998). Feedback in a galaxy is any process that heats or disrupts gas, and hence decelerates star formation, as hot turbulent gas will collapse into stars much more slowly than cold and stationary gas.

Despite from probing the early Universe, QSOs have another important application. The most stable celestial reference frames used so far build on the positions of extragalactic sources such as QSOs. The current IAU standard frame defining the coordinates on the sky is the ICRF-2, built using radio interferometry (Fey et al. 2015). Its accuracy is 100 µas.

Such reference frames are used for astrometry, but also for geodesy and navigation. With the data from the Gaia mission, for the first time an additional µas reference frame, but based on observations in the optical wavelengths, will be available.

The Physics and Evolution of QSOs

Gravitational accretion onto compact objects provides very efficient conversion of potential and kinetic energy into radiation. Such processes give a reasonable explanation for the observed high luminosities and rapid variability of such sources.

When the accretion disk heats up, the ultraviolet and X-ray continuum emission is able to photo- ionize and excite the diffuse cold atomic gas clouds close to the black hole. This leads to the produc- tion of emission lines, which are then broadened due to the high velocities of the clouds,reaching up to 10,000 kms−1 (Peterson 1997).

Despite such accretion processes, there is no evidence up to now what actually gives rise to QSO variability. Large-scale changes in the amount of in-falling material (as discussed e.g. in Hopkins and Beacom 2006) as well as disk instabilities (e.g. Schmidt et al. 2012) are considered as the most probable causes for most of the observed variability.

Other effects being discussed to contribute to the variability are microlensing by the host galaxy (Hawkins 1996; Zackrisson and Bergvall 2003) and starbursts in the host galaxy (Aretxaga et al. 1997). Central SMBHs are widely believed to be found in the centers of most or all galaxies. Furthermore, it is believed that almost all, or even all galaxies undergo active phases for about 107 to several billion years (Hopkins et al. 2005).

Whereas there are rare cases known in which AGN vary by several percent over a few nights (Pollock et al. 1979), most AGN (and thus QSOs) show variation of just a few percent over weeks to years. The fact that some show variations on very sort timescales is an indicator for the existence of a very small region causing the variability, ranging from a few light-months to a few light-days in diameter.