I end this section on AGN unification by discussing work suggesting that AGN may in fact be scaled-up versions of X-ray binaries. X-ray binaries are a pair of orbiting stars where one star is an accreting black hole or neutron star (Remillard & McClintock, 2006; McClintock & Remillard, 2006). The gravity of the compact object strips the larger star of material via tidal forces, and the escaping gas flows into the accretion disc of the compact object (Frank et al., 2002). McHardy et al. (2006) analyse the X-ray variability of a number of X-ray binaries and AGN and find a characteristic variability time scale linking both objects, suggesting that the accretion mechanism may work in exactly the same way for small and larger mass black holes. The X-ray power spectra are modelled as a broken power law with a slope that steepens from approximately -1 to -2 above some characteristic frequency (or below some characteristic time
scale τB). McHardy et al. (2006) find that the fit parameters of both the AGN and galactic
X-ray binaries agree well with each other with a break time scale τB that depends on the
bolometric luminosity of the AGN (or x-ray binary compact object) and black hole mass as
logτB=AlogMBH−BlogLbol+C, (1.3)
where fit coefficientsA=2.10±0.15,B=0.98±0.15 andC =−2.32±0.2 (McHardy et al.,
2006) indicate a roughly quadratic scaling of break time scale with black hole mass, and an inverse linear scaling with bolometric luminosity.
In summary, it is unknown exactly to what extent, and by what mechanism, the various classes of AGN can be unified. Extensive work suggests that unification is real and able to explain many of the observed features of AGN (both in terms of their spectral behaviour and time variability). These mechanisms tend to attribute many of the differences in the observable features of AGN as being due to a combination of black hole parameters (mass and spin), bolometric luminosity and inclination.
1.4
Variability
AGN are known to exhibit variability across all wavelengths (Wanders et al., 1997; Collier et al., 1998; Sergeev et al., 2005; Cackett et al., 2007; Shappee et al., 2014; Edelson et al., 2015; Fausnaugh et al., 2016a). It appears that variability in the line emission is driven by
Chapter 1. Introduction: The AGN Story
continuum emission variability (Blandford & McKee, 1982), thought to originate from the accretion disc (Shakura & Sunyaev, 1973). Emission from the disc itself is believed to be driven by an X-ray emitting corona within 10 gravitational radii of the central super massive black hole (Morgan et al., 2012; Mosquera et al., 2013; Blackburne et al., 2014).
1.4.1
Energetics of Variability
What powers the large energy output from AGN? The common consensus is that this is due to accretion onto a black hole (Frank et al., 2002; Peterson, 1997; Krolik, 1999) . Section 1.1 discusses the energy output of AGN broken down by wavelength regime (X-ray, UV, optical ,IR and radio), but how do these look when stitched together? Figure 1.2 (Mehdipour et al., 2015) shows the Spectral Energy Distribution (SED) for NGC 5548 (more on this object in Chapters 3 and 5). This shows that the accretion disc emission (around 0.01 KeV left side of Figure 1.2) peaks in the UV. As will be discussed in later sections, UV variability often lags behind X-ray variability, suggesting that X-rays may play a role in driving the variability at longer wavelengths. Is this energetically possible? Figure 1.2 suggests that it is since the X-ray part of the SED (1 - 100 KeV Figure 1.2) has at least as much energy as the optical and therefore is certainly energetically capable of driving the optical variations.
1.4.2
Time Scales of Variability
Having introduced the concept of AGN variability, I now review the characteristic time scales governing the variability, and the physical processes for which they are applicable. These are derived in detail in Frank et al. (2002), and summarized below.
Viscous Time Scale
Firstly the viscous time scale is the time over which a thin annulus of accretion disc material will smear out due to the influence of viscous drag forces caused by the Keplarian differential rotation of the accretion disc. Following a (very) lengthy derivation in Frank et al. (2002), it is found that tvisc∼1010 α 0.03 R318/2yr, (1.4)
where R18 = R/1018cm. α is a dimensionless ’viscocity‘ parameter that is observationally
1.4. Variability
Energy (KeV)
Figure 1.2: Spectrum of NGC 5548 showing the reconstructed near IR through to X-ray spectrum based on measurements by the space telescopes listed in the figure (replicated with permission from Mehdipour et al. 2015).
Chapter 1. Introduction: The AGN Story
Dynamical Time Scale
The dynamical time scale measures the time for which surface disturbances on the accretion disc rotate and takes the form
tdyn∼ R vφ = R3 G M 1/2 , (1.5)
where vφ is the Keplarian rotation velocity. For AGN, dynamical time scales can be as little
as 103s for the inner most stable orbit. Due to theR3/2 dependence, this time scale rapidly
increases when we consider variability originating from larger radii.
Hydrostatic Equilibrium Time Scale
This gives the time scale over which fluctuations to hydrostatic equilibrium are transmitted
throughout the vertical direction of the disc. For a vertical scale height H, this time scale is
given by
tz=
H cs
, (1.6)
wherecs is the adiabatic sound speed. (King, 2008) show that for a thin disc with a Keplarian
rotation curve
H cs =
R
vφ, (1.7)
and so the dynamical and hydrostatic equilibrium time scales are similar.
Thermal Time Scale
The thermal time scale is the time scale for which a perturbed accretion disc will readjust itself into thermal equilibrium (Frank et al., 2002). King (2008) show that it relates to the viscous time scale by
tth=
H
R
2