Jet vs Disc: The optical output of Flat-spectrum Radio Quasars.
Matthew Whiting 1, Rachel Webster 2, Paul Francis 3, Alicia Oshlack 2
1 School of Physics, University of New South Wales, Sydney NSW 2052; m.whiting@unsw.edu.au
2 University of Melbourne, 3 Australian National University
IAU General Assembly, Joint Discussion 18, “Quasar Cores and Jets”, Sydney, July 2003.
Power Law Slopes
We use the BVRIJHK photometry of Francis et al. (2000) for sources from the Parkes Half-Jansky Flat-spectrum Sample (Drinkwater et al. 1997).  We fit power laws to the data and find a wide range in power law indices (see Figure 1):
Ø Blue sources: similar to optically-selected quasars in colour and emission line spectrum, with low optical polarisation _ Accretion disc dominates.
Ø Red sources: weaker lines, highly polarised. Seem to have different continuum to blue sources _ Influenced by synchrotron emission?
Ø Others: There are a number of sources that show curvature in their spectra _ Need different emission component(s).
Jet Emission
A large range of turn-over frequencies is observed, spanning the full optical/near-infrared range. However, even the blue quasars have synchrotron emission present (at radio frequencies), and since there is no optical emission, it must have turned over somewhere in the infrared. This should be testable with ground and space-based infrared observatories.
It is likely, though, that we are seeing the upper edge of the turn-over distribution, indicating an upper limit in the particle energies within the relativistic jet. The redshift distributions for sources fit with the two models (Figure 3) indicate that the high redshift sources are overwhelmingly blue. This shows that we are seeing the accretion disc emission and very little synchrotron at the shorter rest-frame wavelengths.
References
• Caccianiga et al. 2002, MNRAS 329, 877
• Drinkwater et al. 1997, MNRAS 284, 85
• Francis, Whiting & Webster 2000, PASA 17, 56
• Marchã et al. 2001, MNRAS 326, 1455
• Mukherjee et al. 1999, ApJ 527, 132
• Whiting, Webster & Francis 2001, MNRAS 323, 718
• Whiting, Majewski & Webster 2003, PASA 20, 196
Accretion in Multi-wavelength Models
The identification of significant accretion disc emission can be important for multi-wavelength modelling of quasar spectra. Figure 4 shows two quasars detected by COMPTEL at gamma-ray energies. The shape of the optical emission of each is very different – one is synchrotron-dominated and red, while the other shows a blue optical slope consistent with an accretion disc. (See Whiting et al. (2003) for more details.)
Both are usually modelled with synchrotron and inverse-Compton models (e.g. Mukherjee et al. 1999), but an accurate knowledge of the strength of the accretion disc emission is vital, to constrain both the synchrotron strength in the optical and the energy density of target photons for inverse-Compton emission. This is something that needs to be considered for future multi-wavelength campaigns.
‘Hidden’ BL Lacs
If a synchrotron component in a BL Lac were to turn over in the infrared, then the BL Lac will be mis-identified. BL Lacs are without significant accretion disc emission, leaving the host galaxy to dominate the optical emission. The radio emission, on the other hand, will still be dominated by the compact jet, and so the source will appear as a flat-spectrum radio galaxy – the BL Lac will then be ‘hidden’ in the centre of the galaxy.
This scenario appears to be increasingly important for surveys at lower flux limits, such as the CLASS survey (Marchã et al. 2001; Caccianiga et al 2002), which has a large fraction of Passive Elliptical Galaxies hosting a flat-spectrum radio core.
Introduction
There are two important elements to the structure of a radio quasar: the accretion disc around the black hole, and the relativistic jet responsible for the radio emission. The optical regime is where the emission from these two elements can compete most strongly.
An understanding of the individual components is important: nature of the synchrotron emission gives insights into the particle energetics and acceleration mechanisms within the jet; while the strength of the accretion disc emission can provide understanding of the actual processes involved in accretion.
Knowledge of their relative strengths meanwhile is important in understanding the overall energetics of the active nucleus, and the relationships between emission components at different wavelengths.
Disc + Jet Model
We now fit a new model to the data and compare to power law fits. Two components:
1. Blue power law – ‘classic’ quasar spectrum, representing accretion disc emission.
2. Synchrotron emission, with exponential turn-over at some critical frequency, representing emission from the relativistic jet.
Examples of the fits are shown in Figure 2, and detailed discussion of the fitting can be found in Whiting et al. (2001). We find that ~40% of the sources show evidence for synchrotron. A similar number are best fit by the power law – these are mostly the blue sources (see the distribution in Figure 3).
Accretion in BL Lacs
What does the modelling tell us about the BL Lacs in the sample? The BL Lacs, on average, have redder colours than the quasars, and all the BL Lacs are best fit with the disc+jet model, with a quite dominant synchrotron component.
Many of the quasars, however, are also fit with a dominant synchrotron component, and some of these exhibit strong emission lines. In fact, the distribution of equivalent widths for the quasars is fairly insensitive to the amount of synchrotron emission present.
The weak emission lines of BL Lacs are thus not wholly due to swamping by a strongly boosted jet component. Rather, the emission from the accretion disc and broad-line region is intrinsically weak, and so cannot compete with the jet emission. We propose two alternative explanations for this:
Ø A different accretion process is taking place in BL Lacs, such as a low efficiency ADAF or ADIOS process. This is hinted at from studies of FR1/FR2 radio galaxies (the parent populations of BL Lacs / FSRQs).
Ø There is a greater amount of outflow into the jet, disrupting the inner (hotter) regions of the accretion disc. This is analogous to the correlated ejection events and hard-X-ray dips seen in Black Hole X-ray Binaries, but scaled according to the larger mass of the BL Lac’s black hole.
Figure 4. Multi-wavelength spectra of two COMPTEL quasars. 0208-512 is dominated in the optical by synchrotron emission, while 0528+134 is instead dominated by accretion disc emission. See Whiting et al. (2003) for details on the data presented.
Figure 3. (Left) Distributions of redshift for sources best fit with the disc+jet model (top) and the power law model (bottom). The green histogram represents those sources with a<-0.8 (the redder ones).  (Right) Power law indices of sources best fit by the power law model (in blue), compared with the distribution from Figure 1.
Figure 4. Multi-wavelength spectra of two COMPTEL quasars. 0208-512 is dominated in the optical by synchrotron emission, while 0528+134 is instead dominated by accretion disc emission. See Whiting et al. (2003) for details on the data presented.
Figure 1. (Left) Histogram of power law indices fitted to the data. Index a defined such that Fn µ na.  (Right) Optical polarisation (taken from literature) as a function of power law index.
Figure 2. Examples of model fits to data.  (Left) A blue power law. (Centre) A model with accretion disc & jet emission of comparable strength.  (Right) An almost pure synchrotron component.