Radio quiet AGN
In radio quiet AGN, the X-rays are thought to be related to the presence of a corona above the disc.
The heating mechanism of the corona is unknown, but for example it could be due to magnetic recon-nection (Poutanen & Fabian 1999) or part of the accretion flow could be intrinsically hot (Narayan &
Yi 1994). The variability of the X-ray emission could be related to the formations of hot spots fol-lowing magnetic reconnection (Czerny & Goosmann 2004) or to disc instabilities (King et al. 2004), and in part due to variable obscuration (Risaliti et al. 2002).
The correlation between the X-ray and the optical/UV emission can provide important clues about how the disc and the corona are related and what is the origin of the variability in these energy bands.
The common models to describe the relation between these bands involve either Comptonisation of the soft disc photons by the corona electrons or reprocessing of the X-ray emission in the disc, which produces the optical/UV emission. If the alternative scenario proposed by Courvoisier & Türler (2005) is considered, the UV and X-ray emission are connected as they are produced during different shock phases of the same clumps of matter accreting onto the black hole. In this model, both a short-lag and a longer-lag correlation are expected.
The X-ray-optical/UV relation has been extensively studied and the results are very different, depending on the objects and on the analysed time scales (see Uttley 2005 for a review). In fact, in some cases a correlation is found (NGC 5548; Uttley et al. 2003), sometimes with a measured time delay that varies with time (NGC 4051; Shemmer et al. 2003), whereas in other cases the correlation is not present (NGC 3516; Maoz et al. 2002). Some of the objects for which a flux-flux correlation is not detected show a correlation between the optical/UV flux and the X-ray spectral index (NGC 7469;
Nandra et al. 2000). These differences might depend on where the optical/UV emission is produced:
the smaller the black hole mass, the hotter the accretion disc, the further from the central engine the optical/UV emission is produced and therefore the more unlikely a correlation with the X-rays is (Uttley 2005). There are however several other reasons to explain the lack of correlation, even within the Comptonisation or reprocessing scenario. For example if the X-rays are due to Comptonisation of the UV photons, the UV-X-ray flux relation could depend on a third parameter, e.g. the X-ray spectral index (Nandra 2001). In addition, the geometry and the size of the X-ray source or of the
Correlation with other wavelengths 99 reprocessing medium could wash out the variations of the observed emission or the joint contribution of reprocessing and Comptonisation could complicate the X-ray-optical/UV relation.
Radio loud AGN
The X-ray variability in radio loud AGN is greatly enhanced due to the aberration effects of plasma moving at relativistic speed in a direction close to the line of sight, giving rise to very extreme vari-ability behaviours in these objects. The mechanisms proposed to explain these variations are different, but usually involving propagation of shocks in the jet (Marscher & Gear 1985) or jet precession (Sil-lanpää et al. 1988).
As the X-ray band can correspond to the peak/end of the synchrotron branch (BL Lac) or to the beginning of the Compton one (FSRQ) in the spectral energy distribution, this emission can be closely related to the emission at lower and higher energies in different ways. HBL objects have been detected up to TeV energies, which correspond to the Compton peak of their SED. In several objects a correlation between X-ray and TeV emission has been detected (e.g. Mrk 501, Krawczynski et al.
2000; Mrk 421, Fossati et al. 2008) confirming the origin of the TeV as synchrotron self-Compton and of the X-rays as synchrotron by the same electron population. Krawczynski et al. (2004) reported though about a TeV flare without an X-ray-flare counterpart for the blazar 1ES 1959+650.
In FSRQ, the X-rays are instead produced by inverse Compton and the radio to UV photons consti-tute the synchrotron branch of the spectrum. It is therefore expected that variations of the synchrotron emission are accompanied by correlated variations of the X-ray/Compton radiation. Such correla-tions have been observed in a number of sources, for example showing quasi-simultaneous X-ray flares correlated to the infrared (3C 273, McHardy et al. 2007) or optical emission (PKS 0537–441, Pian et al. 2007). The dominant emission process, if synchrotron self-Compton or external Compton, varies from object to object, but also as a function of the parameters describing the jet (Ghisellini et al. 2007).
Chapter 7
The quasar 3C 273
Several reasons point to 3C 273 as one of the best candidates for an extensive study of the AGN properties. First of all, 3C 273 is the brightest quasar in the sky, due to its high luminosity (Lbolometric ≃ 6×1046erg/s, assuming isotropic emission) and relatively small distance, being one of the closest quasars to Earth (z=0.158 and a luminosity distance of 630 Mpc when assuming a Hubble constant H0 = 70 km s−1Mpc−1). In addition, this object shows most of the properties characteristic of radio quiet as well as radio loud AGN, such as strong radio emission, polarised during some epochs, a large-scale and a small-large-scale jet, the latter showing blobs with apparent superluminal motion, a broad-band emission characterised by large variability at all wavelengths and an intense emission in the optical-UV range referred to as blue bump (see Courvoisier 1998 for a review).
3C 273 was the first radio source identified as a quasi-stellar object for which the optical emis-sion lines were interpreted as hydrogen lines redshifted by z = 0.158 (Schmidt 1963). In the years following this fundamental discovery, 3C 273 soon revealed its nature of multiwavelength emitter thanks to observations in the infrared (Pacholczyk & Weymann 1968) and in the X-ray bands (e.g.
Bowyer et al. 1970). It was also the first extragalactic source to be detected at gamma-rays by COS –B observations (Swanenburg et al. 1978). Besides the early discovery and the brightness of the source, 3C 273 observations are also made easier by the position of the source near to the celestial equator (RA= 12h29m0.67s, DEC = +0.2◦03′09′′), making observations from most Earth locations possi-ble, and far from the Galactic plane (l=289.95, b= +64.36), reducing the effects of gas and dust in the line of sight.
Fig. 7.1 shows the spectral energy distribution (SED) of 3C 273 from radio to gamma-rays (von Montigny et al. 1995). Even though approximately the same power per frequency decade seems to be emitted along the spectrum, some features can be identified. The two large bumps, peaking for 3C 273 in the IR and MeV bands, are a common characteristic of AGN and correspond to the synchrotron emission from the jet in the radio to IR band and to Compton processes in the X- and gamma-ray domain, respectively. The smaller, but well pronounced hump in between is known as the blue bump and is a common feature of Seyfert galaxies, whereas in blazars it is usually hidden by the stronger jet emission. Part of the blue-bump emission is believed to be related to the accretion disc and the broad line region, but the origin of all this energy release is still unknown. This spectral feature more than others emphasizes that 3C 273 is a very interesting and quite peculiar case, possibly a transition object between different AGN types.
101
Figure 7.1: Spectral energy distribution of 3C 273 in the classicalνFν representation (von Montigny et al.
1995).
7.1 The broad-band emission of 3C 273
Several components with different origins but often similar luminosities contribute to the total energy output of 3C 273 (and of AGN in general). Even in the same energy band, the observed radiation can be the result of the superposition of two or more different processes, as for example in the infrared domain. This complicates the task of disentangling the properties of the observed emission in order to understand the physics behind these objects. It is therefore essential to study the emission along the whole electromagnetic spectrum to identify the different components.
7.1.1 The jet emission
Since its discovery, 3C 273 revealed the presence of a large-scale (dozens of kpc), one-sided jet emitting in the radio, optical and X-ray band. The main properties of this emission are similar in the different bands (for example the extension and direction of the jet), but different structures are present depending on the observed energy domain (Fig. 7.2). Due to the spectral shape and the measured polarisation, the radio and optical emissions are believed to be synchrotron radiation. The generally accepted model to describe synchrotron emission in relativistic jets involves shock waves propagating from the core of the source down the jet (e.g. Marscher & Gear 1985). Three phases can be recognised in the evolution of the shock: the Compton loss phase, the synchrotron loss phase and the adiabatic expansion. As the shock propagates from more to less dense regions, the frequency at which the medium becomes optically thin decreases with time and consequently the frequency of the peak emission decreases. Synchrotron flares are observed up to the IR and optical domains as relatively short and peaked outbursts on a less variable underlying emission. In the radio and mm bands the synchrotron flaring emission dominates the observed flux and the light curves can be decomposed in a series of consecutive flares, with different intensities and durations (Türler et al.
1999a, 2000). These flares follow the typical evolution of synchrotron flares, propagating with time from higher to lower energies and with increasing duration.
The origin of the X-ray emission from the jet is still under debate, as a single synchrotron
compo-The broad-band emission of 3C 273 103
Figure 7.2:Multiwavelength image of the large-scale jet of 3C 273:Spitzer/IRAC image at 3.6µm (top panel), HSTimage at 620 nm withV LAcontours superposed (centre panel),Chandraimage in the 0.4–6 keV band (bottom panel). 3C 273 core is positioned at the origin of the coordinate system (Uchiyama et al. 2006).
nent is not sufficient to describe the radio to X-ray radiation. Even though early Chandra calibration data of the 3C 273 jet showed a good agreement with the jet X-ray emission being inverse Compton on the cosmic microwave background photons (Sambruna et al. 2001), recent multiwavelength obser-vations favour a two-zone model where the X-rays are synchrotron emission of an electron or proton population accelerated by a different process than that responsible for the radio to IR emission (Jester et al. 2006; Uchiyama et al. 2006).
On smaller scales (sub-mas), another jet has been observed with the Very Long Baseline Inter-ferometry (VLBI). It has very different characteristics from the large-scale jet (e.g. the direction) and shows a fine structure composed by blobs moving with apparent superluminal motion (Fig. 7.3).
The outbursts observed in the radio-millimeter emission on larger scale seem to correspond to these VLBI components (Türler et al. 1999a). New blobs have been observed to appear from 3C 273 core and a positive correlation between these events and the enhancement in the gamma-ray emission has been reported (Krichbaum et al. 1996). This suggests that the gamma-ray flares are caused by inverse Compton scattering of relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine (Jorstad et al. 2001a).
7.1.2 The IR emission
The infrared band is an example in the spectrum of 3C 273 where more than one component sig-nificantly contributes to the observed emission. The strong flaring activity in the IR band was first observed by Courvoisier et al. (1988) and is characterised by an increased polarisation compared to the quiescent emission. All this indicates a clear synchrotron origin for this component, confirmed
Figure 7.3: VLBI map of 3C 273 jet at 22 GHz at 4 different epochs (Jorstad et al. 2001b). The A com-ponent indicates the core, whereas the others are the radio blobs whose evolution is shown over 1.5 years of observations.
later by the correlation between these IR and the mm flares (Robson et al. 1993). On the other hand, the presence of a thermal component around 3µm superposed to the non-thermal IR continuum was first noticed by Neugebauer et al. (1979). This was confirmed in 1986, when simultaneous observa-tions revealed that while the sub-mm flux was significantly decreasing, the IR one stayed at a nearly constant level (Robson et al. 1986), excluding a synchrotron origin for most of this IR emission. Rob-son et al. proposed that heated dust close to the sublimation temperature is emitting this IR radiation.
During the historical minimum of the synchrotron emission observed in 2004 in the sub-mm band, Türler et al. (2006) estimated the contribution of the dust in the IR band, modelling the excess over the synchrotron extrapolation from the mm with several thermal components with temperatures of 40 <T <1600 K, possibly associated with dust components distributed on different scales, from pc to several kpc.
7.1.3 The blue bump
The excess emission in the optical-UV region of the spectrum is a common characteristic of Seyfert galaxies and quasars, but it is rather rare in blazars. 3C 273 shows a prominent blue-bump emission (Fig. 7.1) whose origin is still not completely understood. The spectral characteristics of the blue bump are globally consistent with thermal emission from an optically thick and geometrically thin accretion disc, with different temperatures (Malkan 1983). Nevertheless, a few properties of AGN blue bumps cannot be explained by this kind of models, as for example the connection between the shape of the continuum and the luminosities in different objects and the variability of the optical-UV emission (see Sect. 2.1; Courvoisier & Clavel 1991). The latter point is particularly important for
The broad-band emission of 3C 273 105 3C 273 as, when the periods of synchrotron flares are excluded from the study, still different vari-ability properties are observed at different frequencies of the blue bump, i.e. much longer time scales in the optical than in the UV and very short lags in the correlation between optical and UV light curves. Both these aspects point to the presence of two different components superposed to create the blue bump (Paltani et al. 1998b). The “optical” component, showing variations on longer time scales, contributes also to the UV band, where the dominant component, “UV”, is characterised by larger variations and on shorter time scales and it is in turn contributing also to the optical emission.
The total emission would result from the superposition of these two components with different nor-malisations depending on the frequency. Paltani et al. suggest that the “UV” component comes from reprocessing of a hard X-ray, external source on the disc and that it might extend from optical up to soft X-rays. On the other hand, the origin of the “optical” component is still under discussion. Due to the polarised emission observed in the optical, even outside the periods of synchrotron flares (de Diego et al. 1992), and to its spectral shape, it has been proposed that the “optical” component is connected to the emission from the jet, possibly as due to synchrotron radiation, maybe extending down to the near-IR domain.
7.1.4 The high-energy emission
The X- and gamma-ray emission of 3C 273 is characterised by the coexistence of features connected to the disc emission and typical of Seyfert galaxies (the soft-excess and the iron fluorescence line) and others related to the jet and therefore distinctive of blazars (the overall spectral shape from medium X-rays to gamma-X-rays; Haardt et al. 1998; Kataoka et al. 2002; Grandi & Palumbo 2004; Chernyakova et al. 2007). Variability is a common property of all these features, with different intensities and time scales.
The continuum from the medium X-rays (& 2 keV) to the gamma-rays is well represented by a broken power law with break at ∼1 MeV and slopes of Γ = 1.5 −1.8 and Γ = 2.4− 3 before and after this energy. The shape of the spectrum and the break support a non-thermal origin for this emission (Johnson et al. 1995), probably due to inverse Compton processes of a population of relativistic electrons. The overall spectral energy distribution has been fitted with SSC or EC models (Kataoka et al. 2002; Chernyakova et al. 2007), with a combination of the two (Ghisellini et al. 1998) or with the proton-initiated cascade (PIC) model (von Montigny et al. 1997). All these models seem to be in general able to equally well represent the SED of 3C 273.
Superposed to the non-thermal emission, a Seyfert-like component is also observable in the X-ray spectrum of 3C 273. Besides the soft-excess, a weak, variable and large iron Kαemission line at 6.4 keV has been occasionally detected (e.g. Yaqoob & Serlemitsos 2000; Kataoka et al. 2002; Grandi &
Palumbo 2004; Chernyakova et al. 2007), being the signature of X-ray reprocessing by cold material.
Following the Fe line detection, one would expect to see a hump around 30 keV as produced by Compton reflection of the primary continuum, but it is usually not observed in 3C 273, apart from one exception (Grandi & Palumbo 2004).
At soft X-rays, the excess emission over the medium X-ray extrapolation, known as soft-excess, extends up to 1–2 keV for 3C 273. Several different interpretations have been given to the soft-excess emission of AGN (e.g. Mineshige et al. 2000; Gierli´nski & Done 2004; Crummy et al. 2006).
Page et al. (2004) found that 3C 273 soft-excess is best fitted by multiple blackbody components, suggesting that this feature is due to Comptonisation of cool disc photons in a warm corona, possibly being the high-energy tail of the blue bump (Walter et al. 1994). On the other hand, Chernyakova
et al. (2007) found that neither a black body nor a reflection model fits the soft-excess and that the best representation of the soft X-ray spectra is a cut-off power law. Chernyakova et al. (2007) suggest that the soft-excess emission is synchrotron radiation from the compact core of the source. A recent study by Pietrini & Torricelli-Ciamponi (2008) confirms the decomposition between jet- and Seyfert-like components of the X-ray spectrum of 3C 273 and interprets the Seyfert-like contribution as inverse Compton emission of a relativistic non-thermal population of electrons accelerated in the magnetic loops of an active corona above the disc. In addition to describing well 3C 273 X-ray spectrum above 2 keV, their model reproduces the soft-excess emission observed, without the need of any additional component. The soft-excess is in fact produced by SSC mechanism, whereas the higher-energy Seyfert-like component (>2 keV) is rather dominated by EC on the blue-bump photons.
Chapter 8
The 3C 273 on-line database
Covering 17 decades in energy and providing 70 light curves spanning up to 30 years of observations when it was first created (Türler et al. 1999b), the multiwavelength database of 3C 273 still represents the most complete collection of data for an AGN. In order to allow all the scientific community to take advantage of these data and to encourage similar studies for other objects, the database was made publicly available on-line athttp://isdc.unige.ch/3c273/
The database light curves were organised in 6 main groups depending on the energy domain, i.e.
radio, millimeter (mm) and submillimeter (sub-mm), infrared (IR), optical, ultraviolet (UV) and X-and gamma-rays. Each of these groups contained a different number of light curves providing the time in “yyyymmdd.hh” and in fractional format, the frequency and the corresponding wavelength of the observation, the flux in Jy and its error, the instrument that performed the observation and the reference for published data. In addition, for the X-ray band, information about the spectral parameters used to extract the fluxes is also available (see Türler et al. 1999b for further details).
In the years following this publication, 3C 273 monitoring has continued at all wavelengths, but especially in the X-rays a large amount of data has been collected by the numerous high energy missions that had been, or still are, surveying the X-ray sky. Both in the soft and in the hard X-ray domains, the considerable increase of the available data motivated a renewed effort to update the database with the last∼10 years of observations. The collaboration, established when the database was first created and now enlarged to other colleagues, has again provided data and help to accomplish this project. In addition, a research in the on-line databases of different observatories and satellites
In the years following this publication, 3C 273 monitoring has continued at all wavelengths, but especially in the X-rays a large amount of data has been collected by the numerous high energy missions that had been, or still are, surveying the X-ray sky. Both in the soft and in the hard X-ray domains, the considerable increase of the available data motivated a renewed effort to update the database with the last∼10 years of observations. The collaboration, established when the database was first created and now enlarged to other colleagues, has again provided data and help to accomplish this project. In addition, a research in the on-line databases of different observatories and satellites