A panchromatic view of PKS 0558-504: an ideal laboratory to study the disk-jet link

Because of their vicinity and hence their high brightness, the temporal and spectral properties of GBHs are much better known and can be used to infer information on their more powerful, extragalactic analogs. Indeed considerable progress in this field has been made by multi-wavelength correlated studies of GBHs in different spectral states, which allow one to study the link between accretion (generally probed by the X-ray emission) and jet properties (in the radio regime) on “human” timescales. It is now well established that GBHs undergo state transitions, switching between two main states: the low/hard (LS) and the high/soft state (HS) passing through soft and hard intermediate states (IS), which are also named very high states (VHS) when they occur at high values of accretion rate (see McClintock & Remillard 2006;Done et al. 2007 for recent comprehensive reviews on GBHs). Each spectral state is unambiguously characterized by a specific combination of X-ray temporal and spectral properties and by well defined radio features (see Fender et al. 2004).

In the study of the disk-jet link one of the most interesting spectral states is the IS/VHS, which is generally characterized by powerful transient relativistic ejections in the radio coupled with highly variable X-ray emission that is unambiguously associated with the accretion flow, unlike the LS where the origin of the X-rays is still matter of debate (e.g. Markoff et al. 2003;Zdziarski et al. 2004).

Unfortunately, the physical conditions that lead to the transient relativistic ejections during the IS/VHS are still poorly understood mostly because of their short duration in GBHs. Since the dynamical time scales are proportional to the black hole mass, in individual AGNs it is not possible to observe long-term phenomena occurring in GBHs such as the canonical spectral transitions. On the other hand, AGNs may provide better constraints on short-lived GBH phenomena and hence shed light on the jet formation and the interplay between accretion and ejection processes.

In the framework of the AGN-GBH unification, the Narrow-Line Seyfert 1 galaxies (NLS1s) are the best candidates for large-scale analogs of GBHs in the IS/VHS. NLS1s are historically identified by their optical emission line properties: the ratio [O III]/Hβ is less than 3 and FWHM Hβ is less than 2000 km s -1 (Osterbrock & Pogge 1985;Goodrich 1989). They are seldom radio loud (Komossa et al. 2006), although recent studies reveal the existence of several NLS1s characterized by very high radio-loudness (e.g. Yuan et al. 2008). Recently, a few of these very radio-loud NLS1s have been detected at γ-ray energies by the Fermi/LAT collaboration confirming that these sources posses relativistic jets observed at small viewing angles (Abdo et al. 2009a,b,c;Foschini et al. 2009). In the X-rays, NLS1 are generally characterized by steep spectra and strong variability (e.g. Brandt et al. 1999;Leighly 1999a,b;Grupe et al. 2001). Based on these properties, it has been suggested that NLS1 are AGN in their early phase (Grupe et al. 1999;Mathur 2000), characterized by relatively small black hole masses (e.g. Grupe & Mathur 2004), and very high accretion rates in terms of Eddington units (e.g. Boroson & Green 1992;Sulentic et al. 2000;Grupe et al. 2010; see also Marconi et al. 2008;Decarli et al. 2008 for a discording view).

PKS 0558-504 (z = 0.137) is the brightest radio-loud (R = L ν (6 cm)/L ν (B) ≃ 27, Siebert et al. 1999) NLS1 in the X-ray band and therefore one of the best studied at these energies. Previous X-ray observations with different satellites have confirmed that PKS 0558-504 shows the characteristic NLS1 properties: strong variability, steep X-ray spectrum, substantial soft excess, and relatively high luminosity. By comparing the X-ray observations from several satellites over a decade, it is evident that on long timescales the strong X-ray variability of PKS 0558-504 occurs persistently but without being accompanied by significant spectral variability: the spectrum above 2 keV is consistently described by a powerlaw model with photon index Γ ∼ 2.2 (Gliozzi et al. 2000). This conclusion has been confirmed by a recent monitoring campaign with RXTE: the long-term achromatic variability appears to be consistent with the behavior of GBHs during the transition LS-to-HS and inconsistent with the spectral variability typical of jet-dominated AGNs (Gliozzi et al. 2007).

In the last ten years, PKS 0558-504 has been observed repeatedly by XMM-Newton (O’Brien et al. 2001;Glioz

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