BROAD-LINE RADIO GALAXIES OBSERVED WITH FERMI-LAT: THE ORIGIN OF THE GeV γ-RAY EMISSION

A long-debated problem in our understanding of accreting supermassive black holes (SMBHs) in the universe is the unification of different types of active galactic nuclei (AGNs) in a framework ascribing their observed diversity to a relatively few differing parameters and factors. For example, it has been widely argued that the difference between type 1 and type 2 AGNs—i.e., those possessing and lacking broad permitted emission lines in their nuclear spectra, respectively—may be explained by simple geometrical effects involving anisotropic obscuration of the active center viewed at different inclinations with respect to the accretion disk axis (Antonucci 1993). The accretion rate was claimed to play a role in this context as well, since "standard" geometrically thin optically thick accretion disks formed at high accretion rates (>1% Eddington; Shakura & Sunyaev 1973) on the one hand, and radiatively inefficient geometrically thick accretion flows expected to be present at low accretion rates (<1% Eddington; Narayan & Yi 1994, 1995; Abramowicz et al. 1995) on the other hand, can account for different emission spectra and power outputs of high- and low-power AGNs (see an early discussion on this issue by Fabian & Rees 1995 and Lasota 1996, and the more recent one in Ghisellini et al. 2009). The other relevant parameter may also be the mass of the SMBH itself, which seems to determine some physical differences between narrow-line Seyfert 1 and "regular" Seyfert 1 galaxies (e.g., Pogge 2000; Komossa 2008).

None of the aforementioned factors can, however, account for the dichotomy between radio-loud AGNs (which have jets) and radio-quiet AGNs (which do not). One may therefore seek the fundamental difference between AGNs producing luminous radio-emitting outflows and those lacking such outflows in yet another parameter of the accretion disk/black hole system. One possibility is that, for example, radio-loud AGNs harbor rapidly spinning SMBHs with rotational energy extracted electromagnetically via the Blandford & Znajek (1977) mechanism, and converted to the kinetic luminosity of relativistic jets. At the same time, the negligible angular momentum of SMBHs hosted by radio-quiet AGNs precludes the formation of such powerful well-collimated outflows. This so-called spin paradigm (Blandford 1990) has recently been claimed to be supported, after some modifications, by several observational findings and theoretical investigations (e.g., Koide et al. 2002; Sikora et al. 2007; Tchekhovskoy et al. 2010; see also in this context Garofalo 2009). Regardless of this debate, the presence of a relativistic jet constitutes a fundamental distinction between various types of AGNs, simply because an anisotropic and strongly Doppler-boosted jet emission can dramatically affect the observed properties of a source. In this context, geometrical effects play again a major role. In particular, the total radiative output of those radio-loud AGNs which are observed at small viewing angles to the jet axis ("blazars") may be dominated by the broadband jet emission, while those AGNs which are inclined at larger viewing angles (e.g., radio galaxies) may display radiative signatures of both outflowing and accreting matter at comparable levels (see, e.g., Barthel 1989; Urry & Padovani 1995). We note that in addition to such geometrical effects, the age of a radio structure (i.e., the time elapsed after the onset of the jet activity in the nucleus) is yet another factor crucial to understanding unification of radio-loud AGNs (see, e.g., O'Dea 1998).

Interestingly, new deep radio surveys indicate that the radio-loudness parameter—which is defined as a ratio of the jet-related radio flux to the disk-related optical flux, and is considered as a proxy for the jet production efficiency¹⁹—shows a continuous distribution rather than a sharp division between radio-loud and radio-quiet AGNs. This applies to AGNs hosted by early-type galaxies and accreting at high rates (i.e., quasars), which are typically studied in this context (e.g., White et al. 2000), but holds also when other elliptical-hosted AGNs (radio galaxies) spanning a wide range of the accretion rate are taken into account (Sikora et al. 2007). Moreover, unresolved non-thermal radio emission and jet-like structures have been discovered in classes of AGNs considered previously as "radio-quiet," i.e., Seyfert galaxies, although the jets in such systems are non-relativistic and weak when compared to the jets found in "classical" radio galaxies and quasars (e.g., Ulvestad & Wilson 1989; Kukula et al. 1995; Thean et al. 2001; Middelberg et al. 2004, and references therein). The distribution of the radio-loudness parameter in Seyferts, which are typically hosted by late-type (disk) galaxies, is, however, similarly a continuous function of the accretion rate, as demonstrated first by Ho & Peng (2001) and Ho (2002). Yet, it was pointed out by Sikora et al. (2007) that there is a substantial difference in the distribution of the jet production efficiency between disk-hosted and elliptical-hosted AGNs, with Seyfert galaxies being characterized, at any accretion rate, by the radio-loudness parameters orders of magnitude smaller than the analogous parameters characterizing elliptical-hosted radio galaxies or quasars. Clearly, more studies regarding the jet–disk connection in different types of AGNs are needed to understand the jet launching processes and the physics of active SMBHs in general.

In this context, broad-line radio galaxies (BLRGs) seem ideal targets for an in-depth investigation, since this particular class of very radio-loud AGNs exhibits both the disk-related ("Seyfert-like") and the jet-related ("blazar-like") radiative signatures in their broadband spectra. Unlike blazars, the jets in BLRGs are not pointing directly toward the observer, and so the relativistic beaming effects and the related jet dominance are only moderate. Moreover, unlike narrow-line radio galaxies—which are believed to be intrinsically similar but simply inclined at systematically larger jet viewing angles—BLRGs are not generally obscured by large amounts of dust distributed in torus-like structures around the nucleus, and hence radiative properties of the accretion disks and of the circumnuclear gas can be easily accessed in their case. BLRGs show in particular optical/UV continuum and emission-line characteristics very similar to those of luminous Seyfert galaxies, which indicates high accretion rates and the presence of standard Shakura–Sunyaev disks in both classes of objects. Some fundamental differences in the X-ray spectra between BLRGs and high-accretion-rate Seyferts have been noted however. Specifically, even though the observed X-ray/soft γ-ray emission of BLRGs still seem dominated by the moderately absorbed emission by the accreting plasma (i.e., disks and disk coronae), rather than by the jets, the 1–100 keV continua of BLRGs are flatter, and their reflection components (as well as the fluorescent Fe Kα lines) are weaker than in the case of luminous Seyfert galaxies (e.g., Maraschi et al. 1991; Wozniak et al. 1998; Sambruna et al. 1999, 2002; Eracleous et al. 2000; Zdziarski & Grandi 2001; Ballantyne 2007). Because of such differences, several authors in the past have speculated about the non-negligible jet contribution to the X-ray emission of BLRGs, diluting the accretion-related radiative output in the X-ray domain (Wozniak et al. 1998; Eracleous et al. 2000; Grandi et al. 2002). This idea was subsequently examined in various different approaches using most recent broadband X-ray data obtained with BeppoSAX (Grandi et al. 2006; Grandi & Palumbo 2007), Suzaku, and Swift (Kataoka et al. 2007; Sambruna et al. 2009).

Grandi & Palumbo (2007) attempted to disentangle the jet and the disk contributions to the X-ray spectra of three of the brightest BLRGs (the approach first applied to the case of the quasar 3C 273; see Grandi & Palumbo 2004). For simplicity, they assumed that the accretion disks in BLRGs and Seyfert 1 galaxies produce similar emission continua and reprocessing features, and subsequently allowed for a presence of the Doppler-enhanced jet radiation at an arbitrary level. The fits obtained for 3C 120, 3C 390.3, and 3C 382 showed that the data are indeed consistent with a combination of a thermal component (in a first approximation associated with an accretion disk) and a non-thermal component associated with the beamed radiation (due to a jet). Grandi & Palumbo concluded however that jets make only minimal contribution to the X-ray continuum emission of BLRGs, in agreement with the previous findings by Wozniak et al. (1998). More recently, Sambruna et al. (2009) also proposed that BLRGs may be just clustered at one end of the distribution of the X-ray spectral parameters (e.g., photon indices and reflection albedos) characterizing Seyfert galaxies with significant overlap. Interestingly, both Grandi & Palumbo (2007) and Sambruna et al. (2009) suggested that the emission of the underlying jet may instead dominate the radiative output of BLRGs at high-energy γ-rays, and in particular in the GeV regime.

With the successful launch of the Fermi Gamma-ray Space Telescope, we now have an unprecedented opportunity to study in detail the γ-ray emission from different types of extragalactic sources—not only blazars, but also radio galaxies (Abdo et al. 2009a, 2009b, 2010c, 2010d; Kataoka et al. 2010), and other classes of AGNs as well (such as narrow-line Seyfert 1 galaxies, for example; Abdo et al. 2009c). During the first 15 months of the Fermi mission, 11 non-blazar AGNs have been detected in the GeV photon energy range (Abdo et al. 2010a, 2010d) by the Fermi Large Area Telescope (LAT; Atwood et al. 2009; Abdo et al. 2010b). This "misaligned AGN sample" includes seven Faranoff–Riley type I (low-power, hereafter FR I) radio galaxies, and four Faranoff–Riley type II (high-power, hereafter FR II) radio sources consisting of two radio galaxies and two steep spectrum radio quasars. The two luminous radio galaxies detected in γ-rays are, in fact, X-ray bright objects classified spectroscopically as BLRGs: 3C 120 (FR I radio morphology), detected for the first time with Fermi-LAT, and 3C 111 (FR II radio morphology), whose γ-ray detection was initially reported by EGRET (Hartman et al. 2008) and confirmed by Fermi-LAT. None of the X-ray bright Seyfert 1 galaxies appear to be detected in γ-rays thus far.

In this paper we report on a detailed investigation of the γ-ray emission from 18 hard X-ray brightest BLRGs, as well as a comparison sample of high-accretion-rate Seyfert 1 galaxies selected as their supposed radio-quiet counterparts (in the framework of the AGN unification scheme). Our primary goals are to examine the γ-ray properties of BLRGs as potential "γ-ray-loud" AGNs, to study the high-energy jet emission in the selected sources, in particular, investigating the relative contributions of the nuclear jet and accretion disk emission in the broadband spectra of BLRGs. The two years of Fermi-LAT exposure provides us with a rather deep all-sky survey reaching the flux limit of typically a few × 10⁻¹² erg cm⁻² s⁻¹ (95% confidence level) between the observed photon energies 100 MeV and 10 GeV. In Section 2, we describe the Fermi-LAT observations and data reduction procedure. The results of the analysis are given in Section 3. The discussion and final conclusions are presented in Section 4. Throughout this paper, a ΛCDM cosmology with H₀ = 71 km s⁻¹ Mpc⁻¹, Ω_Λ = 0.73, and Ω_m = 0.27 is adopted (Komatsu et al 2009).

2.1. The Sample

2.2. Fermi-LAT Observations and Data Analysis

Table 2 summarizes the results of the Fermi-LAT data analysis of the 18 BLRGs and 9 Seyfert galaxies. For each source considered, Table 2 provides (1) name, (2) statistical significance TS of the Fermi-LAT detection, (3) γ-ray photon index Γ_γ evaluated for the photon energy range 0.1–10 GeV, (4) the integrated photon flux above 100 MeV, F_>0.1 GeV, (5) γ-ray flux [νF_ν]_{0.1–10 GeV}, (6) the corresponding γ-ray luminosity L_γ = 4πd²_L[νF_ν]_{0.1–10 GeV}, (7) total accretion-related luminosity L_acc derived from the spectral fitting as described below, and (8) "mixing" parameter η discussed in the next section. Only two BLRGs (3C 111 and 3C 120) are detected at sufficiently high significance levels, i.e., TS ⩾ 25, in the accumulated two-year Fermi-LAT data set. For these, the γ-ray fluxes and luminosities are evaluated straightforwardly.²⁹ For the targets detected at lower significance levels, TS < 25, the corresponding 95% confidence level flux upper limits are calculated using the dedicated software UpperLimit.py.

For 3C 120, the results presented here are consistent with those reported in Abdo et al. (2010d), but with reduced uncertainties in the flux and photon index due to the improved photon statistics based on the two-year accumulation of the Fermi-LAT data. We note however that the TS value increased only slightly between the 15 month and 24 month Fermi-LAT data sets (cf. TS = 34 found here versus TS = 32 reported in Abdo et al. 2010d). In contrast, the flux and photon index uncertainties increased in the case of 3C 111, and the corresponding TS value decreased between the 15 month and 24 month Fermi-LAT data sets (cf. TS = 31 found here versus TS = 34 reported in Abdo et al. 2010d). The reason for this behavior is twofold. First, the likelihood analysis was limited here to the photon energy range 0.2–100 GeV, whereas the energy range 0.1–100 GeV was adopted in Abdo et al. (2010d). The difference in energy selection is relevant since 3C 111 is located at a relatively low Galactic latitude (see footnote 26), and as such is heavily affected by the contamination from the Galactic diffuse emission especially below 200 MeV.³⁰ If we lower the photon energy threshold of the likelihood analysis to 100 MeV, the significance of the 3C 111 detection in the 24 month data increases (TS = 59), as expected. Second, as noted in Abdo et al. (2010d), the considered galaxy seemed variable in the GeV range, being in particular brighter at the very beginning of Fermi-LAT observations. Here we investigate this issue in more detail, showing in Figure 1 (top panel) the temporal variations of the γ-ray flux of 3C 111 above 100 MeV using the two-year accumulation of the Fermi-LAT data binned in the three-month-long periods. The fluxes are plotted only when the detection significance reached TS > 10 in a given time bin; in the case of 3C 111 such a criterion was fulfilled only in three time intervals out of eight total. Even lower γ-ray duty cycle emerges from the 3C 120 light curve, as also shown in Figure 1 (bottom panel). All in all, we conclude that both BLRGs detected by Fermi-LAT are variable in the GeV photon energy range, even though the significance of the variability can hardly be evaluated due to the insufficient photon statistics. A dedicated analysis of all EGRET data also indicated plausible variability of the MeV/GeV emission in 3C 111 (Hartman et al. 2008). Note also that over the past two years both BLRGs have declined in flux by ∼30% at centimeter wavelengths as observed by the University of Michigan Radio Astronomy Observatory.³¹ In 3C 111 specifically, a bright submillimeter (230 GHz) flare was observed toward the end of 2008 (Chatterjee et al. 2011), coinciding with the period of the initial ∼0.5 years of LAT observations, with a subsequent decline in the submillimeter flux over the next ∼1.5 years.

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At this point let us also comment on the particular case of Pictor A galaxy, for which a relatively high TS value has been found (see Table 2), yet below the critical value of 25 required for claiming the detection. In fact, in the analysis procedure described above (i.e., assuming a single point-like source at the position of the galaxy, R.A. = 79957 and decl. = −45779), one can find formally TS = 45 for Pictor A. However, a close inspection of the spatial distribution of TS for the whole source region (hereafter the "TS map") reveals a rather complex structure elongated substantially in the R.A. direction, which is inconsistent with a single point source. Figure 2 shows TS map thus obtained, where contamination from nearby sources (listed in the 1FGL and in the internal LAT collaboration catalog using 18 months of data) are modeled as part of the background that also includes Galactic/extragalactic diffuse γ-ray emission. This suggests the presence of multiple γ-ray sources in the field. More exactly, in a careful examination of the TS map we found three emission peaks (each about a degree apart), one of which coincides almost exactly with the position of Pictor A, being characterized by TS = 20. Also, one of the other two emission peaks located at R.A. = 7909, decl. = −4608 is positionally coincident with a blazar BZQ J0515−4556 (R.A. = 7894, decl. = −4595) with a TS value of 19. The last peak seen to the east of Pictor A is less prominent (TS value of 11) and located at R.A. = 8135, decl. = −4578. Sources at the positions of the two TS peaks just mentioned were included in the model when the TS for Pictor A listed in Table 2 was evaluated. Hence, one may conclude that there are some indications for the GeV emission of Pictor A galaxy close to the upper limits provided in Table 2, and that the formal detection of this object by Fermi-LAT in the near future is quite likely.

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No statistically significant detection of any Seyfert 1 galaxy analyzed was found in the 24 month Fermi-LAT data set. Interestingly, IC 4329A shows a relatively high TS value of 15 (see Table 2), but this could be due to a contamination from a nearby source. This nearby source was found only recently, and has been associated tentatively with a blazar in the 2FGL catalog (the Fermi-LAT Collaboration 2011, in preparation). Nevertheless, the provided upper limits for the γ-ray emission of the considered sample of Seyfert galaxies—typically at the flux level of a few × 10⁻¹² erg cm⁻² s⁻¹ between 0.1 and 10 GeV, depending on the degree of the contamination from the Galactic diffuse emission and on the presence of nearby bright γ-ray sources—provide interesting constraints as discussed below.

The analysis of the two-year Fermi-LAT data for the selected 18 X-ray bright BLRGs confirmed the previously reported detections of the two sources (3C 120 and 3C 111), at the significance levels not much different between the 15 month and 24 month data sets. This may indicate that the observed γ-ray emission is variable on month-to-year timescales, with a relatively low duty cycle, as revealed by a closer inspection of the corresponding light curves (Figure 1). In fact, the >100 MeV γ-ray flux of 3C 111 observed with Fermi-LAT is (3.5 ± 1.2) × 10⁻⁸ ph cm⁻² s⁻¹, which is about ∼20 times smaller than the maximum recorded by EGRET in the same energy range (64 × 10⁻⁸ ph cm⁻² s⁻¹; Hartman et al. 2008), thus suggesting significant variability in the decade timescale between the EGRET and Fermi-LAT observations. Moreover, we found some hints for γ-ray emission from Pictor A (TS = 20), even though we cannot claim a formal detection at the moment. These results suggest that other BLRGs could be promising candidates for Fermi-LAT detections in the near future, and that BLRGs in general could potentially constitute a class of γ-ray-loud AGNs. Such a statement is difficult to quantify, however, because of the aforementioned variability of the GeV fluxes from 3C 120 and 3C 111 (assuming that both galaxies are representative for the whole class). On the other hand, the observed flux changes from these two detected galaxies imply that, analogous to blazar sources, the GeV continua of BLRGs are produced predominantly in the inner parts of their nuclear outflows (jets on scales less than tens or hundreds of parsecs). In fact, as noted in the previous sections, several authors have previously suggested that the GeV radiation from BLRGs, if detected, should most likely be due to non-thermal and beamed blazar-type emission, but only observed at intermediate inclinations (jet viewing angles, say, 10° ≲ θ_j ≲ 30°, versus θ_j ≲ 10° expected for blazars; see Grandi & Palumbo 2007; Sambruna et al. 2009). Interestingly, unlike in the luminous blazars, the X-ray/soft γ-ray spectra of BLRGs (up to the observed photon energies of ∼100 keV) have been argued to be dominated by the thermal radiation of the accreting matter (Wozniak et al. 1998; Sambruna et al. 1999; Eracleous et al. 2000; Zdziarski & Grandi 2001), with only a little (if any) jet contribution (Grandi & Palumbo 2007; Kataoka et al. 2007; Sambruna et al. 2009). Hence, it is clear that BLRGs are truly ideal targets for investigating the AGN jet–disk connection in the X-ray/γ-ray regime. Here we present a few considerations regarding this issue.

Figure 3 presents spectral energy distributions (SEDs) of the two aforementioned γ-ray-detected BLRGs; the SEDs of all the remaining objects, for which only the upper limits in the Fermi-LAT range were derived, are given in Figures 8 and 9 further below (see the Appendix). In the figures, the Fermi-LAT data are indicated by red circles (or arrows), black squares represent the historical data from NED, and the magenta squares show the 5 GHz radio fluxes of the unresolved nuclei. Even though the broadband spectra vary to some degree between 3C 120 and 3C 111 (e.g., with respect to the ratio between the total and the nuclear radio luminosities), one can gauge the main similarities and differences between the BLRG-type and Seyfert-type SEDs. In particular, while the infrared-to-hard X-ray continua of the two BLRGs are very similar to the one characterizing Seyfert galaxies considered here, the former objects are significantly brighter than the representative Seyfert galaxy in the radio and γ-ray domains. Such a dramatic difference in the radio loudness is related to the presence of a relativistic jet, as discussed above, and here we suggest that same is true regarding the γ-ray loudness. To investigate it further more quantitatively (yet still illustrative), we consider a simple phenomenological "hybrid" model for the broadband emission of a type 1 AGN, consisting of individual thermal and the non-thermal emission components (the approach analogous to the one adopted in Grandi & Palumbo 2004, 2007). The thermal component, related to the radiative output of the accreting and circumnuclear matter (accretion disk, disk corona, dusty torus), is approximated here by the template Seyfert spectrum given by Koratkar & Blaes (1999). For the non-thermal broadband emission component of the nuclear (blazar-type) relativistic jet observed at intermediate viewing angles, we use the well-constrained SED of the radio-loud quasar 3C 273 (from Soldi et al. 2008) as a template.³² Both templates, plotted in Figure 3 (as well as in Figures 8 and 9) as green and blue curves, respectively, are rescaled to match the data points for the analyzed sources. If Fermi-LAT did not detect the source and only upper limits on γ-ray flux were derived, its non-thermal component is entirely determined by the 5 GHz nuclear flux (in this context, see Table 3 of Ghisellini et al. 2005, who predicted γ-ray fluxes of 3CR FR I radio galaxies from 5 GHz core fluxes). This allows us to approximate the relative contribution of the non-thermal and thermal emission components to the observed SED of each object, in terms of a "mixing" parameter (η). Under the adopted definition, this parameter increases with the increasing contribution of the jet-like component, and equals unity when both the 3C 273 and the Seyfert template spectra provide comparable contributions to the observed UV flux of a source. Note that the direct radiation of the standard (Shakura–Sunyaev) AGN accretion disk is expected to peak in the UV regime (photon energies ∼10 eV).

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Obviously, the adopted model is an oversimplification of a realistic situation, as it ignores several complications which may be potentially relevant. For example, it is at some level questionable to use the broadband spectrum of the radio-loud quasar 3C 273 as a template for the jet-related emission of Seyfert galaxies in general. Moreover, possible (or even likely) temporal variability of each source considered, as well as the ad hoc adopted procedure of matching the assumed templates to the collected data points, precludes any robust statistical analysis (e.g., χ² fitting) which would allow for the values and errors of the η parameter to be determined more accurately. Nevertheless, it is worth noting that this simple model seems to match the SEDs of the two BLRGs well from radio to γ-ray frequencies, and that in both cases the emerging values of the η parameter are similar (≃ 0.15–0.35), being in addition consistent with the ones following from the analysis by Grandi & Palumbo (2007). Importantly, the model when applied to the other BLRGs analyzed here returns similar values for the η parameter at the level of few tens of percent at most (as determined by the nuclear radio fluxes), without violating the Fermi-LAT upper limits (see Column 8 in Table 2 and the SEDs presented in Figure 8). Figure 4 presents the distribution of the mixing parameter η emerging from the SED matching. This distribution suggests that, even though the particular frequency ranges may be dominated by one of the two main emission components, the total observed luminosities of the nuclear jets in BLRGs constitute on average not less than 1% of the accretion-related luminosities (median η ≈ 0.10), and that in the case of a few particular objects the jet-to-disk luminosity ratio may even approach unity. An interesting implication of the above statement, strengthened thanks to the inclusion of the Fermi-LAT data in our modeling, is that one should expect a non-negligible non-thermal emission component in BLRGs in the MeV energy range, constituting as much as ∼1%–10% of the total power emitted in the hard X-ray domain. If correct, this may be of importance for understanding the recently debated origin of the extragalactic MeV background (see the discussions in Inoue et al. 2008 and Ajello et al. 2009). On the other hand, the contribution of the jet emission to the total radiative output of the Seyfert 1 galaxies are in general very small (median η ≈ 5 × 10⁻⁴; see Figure 4 and the SEDs shown in Figure 9).

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To investigate further the collected data set, we plot the optical [νF_ν]_B versus X-ray [νF_ν]_2–10 keV fluxes for all the sources included in the sample in Figure 5 (the top panel), with BLRGs denoted by blue filled circles and Seyfert galaxies by red crosses. The bottom panel of Figure 4 presents instead the radio [νF_ν]_5 GHz versus X-ray fluxes. Here, BLRGs are plotted as blue filled circles when total radio fluxes are used, and as cyan open circles when nuclear radio fluxes (obtained predominantly from very long baseline interferometry (VLBI) observations where available, supplemented by Very Large Array measurements in a few cases) are considered. In both panels, large symbols indicate the two objects detected by Fermi-LAT (3C 111 and 3C 120), while the medium-size symbols represent Pictor A galaxy which, even though not formally detected, is characterized by the third-highest TS in the analyzed two-year Fermi-LAT data set. These flux–flux plots show that the optical and X-ray fluxes of BLRGs are roughly linearly correlated, as expected, though with a substantial (order-of-magnitude) scatter. In addition, however, it is revealed that 3C 111, 3C 120, and also Pictor A are at the same time the brightest in radio, but only when the nuclear (and not the total) radio fluxes are taken into account. Moreover, the nuclear radio fluxes seem well correlated with the X-ray fluxes for the whole BLRG population. Hence, one may conclude that Fermi-LAT detects preferentially those BLRGs which are characterized by the brightest radio and X-ray nuclei. This supports the idea stated previously that the GeV emission of BLRGs is dominated by the innermost parts of their jets, and is therefore "blazar-like," being dependent on the jet luminosity and viewing angle. As argued above, the quasar 3C 273, for which the GeV luminosity is about 100 times higher than the nuclear radio luminosity, should provide a reasonably accurate template for such an emission. Indeed, as illustrated in Figure 6, the γ-ray-to-radio energy flux ratios for the two BLRGs detected by Fermi-LAT are of the order of ≃ 100, while the corresponding upper limits for all the other objects from the sample (including Seyfert galaxies) are above or much above this value. This indicates that the detections of BLRGs in γ-rays are at present limited by the sensitivity of the Fermi-LAT instrument. In fact, from visual inspection of Figures 8 and 9, the predicted γ-ray flux from the template is very close to the Fermi-LAT upper limits for Pictor A, 3C 390.3, 3C 445, B3 0309+411B, and PKS 2153−69.

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Also it is important to note that the parsec-scale components of the radio jets in both 3C 111 and 3C 120 are characterized by apparent superluminal motions with maximum velocities β_app ≃ 5.9 and 5.3, respectively, from MOJAVE monitoring data (Lister et al. 2009). The corresponding upper limits to the jet viewing angles are thus θ_j ≲ 20°. Interestingly, mildly relativistic velocities are inferred for 3C 390.3 and Pictor A (β_app up to ≃ 2.2 and 1.6, respectively; Kellermann et al. 2004; Tingay et al. 2000), both of which are good candidates for future Fermi-LAT detections (see Table 2 and Figure 8). These four BLRGs, not coincidentally, have the brightest radio/VLBI cores in the analyzed sample, consistent also with a relatively large degree of relativistic beaming (see the comment in the Appendix). Note also that 3C 303 and 3C 382 are other BLRGs with possible mildly relativistic parsec-scale VLBI jets (Giovannini et al. 2001).

But what else—besides the nuclear radio jets—could be the source of high-energy emission in the considered objects? Clearly, large-scale structures (i.e., extended lobes, large-scale jets and terminal hotspots), which are particularly prominent in BLRGs, are expected to contribute at least at some level to the γ-ray emission in the GeV band, since the non-thermal synchrotron continua of these structures extend to the highest radio frequencies and even into the X-ray energies in some cases (see in this context Fermi-LAT detection of the extended lobes in the low-power radio galaxy Centaurus A; Abdo et al. 2010e). Such structures are on the other hand almost absent in Seyfert galaxies, due to much less prominent jet activity in those objects. Other possible γ-ray emission sites in both BLRGs and Seyfert galaxies are their accretion disks and disk coronae. Several related (and rather preliminary) studies presented in the literature, even though not directly applicable to the types of astrophysical objects discussed here, indicate that other additional processes than those typically considered for generating high-energy photons within the accreting matter (e.g., proton–proton interactions) may be relevant in this context. In these scenarios, the resulting disk-related γ-ray emission may strongly depend on the main parameters of a black hole/accretion disk system (such as the black hole mass, spin, accretion rate, and disk inclination; see, e.g., Mahadevan et al. 1997; Oka & Manmoto 2003; Niedzwiecki et al. 2009). Hence, it would be interesting to look into this issue in more detail for all the sources included in our sample.

Figure 7 presents therefore the dependence of the γ-ray luminosities (or upper limits for such), expressed in the Eddington units (L_γ/L_Edd) or as a ratio of the GeV and X-ray monochromatic luminosities (L_γ/L_X), on (1) the black hole mass $\mathcal {M}_{\rm BH}$ (top left panel), (2) the accretion ratio L_acc/L_Edd determined from the model fitting (top right panel), (3) the proxy of the radio-loudness parameter L_R/L_B (bottom left panel), and (4) the observed proxy of the accretion rate L_B/L_Edd (bottom right panel). In the figure, BLRGs and Seyfert galaxies are denoted by blue and red symbols, respectively. Large filled circles represent the two BLRGs detected by Fermi-LAT (3C 111 and 3C 120), while the medium-size open circles represent Pictor A. In the bottom left panel, when nuclear radio fluxes are used instead of the total radio fluxes, BLRGs are denoted by cyan symbols, and Seyfert galaxies by magenta symbols. A ratio of the GeV and X-ray monochromatic luminosities (L_γ/L_X) expected from a hybrid model is given as dashed lines in the bottom right panel, for different values of a mixing parameter η = 10⁻², 10⁻¹, and 1.

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Note that the monochromatic luminosity ratio L_R/L_B is simply proportional to the "standard" radio-loudness parameter $\mathcal {R}$, and in particular that $L_{\rm R} / L_{\rm B} = (\nu _{\rm R} / \nu _{\rm B}) \times \mathcal {R} \sim 10^{-5} \, \mathcal {R}$. Also, for the high-accretion-rate objects analyzed in this paper one expects the accretion luminosity L_acc ≃ L_tot ≃ 10 × L_B. As shown, the two galaxies which are the brightest in γ-rays, and also Pictor A, are not characterized by any outstanding values of the radio loudness, accretion rate, or black hole mass, but are in fact quite representative for the other BLRGs included in the sample. This supports once again our conclusion that the detected GeV emission of BLRGs is related predominantly to nuclear jets, and not the other possible nuclear emission components. What should be noted for constraining theoretical models regarding the high-energy emission of accretion disks in AGNs is the fact that, at least for a number of sources, the Fermi-LAT upper limits evaluated here already probe the GeV emission of BLRGs and Seyfert galaxies down to the levels of 1% of the X-ray (disk-related) luminosity, or equivalently 0.01% Eddington luminosity (see Figure 7).

The Fermi-LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council, and the Swedish National Space Board in Sweden.

Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d'Études Spatiales in France.

This research has made use of data from the University of Michigan Radio Astronomy Observatory, which has been supported by the University of Michigan and by a series of grants from the National Science Foundation, most recently AST-0607523.

Ł.S. acknowledges the support from the Polish MNiSW through the grant N-N203-380336. We thank the anonymous referee for critical comments which helped to improve the paper.

Here we present all the SEDs of the BLRGs (Figure 8) and Seyfert 1 galaxies (Figure 9) analyzed in this paper which are not detected by Fermi-LAT, together with the hybrid model fits described in the main text. The figures illustrate the expected level of the GeV emission of each source considered, in comparison with the current upper limits provided by two years of Fermi-LAT data. Note in particular that the expected γ-ray fluxes of Pictor A, 3C 390.3, 3C 445, B3 0309+411B, and PKS 2153−69 are close to the Fermi-LAT limits derived in this paper (cf. Table 2). In contrast to the BLRGs (or at least the brightest and most beamed examples of such considered in this work), we speculate that the jet-related non-thermal emission of all the analyzed Seyfert 1 galaxies within the GeV photon energy range are in general beyond the level of detectability with Fermi-LAT.

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