We report the discovery by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope of high-energy gamma-ray emission from the peculiar quasar PMN J0948+0022 (z=0.585+/-0.001). Contrary to the expectations, the optical spectrum of this quasar shows only narrow lines [FWHM(Hbeta) ~ 1500 km/s] and the typical characteristics of narrow-line Seyfert 1 type galaxies. However, the strong radio emission and the flat spectrum suggest the presence of a relativistic jet, which can now be confirmed by the detection of MeV-GeV photons. PMN J0948+0022 is therefore the first radio-loud narrow-line Seyfert 1 quasar to be detected at gamma-rays and the third type of gamma-ray emitting AGN, after blazars and radiogalaxies.
It is known that there are two types of γ-ray emitting active galactic nuclei (AGN): blazars and radiogalaxies. Their spectral energy distribution (SED) has typically two broad peaks: one, at low frequencies, is due to the radiation emitted by synchrotron processes; the second, at high frequencies, is thought to be due to inverse-Compton scattering (IC) of high-energy electrons off ambient seed photons. The underlying physical mechanism generating such a type of SED is supposed to be the same: a relativistic jet observed with different viewing angles, very small in the case of blazars and larger for radiogalaxies (see Fossati et al. 1998 andDonato et al. 2001 for blazars; see Ghisellini et al. 2005, Tavecchio & Ghisellini 2008 for radiogalaxies).
The SEDs of blazars seem to have different shapes depending on the emitted power and are organized in the so-called “blazar sequence” (Fossati et al. 1998).
In particular, high-luminosity blazars have the peaks at low frequencies (“red” blazars), while as the luminosities of the objects decrease, the peaks shift to higher frequencies, so that the lowest luminosity blazars are detected even at TeV energies (“blue” blazars).
The blazar sequence can be interpreted in terms of changes of the seed photons for the IC processes (Ghisellini et al. 1998). Blue blazars have no or weak emission lines (equivalent width EW < 5 Å) and the IC seed photons are those from the synchtrotron radiation (e.g. Ghisellini et al. 1985, Band & 2 Grindlay 1985). Instead, red blazars have strong (EW > 5 Å) emission lines and the seed photons are from the broad-line region (BLR) or accretion disk or even from the molecular torus (e.g. Dermer et al. 1992, Sikora et al. 1994, B lażejowski et al. 2000). It is worth noting that all the permitted emission lines -with weak or strong intensities -are broad, i.e. with F W HM > 2000 km/s (see, e.g., Wills & Browne 1986, Wang et al. 2009).
This was, roughly speaking, the scenario before of the launch of Fermi.
The surprise came with the detection by the Large Area Telescope (LAT, Atwood et al. 2009), onboard the Fermi satellite, of a bright γ-ray source associated with the quasar PMN J0948+0022 (Abdo et al. 2009a,b,c). This quasar is known to be a radio-loud narrow-line Seyfert 1, with narrow permitted lines, bump of FeII and the flux ratio between [OIII] and Hβ smaller than 3 (Zhou et al. 2003, Komossa et al. 2006, Yuan et al. 2008). Particularly, the FWHM of Hβ is about 1500 km/s (Zhou et al. 2003, Yuan et al. 2008), the narrowest permitted line ever detected in a γ-ray emitting AGN. Two observations at different epochs (28 February and 27 March 2000) were available at the Sloan Digital Sky Survey (SDSS1 ), indicating a change in the intensity (EW) of the Hβ from 16 Å to 21 Å within about one month. This “surprise” was somehow expected, since radio observations of PMN J0948+0022 have shown a compact source, with flat spectrum and high brightness temperature, suggesting the presence of a relativistic jet (Doi et al. 2006, Yuan et al. 2008). The γ-ray detection by Fermi confirmed this suggestion and allowed to build a complete SED: it was found that PMN J0948+0022 has the characteristics of flat-spectrum radio quasars (FSRQ), but with low power, relatively small mass (1.5 × 10 8 M ⊙ ) and high accretion (40% the Eddington value). More details can be found in Abdo et al. (2009c). We take the opportunity of this work to deal with more details on the classification. Indeed, this will be the first source of a new population of γ-ray emitting AGN and, therefore, it is necessary to address carefully all the known possible issues and doubts.
Fig. 1 shows the SED of PMN J0948+0022 compared with the blazar sequence (continuous lines of different colors) and a few of the most powerful radiogalaxies (Cen A, M 87, NGC 6251). It is immediately evident that PMN J0948+0022 is in the region of blazars, with the observed emitted power well above the radiogalaxies region. This is an observational evidence, which does not need for any further explanation.
The study of the morphology of the source at radio frequencies shows a very compact source from 1.7 to 15.4 GHz (see Doi et al. 2006), except for the data at 1.4 GHz from the NVSS (Fig. 2). In this case, there seems to be an extended structure, but it is likely to be an artifact of low angular resolution of NVSS (F W HM = 45 ′′ , Condon et al. 1998). Indeed, images at higher resolution (F W HM = 5 ′′ , Becker et al. 1995) from the FIRST survey (1.4 GHz) indicated the presence of two resolved sources, 1 ′ .2 distant each other, one of which is the core of PMN J0948+0022 (with flux 107.5 ± 0.1 mJy), while the second one is unknown and has a radio flux of 8.0 ± 0.1 mJy.
No optical data are available in any public catalog. Swift/UVOT observations found no source at any filter with these upper limits (3σ, in units of 10 -13 erg cm -2 s -1 ): V < 3.7 , B < 2.5, U < 1.5, U V W 1 < 0.9, U V M 2 < 0.9, U V W 2 < 0.6. No
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