1.2.2.1 Identification from optical and multiwavelength properties
Osterbrock & Pogge (1985) identified a subset of ‘narrow-line’ Seyfert 1s (NLS1s),
in which the broad permitted lines were relatively narrow. Sometimes the FWHMs of the Balmer lines are only a few hundred km s−1 (similar to typical forbidden line widths and the permitted lines of S2s), as in the case of the first NLS1 to be discovered, Mrk 359 (Davidson & Kinman 1978). The distinction of NLS1s from broad-line Seyferts and S2s is often made with reference to three optical spectral properties set out byGoodrich (1989):
• a FWHM of permitted emission lines (usually Hβ) < 2000 km s−1;
1.2. Putting it all together 28
• strong Fe ii emission.
The first criterion distinguishes NLS1s from their more common broad-line coun- terparts. The second and third criteria separate NLS1s from S2s, which also have narrow permitted lines. Because the emission from the BLR is obscured in S2s, their total permitted line luminosities are lower than those in S1s. Forbidden line emission from the NLR is visible in the spectra of both S1s and S2s, so S2s will have lower permitted to forbidden line ratios than S1s. The line ratio above is commonly used to divide S1s and S2s (Shuder & Osterbrock 1981). Fe ii emission is thought to
originate from the BLR and so is absent in the spectra of S2s. Being type 1 AGN with unobscured nuclei, the accretion disc continuum emission can be seen in the optical spectra of NLS1s, but not S2s (see Figure 1.10).
Boroson & Green(1992) conducted a principal component analysis of a sample of 87
quasars, exploring how the optical spectral parameters varied together, and with the radio and X-ray properties. They identified a number of trends in the correlation space (see Figure1.12, left). Firstly, a strong anticorrelation of Fe ii9 and [O iii] λ5007
strength was evident. These relations are said to describe ‘Eigenvector 1’10. It is possible to see a ‘main sequence’ connecting quasars across the Eigenvector 1 optical plane (see Figure 1.12, right). Sulentic et al. (2000) noted that the properties of NLS1s were continuous with those of broad-line sources, so NLS1s do not represent a distinct population of AGN, but the extreme end of this sequence.
The widths of the permitted lines in NLS1s imply low SMBH masses (see Section2.1). Indeed,Rakshit et al. (2017) found that a sample of 11101 NLS1s had hlog MBHi = 6.9 M , compared with hlog MBHi = 8.0 M for BLS1s. It follows that since NLS1s have similar luminosities to broad-line AGN, but lower masses, they must have a greater L/LEdd. It has been suggested that L/LEdd is one of the main drivers of Eigenvector 1 (e.g. Sulentic et al. 2000).
9The Fe ii strength is often characterised as R
Fe ii, the ratio of the integrated flux of the Fe ii λ4570
complex to the flux of broad Hβ.
10A second eigenvector relates the optical luminosity with the strength of He ii and the optical-
1.2. Putting it all together 29
Figure 1.12: The optical plane Eigenvector 1 and the quasar main sequence. Left: Correlations between FWHM(Hβ), soft X-ray photon index Γ, RFe ii and the equi- valent width of Fe ii λ4570. From Sulentic et al. (2000). Right: A dark horizontal dot-dashed line separates broad-line (FWHM(Hβ)> 4000 km s−1) Population B sources from narrower-line Population A sources. The light horizontal dot-dashed line indicates the 2000 km s−1 limit for NLS1s. NLS1s with strong Fe ii emission (RFe ii > 1) form part of the extreme tail of Population A sources (xA). FromMarziani
et al.(2018).
Boroson & Green (1992) found that sources with strong Fe ii emission have soft
X-ray photon indices and narrower broad Hβ (see Figure 1.12). These observations have been supported by other studies. For example,Leighly(1999) studied a sample of NLS1s with ASCA and found that their X-ray spectral indices were significantly steeper Γ ≈ 2–2.5 than those measured in other Seyferts. A correlation between the broad Hβ line width and X-ray photon index was also noted.
With regard to the radio properties, Boroson & Green (1992) found a distinction between radio-loud and radio-quiet sources with respect to their Fe ii emission. Radio- loud sources tended to have weak Fe ii, while the opposite was true for radio-quiet sources. The incidence of radio-loudness among NLS1s has been found to be very low (≈ 7 per cent,Komossa et al. 2006 or ≈ 5 per cent, Rakshit et al. 2017). This compares with a radio-loud fraction of 15–20 per cent among quasars generally
1.2. Putting it all together 30
It should be noted that NLS1s were first identified from observations of luminous AGN in the local Universe (z ∼ 0.01, Osterbrock & Pogge 1985). As noted by
McHardy et al. (2006), that NLS1s are found to have low SMBH masses is a selec-
tion effect, because AGN with greater MBH would have to be accreting above the Eddington limit to be luminous enough to produce broad emission lines narrower than 2000 km s−1 (see also e.g. Czerny et al. 2018). Indeed, some AGN have been found to have many similar characteristics as NLS1s, but do not qualify as such because of their broader emission lines, resulting from their higher SMBH mass; PDS 456 is one such example (Reeves et al. 2003). McHardy et al. (2006) point out the linewidth depends not upon ˙m or MBHalone, but their ratio ˙m/MBH. This may motivate the introduction of ‘NLQ1s’: higher-mass, broader-line quasar analogues of NLS1s. Perhaps instead it would be better to do away with arbitrarily classifying type 1 AGN by linewidth altogether.
1.2.2.2 The γ-ray emitters
As noted above, the incidence of radio-loudness among NLS1s is rare, and their radio- loud fraction is much smaller than is found for BLS1s and AGN in general. However,
Zhou et al. (2007), Yuan et al. (2008), Foschini et al.(2009) and others noted that
several of the radio-loudest NLS1s exhibited blazar-like properties (e.g. flat radio spectra, compact one-sided jet structures seen in radio images and strong optical, UV and X-ray variability behaviour), suggesting the presence of a relativistic jet. Foschini
et al.(2009) thought that the evidence for relativistic jets in NLS1s was compelling
but not conclusive; however, they said that the detection in γ-rays by the Fermi
Gamma-Ray Space Telescope, which had recently been launched (see Section 3.5.1),
would provide confirmation of the blazar nature of these sources. Indeed, γ-ray emission from the NLS1 PMN 0948+0022 was detected with high significance by
Fermi shortly after its launch in 2008 (Abdo et al. 2009a). This discovery was soon
followed by the γ-ray detection of three more NLS1s, 1H 0323+342, PKS 1502+036 and PKS 2004−447 (Abdo et al. 2009b).
1.2. Putting it all together 31
Table 1.2: Known γ-ray emitting narrow-line Seyfert 1 galaxies Source name RA Dec. z Reference
1H 0323+342 03 24 41.16 +34 10 45.8 0.0625 Abdo et al. (2009b) SBS 0846+513 08 49 75.983 +51 08 29.08 0.5840 Donato & Perkins(2011) J0932+5306 09 32 41.1 +53 06 33.3 0.5970 Paliya et al. (2018) J0937+5008 09 37 12.33 +50 08 52.1 0.2754 Paliya et al. (2018) SDSS J0946+1017 09 46 35.06 +10 17 06.11 1.004 Yao et al. (2019) PMN J0948+0022 09 48 57.319 +00 22 25.52 0.5836 Abdo et al. (2009a) B2 0954+25A 09 56 49.880 +25 15 16.05 0.7076 Calderone et al. (2012) J0958+3224 09 58 20.9 +32 24 01.6 0.5306 Paliya et al. (2018) J1102+2239 11 02 23.36 +22 39 20.7 0.453 Foschini et al. (2015) J1146+3236 11 46 54.30 +32 36 52.2 0.465 AB
PKS J1222+0413 12 22 22.548 +04 13 15.75 0.9662 Yao et al. (2015a) J1238+3942 12 38 52.15 +39 42 27.6 0.623 Foschini et al. (2015) SDSS J1246+0238 12 46 34.649 +02 38 09.02 0.3623 Foschini et al. (2015) J1305+5116 13 05 22.749 +51 16 40.26 0.7876 FL8Y
U1575-03416792 14 10 45.83 +74 05 11.1 0.429 Marchesini et al.(2016) J1421+2824 14 21 14.07 +28 24 52.2 0.538 Paliya et al. (2018) B3 1441+476 14 43 18.561 +47 25 56.71 0.6972 D’Ammando et al. (2016) PKS 1502+036 15 05 06.476 +03 26 30.83 0.4083 Abdo et al. (2009b) B3 1518+423 15 20 39.610 +42 11 08.90 0.4847 Paliya et al. (2018) J1641+3452 16 41 00.11 +34 54 52.68 0.1641 Lähteenmäki et al. (2018) FBQS J1644+2619 16 44 42.536 +26 19 13.25 0.1440 D’Ammando et al. (2015) PKS 2004−447 20 07 55.18 −44 34 44.4 0.240 Abdo et al. (2009b) PMN J2118+0013 21 18 17.4 +00 13 16.8 0.4629 Paliya et al. (2018) J2118−0732 21 18 52.97 −07 32 27.55 0.260 Yang et al. (2018a)
Notes: FL8Y is the Fermi LAT 8-year source list. AB: The γ-ray detection of this
RL-NLS1 was made by Dr. Anthony Brown of the Gamma Ray Astronomy Group at Durham University.
Over the last decade, more γ-ray emitting NLS1s (γ-NLS1s) have been found. The increasing depth of Fermi data over time has made the detection of fainter γ-NLS1s possible, and allowed more time for usually faint sources to flare above the detection threshold. In the case of PKS J1222+0413, the optical counterpart of a known γ-ray source was identified as a NLS1 (Yao et al. 2015a). To date, only 24 γ-NLS1s are known; a list of those sources is given in Table1.2.
Early studies (e.g.Laor 2000; McLure & Dunlop 2001) found no evidence of a RL- AGN population with BH masses MBH . 108 M and RL-AGN were not among the Population A (FWHM(Hβ) < 4000 km s−1) sources of Marziani et al. (2001). High-mass SMBHs are almost exclusively found in elliptical galaxies with large bulges, leading to ideas that there is something about the evolutionary history of
1.2. Putting it all together 32
these systems which triggers jet production such as SMBH spin (Blandford & Znajek 1977) or the history of concentration of magnetic flux (Sikora & Begelman 2013), or both. The discovery of γ-NLS1s and lower-mass RL-AGN (Ho 2002;Yuan et al. 2008) which are hosted by spiral galaxies has broken this simple paradigm. This has opened up a new region of parameter space in which to explore the relationship between jets and accretion discs.
In this thesis, detailed multiwavelength studies of the nearest (1H 0323+342, z = 0.063) and second-most distant (PKS J1222+0413, z = 0.967) γ-NLS1s are made in Chapter 4and Chapter 5, respectively.
Chapter 2
Methods and models
In this chapter I outline some of the methods and models which are used throughout this thesis. Section 2.1 discusses different methods to measure SMBH masses; the single-epoch virial method (Section 2.1.3) is most commonly used in the following chapters. Section2.2 describes methods to deal with observations in which the flux is attenuated by reddening or absorption. Sections 2.3and 2.4describe two of the models which are applied to multifrequency spectral energy distributions in order to determine the physical parameters of the system.