Cherenkov Telescope Array

When γ-rays strike the Earth’s atmosphere they cause a cascade of subatomic particles. Some of these particles travel faster than the speed of light in air and consequently emit Cherenkov radiation (analogous to the ‘sonic boom’ of objects

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travelling faster than the speed of sound). It is these faint flashes of blue light which are detected by Cherenkov telescopes and used to determine the energy and direction of the incident γ-ray.

The Cherenkov Telescope Array (CTA) comprises two ground-based telescope arrays capable of detecting very high energy γ-rays (20 GeV to 300 TeV). The Northern hemisphere site is at the Roque de los Muchachos Observatory in La Palma and the Southern hemisphere site is at the European Southern Observatory (ESO) on Cerro Paranal in Chile. The arrays consist of three different sizes of telescope which give sensitivity in different energy bands. 4 large telescopes (23 m diameter) at each site provide sensitivity in the low energy range, 20 GeV–150 GeV. 40 medium-sized telescopes (12 m) shared between the two sites will provide coverage in the core energy range of 150 GeV–5 TeV. The array of 70 small-sized telescopes (4 m) will be installed at the Southern Site only and give sensitivity to the highest energies (5–300 TeV). Construction is currently planned to be completed in 2025, although preliminary science may be conducted as early as 2023, using only part of the array

Chapter 4

The γ-ray emitting narrow-line

Seyfert 1 1H 0323+342

γ-ray emitting narrow-line Seyfert 1 galaxies (γ-NLS1s) are a very rare class of blazar- like AGN with powerful relativistic jets. Unlike the vast majority of blazars (BL Lacertae objects: ‘BL Lacs’ and flat spectrum radio quasars: ‘FSRQs’), γ-NLS1s have been found to have low supermassive black hole (SMBH) masses and high accretion rates. It has been shown that blazar jet and accretion flow powers are well-correlated, so there must be some connection between the accretion and ejection process within blazars. The γ-NLS1s therefore provide us with the opportunity to investigate this connection at much lower SMBH masses and higher accretion rates than the general blazar population. This is important because it is still poorly understood how jets are launched and powered and why some AGN have powerful jets whereas the majority do not.

In this chapter, I present a case study of the source 1H 0323+342 which is currently the nearest γ-NLS1. This source a superb laboratory in which to explore the disc-jet connection. It has been extensively studied in the literature and has an extremely well-sampled multiwavelength data set which I exploit to model the ambient photon field produced by the accretion flow. I self-consistently apply this within a jet emission model to fit the entire broadband spectrum. I explore its mutliwavelength

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properties and discuss the nature of the source as both a NLS1 and a blazar. I test simple scaling relations to examine whether this source can be interpreted as a ‘mini-FSRQ’: a typical blazar, but with much lower SMBH mass.

4.1

Introduction

The detection of several radio-loud narrow-line Seyfert 1 (NLS1) galaxies by the

Fermi Gamma-Ray Space Telescope hints at the existence of a rare, new class of γ-ray emitting active galactic nuclei with low SMBH masses. Like FSRQs, their γ-ray emission is thought to be produced via the external Compton mechanism

whereby relativistic jet electrons upscatter a photon field external to the jet, e.g. from the accretion disc, broad line region (BLR) and dusty torus, to higher energies. Unlike FSRQs, which have high-mass SMBHs with log(M_BH/M) ≈ 8–10, NLS1s are powered by accretion onto a SMBH of much lower mass (Abdo et al. 2009b;

Foschini 2011).

The mechanisms by which relativisitic jets are launched and accelerated remain poorly understood. However, these γ-NLS1s can provide new insights on how these processes might scale with BH mass. In terms of AGN unification schemes it is insightful to investigate whether γ-NLS1s represent the low-mass, low-power tail of FSRQs in this sequence, or whether they constitute a genuinely new class of their own.

In this chapter I present a detailed study of the nearest γ-NLS1, 1H 0323+3421 (RA: 03 24 41.16, Dec: +34 10 45.8), at a redshift of z = 0.0625 (Landt et al. 2017). High-energy γ-ray emission has been associated with its radio counterpart with high significance, and was first reported byAbdo et al. (2009b).

I assemble an unprecedentedly well-sampled SED containing several relatively high S/N spectra as well as complementary photometry. SEDs for this object have

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previously been presented in e.g.Abdo et al. (2009b), Paliya et al. (2014) and Yao

et al. (2015a), but here I include much more spectral and photometric data to

assemble a more detailed and quasi-simultaneous SED.

I take a new approach to investigating the disc-jet connection. I observationally constrain the external photon field using quasi-simultaneous near-IR, optical and X-ray spectroscopy. Applying a one-zone leptonic jet model, I simulate the range of jet parameters for which this photon field, when Compton scattered to higher energies, can explain the γ-ray emission. I find that the site of the γ-ray emission lies well within the BLR and that the seed photons mainly originate from the accretion disc. The jet power that we determine, 1.0 × 1045 erg s−1_{, is approximately half} the accretion disc luminosity. I show that this object is not simply a low-mass FSRQ, its jet is intrinsically less powerful than predicted by scaling a typical FSRQ jet by SMBH mass and accretion rate. That γ-ray emitting NLS1s appear to host underpowered jets may go some way to explaining why so few have been detected to date.

This chapter is organised as follows: in Section 4.2 I present the multiwavelength data set I have assembled for this source. The mass of the source is determined from NIR and optical spectroscopic data in Section 4.3. In Section 4.4 I provide a detailed analysis of the XMM-Newton X-ray spectrum. I describe how I use the multiwavelength data to determine the ambient photon field contributions from the accretion disc, X-ray corona, BLR and torus in Section 4.5.1. In Section 4.5.2 I self-consistently apply this ambient photon field within a jet model to reproduce the entire broadband SED from radio to γ-rays. I test simple scaling relations for the jet with the SMBH mass and accretion rate and I determine the power in the jet and compare this to the accretion power and the jet power of other blazars. The discussion and conclusions are presented in Section4.6and Section4.7. In particular, I investigate the nature of this source as both a NLS1 and a blazar.

For my adopted cosmological parameters, the redshift z = 0.0625 implies a luminosity distance of 280 Mpc and a flux-to-luminosity conversion factor of 9.41 × 1054 cm2.