Universität zu Köln

Ultra-high resolution observations of selected blazars

Hodgson, Jeffrey Adam (2015) Ultra-high resolution observations of selected blazars. PhD thesis, Universität zu Köln.

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    Abstract

    Active Galactic Nuclei are the luminous centres of active galaxies that produce powerful relativistic jets from central super massive black holes (SMBH). When these jets are oriented towards the observer’s line-of-sight, they become very bright, very variable and very energetic. These sources are known as blazars and Very Long Baseline Interferometry (VLBI) provides a direct means of observing into the heart of these objects. VLBI performed at 3 mm with the Global mm-VLBI Array (GMVA) and 7 mm VLBI performed with the Very Long Baseline Array (VLBA), allows some of the highest angular resolution images of blazars to be produced. In this thesis, we present the first results of an ongoing monitoring program of blazars known to emit at γ-ray energies. The physical processes that produce these jets and the γ-ray emission are still not well known. The jets are thought to be produced by converting gravitational energy around the black hole into relativistic particles that are accelerated away at near the speed of light. However, the exact mechanisms for this and the role that magnetic fields play is not fully clear. Similarly, γ-rays have been long known to have been emitted from blazars and that their production is often related to the up-scattering of synchrotron radiation from the jet. However, the origin of seed photons for the up-scattering (either from within the jet itself or from an external photon field) and the location of the γ-ray emission regions has remained inconclusive. In this thesis, we aim to describe the likely location of γ-ray emission in jets, the physical structure of blazar jets, the location of the VLBI features relative to the origin of the jet and the nature of the magnetic field, both of the VLBI scale jet and in the region where the jet is produced. We present five sources that have been monitored at 3 mm using the GMVA from 2008 until 2012. These sources have been analysed with near-in-time 7 mm maps from the Very Long Baseline Array (VLBA), γ-ray light curves from the Fermi/LAT space telescope and cm to mm-wave total-intensity light curves. In one source, OJ 287, the source has additionally been analysed with monthly imaging at 7 mm with the VLBA and near-in-time 2 cm VLBI maps. We use these resources to analyse high angular resolution structural and spectral changes and see if they correlate with flaring (both radio and γ-ray) activity and with VLBI component ejections. By spectrally decomposing sources, we can determine the spatially resolved magnetic field structure in the jets at the highest yet performed resolutions and at frequencies that are near or above the turnover frequency for synchrotron self-absorption (SSA). We compute the magnetic field estimates from SSA theory and by assuming equipartition between magnetic fields and relativistic particle energies. v All sources analysed exhibit downstream quasi-stationary features which sometimes exhibit higher brightness temperatures and flux density variability than the VLBI “core”, which we interpret as being recollimation or oblique shocks. We find that γ-ray flaring, mm-wave radio flaring and changes in opacity from optically thick to optically thin, is in many cases consistent with component ejections past both the VLBI “core” and these quasi-stationary downstream features. We find decreasing apparent brightness temperatures and Doppler factors as a function of increased “core” separation, which is interpreted as consistent with a slowly accelerating jet over the de-projected inner ∼10-20 pc. Assuming equipartition between magnetic energy and relativistic particle energy, the magnetic field strengths within the jets at these scales are, on average, between B ∼ 0.3 − 0.9 G, with the highest strengths found within the VLBI “core”. From the observed gradient in magnetic field strengths, we can place the mmwave “core” ∼1-3 pc downstream of the base of the jet. Additionally, we estimate the the magnetic field is Bapex ∼ 3000 − 18000 G at the base of the jet. We computed theoretical estimates based on jet production under magnetically arrested disks (MAD) and find our estimates to be consistent. In the BL Lac source OJ 287, we included monthly 7 mm and near-in-time 2 cm VLBA maps to provide full kinematics and increased spectral coverage. Following a previously reported radical change in inner-jet PA of ∼100◦we find unusually discrepant PAs compared with the previous jet direction, that follow very different trajectories. The source exhibits a downstream quasi-stationary feature that at times has higher brightness temperatures than the “core”. The source also exhibited a large change in apparent component speeds as compared with previous epochs, which we propose could be due to changes in jet pressure causing changes in the location of downstream recollimation or oblique shocks and hence their line-of-sight viewing angle. The addition of 2 cm VLBA data allows for a comparison of magnetic fields derived from SSA and equipartition. The magnetic field estimates are consistent within 20%, with BSSA ≥ 1.6 G and Bequi ≥ 1.2 G in the “core” and BSSA ≤ 0.4 G and Bequi ≤ 0.3 G in the stationary feature. Gamma-ray emission appears to originate in the “core” and the stationary feature. The decrease in magnetic field strengths places the mmwave “core” downstream of the jet base by ≤6 pc and likely outside of the broad line region (BLR). This, combined with the results in other sources are consistent with γ-rays being produced in the vicinity of the VLBI “core” of in further downstream stationary features, which are likely over a parsec downstream of the central black hole, favouring the scenario of photons being up-scattered within the relativistic jet.

    Item Type: Thesis (PhD thesis)
    Creators:
    CreatorsEmail
    Hodgson, Jeffrey Adamjhodgo@gmail.com
    URN: urn:nbn:de:hbz:38-64925
    Subjects: Natural sciences and mathematics
    Uncontrolled Keywords:
    KeywordsLanguage
    AGNEnglish
    blazarsEnglish
    VLBIEnglish
    Faculty: Mathematisch-Naturwissenschaftliche Fakultät
    Divisions: Mathematisch-Naturwissenschaftliche Fakultät > I. Physikalisches Institut
    Language: English
    Date: 28 January 2015
    Date Type: Publication
    Date of oral exam: 27 January 2015
    Full Text Status: Public
    Date Deposited: 29 Dec 2015 14:09:42
    Referee
    NameAcademic Title
    Eckart, AndreasProf. Dr.
    Zensus, AntonProf. Dr.
    URI: http://kups.ub.uni-koeln.de/id/eprint/6492

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