Publications by James Matthews

Cosmic Ray Acceleration in Hydromagnetic Flux Tubes

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2019)

A BELL, J Matthews, K Blundell, A Araudo

We find that hydromagnetic flux tubes in back-flows in the lobes of radio galaxies offer a suitable environment for the acceleration of cosmic rays (CR) to ultra-high energies. We show that CR can reach the Hillas (1984) energy even if the magnetised turbulence in the flux tube is not sufficiently strong for Bohm diffusion to apply. First-order Fermi acceleration by successive weak shocks in a hydromagnetic flux tube is shown to be equivalent to second-order Fermi acceleration by strong turbulence.

The origin of radio emission in broad absorption line quasars: Results from the LOFAR Two-metre Sky Survey

Astronomy and Astrophysics EDP Sciences 622 (2018) A15

L Morabito, J Matthews, G Gurkan, P Best, M Jarvis, I Prandoni, K Duncan, M Kunert-Bajraszewska, M Hardcastle, A Mechev, J Sabeter, S Mooney, H Rottgering, T Shimwell, D Smith, C Tasse, W Williams

We present a study of the low-frequency radio properties of broad absorption line quasars (BALQSOs) from the LOFAR Two-metre Sky-Survey Data Release 1 (LDR1). The value-added LDR1 catalogue contains Pan-STARRS counterparts, which we match with the Sloan Digital Sky Survey (SDSS) DR7 and DR12 quasar catalogues. We find that BALQSOs are twice as likely to be detected at 144 MHz than their non-BAL counterparts, and BALQSOs with low-ionisation species present in their spectra are three times more likely to be detected than those with only high-ionisation species. The BALQSO fraction at 144 MHz is constant with increasing radio luminosity, which is inconsistent with previous results at 1.4 GHz, indicating that observations at the different frequencies may be tracing different sources of radio emission. We cross-match radio sources between the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) survey and LDR1, which provides a bridge via the LDR1 Pan-STARRS counterparts to identify BALQSOs in SDSS. Consequently we expand the sample of BALQSOs detected in FIRST by a factor of three. The LDR1-detected BALQSOs in our sample are almost exclusively radio-quiet (log(R144 MHz) <2), with radio sizes at 144 MHz typically less than 200 kpc; these radio sizes tend to be larger than those at 1.4 GHz, suggesting more extended radio emission at low frequencies. We find that although the radio detection fraction increases with increasing balnicity index (BI), there is no correlation between BI and either low-frequency radio power or radio-loudness. This suggests that both radio emission and BI may be linked to the same underlying process, but are spatially distinct phenomena.

Ultra-high energy cosmic rays from shocks in the lobes of powerful radio galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 482 (2018) 4303–4321-

J Matthews, B Bell, K Blundell, AT Araudo

The origin of ultra-high energy cosmic rays (UHECRs) has been an open question for decades. Here, we use a combination of hydrodynamic simulations and general physical arguments to demonstrate that UHECRs can in principle be produced by diffusive shock acceleration (DSA) in shocks in the backflowing material of radio galaxy lobes. These shocks occur after the jet material has passed through the relativistic termination shock. Recently, several authors have demonstrated that highly relativistic shocks are not effective in accelerating UHECRs. The shocks in our proposed model have a range of non-relativistic or mildly relativistic shock velocities more conducive to UHECR acceleration, with shock sizes in the range 1 − 10 kpc. Approximately 10% of the jet’s energy flux is focused through a shock in the backflow of M > 3. Although the shock velocities can be low enough that acceleration to high energy via DSA is still efficient, they are also high enough for the Hillas energy to approach 1019−20 eV, particularly for heavier CR composition and in cases where fluid elements pass through multiple shocks. We discuss some of the more general considerations for acceleration of particles to ultra-high energy with reference to giant-lobed radio galaxies such as Centaurus A and Fornax A, a class of sources which may be responsible for the observed anisotropies from UHECR observatories.

Cosmic ray acceleration by shocks: spectral steepening due to turbulent magnetic field amplification

Monthly Notices of the Royal Astronomical Society Oxford University Press 488* (2019) 2466-2472

A Bell, J Matthews, K Blundell

We show that the energy required to turbulently amplify magnetic field during cosmic ray (CR) acceleration by shocks extracts energy from the CR and steepens the CR energy spectrum.

Hot, dense He II outflows during the 2017 outburst of the X-ray transient Swift J1357.2−0933

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 489 (2019) L47–L52-

P Charles, J Matthews, P Gandhi, D Buckley, E Kotze, J Paice

Time-resolved SALT spectra of the short-period, dipping X-ray transient, Swift J1357.2−0933, during its 2017 outburst has revealed broad Balmer and He II λ4686 absorption features, blueshifted by ∼600 km s−1. Remarkably these features are also variable on the ∼500 s dipping period, indicating their likely association with structure in the inner accretion disc. We interpret this as arising in a dense, hot (≳30 000 K) outflowing wind seen at very high inclination, and draw comparisons with other accretion disc corona sources. We argue against previous distance estimates of 1.5 kpc and favour a value ≳6 kpc, implying an X-ray luminosity LX ≳ 4 × 1036 erg s−1. Hence it is not a very faint X-ray transient. Our preliminary 1D Monte Carlo radiative transfer and photoionization calculations support this interpretation, as they imply a high intrinsic LX, a column density NH ≳ 1024 cm−2, and a low covering factor for the wind. Our study shows that Swift J1357.2−0933 is truly remarkable amongst the cohort of luminous, Galactic X-ray binaries, showing the first example of He  II λ4686 absorption, the first (and only) variable dip period and is possibly the first black hole ‘accretion disc corona’ candidate.

Cosmic ray acceleration to ultrahigh energy in radio galaxies

EPJ Web of Conferences EDP Sciences (2019)

JH Matthews, AR Bell, AT Araudo, KM Blundell

The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In this proceeding, we first review the general physical requirements that a source must meet for acceleration to 10-100 EeV, including the consideration that the shock is not highly relativistic. We show that shocks in the backflows of radio galaxies can meet these requirements. We discuss a model in which giant-lobed radio galaxies such as Centaurus A and Fornax A act as slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a more powerful past episode. We also show that Centaurus A, Fornax A and other radio galaxies may explain the observed anisotropies in data from the Pierre Auger Observatory, before examining some of the difficulties in associating UHECR anisotropies with astrophysical sources.

Do reverberation mapping analyses provide an accurate picture of the broad-line region?

Monthly Notices of the Royal Astronomical Society Oxford University Press 488 (2019) 2780–2799-

SW Mangham, C Knigge, P Williams, K Horne, A Pancoast, J Matthews, KS Long, N Higginbottom

Reverberation mapping (RM) is a powerful approach for determining the nature of the broad-line region (BLR) in active galactic nuclei. However, inferring physical BLR properties from an observed spectroscopic time series is a difficult inverse problem. Here, we present a blind test of two widely used RM methods: MEMECHO (developed by Horne) and CARAMEL (developed by Pancoast and collaborators). The test data are simulated spectroscopic time series that track the Hα emission line response to an empirical continuum light curve. The underlying BLR model is a rotating, biconical accretion disc wind, and the synthetic spectra are generated via self-consistent ionization and radiative transfer simulations. We generate two mock data sets, representing Seyfert galaxies and QSOs. The Seyfert model produces a largely negative response, which neither method can recover. However, both fail ‘gracefully', neither generating spurious results. For the QSO model both CARAMEL and expert interpretation of MEMECHOś output both capture the broadly annular, rotation-dominated nature of the line-forming region, though MEMECHO analysis overestimates its size by 50 per cent, but CARAMEL is unable to distinguish between additional inflow and outflow components. Despite fitting individual spectra well, the CARAMEL velocity-delay maps and RMS line profiles are strongly inconsistent with the input data. Finally, since the Hα line-forming region is rotation dominated, neither method recovers the disc wind nature of the underlying BLR model. Thus considerable care is required when interpreting the results of RM analyses in terms of physical models.

The luminosity dependence of thermally driven disc winds in low-mass X-ray binaries

Monthly Notices of the Royal Astronomical Society 484 (2019) 4635-4644

N Higginbottom, C Knigge, KS Long, JH Matthews, EJ Parkinson

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. We have carried out radiation-hydrodynamic simulations of thermally driven accretion disc winds in low-mass X-ray binaries. Our main goal is to study the luminosity dependence of these outflows and compare with observations. The simulations span the range 0.04 ≤ L acc /L Edd ≤ 1.0 and therefore cover most of the parameter space in which disc winds have been observed. Using a detailed Monte Carlo treatment of ionization and radiative transfer, we confirm two key results found in earlier simulations that were carried out in the optically thin limit: (i) the wind velocity - and hence the maximum blueshift seen in wind-formed absorption lines - increases with luminosity; (ii) the large-scale wind geometry is quasi-spherical, but observable absorption features are preferentially produced along high-column equatorial sightlines. In addition, we find that (iii) the wind efficiency always remains approximately constant at skew4dotM-rm wind/skew4dotM-rm acc simeq 2, a behaviour that is consistent with observations. We also present synthetic Fe xxv and Fe xxvi absorption line profiles for our simulated disc winds in order to illustrate the observational implications of our results.

On the maximum energy of non-thermal particles in the primary hotspot of Cygnus A

Monthly Notices of the Royal Astronomical Society Oxford University Press 473 (2017) 3500–3506-

AT Araudo, AR Bell, KM Blundell, JH Matthews

We study particle acceleration and magnetic field amplification in the primary hotspot in the northwest jet of radiogalaxy Cygnus A. By using the observed flux density at 43 GHz in a well resolved region of this hotspot, we determine the minimum value of the jet density and constrain the magnitude of the magnetic field. We find that a jet with density greater than $5\times 10^{-5}$ cm$^{-3}$ and hotspot magnetic field in the range 50-400 $\mu$G are required to explain the synchrotron emission at 43 GHz. The upper-energy cut-off in the hotspot synchrotron spectrum is at a frequency < $5\times 10^{14}$ Hz, indicating that the maximum energy of non-thermal electrons accelerated at the jet reverse shock is $E_{e, \rm max} \sim 0.8$ TeV in a magnetic field of 100 $\mu$G. Based on the condition that the magnetic-turbulence scale length has to be larger than the plasma skin depth, and that the energy density in non-thermal particles cannot violate the limit imposed by the jet kinetic luminosity, we show that $E_{e,\rm max}$ cannot be constrained by synchrotron losses as traditionally assumed. In addition to that, and assuming that the shock is quasi-perpendicular, we show that non-resonant hybrid instabilities generated by the streaming of cosmic rays with energy $E_{e, \rm max}$ can grow fast enough to amplify the jet magnetic field up to 50-400 $\mu$G and accelerate particles up to the maximum energy $E_{e, \rm max}$ observed in the Cygnus A primary hotspot.

Radiation-hydrodynamic simulations of thermally-driven disc winds in X-ray binaries: A direct comparison to GRO J1655-40

Monthly Notices of the Royal Astronomical Society (2018)

N Higginbottom, C Knigge, KS Long, JH Matthews, SA Sim, HA Hewitt

Cosmic ray acceleration by relativistic shocks: Limits and estimates

Monthly Notices of the Royal Astronomical Society Oxford University Press (2017)

AR Bell, AT Araudo, KM Blundell, JH Matthews

We examine limits to the energy to which cosmic rays can be accelerated by relativistic shocks, showing that acceleration of light ions as high as 100 EeV is unlikely. The implication of our estimates is that if ultra-high energy cosmic rays are accelerated by shocks, then those shocks are probably not relativistic.

The reverberation signatures of rotating disc winds in active galactic nuclei


SW Mangham, C Knigge, JH Matthews, KS Long, SA Sim, N Higginbottom

Amplification of perpendicular and parallel magnetic fields by cosmic ray currents

Monthly Notices of the Royal Astronomical Society Oxford University Press 469 (2017) 1849-1860

JH Matthews, A Bell, KM Blundell, AT Araudo

Cosmic ray (CR) currents through magnetized plasma drive strong instabilities producing amplification of the magnetic field. This amplification helps explain the CR energy spectrum as well as observations of supernova remnants and radio galaxy hotspots. Using magnetohydrodynamic simulations, we study the behaviour of the non-resonant hybrid (NRH) instability (also known as the Bell instability) in the case of CR currents perpendicular and parallel to the initial magnetic field. We demonstrate that extending simulations of the perpendicular case to 3D reveals a different character to the turbulence from that observed in 2D. Despite these differences, in 3D the perpendicular NRH instability still grows exponentially far into the non-linear regime with a similar growth rate to both the 2D perpendicular and 3D parallel situations. We introduce some simple analytical models to elucidate the physical behaviour, using them to demonstrate that the transition to the non-linear regime is governed by the growth of thermal pressure inside dense filaments at the edges of the expanding loops. We discuss our results in the context of supernova remnants and jets in radio galaxies. Our work shows that the NRH instability can amplify magnetic fields to many times their initial value in parallel and perpendicular shocks.

Quasar emission lines as probes of orientation: implications for disc wind geometries and unification


JH Matthews, C Knigge, KS Long

Testing quasar unification: radiative transfer in clumpy winds

Monthly Notices of the Royal Astronomical Society 458 (2016) 293-305

JH Matthews, C Knigge, KS Long, SA Sim, N Higginbottom, SW Mangham



F Shankar, G Calderone, C Knigge, J Matthews, R Buckland, K Hryniewicz, G Sivakoff, X Dai, K Richardson, J Riley, J Gray, F La Franca, D Altamirano, J Croston, P Gandhi, S Hoenig, I McHardy, M Middleton

The impact of accretion disc winds on the optical spectra of cataclysmic variables


JH Matthews, C Knigge, KS Long, SA Sim, N Higginbottom



N Higginbottom, D Proga, C Knigge, KS Long, JH Matthews, SA Sim

A simple disc wind model for broad absorption line quasars


N Higginbottom, C Knigge, KS Long, SA Sim, JH Matthews



R Di Stefano, J Matthews, S Lepine