Publications by Katherine Blundell

Particle acceleration in astrophysical jets

New Astronomy Reviews Elsevier (2020)

J Matthews, A Bell, K Blundell

In this chapter, we review some features of particle acceleration in astrophysical jets. We begin by describing four observational results relating to the topic, with particular emphasis on jets in active galactic nuclei and parallels between different sources. We then discuss the ways in which particles can be accelerated to high energies in magnetised plasmas, focusing mainly on shock acceleration, second-order Fermi and magnetic reconnection; in the process, we attempt to shed some light on the basic conditions that must be met by any mechanism for the various observational constraints to be satisfied. We describe the limiting factors for the maximum particle energy and briefly discuss multimessenger signals from neutrinos and ultrahigh energy cosmic rays, before describing the journey of jet plasma from jet launch to cocoon with reference to the different acceleration mechanisms. We conclude with some general comments on the future outlook.

Uncovering the orbital dynamics of stars hidden inside their powerful winds: application to $η$ Carinae and RMC 140

Monthly Notices of the Royal Astronomical Society Oxford University Press 494 (2020) 17-35

D Grant, K Blundell, J Matthews

Determining accurate orbits of binary stars with powerful winds is challenging. The dense outflows increase the effective photospheric radius, precluding direct observation of the Keplerian motion; instead the observables are broad lines emitted over large radii in the stellar wind. Our analysis reveals strong, systematic discrepancies between the radial velocities extracted from different spectral lines: the more extended a line's emission region, the greater the departure from the true orbital motion. To overcome these challenges, we formulate a novel semi-analytical model which encapsulates both the star's orbital motion and the propagation of the wind. The model encodes the integrated velocity field of the out-flowing gas in terms of a convolution of past motion due to the finite flow speed of the wind. We test this model on two binary systems. (1), for the extreme case $\eta$ Carinae, in which the effects are most prominent, we are able to fit the model to 10 Balmer lines from H-alpha to H-kappa concurrently with a single set of orbital parameters: time of periastron $T_{0}=2454848$ (JD), eccentricity $e=0.91$, semi-amplitude $k=69$ km/s and longitude of periastron $\omega=241^\circ$. (2) for a more typical case, the Wolf-Rayet star in RMC 140, we demonstrate that for commonly used lines, such as He II and N III/IV/V, we expect deviations between the Keplerian orbit and the predicted radial velocities. Our study indicates that corrective modelling, such as presented here, is necessary in order to identify a consistent set of orbital parameters, independent of the emission line used, especially for future high accuracy work.

Classical Nova Carinae 2018: Discovery of circumbinary iron and oxygen

Monthly Notices of the Royal Astronomical Society Oxford University Press 494 (2020) 743-749

D McLoughlin, K Blundell, S Lee

We present time-lapse spectroscopy of a classical nova explosion commencing 9 days after discovery. These data reveal the appearance of a transient feature in Fe ii and [O i]. We explore different models for this feature and conclude that it is best explained by a circumbinary disc shock-heated following the classical nova event. Circumbinary discs may play an important role in novae in accounting for the absorption systems known as THEA, the transfer of angular momentum, and the possible triggering of the nova event itself.

Probing the non-thermal emission in the Perseus cluster with the JVLA

Proceedings of the International Astronomical Union (2020) 44-52

M Gendron-Marsolais, J Hlavacek-Larrondo, RJ Van Weeren, T Clarke, AC Fabian, HT Intema, GB Taylor, KM Blundell, JS Sanders

© International Astronomical Union 2020. We present deep low radio frequency (230-470 MHz) observations from the Karl G. Jansky Very Large Array of the Perseus cluster, probing the non-thermal emission from the old particle population of the AGN outflows. Our observations of this nearby relaxed cool core cluster have revealed a multitude of new structures associated with the mini-halo, extending to hundreds of kpc in size. Its irregular morphology seems to have been influenced both by the AGN activity and by the sloshing motion of the cluster' gas. In addition, it has a filamentary structure similar to that seen in radio relics found in merging clusters. These results illustrate the high-quality images that can be obtained with the new JVLA at low radio-frequencies.

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.

Cosmic ray acceleration in hydromagnetic flux tubes

Monthly Notices of the Royal Astronomical Society Oxford University Press 487 (2019) 4571-4579

AR Bell, J Matthews, KM Blundell, AT 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 magnetized 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.

Cosmic ray acceleration to ultrahigh energy in radio galaxies

EPJ Web of Conferences EDP Sciences 210 (2019) 04002

J 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.

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.

Studying the H-alpha line of the B[ e ] supergiant binary GG Carinae using high-cadence optical spectroscopy

Proceedings of the International Astronomical Union (2019) 123-124

A Porter, K Blundell, S Lee

© International Astronomical Union 2019. We present a case study of GG Carinae (GG Car), a Galactic B[ e ] supergiant binary having significant eccentricity (0.28), based on Global Jet Watch spectroscopy data which has been collecting high-time-sampled optical spectra since early 2015. GG Car has so far not been observed in the X-ray band, however it is of similar phenomenology to known X-ray binaries and may therefore be an obscured X-ray source. We have discovered that the absorption component of the H-alpha line displays a '1/462-478-day period in both equivalent width and wavelength centroid indicating cycles in the dynamics of the circumstellar environment, such as precession of the circumbinary or circumprimary disk. Circumbinary disk precession is an as-of-yet underexplored origin of super-orbital variations in the X-ray flux of X-ray binaries, since the rate of precession is generally much longer than the orbital period of the inner binary.

On the maximum energy of protons in the hotspots of AGN jets

ULTRA HIGH ENERGY COSMIC RAYS 2018 (UHECR 2018) 210 (2019) ARTN 04006

AT Araudo, AR Bell, J Matthews, K Blundell

SS433's Jet Trace from ALMA Imaging and Global Jet Watch Spectroscopy: Evidence for Post-launch Particle Acceleration


KM Blundell, R Laing, S Lee, A Richards

Fornax A, Centaurus A and other radio galaxies as sources of ultra-high energy cosmic rays

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 479 (2018) L76-L80

JH Matthews, AR Bell, KM Blundell, AT Araudo

The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starbust galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-lobed radio galaxies such as Centaurus A and Fornax A can explain the data.

LOFAR 150-MHz observations of SS 433 and W 50


JW Broderick, RP Fender, JCA Miller-Jones, SA Trushkin, AJ Stewart, GE Anderson, TD Staley, KM Blundell, M Pietka, S Markoff, A Rowlinson, JD Swinbank, AJ van der Horst, ME Bell, RP Breton, D Carbone, S Corbel, J Eisloeffel, H Falcke, J-M Griessmeier, JWT Hessels, VI Kondratiev, CJ Law, GJ Molenaar, M Serylak, BW Stappers, J van Leeuwen, RAMJ Wijers, R Wijnands, MW Wise, P Zarka

Evidence that particle acceleration in hotspots of FR II galaxies is not constrained by synchrotron cooling

Nuclear and Particle Physics Proceedings 297-299 (2018) 242-248


© 2018 We study the hotspots of powerful radiogalaxies, where electrons accelerated at the jet termination shock emit synchrotron radiation. The turnover of the synchrotron spectrum is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the shock is ≲ TeV for a canonical magnetic field of ∼100 μG. We show that this maximum energy cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonably large and most of the jet upstream energy goes to non-thermal particles. We test this result by considering a sample of hotspots observed at radio, infrared and optical wavelengths.

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.

Cosmic ray acceleration by relativistic shocks: Limits and estimates

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

AR Bell, AT Araudo, JH Matthews, KM Blundell

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.

Deep 230-470 MHz VLA Observations of the mini-halo in the Perseus Cluster

MNRAS 469 (2017) 2017-2017

M Gendron-Marsolais, J Hlavacek-Larrondo, RJV Weeren, T Clarke, AC Fabian, HT Intema, GB Taylor, KM Blundell, JS Sanders

We present a low-frequency view of the Perseus cluster with new observations from the Karl G. Jansky Very Large Array (JVLA) at 230-470 MHz. The data reveal a multitude of new structures associated with the mini-halo. The mini-halo seems to be influenced both by the AGN activity as well as by the sloshing motion of the cool core cluster's gas. In addition, it has a filamentary structure similar to that seen in radio relics found in merging clusters. We present a detailed description of the data reduction and imaging process of the dataset. The depth and resolution of the observations allow us to conduct for the first time a detailed comparison of the mini-halo structure with the X-ray structure as seen in the Chandra X-ray images. The resulting image shows very clearly that the mini-halo emission is mostly contained behind the cold fronts, similar to that predicted by simulations of gas sloshing in galaxy clusters. However, due to the proximity of the Perseus cluster, as well as the quality of the data at low radio frequencies and at X-ray wavelengths, we also find evidence of fine structure. This structure includes several radial radio filaments extending in different directions, a concave radio structure associated with the southern X-ray bay and sharp edges that correlate with X-ray edges. Mini-halos are therefore not simply diffuse, uniform radio sources, but are rather filled with a rich variety of complex structures. These results illustrate the high-quality images that can be obtained with the new JVLA at low radio-frequencies, as well as the necessity to obtain deeper, higher-fidelity radio images of mini-halos and halos in clusters to further understand their origin.

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.

Fast launch speeds in radio flares, from a new determination of the intrinsic motions of SS 433's jet bolides

Monthly Notices of the Royal Astronomical Society Oxford University Press 461 (2016) 312-320

K Blundell, RM Jeffrey, SA Trushkin, AJ Mioduszewski

We present new high-resolution, multi-epoch, VLBA radio images of the Galactic microquasar SS 433. We are able to observe plasma knots in the milliarcsecond-scale jets more than 50 days after their launch. This unprecedented baseline in time allows us to determine the bulk launch speed of the radio-emitting plasma during a radio flare, using a new method which we present here, and which is completely independent of optical spectroscopy. We also apply this method to an earlier sequence of 39 short daily VLBA observations, which cover a period in which SS 433 moved from quiescence into a flare. In both datasets we find, for the first time at radio wavebands, clear evidence that the launch speeds of the milliarcsecondscale jets rise as high as 0.32c during flaring episodes. By comparing these images of SS 433 with photometric radio monitoring from the RATAN telescope, we explore further properties of these radio flares.

Evidence that the maximum electron energy in hotspots of FR II galaxies is not determined by synchrotron cooling

Monthly Notices of the Royal Astronomical Society (2016) stw1204-stw1204

AT Araudo, AR Bell, A Crilly, KM Blundell