Transients, pulsars and high-energy astrophysics

 Supervisors: Boon Kok-Tan and Gianluca Gregori

Since the invention of the chirped pulse amplification technique by Strickland and Mourou (2018 Nobel prize in Physics), high intensity lasers focused onto solid foils are now able to accelerate electrons in the matter to relativistic velocities by their strong electric fields. These electrons then interact with the nuclei and produce copious electron-positron pair jets. These jets mimic properties of gamma-ray fireballs and can be used to investigate the microphysics of extreme astrophysical phenomena as well as tools for fundamental physics investigations. The goal of this project is to develop a novel gamma-ray detector using superconducting quantum technologies to study the high-energy gamma ray emission during pair production in order to optimise the jet emission and characterise its properties. The developed detectors can also be used for detecting gamma-ray from other non-astronomical sources such as lab-based radiometry, as long as it is within the designated mass range.

There are several promising candidates for developing such novel superconducting quantum gamma-ray detector. For this project, we expect to explore the possibility of using superconducting tunnel junctions (STJs) and/or Kinetic Inductance Detectors (KIDs) technology as gamma-ray detector. Both technologies have been widely used in astronomy in the past. STJs have been one of the main workforces for millimetre and sub-millimetre astronomy, while KIDs have been deployed for detecting photon ranging from microwave up to X-ray regime. Here, the student will first investigate the feasibility of using one of these technologies for gamma-ray detection with high energy resolution. Once the most suitable technology is identified, the student will proceed to design and fabricate the devices, along with setting up the experiment arrangement required to test the performance of the gamma-ray detector. 

This programme is comprising two complementary science topics. First, a focus on the development of the superconducting quantum gamma-ray detectors, and second using the developed detector to understand the microphysics of extreme astrophysical phenomena. The project will suit a student who enjoys reading and understanding the underlying theoretical work of quantum sensors, superconducting electromagnetism, as well as state-of-the-art astrophysics development while enjoying coding, lab-based experimental works and data analysis. We have a state-of-the-art cryogenic detector laboratory comprising several sub-Kelvin dilution refrigerators and many high-end test and measurement equipment. The student will also be supported by a technician and postdocs in addition to the supervisors. He/she will also have access to commercial and our own software/code in order to perform the research. 

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.105.015003

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8453/84532N/High-resolution-gamma-ray-detection-using-phonon-mediated-detectors/10.1117/12.926829.full?SSO=1

https://link.springer.com/article/10.1007/s11432-020-2932-8

https://www.sciencedirect.com/science/article/pii/S0921453417300643

Supervisor: Katherine Blundell

Nova explosions occur much more frequently than supernova events and arise as the result of a thermonuclear runaway on the surface of a white dwarf. The recent discovery of jets being ejected at the onset of a nova explosion, which have speeds of a few thousand km/s, suggests an important means by which the inter-stellar medium can be enriched by the products of nucleosynthesis that take place on the surface of the white dwarf.  The goal is to investigate the mechanisms by which these processes take place, and the efficacy of enrichment of the ISM by jets from nova explosions, using time-lapse data from the Global Jet Watch (PI K Blundell; www.GlobalJetWatch.net).

Supervisors: Rob Fender and Ian Heywood (Oxford Astrophysics) in collaboration with Michael Kramer (Max Planck Institute for Radio Astronomy, Bonn)

Astrophysical transients represent the sites of the most extreme physics in the universe since the Big Bang. Strong and variable emission in radio waves from these events allows us to track both the (huge!) kinetic energy output of the transient, and to pinpoint with the highest possible accuracy the physical site of the event. Our team in Oxford leads the world in both high cadence temporal observations of radio emission from such events, and deep studies to unearth the faintest, most distant and rarest of such transients. In recent years we have had huge success using the MeerKAT radio telescope in South Africa to study radio transients, in a 5-year approved-time project led by Professor Fender. MeerKAT is about to begin a dual upgrade path in which new, higher-frequency receivers are being installed, and new antennas are being added on longer physical baselines. These upgrades will dramatically improve the sensitivity, frequency agility and angular resolution of the telescope. At Oxford, Dr Heywood, co-supervisor, is the world leader in the analysis of data from this telescope, and is responsible for the iconic MeerKAT images of the Galactic centre region. The project will be conducted in close collaboration with the Max Planck Institute for Radio Astronomy in Bonn, Germany, which has guaranteed time with both the new high-frequency receivers and increased angular resolution array. As a result, the successful applicant will be the amongst the first in the world to utilise the new, upgraded MeerKAT array to study astrophysical transients.

Potential targets include outbursts from black holes and neutron stars within our own galaxy, gamma-ray bursts and supernovae at cosmological distances and - most excitingly - the propsect of radio follow-up of gravitational-wave neutron star-neutron star merger events when LIGO and VIRGO recommence operations next year. Based upon recent successes we can be very confident that significant discoveries and breakthroughs lie ahead. We encourage applications from candidates who are keen to work in a large and dynamic international team, and who are enthusiastic to work both with data and their interpretation.

Supervisors: Gianluca Gregori, Archie Bott, and Alexander Schekochihin

Gamma-ray bursts (GRBs) are among the most energetic events in the Universe. They occur at cosmological distances and are the result of the collapse of massive stars or neutron stars mergers, with emission of relativistic “fireballs" of electron-positron pairs. From astrophysical observations, a wealth of information has been gleaned about the mechanism that leads to such strong emission of radiation, with leading models predicting that this is due to the disruption of the beam as it blasts through the surrounding plasma. This produces shocks and hydromagnetic turbulence that generate synchrotron emission, potentially accelerating to ultra-high energies the protons which are observed on Earth as cosmic rays. However, there is no direct evidence of the generation of either magnetic fields or cosmic rays by GRBs. Estimates are often based on crude energy equipartition arguments or idealized numerical simulations that struggle to capture the extreme plasma conditions. We propose to address this lacuna by conducting laboratory experiments at large laser and accelerator facilities to mimic the jet propagation through its surrounding plasma. Such experiments will enable in situ measurement of the plasma properties, with exquisite details that cannot be achieved elsewhere. The experiments also complement numerical simulations by providing long measurement times extending into the non-linear regime where numerical simulations are not possible today. The proposed experiments will study fundamental physics processes, unveil the microphysics of GRBs, and provide a new window in high energy astrophysics using novel Earth-based laboratory tools.

Background Reading:

1. C. D. Arrowsmith et al., "Generating untradense pair beams using 400 GeV/c protons," Phys. Rev. Res. 3, 023103 (2021)

Supervisor: Aris Karastergiou

Please click here for a video description.

Project description: Using pulsars for experiments in fundamental physics relies more and more on our understanding of the objects themselves. What are their orientations in space? What is the origin of their radio emission and how does it relate to the rotating star? Modern surveys with the MeerKAT telescope in South Africa will provide the data for measurements of radio emission also at multiple epochs, providing an additional axis to separate those effects that are intrinsic to the star from those related to propagation of the radiation through the intervening media. The student will work within an international collaboration (www.meertime.org) to explore the characteristics of a large population of pulsars, monitored through the so-called Thousand Pulsar Array. Results from this project directly feed into our understanding of the cold and dense nuclear matter in neutron star interiors, the plasma physics processes the occur in pulsar magnetospheres, the properties of the ionized and magnetized interstellar medium, the birth and evolution of neutron stars, and interpretation of the neutron star population in the context of modern results in gravitational wave astrophysics.

Supervisor: Bence Kocsis

The recent discovery of gravitational waves opened new horizons for understanding the Universe. The measurements have unveiled an abundant population of stellar mass black hole mergers. The great challenge is to understand the possible astrophysical mechanisms that may lead to mergers. The existing theoretical models of their astrophysical origin are currently either highly incomplete or in tension with data (Barack+ 2019).

An interesting possibility is that the observed black hole mergers are generated in dense stellar systems such as globular clusters or galactic nuclei harboring a supermassive black hole with or without a gaseous accretion disk. In these systems, stellar mass black holes sink to the center of the cluster and undergo frequent dynamical encounters, forming binaries, which may eventually merge and produce gravitational wave emission.

In this project, the student will work with Prof. Bence Kocsis to build a comprehensive dynamical model of dense stellar systems with a population of stellar mass black holes. The black holes are expected to settle to a flattened disk-like structure which gets twisted and warped due to the fluctuating anisotropy of the otherwise spherical surrounding star cluster. We will use a combination of analytic and numerical methods including statistical mechanics, kinetic theory, and N-body simulations to determine if such subsystems may be long-lived, how it affects the evolution of the cluster, study the formation and evolution of binaries, and examine the implications for electromagnetic and gravitational wave observatories.

Abbott R. et al., 2021, ApJL 913, 7 (https://iopscience.iop.org/article/10.3847/2041-8213/abe949)

Barack L. et al., 2019, Classical and Quantum Gravity, Volume 36, Issue 14, article id. 143001 (https://arxiv.org/abs/1806.05195)

Samsing et al. 2022, 603, 7900, 237 

Szolgyen A. & Kocsis B. 2018, PRL, 121, 101011 (https://arxiv.org/abs/1803.07090)

 Supervisor: James Matthews

Supermassive black holes are thought to lie at the centre of virtually every galaxy, and many of them are "active", in the sense that they accrete matter from their surroundings and give of prodigous amounts of radiation. These Active Galactic Nuclei (AGN) are fascinating and astrophysically important objects.  Remarkably, the accretion process also expels outflowing material, a process that can transport huge amounts of energy and momentum to vast distances, allowing the AGN to influence physical proceedings far from its gravitational sphere of influence. These outflows are split into two broad classes -- narrow beams of relativistic material called "jets", and slower, wider-angle flows called "winds". Both winds and jets are important in influencing their surroundings, but also seem to be intimately connected to the underlying AGN accretion disc. Additionally, the outflowing material dissipates its energy in shocks and turbulence which can accelerate particles to high energy, producing radio gamma-ray emission as well as other messengers like neutrinos and cosmic rays. Understanding the details of this energy dissipation process, the particle acceleration physics, and the connection between observable quantities and the underlying physics is an important goal of modern high-energy astrophysics.

Oxford takes a prominent role in a range of international projects relevant for particle acceleration in AGN outflows, with prominent examples being the Cherenkov Telescope Array (CTA), a next-generation ground-based gamma-ray observatory, and MeerKAT, a powerful radio telescope which acts as a precursor for the Square Kilometre Array. As a result, this is an exciting time to be working on the topic, and the project may involve collaboration with Profs. Rob Fender and Garret Cotter to exploit synergies with other research groups.

Movies available here.

The exact direction of the DPhil project will be flexible, but the main aim of the project is to provide the theoretical insight and numerical modelling needed to maximise the science output from the next generation of instruments. In particular, we will use hydrodynamic simulations combined with particle acceleration physics to predict observational signatures from AGN outflows and study the link between particle acceleration and observational signatures. An important part of the project will be the development of a new particle module (or adapting an existing one) so that we can predict radio and gamma-ray spectra and images from numerical simulations. There are various astrophysical applications for this tool. One possibility is to model nonthermal emission from AGN wind shocks, produced as winds from the accretion disc propagate into the surrounding galaxy. Another is to conduct simulations of AGN jets on large scales, with the aim of predicting combined observational signatures for the next generation of gamma-ray and radio telescopes. This DPhil project may also involve working on ultrahigh energy cosmic rays, and the theory of particle acceleration at shocks, and interested students may want to take a look at my other project on cosmic ray origins.

Supervisor: James Matthews

Cosmic rays (CRs) are high energy particles bombarding the Earth's atmosphere from space. The highest energy CRs, known as ultrahigh energy cosmic rays (UHECRs) have jaw-dropping energies of up to 1e20 electron Volts, orders of magnitude higher than the maximum particle energies attainable in the LHC and corresponding to Lorentz factors of ~1e11. The origin of these particles is not yet known, but there is strong evidence that they must be produced in extragalactic astrophysical sources. Attempting to correlate their arrival directions with potential sources is challenging, because the UHECRs undergo large deflections in astrophysical magnetic fields. However, this is an exciting time to be studying UHECRs, because the state of the art Pierre Auger Observatory has recently detected anisotropies in the arrival directions, with potential correlations with nearby galaxies at the highest energies. In addition, together with neutrino and gamma-ray observatories like IceCube, HESS and CTA (the latter two with Oxford involvement), we now have a truly 'multimessenger' view of the high-energy sky.

In our work, we will explore scenarios in which jets from supermassive black holes accelerate UHECRs, and use a combination of theory and numerical modelling to test these scenarios against the observed data. One model I'm particularly excited about is the so-called "echoes" model, in which UHECRs reverberate off nearby magnetic structures within a few Megaparsecs of the Milky Way, before propagating towards Earth. More detailed simulations are needed to test this model, and the topic is inherently multi-disciplinary, spanning topics from the circumgalactic medium around galaxies to the physics of plasma instabilities in shock waves.

This project is flexible, and may involve collaboration with Prof. Garret Cotter to exploit synergies with other research groups. Interested students may want to take a look at my other project on particle acceleration in outflows, which has substantial overlap with this project. 

Supervisor: Stephen Smartt

Sky surveys in the optical can now cover the whole sky every 24 hours, allowing association of optical transients with high energy sources, radio emission and gravitational waves from compact binary mergers. Supernovae from the death of massive stars are still primarily discovered by optical surveys but some can emit non-thermal radiation from the x-ray to radio.  Kilonovae are theoretically predicted electromagnetic transients from the merger of neutron stars. Such mergers produce gravitational wave sources and in one case the two have been linked.

We lead searches for supernovae from the most massive stars and the nature of the most luminous supernovae known. Our team also searches for kilonovae both with and without associated gravitational wave emission. We have access to the ATLAS sky survey project, the Pan-STARRS system, several follow-up programmes at ESO and lead the development of the UK's broker system to find transients in the Rubin Observatory's Legacy Survey of Space and time. Projects will focus on an aspect of transient astrophysics linking these facilities and applying simple models to understand the energetics of the explosions.

Further reading:

https://ui.adsabs.harvard.edu/abs/2017Natur.551...75S/abstract

https://ui.adsabs.harvard.edu/abs/2018ApJ...865L...3P/abstract

https://ui.adsabs.harvard.edu/abs/2019ApJ...885L..23M/abstract

https://ui.adsabs.harvard.edu/abs/2020A%26A...643A.113A/abstract