Transients, pulsars and high-energy astrophysics

Supervisors: Chris Lintott, Steve Croft

The large, wide-field, multipurpose LSST survey conducted with the Vera C. Rubin Observatory will bring unprecedented opportunities for scientific discovery. A census of the solar system will result in the discovery of millions of new small bodies, the transient survey will find thousands of new supernovae and other transients each year and deep images will show the low surface brightness universe for the first time. In each of these domains, there exists the opportunity for finding the truly unexpected and unusual. 

Such anomaly detection requires the development of new machine learning methods capable of highlighting objects or events worthy of attention. Amongst the data, there exists the possibility of technosignatures - evidence of intelligent life in the Universe - which is a particular focus for this work. 

This DPhil, which overlaps with the first data releases from the survey, will develop such techniques for a broad set of science cases. With the opportunity to follow up objects of interest via facilities around the world, this project would suit a student with a broad interest in observational astronomy and a desire to develop skills in citizen science, machine learning and beyond. 

Black holes are the sites of the most extreme physics in the universe since the Big Bang. Their most poorly understood characteristic is the formation of powerful collimated outflows of mass and energy, moving at extremely high speeds away from the region of the event horizon. These ‘relativistic jets’ can have a huge impact on their surrounding environments, even regulating the growth of the most massive galaxies on cosmic scales.

 Black holes also span an enormous range in masses and hence timescales. In the lowest mass black holes, stellar-mass black holes in ‘X-ray binary’ systems, we can track the jets from their formation and launch at just a handful of event horizon radii to their eventual deceleration and termination in the interstellar medium a year or so later. Our team in Oxford leads the world in studying the connection between these jets and the inner accretion flow, as well as their late-time deceleration phases, where they finally dissipate their launch energy in the ambient interstellar medium. We lead major guaranteed-time radio observing programmes of black holes on a number of radio telescope arrays, including MeerKAT in South Africa, as well as working at the interface between observation, interpretation and numerical modelling.

We have two DPhil positions available to work on aspects of black hole accretion and jet formation. These projects are funded as part of a large European project, Blackholistic, a partnership between The University of Oxford, The University of Amsterdam and Radboud University. The project is affiliated to the large Event Horizon Telescope collaboration. As a result 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.

Project 1: Relativistic jets from black holes from birth to termination

Supervisors: Rob Fender and Ian Heywood

In this project we will complete our understanding of the entire life cycle of these jets, using very long baseline interferometry (VLBI) to image relativistic jets from stellar mass black holes within minutes of their launch. These will be synchronised to later observations on larger scales with radio arrays such as MeerKAT to provide comprehensive monitoring from launch to termination. We will push on the existing state of the art by going to high frequencies, and coordinating with multiple facilities at other wavelengths. In the second half of the project we will explore how the Event Horizon Telescope can be used in a rapid-response mode to directly resolve these dynamically evolving jets closer to the black hole than ever previously possible.

Project 2: Scaling black hole accretion and jet formation from stellar-mass to supermassive black holes

Supervisors: Rob Fender and James Matthews

In this project we aim to understand how we can test models of jet formation in the accretion flow and how these processes scale across the black hole mass range from stellar-mass to supermassive black holes. In particular we will work with teams in The Netherlands to see what observational tests can be made of their general relativistic MHD jet launch modelling, will work on propagating jets from launch to termination in our own relativistic HD simulations, and will test - via a combination of observational interpretation and modelling - how these processes scale with mass and the properties of the ambient environment. This problem is inherently multi-scale and will connect the small scale black hole and accretion physics with the large scale dynamics of astrophysical jets, touching on accretion, jet formation, particle acceleration and shocks. The successful applicant will be prepared to work at the interface of observation, theory and modelling, and will be comfortable working in a large international collaboration.

Supervisors : Maria Vicenzi and Stephen Smartt

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will start in January 2025 and is expected to completely revolutionise transient astronomy and, in particular, cosmological measurements using Type Ia supernovae. 

Rubin and LSST involve a wide network of international scientists, and our team is leading the preparation for the beginning of the survey and will be heavily involved in the analysis of the first transient data. We work on a wide range of projects (both observations oriented and simulations oriented) that will have a high impact on LSST science. These projects revolve around the development and testing of transient discovery and classification pipelines, design and optimisation of LSST spectroscopic follow-up programs, and the design of new strategies to use and compile cosmological samples using LSST supernova data.

 We are also leading various analyses of recently concluded supernova surveys like the Dark Energy Survey supernova sample (the largest and deepest supernova survey from a single telescope in the pre-LSST era) and current/upcoming SN surveys with the space telescopes Euclid (an ESA led facility, now working) and Nancy Grace Roman (a future NASA mission). These surveys will provide tremendous datasets to tackle some of the most pressing questions in supernova cosmology and astrophysics. The DPhil project can focus on either the astrophysics of the explosions or the application to cosmology. On the astrophysics side, we aim to answer the question of how do white dwarfs explode to give type Ia supernovae and how many possible stellar evolutionary channels lead to this end point. How they are affected by dust extinction and their host galaxy properties critically affect their use as cosmological probes.

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).

Supervisor: Katherine Blundell

In recent years the existence and significance of circumbinary discs, orbiting outside of pairs of binary stars in orbit around one another, has emerged. Not only are these purported to have significant dynamical back-reactions on their inner binary stars (and hence their evolution) and in the development of nova explosions but they are in some cases likely to be the breeding ground of Tatooine-like circumbinary planets. The goal is to explore and understand the nature of binary star systems that host such circumbinary structures, using data from the Global Jet Watch telescopes (PI K Blundell; www.GlobalJetWatch.net).

Supervisors: Joe Bright and Ian Heywood in collaboration with Wael Farah (SETI Institute)

A new generation of telescopes is transforming our view of the radio sky. The large field of view, high time and frequency resolution, capabilities for commensal observations, and innovative beamforming and imaging pipelines, have enabled new insights into radio transients and their progenitors, led to the discovery of new sources such as slow pulsars, and provided new constraints on the abundance and properties of technosignatures (indicators of technology as a proxy for extraterrestrial intelligence).

Two such telescopes, the Allen Telescope Array in Northern California, and the MeerKAT Array in South Africa, utilise advanced digital signal processing to perform surveys covering huge volumes of parameter space in frequency, time, sky coverage, and sensitivity. This project will help to develop new pipelines for fast imaging at both telescopes, leverage the enormous amounts of data generated to undertake characterisation of the radio frequency environment at both sites, and apply machine learning and citizen science techniques to help find “needle in a haystack” signals, which could be indicative of extreme astrophysics or even of the presence of intelligent life beyond Earth.

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.

Supervisors Dmitri Uzdensky and Alexander Schekochihin

Accreting supermassive black holes (SMBHs) residing in active galactic nuclei (AGN) at the centres of many galaxies, including our own, are rightfully regarded as some of the most fascinating and enigmatic objects in the Universe. These black holes, and the powerful relativistic jets emanating from them, often exhibit spectacular, violent phenomena, such as bright high-energy flares with nonthermal X-ray and gamma-ray spectra. They are also viewed as the most likely generators of highly energetic non-electromagnetic observational "messengers": extremely relativistic cosmic rays and ultra-high-energy neutrinos, detected on Earth with dedicated ground-based observatories. All these observational signals indicate that BH environments are very efficient relativistic particle accelerators. Understanding how these cosmic accelerators work is an outstanding problem in modern high-energy astrophysics. Nonthermal particle acceleration is believed to be a natural product of collective kinetic nonlinear plasma processes, such as magnetic reconnection, shocks, and magnetised plasma turbulence taking place in weakly collisional plasmas. How these processes operate under the extreme physical conditions expected in the exotic relativistic plasma environments of accreting black holes is, however, not well understood. The situation is greatly complicated by special- and general-relativistic effects, by strong interaction of the energetic emitting plasma particles with radiation, and by quantum-electrodynamic effects such as pair creation. Elucidating the complex interplay of these effects with collective plasma processes and, consequently, their impact on particle acceleration is an active area of today's plasma astrophysics research, presenting a number of interesting, nontrivial theoretical challenges. This project will address these fundamental plasma-theoretical questions and their observational implications through a combination of analytical and computational methods, with the balance between theory and computation to be decided based on the student's preferences.