Transients, Pulsars and High-Energy Astrophysics

Understanding the population of radio pulsars.
Aris Karastergiou

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.

Pulsar evolution through Radio- X-ray correlations.
Aris Karastergiou & Bettina Posselt

This project will use high quality data from Radio and X-ray observatories to explore how Pulsars evolve. Existing observations of X-Ray - Radio correlations reveal connections between the emission processes, but this work has so far been limited to a very small number of sources. Extending observations of such correlations to Pulsars of different age, and perhaps more importantly, different spin-down energy (Edot), this project aims to address questions regarding the orientation, structure, and physical properties of pulsar magnetospheres, and the underlying magnetic field. How do these properties evolve with Pulsar age? Radio data will mostly come from the MeerKAT Thousand Pulsar Array survey, which has the potential to reveal a new population of "interesting" pulsars that can also be studied through XMM-Newton or NICER data in X-rays. The student will undertake original research attempting to straddle the gap between Radio and X-ray Pulsar science, with the support of local experts and international collaborators.

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Relativistic Astrophysics with the world’s most powerful radio telescope.
Rob Fender, Ian Heywood and Peter Braam(Oxford), and
Michael Kramer (Max Planck Institute for Radio Astronomy, Bonn)

The most extreme phenomena in the Universe since the Big Bang are associated with explosive, transient astrophysical phenomena. The underlying physics of these events ranges from the release of gravitational potential energy as matter falls into black holes, to the explosive merger of two neutron stars and their associated electromagnetic and gravitational wave burst.
Radio emission from such events results from the spiralling of shock-accelerated, relativistic electrons around magnetic field lines, via the synchrotron process. The shock acceleration of the particles and compression and enhancement of the magnetic field result in turn from the supersonic outflow of matter from these transients, usually in the form of highly relativistic, collimated ‘jets’. Although having very large energy content, this synchrotron emission is usually unobservably faint at higher frequencies. Observations with radio telescopes are therefore the only way to study the kinetic output from such transient events, and have revealed it to be a fundamental and powerful channel for the dissipation of the released energy. A prime recent example is the relativistic jet, observed at radio wavelengths, from the neutron star merger and gravitational wave burst (LIGO) event GW170817, in which our team are heavily involved.
MeerKAT is the most powerful radio telescope in the southern hemisphere, and both Oxford and Bonn co-lead projects to study transients with the array over the next five years (via the ThunderKAT and TRAPUM projects). In addition, during this project the telescope will be upgraded with new high-frequency receivers, and expanded to a greater sensitivity and angular resolution, via direct investment from the Max Planck institute in Bonn. The successful applicant will be one of the first users of this extended MeerKAT, helping to commission it and using its upgraded sensitivity to study the radio universe in unprecedented detail. We are furthermore working at the cutting edge of software development for radio astronomy, and are looking to Machine Learning to dramatically improve our scientific analysis of the data. Major discoveries await!

Two projects are potentially available:
The first project involves the successful applicant working both at Oxford Astrophysics and the Max Planck Institute in Bonn, and will make use of the extended MeerKAT radio telescope array in South Africa. As a result, the student will spend at least one year of their studies based in Bonn, working in the relativistic astrophysics group there, and will be expected to make multiple visits to South Africa. In addition, there will be frequent additional travel to collaboration meetings and conferences.

The second project will investigate the application of Machine Learning techniques to the problems of imaging radio astronomical data, and also to the detection and classification of radio transients found in images. The new computational techniques developed will be applied to the data from the extended MeerKAT array, providing the student with early access to the world's most sensitive radio astronomical data. This may lead to a breakthrough in the way that interferometric imaging is processed, while in parallel leading to a guaranteed yield of exciting radio transient discoveries.

The link between black hole accretion flows and their jets
Adam Ingram & Rob Fender

In a black hole X-ray binary system, a stellar-mass black hole strips gas from its companion star. The accreting material glows brightly in X-rays as it approaches the black hole, with some eventually disappearing forever beyond the event horizon and the rest thrown out at high speed into narrow outflows called jets, which are observed at infrared and radio wavelengths. This project will be focused on understanding how fluctuations in the relativistic accretion flow onto the black hole -- which cause variations in the observed X-ray brightness -- propagate up the jet and cause fluctuations in the optical, IR and radio brightness. It has long been known that the X-ray variations follow a so-called linear rms-flux relation, meaning that the amplitude of rapid variability is larger in brighter epochs of X-ray emission. This is accepted to be a fundamental property of accretion powered radiation, resulting from fluctuations from the outer accretion flow propagating inwards and modulating faster fluctuations in the inner flow. Recently, a linear rms-flux relation has also been discovered in a few IR and radio observations. The candidate will explore the rms-flux relation in further IR and radio datasets and develop theoretical models to explain these new observations in the context of propagation of fluctuations down the accretion flow and up the jet. They will also explore the newly available diagnostic of X-ray polarisation by analysing data from the Imaging X-ray Polarimetry Explorer (IXPE), due to launch in September 2021. Specifically, we will look for time lags between modestly polarized X-rays emitted by the inflow and highly polarized X-rays emitted from the base of the jet.

Discovery of new microquasars.
Katherine Blundell

The proposed project will make use of the Many Commensal Cameras (MC2) data streams from the worldwide Global Jet Watch observatories that are making sensitive, successive observations of the signatures of precessing jets exhibited by microquasars in our Galaxy. Promising candidates will be followed-up and investigated via spectroscopy to discern dynamical confirmation of precessing jets moving at relativistic speeds. Unique data streams for this student project are assured, from the five Global Jet Watch observatories around the planet (PI K Blundell; www.GlobalJetWatch.net) although in addition there will be scope to obtain complementary data at radio wavelengths with milli-arcsec-scale angular resolution.

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Very high-energy gamma ray astrophysics.
Garret Cotter

Very high-energy (VHE) gamma-ray astrophysics is an exciting field spanning fundamental physics and extreme astrophysical processes. We detect the Cherenkov light emitted when VHE gamma rays from space hit the Earth’s atmosphere, using optical telescopes on the ground fitted with high-speed detectors similar to those used by the Large Hadron Collider, and also with water-filled Cherenkov detectors at high-altitude sites. Our science goals include :
• Understanding the natures and variety of particle acceleration around black holes.
• Understanding the origin of cosmic rays and their role in the Universe.
• Searching for the ultimate nature of matter and physics beyond the Standard Model

At Oxford we are involved in three major international experiments. The High Energy Stereoscopic System (H.E.S.S.) consists of five telescopes in the Khomas Highlands of Namibia, one of the darkest sites for astronomy in the world. H.E.S.S. is at present the world’s largest gamma-ray observatory. The international Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory, which has just commenced construction. It will have two arrays, totalling one hundred telescopes. One array is in the Northern Hemisphere, on La Palma in the Canary Islands, and one in the Southern Hemisphere, at Cerro Paranal in Chile. Then at the very highest photon energies, the proposed Southern Wide-Field Gamma-ray Observatory (SWGO) will have water Cherenkov detectors at an extremely-high-altitude site in the Andes.

There are openings for both experimental and theoretical work. We are developing software and data analysis techniques for CTA’s small-sized unit telescopes. These will have 2k pixel detectors and front-end amplifiers which feed into custom high-speed electronics. This gives a system that can image at a rate of a billion frames per second. We are also leading the efforts on machine learning techniques for the large volumes of data that will be generated when the new experiments become operational. These techniques allow us to use large datasets, in large-dimensioned parameter spaces, with very high entropy, and still undertake precise calibration of the astrophysical signals.
On the theoretical/observational side of the programme, recently we have developed new theoretical models for the broad-spectrum emission from steady-state jets that let us use the gamma-ray observations and those at other wavelengths to investigate the physical properties of the jet and the black hole at its base. We now propose to extend these models to look in particular at the entrainment of heavy particles as the jets propagate through their host galaxy, and the resulting possibility of hadronic particle processes within the jets. We will investigate the physical conditions that lead to flaring and the presence, and extent, of emission from hadronic processes.

We will offer the possibility of joint supervision with the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg. All students will have the opportunity to gain experience on observing shifts at the H.E.S.S. site, and opportunities are available to participate in our projects on education, outreach and socio/economic capacity building in Namibia.

H.E.S.S. website https://www.mpi-hd.mpg.de/hfm/HESS/
CTA website https://www.cta-observatory.org
SWGO website https://www.swgo.org/
Background reading: https://www.worldscientific.com/doi/pdf/10.1142/10986

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Left: H.E.S.S. 28-m telescope in Namibia (Credit: G.Cotter) Right: Artist’s impression of CTA-South site in Chile (Credit: Gabriel Pérez Diaz (IAC)/Marc-André Besel (CTAO)/ESO/ N. Risinger (skysurvey.org)

For more information please contact Professor Garret Cotter (garret.cotter@physics.ox.ac.uk)