Research Projects for PhDs starting in 2017
Several PhD places will be available in the astrophysics group this year, with projects chosen from among those listed below. Candidates need not express a preference for project/supervisor before interviews are held. Applications will be reviewed at the end of January and successful candidates invited for interview shortly afterwards. Late applications may be considered. We also offer the possibility to co-host PhD studentships in collaboration with ESO, Europe’s flagship observatory. PhD projects will typically be supervised by a member of staff of the astro group and one at ESO in Garching near Munich (Germany) or Santiago (Chile). Students will have the opportunity to work two years in an international environment in Germany or Chile before finishing their last year at the University of Southampton. Please contact a member of staff if you are interested.
Projects internally funded
High-energy emission from black hole and neutron star systems using NuStar (Diego Altamirano)
The Nuclear Spectroscopic Telescope Array (NuStar) mission was launched in June 2012, and it is the very first (and so far only) focusing high-energy X-ray telescope in orbit. NuStar operates in the 3 - 80 keV band, extending the sensitivity of focused (hard) X-rays far beyond the ~10 keV high-energy cut-off achieved by all previous X-ray satellites. Using an unprecedented combination of sensitivity and spatial and spectral resolution, NuStar is suited to address long-standing questions on a variety of topics, including obscured active galactic nucleus, ultra-luminous X-ray sources, non-thermal radiation in young supernova remnants, and galactic sources such as black holes and neutron stars (both pulsars and not pulsars). New NuStar data are becoming publicly available every month since August 2013, giving a unique opportunity to analyse and study almost unexplored data from a variety of sources.
The PhD student will use NuStar data to study galactic black holes and neutron stars in binary systems (the so-called low-mass X-ray binaries). Building on our groups recent successful work on complex modelling of X-ray spectra we will
use simultaneous XMM-Newton/NuStar observations to tackle questions related to the relativistic iron emission line found in the energy spectra of black-hole and neutron-star systems. Furthermore, together with the PhD student will explore
alternative methods to test the nature of the so-called quasi-periodic oscillations (QPOs), with particular focus on the Lense-Thirring precession.
Informal enquiries: D.Altamirano@soton.ac.uk
Accretion processes in HMXB (Antony Bird & Malcolm Coe)
The accretion processes in HMXB are dominated by two effects: (i) inhomogeneous and time-varying winds from the donor star, and (ii) the magnetic field around the neutron star. These primary effects may also be heavily modified by orbital modulation in eccentric systems. These time-dependent effects modify both the X-ray signature (from accretion) and the optical/IR signature (from the mass as it transfers between the binary components), and multi-wavelength observations allow us to study the geometry and physical processes of the detected systems. Models of the mass transfer processes can now be implemented in SPH simulation, providing a new level of support in the interpretation of the observational data, and new source detections can shed new light on individual systems, challenging and refining existing models.
Informal enquiries: A.J.Bird@soton.ac.uk
Hunting new hard X-ray transients (Antony Bird)
Current generation hard X-ray telescopes rely on long integration times, of typically 10-100 ks, to make an observation of a field with the intention to detect faint persistent sources. This has been the basis of catalogs produced by the INTEGRAL and Swift teams, surveying the hard X-ray sky. It is much more challenging to look for short-lived emission which may be lost in long observations, but the event-mode data collected by modern telescopes contains much more information which is currently not being fully exploited. The aim of this project is to radically improve existing source detection algorithms by implementing a system that searches all points on the sky on all timescales for significant emission. This is a computationally intensive problem, and also requires statistical analysis to rigorously establish the significance of results and ensure resulting catalogs are robust. Early version of these analyses already found a new class of objects - the SFXTs - but those methods are too expensive to employ on the new, much larger datasets being produced today. We know that the hard X-ray sky is a tremendously variable place, and this project aims to fully explore those properties in an unbiased way for the first time.
Informal enquiries: A.J.Bird@soton.ac.uk
Near field 3-D gamma-ray imaging with coded apertures for medical and security applications (Antony Bird)
The coded aperture imaging technique has become standard for hard X-ray astronomical imaging in missions such as INTEGRAL and Swift, providing good imaging over a wide field of view. In the context of astronomy, the technique has been optimised for point sources at infinity in a low signal-to-noise environment. The aim of this project is to transform the coded aperture technique to address the much more challenging near-field case found in medical and security applications. Here, sources may be at an unknown distance (or same order as the detector-aperture distance, and may be extended in both lateral and depth axes). In such a case, the aperture design may differ dramatically from those used for astronomy, and must take into account recent advances in detector technology which make depth-sensing practical for the first time. This project will aim to develop new reconstruction algorithms for near-field imaging of complex sources, and investigate the optimum design of an aperture for this application. The project is in collaboration with an industrial partner that is currently developing the detector technology, and there will be an opportunity to develop and test a prototype imaging system if progress allows it.
Informal enquiries: A.J.Bird@soton.ac.uk
BHollywood : Black hole physics caught on high-speed cameras (Poshak Gandhi)
An exciting opportunity has arisen for a student to work on data from the AstroSat mission - the most sensitive fast timing multiwavelength space telescope in-orbit. AstroSat was launched by India in 2015 and its operations are now in full swing. The student will spend at least 1 year at Southampton and up to 2.5 years at IUCAA in Pune, India, working with AstroSat team members and coordinating multiwavelength observations of outbursting black holes. Frequent collaboration between UK/India/South Africa/Chile/space telescopes is envisaged.
Black holes are the most compact objects known. So, physical conditions in their surroundings can change rapidly. The aim of this project is to capture these rapid changes by making state-of-the-art high frame-rate multi-colour 'movies' of black holes in outburst. Observations across the electromagnetic spectrum including X-rays, optical, infrared and radio data are needed to capture the full energy release of black hole emissions both from the accreting material, and from outflowing fast relativistic jets.
You will have the opportunity to plan and execute multiwavelength observations, analyse and interpret the data, and to constrain theories of black hole growth and launching mechanisms for relativistic jets. For more details on this growing field of rapid time domain astronomy, please see the following links:
Informal enquiries: email@example.com
Fast black hole movies:
IUCAA, Pune, India:
Ultraluminous X-ray sources with eROSITA (Matthew Middleton)
Ultraluminous X-ray sources (ULXs) represent the extremes of accretion physics, with their remarkable X-ray luminosities resulting from either accretion at rates below the classical Eddington limit onto intermediate mass black holes (IMBHs) or accretion at rates in excess of the Eddington limit onto neutron stars and black holes (with masses < 100 times that of the Sun). Whilst evidence now strongly points towards the latter scenario, there is still the possibility that IMBHs - distinguished by distinct behaviours in the energy and time domains - may yet be found. Studying ULXs en-masse will provide clues as to how super-Eddington accretion operates in practice and - should IMBHs be found - how the seeds for supermassive black holes - present in the centres of all galaxies - may have formed.
eROSITA is a joint German/Russian mission launching on the Russian Spectrum-Roentgen-Gamma (SRG) satellite late 2017 or early 2018. As external partner I (and by extension my PhD student) will have access to the data for ULX science projects. This will provide an opportunity to take part in a new, cutting-edge mission that will scan the sky finding new sources. In particular we will search for new 'ULX pulsars', create a catalogue of ULXs to perform demographic tests which will reveal important accretion physics, and will explore unusual sources (with the possibility of finding IMBHs). The project will employ a range of tools including spectral and timing analysis which the student will become fully versed in.
Informal enquiries: M.J.Middleton@soton.ac.uk
Ultraluminous X-ray sources with eROSITA (Matthew Middleton)
Accretion of material onto black holes releases potential energy in the form of radiation, at frequencies extending from the radio through to the high energy gamma-rays. The X-ray emission originates from the material closest to the black hole and therefore contains imprints of general relativity. Traditional approaches to understanding the details of accretion rely on studying the energy spectrum or the temporal properties (usually the Fourier power spectrum) in isolation. New techniques allow the confluence of the time and energy domain (the cross-spectrum) and promise to reveal far more than either technique will by itself.
The PhD student will develop approaches related to the cross-spectrum and apply these to obtaining a greater understanding of accretion from the wealth of high quality X-ray data on Galactic black hole binaries (using data from multiple satellites including XMM-Newton, Chandra and RXTE). To determine how the accretion flow connects to astrophysical jets, the student will use a multi-wavelength, broadband approach to include optical/IR and potentially radio data.
Informal enquiries: M.J.Middleton@soton.ac.uk
The Next Generation of Supernova Surveys (Mark Sullivan)
Exploding stars, or supernovae, impact upon many diverse areas of astrophysics, from galaxy formation, to stellar evolution, to cosmology and studies of dark energy. The next few years will see a revolution in this field, with the numbers of objects available to study rising from the hundreds to thousands and tens of thousands per year. In particular, two major new facilities will revolutionise the study of supernovae: the first is the billion-dollar Large Synoptic Survey Telescope (LSST), an 8-m survey telescope that will image the whole sky every 3 days, and which will find new supernova explosions at an unprecedented rate. The second is the 4MOST multi-object spectrograph, which will study thousands of supernova explosions in great detail as part of its TIme Domain Extragalactic Survey (TIDES). This combination will provide the ultimate cosmological sample of type Ia supernovae, probing completely new parts of time-domain parameter space, and Southampton is involved in this key work.
This project will use scientific results based on existing samples of supernovae - from the Dark Energy Survey, the OzDES survey, and the Palomar Transient Factory - to prepare for the advent of these new facilities. This will involve developing new techniques to classify large samples of supernova events based only on photometric data (with immediate application to existing Dark Energy Survey data), and the calculation of the rate of occurrence of exotic supernova explosions. This is the perfect opportunity to get involved in two major new surveys from the start of their operations.
Informal enquiries: firstname.lastname@example.org
VEILS: The first infrared extragalactic time domain survey (Mark Sullivan)
Type Ia supernovae can be used to answer one of Physics's most pressing questions: "What is the nature of dark energy?". These supernovae were responsible for the initial detection of the accelerating universe nearly 20 years ago, and provide the best current measurement of the properties of dark energy. However, questions remain about the effect of dust extinction on the luminosities of type Ia supernovae, and hence on the reliability to which these objects can be calibrated to measure distances. Southampton is leading a major new ESO public survey using the ESO VISTA telescope - VEILS - to obtain near-infrared (IR) imaging of around 300 high-redshift type Ia supernovae, with complementary optical data from the Dark Energy Survey. These near-IR data are less susceptible to extinction by dust, and will therefore provide an improved cosmological standard candle.
This PhD project will be working on the VEILS survey to study these new high-redshift supernovae. This will include analysing imaging data to make light curves, and spectroscopy to study the supernova chemistry and physics. The project will also use these data to make the largest ever sample of distant type Ia supernovae with near-IR data, and will make precise distance measurements based on these data.
Informal enquiries: email@example.com
Projects for applicants with external funding
Spatial and temporal structure of the Earth's magnetotail (Robert Fear)
The Earth's magnetic field forms a cavity in the solar wind called the magnetosphere; the interaction between the solar wind and the magnetosphere is ultimately responsible for dynamics of near-Earth space, including variations in the intensity of the radiation belts and the most spectacular displays of the aurora (the northern and southern lights). The night side of the Earth's magnetosphere forms an extended magnetotail, which consists of a plasma sheet sandwiched between two low density regions called the lobes. However, several aspects of its structure are unclear. First, whilst it is known that the interplanetary magnetic field affects the orientation of the magnetotail magnetic field through a process of "penetration" into the tail, the timescale for this process is hotly debated. Secondly, in textbook picture of the magnetotail, the low latitude "plasma sheet" is hot, whereas the plasma in the lobes is very cold and usually low in density. However, a recent series of papers have begun to challenge this paradigm, and have found that under certain conditions uncharacteristically hotter/higher density plasma can be observed in the lobes. The mechanisms causing this are at times unclear. The aim of this project is to use in situ satellite data from spacecraft such as the European Space Agency's Cluster mission to investigate the timescales on which the magnetotail is controlled by the interplanetary magnetic field, and to determine and explain its structure during more complex intervals. At this time only applicants with external funding can be accepted for this project.
Informal enquiries: R.C.Fear@soton.ac.uk
X-ray/UV/Optical Variability in AGN: Reverberation Mapping of the inner regions of AGN (Ian McHardy)
Matter accretes onto the supermassive black hole at the centre of an active galactic nucleus (AGN) via an accretion disc. Following the release of gravitational potential energy as heat, the disc is hotter near the black hole than further out. Thus it emits UV radiation from small radii and optical/IR from further out. This emission varies and
a long standing question in AGN astrophysics is 'What causes the UV and optical variability of AGN?'. The variations could be caused by variations in accretion rate or by variable X-ray heating from the X-ray source around the black hole.
Recent observations using the NASA Swift X-ray/UV/optical space observatory by ourselves (eg McHardy et al, 2014, MNRAS, 444, 1469) and later by others, shows that the UV/optical emission lags behind the X-ray emission in a way which is broadly consistent with variable X-ray heating. However these observations produced a number of new questions.
The lags between the various UV and optical bands map out the temperature structure of the disc and
tell us about the size of the disc. This process is known as 'reverberation mapping'. However these lags are
about 3x longer than expected theoretically, based on the standard accretion disc model of Shakura and Sunyaev (1973, A\&A, 24, 337). Also there is sometimes, but not always, a much longer lag between the X-rays and the UV than expected, even for a 3x larger than expected disc. There is also evidence that some of the UV and optical variations come not from the accretion disc but from the surrounding gas which produces the broad optical emission lines.
In order to solve these problems we must first measure lags in more AGN of different black hole masses and accretion rates to determine which physical parameters are causing the problems. At Southampton we lead much of the world observational activity in reverberation mapping. We lead most of the major observational programmes. These programmes are mainly made with the NASA Swift observatory and with the ESA XMM-Newton observatory. Opportunities are available to work on these very extensive datasets both to measure the lags and also to explain them using our computer model of an accretion disc and surrounding gas. The aim is to understand the inner geometry of AGN. In particular, a huge observational program with Swift will occur in Autumn 2017, ideally timed for a new student.
Informal enquiries: Ian McHardy (firstname.lastname@example.org).
Modelling of X-ray Source Geometries around Black Holes (Ian McHardy)
Super-massive black holes (SMBHs), when accreting large masses of material, are the most luminous objects in the universe (QSOs), particularly in the X-ray waveband. There is one in the middle of every galaxy and their emission can halt starformation and affect the whole evolution of the galaxy and the appearance of the universe. They are luminous enough to be seen back almost to the start of the universe and we can use their emission to measure the geometry of the distant universe. On a smaller scale, emission from accretion black holes of solar mass size are the most luminous objects within our own Galaxy.
A major uncertainty, however, is the geometry of the X-ray source. Is it a sphere around the black hole, or is it flat, extending over the disc of material accreting onto the black hole? Similarly, we do not know the shape of the accretion disc, ie is it flat everywhere or is it inflated near to the black hole? How close to the black hole does it extend (which depends on the spin of the black hole)? These are currently key questions in astrophysics
and answering them forms a major part of the science case of almost all new proposed high energy astrophysics observatories, eg the recently approved ESA Athena X-ray observatory.
We can study the geometry of these systems by measuring the time lag between radiation which comes to us directly from the X-ray source and from radiation which is first reprocessed to lower energies by the surrounding disc before travelling to us. Lags measured from fast variations tell us about smaller scale sizes than lags measured from slow variations so, using straightforward Fourier techniques, we measure the lags as a function of Fourier frequency, producing a 'lag spectrum'. We use observations from major X-ray observatories such as the ESA XMM-Newton Observatory.
To properly interpret the observed lag spectrum and measure black hole mass, X-ray source size and disc geometry, it is then essential to make a computer model of the X-ray source and surrounding disc to produce predicted, or model lag spectra, to compare with the observations. So far almost all researchers use only the very simplest, and almost certainly unrealistic, models. These models usually represent the X-ray source as just a point source above the black hole, on its spin axis (the 'lampost' model).
Within the context of the Mathematica computing framework we already have a computer model consisting of an accretion disc with a variable inner disc radius, and a single point X-ray source. We trace the path of multiple individual X-rays leaving the source in all directions, some hitting the disc and others traveling directly to the observer. This model is fully General Relativistically kinematically correct. We have already published results of modelling observed lag spectra with a simple point source model (Emmanoulopoulos et al 2014, MNRAS, 439, 3931).
The aim of the present project is to make realistic 3D X-ray source geometries by combining together many point source models, each generated from many individual rays. These models will include spherical models and also models where the X-ray source extends out over the inner edge of the accretion disc. We will also explore models where part of the emission is in the form of a jet along the black hole spin axis. The resulting model lag predictions will be compared with the current observations, eg from XMM-Newton. The predictions will also be very important for designing key observing programs with the next generation of X-ray observatories such as Athena.
This work will be supervised jointly by Professor Leor Barack in the General Relativity Group of the Mathematics Department and Professor Ian McHardy in the Astronomy Group in the Physics and Astronomy Department of Southampton University.
Informal enquiries: contact either Professor Barack (email@example.com) or Professor McHardy (firstname.lastname@example.org).
Constraining the key physical processes governing the formation and evolution of lenticular galaxies (Francesco Shankar)
Lenticular (S0) galaxies are a special class of bulged galaxies in between ellipticals and spirals. They are often characterized by lower specific star formation rates and no clear spiral patterns, with a broad variety of structural properties, going from galaxies with very big bulges to others dominated by the disc component. After almost one century since their definition, the origin of lenticular galaxies is still a matter of debate. This project aims at probing the formation and evolution of bulged galaxies, with a specific focus on lenticulars/S0s, using a cutting-edge methodology based on extensive, advanced semi-empirical models which make use of sub-halo abundance matching and halo occupation distribution techniques.
The main objectives of this proposal are the following:
- Analyse in a cosmological context through advanced semi-empirical models, an array of key physical processes to form and evolve lenticulars, such as mergers, bar/disc instabilities, disc regrowth, clumpy accretion, morphological and/or halo and/or environmental quenching. Several of the latter physical processes are still nearly unexplored in this context.
- Compare the outputs of each different model with the statistical, spectral, morphological, structural, and environmental properties of lenticulars.
We will make extensive use of new, unique, and comprehensive data sets available to our group from, e.g., SDSS and COSMOS, specifically catalogued for S0 galaxies in different environments and redshifts.
Informal enquiries: F.Shankar@soton.ac.uk