Are you fascinated by black holes and neutron stars? Do you want to learn how they swallow gas from their surroundings and spit it back into space? Do you want to find our how this can be achieved using astronomical observations obtained at different wavelengths? If you answer yes to these questions, a masterproject in my group might be something for you!
Within the overarching theme of “observational studies of accreting neutron stars and black holes” there is a wealth of possibilities for exciting projects using different analysis techniques. Think of high-resolution X-ray spectroscopy to study wind outflows, radio observations to study jet outflows, X-ray reflection studies to zoom in on the hot gas circling closest to black holes and neutron stars, and linking the X-rays to UV, optical or infrared emission to understand how the gas gets to the neutron star or black hole. To get a general idea of the research done within my group, visit my research page.
Below are some ideas for projects that I would love to dive into with a Master student. However, we can also tailor a project to your specific interests. Just stop by for a chat or drop me an e-mail if you’re interested in exploring possibilities.
Here are my slides of the MSc project presentations of April 2021 (~50 MB).
Possible Master projects
- The quiescent emission of the famous neutron star X-ray binary SAX J1808.4-3658
Low-mass X-ray binaries (LMXBs) and millisecond radio pulsars (MSRPs) are two different manifestations of neutron stars in binary systems. It is thought that LMXBs evolve into MSRPs once a neutron star is done feasting on its companion star. However, many questions about the connection between these source types remain. The famous neutron star LMXB SAX J1808.4-3658 has long been suggested to switch on as a MSRP during times that it is not accreting from its companion star. However, despite extensive efforts, radio pulsations have never been detected from it. Possibly, there is still matter flowing towards the neutron star during its apparent “off” states, which prevents radio pulsations to switch on, or to be observed.
In this project, you will investigate what happens to SAX J1808 when its accretion appears to switch off, by studying its UV and optical emission. You will use UV imaging data from the Hubble Space Telescope and optical data from the 4-m SOAR telescope. Specifically, you will test whether its UV emission can be explained by a cold accretion disk, or whether the UV emission is coming from the surface of the neutron star. The latter could be an exciting alternative explanation that potentially allows us to measure the temperature of this unusually cold neutron star. Such a measurement could have important implications for the physics of ultra-dense matter.
This is an observational project in which you will learn to work with UV and optical imaging data from different ground-based and space-based telescopes. The science topics involved are X-ray binaries, accretion, the evolution of neutron stars in binary systems and the physics of ultra-dense matter.
Keywords: Observational, UV, optical, photometry, neutron stars, accretion, dense matter
- The origin of the quiescent X-ray emission of neutron stars
Neutron stars and black holes are the gravitationally-collapsed remains of once massive stars that ended their life in a supernova explosion. In X-ray binaries, these stellar corpses accrete gas from a companion star, which makes them shine bright at X-rays and other wavelengths. However, neutron stars and black holes often only actively accrete during outbursts that last a few weeks to months. During intervening quiescent episodes little or no matter is being accreted, causing X-ray binaries to dim by orders of magnitude.
Whereas X-ray binaries are very dim in quiescence, their X-ray emission is often still detectable with sensitive X-ray telescopes like Chandra and XMM-Newton. For black holes, it is generally accepted that their quiescent X-ray emission is originating from a residual accretion flow. For neutron stars, on the other hand, it is not fully established what causes the quiescent X-ray emission. If the neutron star is hot, it may emit detectable thermal (black-body like) X-ray emission. If accretion is absent, this surface emission can be used to measure the radius of the neutron star and even to obtain information on the interior properties of neutron stars. However, quiescent neutron stars often display a second X-ray emission component that can be described by a power-law model and may either be generated in a residual accretion flow, or due to processes that involve the magnetosphere of the neutron star.
In this project, you will analyze a large sample of X-ray observations of quiescent neutron star X-ray binaries obtained with the Chandra and XMM-Newton satellites. The aim is to perform a systematic analysis of the X-ray spectra to determine if the power-law emission component has the same characteristic in every source or if there are clear differences between sources. This will give important insight into the origin of this emission component and will have implications for using quiescent X-ray spectra to determine the fundamental properties of neutron stars.
Keywords: Observational, X-ray, neutron stars, accretion
- Connecting X-ray reflection to the radio properties of accreting neutron stars
Whenever accretion occurs, it is inevitable that collimated outflows, called jets, are produced. Despite their ubiquity, however, it remains elusive exactly how jets are formed, and what determines their power.
X-ray binaries, in which a black hole or a neutron star accretes gas from a companion star, are routinely used to study jets. There is a very strong correlation between the X-ray properties and radio properties of X-ray binaries. This demonstrates that there is a very close connection between how gas flows in (traced by the X-rays) and how it is spit out (traced by the radio).
As expected by theoretical models, the jets of black hole X-ray binaries are strongly suppressed when the X-ray luminosity (or equivalently the rate at which matter is accreted) becomes high. Similar behavior is also observed for a number of neutron stars. Surprisingly, however, there are also a few neutron stars that seem able to sustain their jets despite accreting at a very high X-ray luminosity. A systematic study of neutron stars and their jets at high X-ray luminosity has the potential to reveal important information about the formation and destruction of jets, which has direct implications for all accreting systems.
This project will focus on linking the radio properties of X-ray bright neutron stars to their detailed X-ray spectral properties. You will use X-ray spectral data obtained with the NuSTAR and Swift satellites of several accreting neutron stars. The results from your analysis will be compared with their radio properties reported in the literature.
Keywords: Observational, radio, X-ray, reflection and broad-band spectroscopy, neutron stars, black holes, accretion, jets
- Understanding accretion onto neutron stars with X-ray and infrared observations
Accretion is a very common process in the universe wherein an object attracts surrounding material through its gravity. For instance, in binary systems a neutron star can pull gas from a companion star and accrete this. Such systems are very interesting: we can study the stream of material as it accretes, test general relativity as the gas gets close to the neutron star, and probe the poorly understood interior of the neutron star when the gas hits the surface. This project will focus on understanding the accretion process itself, by comparing the radiation it emits in X-rays with the infrared emission coming from the entire binary system.
The project will be an observational one, meaning that you will work hands-on with astronomical data. You will analyse near-infrared images obtained for many different neutron stars with the 6.5-m Magellan telescope located in Chile, and X-ray spectra obtained for these neutron stars with different telescopes. By combining the near-infrared and X-ray results, you will investigate what the dominant emission processes are, and whether this changes depending on how fast the neutron star is accreting. You also compare your results for this large sample of neutron stars with the X-ray and near-infrared properties of accreting black holes, to see what the differences and similarities are.
Keywords: Observational, X-ray+near-infrared, imaging, spectroscopy, neutron stars, black holes, accretion
Related research blog: Changing emission mechanisms
- Mapping the UV properties of X-ray binaries
Neutron stars and black holes are extreme objects that are involved in the most explosive and energetic phenomena observed in the universe. When located in binary star systems, black holes and neutron stars can pull off and swallow gas from their companion star. This process, called accretion, converts enormous amounts of gravitational energy into electromagnetic radiation. These so-called X-ray binaries can therefore be observed at radio, infrared, optical, ultra-violet (UV), X-ray, and sometimes even gamma-ray, wavelengths.
Whereas X-ray binaries have been known and extensively studied for over 5 decades, little is known about their UV properties. However, in recent years several X-ray binaries have been observed at UV wavelengths with the Hubble Space Telescope and the Swift satellite. This observational project will focus on characterizing the UV properties of X-ray binaries, linking it to their X-ray properties, using astronomical data. You will work with satellite data obtained with the following observatories: Hubble Space Telescope, Swift, Chandra, XMM-Newton.
Keywords: Observational, UV+X-rays, imaging, spectroscopy, neutron stars, black holes, accretion
Related research blog: Changing emission mechanisms
- Blown away by a thermonuclear X-ray burst
Neutron stars that are located in X-ray binaries are pulling off gas from their companion star. The stripped material forms a gaseous disk around the neutron star, where it spirals in at increasingly high speed until it finally plunges on to the neutron star. The plasma that accumulates on the surface of a neutron star undergoes thermonuclear reactions, which can cause an extremely energetic explosion and result in a bright flash of X-ray emission. Such “X-ray bursts” are extremely common: tens of thousands of such events have been recorded to date with different X-ray detectors. X-ray bursts typically last for a few minutes, up to a few hours, and on some neutron stars the explosions repeat every few hours.
X-ray bursts can be very bright and powerful. Possibly, X-ray bursts can be so powerful that it blows off material from the accretion disk that surrounds the neutron star. Such a “disk wind” may be observable as narrow absorption features in high-resolution X-ray and UV spectra. The aim of this project is to analyze high-resolution spectral data of neutron stars that show thermonuclear bursts, to search for evidence of such burst-driven disk winds. This will give valuable insight into the physics of the accretion process.
Keywords: Observational, X-rays, UV, high-resolution spectroscopy, neutron stars, accretion, outflows