Neutron stars are stellar remnants that form in supernova explosions of massive stars. Each of these objects contains a mass comparable to the Sun's within a radius of about ten kilometres and exhibits high densities, fast rotation and large magnetic fields. Such conditions cannot be recreated on Earth, making neutron stars amazing cosmic laboratories to study matter under extreme conditions.
While I am interested in many aspects of neutron stars, my work can be broadly separated into two parts: The first focuses on the interface between astrophysics and condensed matter physics, while my second research area concerns the population synthesis of isolated neutron stars. More details can be found here:
One of the most exciting aspects of neutron stars is that their interiors are strongly influenced by quantum mechanics. To better understand their behaviour, I study so-called superfluid and superconducting components.
Much like the Earth, neutron stars are composed of distinct layers. They have a solid crust and a fluid interior that contain neutrons, protons, electrons and possibly exotic particles. In terms of their high densities, neutron stars are very cold, giving the protons and neutrons the special ability to flow without friction: The charged protons form a superconductor, whereas the neutrons are referred to as a superfluid.
These two are exotic versions of quantum states that are observed in experiments on Earth. From their laboratory counterparts, we know that superconductors and superfluids create vortices that can be envisaged as tiny, rapidly rotating tornadoes. These small structures interact with their surroundings, affecting the large-scale dynamics of the star.
I research different approaches to include these small-scale effects into theoretical models of neutron stars. Using techniques that are well-known from standard magnetohydrodynamics, I have for example studied the evolution of the magnetic field in the interior of superconducting neutron stars. I have also presented novel ways that low-temperature laboratory experiments could be used to make progress in understanding neutron star astrophysics.
Furthermore, I have been analysing how coupling processes in the interior affect the star's response after a so-called glitch. These sudden spin-ups interrupt the regular spin-down of pulsars and are thought to be a macroscopic manifestation of superfluidity. By connecting the physics on different length scales, I developed a predictive model of the glitch rise showing that assumptions about the microphysics of vortices crucially affect the star's rotational behaviour. Comparing my predictions to the first pulse-to-pulse glitch observations, reported by Palfreyman et al. (2018), I derived constraints on the strength of the frictional mechanisms in the star's interior. An improved analysis of the data, revealing novel details about the internal components of the star, was recently published in Nature Astronomy.
Using techniques well-known from the studies of laboratory superconductors, so-called Ginzburg-Landau models, I have also been exploring the micro-scale characteristics of the superconducting protons in the neutron star core. Their properties are poorly understood but could have a significant impact on the stellar magnetism and are thus crucial to understand the macroscopic magnetic field properties of compact objects. By adapting the Ginzburg-Landau description to the neutron star interior and connecting it with realistic superfluid parameters and equations of state, my collaborators and I have constructed superconducting phase diagrams and found that the outer core of neutron stars exhibits so-called type-1.5 superconductivity, rather than type-II superconductivity as generally assumed.
In addition to focussing on the neutron star interior, I also investigate the global population of isolated neutron stars in our Milky Way.
Although about a billion neutron stars are expected to exist in our own galaxy, observational constraints limit us to only detecting a small fraction of them; we only know a few thousand of these compact objects to date. To overcome this gap, so-called population synthesis approaches are used to theoretically model the full population. Based on our current knowledge of input physics, these approaches focus on simulating a synthetic neutron star population. Once the simulated sample is created, we compare it to real observations to identify discrepancies and subsequently adjust our theoretical models. This kind of global study, thus, allows us to better constrain the input physics, i.e., learn more about neutron stars on an individual level.
I am personally interested in using new computational techniques, specifically machine learning, to perform the comparison between the synthetic sample and the observed characteristics. Machine learning is an implementation of artificial intelligence that gives systems the ability to automatically improve and learn from previous experiences without being explicitly told how to do so. These techniques have seen a lot of interest in the astronomy and astrophysics community, where it is often no longer possible to evaluate large amounts of data by hand.
Our aim is to use machine learning frameworks to improve our understanding of the physics of compact objects in the Milky Way, in particular a class of neutron stars with strong magnetic fields, so-called magnetars, of which we currently only know about a dozen.
A full list of papers can be found on the online databases
arXiv or ORCID.
Alternatively, a list of selected publications is given below.
M. Ronchi, V. Graber, A. Garcia-Garcia, J. A. Pons, and N. Rea, Analyzing the Galactic pulsar population with machine learning, (submitted to Monthly Notices of the Royal Astronomical Society) eprint arXiv:2101.06145
T. S. Wood, V. Graber, and W. G. Newton, Superconducting phases in a two-component microscale model of neutron star cores, (submitted to Physical Review C) eprint arXiv:2011.02873
P. Esposito, N. Rea, A. Borghese, F. Coti Zelati, D. Viganò, G. L. Israel, A. Tiengo, A. Ridolfi, A. Possenti, M. Burgay, D. Götz, F. Pintore, L. Stella, C. Dehman, M. Ronchi, S. Campana, A. Garcia-Garcia, V. Graber, S. Mereghetti, R. Perna, G. A. Rodríguez Castillo, R. Turolla, and S. Zane, A very young radio-loud magnetar, Astrophysical Journal Letters, vol. 896, L30 (2020)
G. Ashton, P. D. Lasky, V. Graber, and J. Palfreyman, Rotational evolution of the Vela pulsar during the 2016 glitch, Nature Astronomy, vol. 3, 1143 (2019)
V. Graber, A. Cumming, and N. Andersson, Glitch rises as a test for rapid superfluid coupling in neutron stars, Astrophysical Journal, vol. 865, 23 (2018)
V. Graber, Fluxtube dynamics in neutron star cores, Astronomische Nachrichten, vol. 338, 1090 (2017)
W. C. G. Ho, N. Andersson, and V. Graber, Dynamical onset of superconductivity and retention of magnetic fields in cooling neutron stars, Physical Review C, vol. 96, 065801 (2017)
V. Graber, N. Andersson, and M. Hogg, Neutron stars in the laboratory, International Journal of Modern Physics D, vol. 26, 1730015 (2017)
V. Graber, N. Andersson, K. Glampedakis, and S. K. Lander, Magnetic field evolution in superconducting neutron stars, Monthly Notices of the Royal Astronomical Society, vol. 453, 671 (2015)
A. Markowsky, A. Zare, V. Graber, and T. Dahm, Optimal thickness of rectangular superconducting microtraps for cold atomic gases, Physical Review A, vol. 86, 023412 (2012)
I taught the undergraduate module PHYS 434 Optics during the winter term 2019 at McGill University. Below you can find general information on the course as well as the lecture notes, I created.
Classes started on Monday, January 7 and took place every Monday and Wednesday from 2:35pm to 3:55pm in Rutherford RPHYS 114. General information about the course, teaching assistants and an overview of the course content, prerequisites, evaluation and reading materials can be found in the syllabus. Individual lecture topics, assigned reading materials and important dates are given in the course calender. Note that both were subject to change throughout the term.
PART I – Electromagnetism and Light Propagation
PART II – Geometric Optics
PART III – Superposition, Polarisation and Interference
PART IV – Diffraction, Fourier Optics and Modern Optics
The Centre for Research in Astrophysics of Quebec (CRAQ) hosted its annual summer school in June, 2019 in Montreal. The 2019 topic was Stellar Astrophysics and I covered Neutron Stars during the Stellar Death section.
General information about the summer school can be found here. My presentation slides and a Jupyter notebook to calculate mass-radius relations for two simple neutron star model equation of state can be downloaded below.
While working as a postdoctoral fellow at the McGill Space Institute in Montreal, I was part of the AstroMcGill outreach team. You can find information about past and upcoming events on facebook and twitter. We regularly hosted a Public AstroNight, a free monthly lecture typically attracting several hundred people, as well as Astronomy on Tap, a world-wide initiative combining your two favourite things: astronomy and beer. AstroMcGill usually runs several astronomy related games and short talks at the AoT events. In September 2017, I had fun talking about 'Neutron stars - a space oddity' .
In December 2018, I gave the public monthly lecture jointly organised by AstroMcGill and PhysicsMatters, the McGill Physics Outreach group. My Public AstroPhysicsNight talk was titled 'Neutron Stars: Extraordinary Cosmic Laboratories for Physicists' and provided a non-specialist introduction to my research (no equations, I promise). You can watch a recording of the lecture here.
Besides its regular programme, AstroMcGill also organises and participates at special events. In August 2017, for example, we hosted several thousand people at the McGill University campus for a viewing party of the partial solar eclipse. Seeing so many people exciting about astronomy was an amazing experience.
Besides participating at outreach events in person, I occasionally write for blogs that focus on different topics related to science and academia. You can, for example, read an interview with me about the importance of women in research here.
If you are interested in learning more about my experience at the 69th Nobel Laureate Meeting dedicated to Physics, which I participated at in the summer of 2019, you can read two posts I wrote here and here. One aspect of the Lindau Meeting is a poster exhibition, where a pre-selected group of thirty young scientists gets the chance to present their research. I was one of the lucky participants and my poster won the shared first prize, by public vote of all the meeting attendees.
Demonstrations are an excellent way to get people of all ages interested in science. As a PhD student, I was involved in constructing simple hands-on experiments for open days and public events that help to illustrate complex physical concepts. A few examples that have proven particularly successful over the years:
Combining an old trampoline, striped lycra fabric and marbles of different masses provides a fantastic set-up to visualise the workings of gravity and the concept of space-time.
Wave propagation, reflection and interference can be playfully illustrated using a `jelly baby wave machine'. To build your own you need jelly babies, duck tape and kebab sticks. Idea first spotted here.
Following the first direct detection of gravitational waves, I constructed a table-top Michelson interferometer (my all-time favourite physics experiment) to visualise the concepts employed by interferometric gravitational wave detectors. I followed instructions provided by the LIGO outreach team.
In December 2018, while working as a postdoctoral fellow at the McGill Space Institute in Montreal, I gave the public monthly lecture jointly organised by AstroMcGill and PhysicsMatters, the McGill Physics Outreach group. My Public AstroPhysicsNight talk was titled 'Neutron Stars: Extraordinary Cosmic Laboratories for Physicists' and provided a non-specialist introduction to my research (no equations, I promise). You can watch a recording of the lecture here.
In July 2020, I contributed to the Faszination Astronomie Online initiative organised by the Haus der Astronomie. The Haus der Astronomie, which literally translates to 'House of Astronomy', is a Centre for Astronomy Education and Outreach in Heidelberg, Germany, that runs events for the general public, as well as workshops for students, teachers, and science communicators. In response to the Covid-19 pandemic, the centre moved its German public talk series online and has been regularly streaming about fascinating astronomy topics on its own YouTube-channel. My thirty minute-long talk on pulsar glitches, titled 'Wenn Neutronensterne Schluckauf haben', can be viewed here.
In addition to making scientific content more accessible to the general public, I have also participated in events that aim to make scientists themselves more relatable. One great way of achieving this is via storytelling and in November 2018, I performed in front of an amazing audience at a Science Story Slam hosted by Broad Science and Confabulation.
In October 2018, I visited a secondary school in Tuttlingen, Germany, to tell the students about the wonders of the solar system and answer all their questions about what it means to be a scientist. We also played a game that the AstroMcGill outreach team has been using at their events. It's called 'Moon or Frying Pan' (idea first spotted here). Try it out! It's actually a lot harder than it seems, but the kids loved it.
While working at the McGill Space Institute in Montreal, together with other AstroMcGill and PhysicsMatters members, I also volunteered for the Inquiry Institute. The project aims to connect physicists with Montreal school teachers to introduce the educators to simple experiments that can be repeated in the classroom and specifically highlight the importance of critical and structural thinking. We, for example, worked on a demonstration that combines a hula hoop with painted table tennis balls to illustrate the concepts of moon phases and solar eclipses; a set-up that has proven useful to explain how to encourage pupils to ask critical questions.
I am currently a senior postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC) in Barcelona, Spain, working with Nanda Rea and others on the newly funded ERC project MAGNESIA that focuses on providing a census of galactic magnetars (a class of highly magnetised neutron stars). Further, I am a member of the European COST Action network PHAROS, which focuses on the multi-messenger physics and astrophysics of neutron stars and is a continuation of the NewCompStar network. I am also part of the Square Kilometre Array (SKA) Pulsar Working Group as well as the NSF Physics Frontiers Center JINA-CEE and the International Research Network for Nuclear Astrophysics (IReNA), which are dedicated to the study of nuclear astrophysics and the formation of elements.
The first 18 years of my life I was lucky enough to be living by Lake Constance in the South of Germany. Having seen many beautiful places over the years, I can say that the Swabian Sea (as the lake is often nicknamed) is one of the most magnificent spots I have been to and I try to go back as often as possible.