The key objective of this project is to carry on fundamental physics research by using a unique astrophysical laboratory: neutron stars. With these objects one can study still unexplored regimes of two of the four fundamental forces of Nature: the strong force and gravity. The study of neutron stars and their enigmatic behaviour will be done by using modern ground based and space astronomical observatories that are able to record radiation from the radio waveband up to X-rays. My group and I will also use state-of-the-art numerical codes to perform magneto-hydrodynamic simulations of accreting neutron stars.
The ultimate goal of this project is to rigorously identify those signatures in the spin evolution of neutron stars that can reveal new physics. In practical terms, which mechanisms set a speed limit to neutron stars and how their spin evolution proceeds in time. The foreseeable mechanisms and immediate implications are the following:
- Gravitational waves: accreting neutron stars are efficient emitters meaning that they are good candidates for direct gravitational wave searches with next generation laser interferometers (Advanced LIGO and VIRGO, starting their operations in 2015/2016).
- Phase transition: if a phase transition is currently ongoing, it will be possible to place strong constraints on the dense/low-temperature side of the quantum chromodynamics phase diagram.
- Magnetic centrifugal barrier: no fast screening has to be possible for most accreting neutron stars, implying that an extended magnetosphere has to be present in at least some systems.
This is a personal project, but I will be surrounded by exceptional collaborators that will contribute to achieve this important target. The main players are:
- Prof. Nils Andersson (Southampton University)
- Dr. Jon Braithwaite (University of Bonn)
- Prof. Marina Romanova (Cornell University; check her amazing simulations here !)
Accreting Millisecond X-Ray Pulsars
This is the main topic of my research and my true scientific love (see my review here). I have mostly worked on this topic for the past 9 years and I have published a large amount of papers on X-ray observations of AMXPs, but I have also some phenomenological and theoretical work on these systems. Some neutron stars are born in binary systems with a star like our Sun as a companion. As the star ages and the system evolves, the neutron star gravity can start pulling gas off the outer layer of the companion star. As the neutron star devours its companion, three different phenomena happen. The first is the formation of a so-called accretion disc, an enormous disc of plasma spiralling around the neutron star. The disc is truncated close to the neutron star, by a very strong magnetic field that the neutron star possesses. The plasma flows along the field lines and hits the magnetic poles creating X-ray radiation modulated at the spin frequency of the neutron star. This second phenomenon, the formation of X-ray pulsations, allows the study of these objects via X-ray timing observations carried with X-ray space telescopes (RXTE, Swift/XRT, XMM-Newton, Chandra). Since these neutron stars pulsate they are called accreting X-ray pulsars. The third phenomenon at work is the spin up of the neutron star. As the gas flows on the mangetosphere of the accreting pulsar, it transfers its specific angular momentum to the neutron star, spinning it up from rotational periods of about 1 second down to milliseconds. These are the so-called Accreting Millisecond X-Ray Pulsars. The reasons to study them are several: understand how the spin up process operates, understand how fast neutron stars can become and thus constrain the equation of state of ultra-dense matter (read: constrain the nuclear force at supra-nuclear densities), understand the evolution and decay of ultra-strong magnetic fields and perform studies of stellar binary evolution.
X-ray binaries are binaries composed by a neutron star or a black hole and a normal star (where “normal” means a non-collapsed object). I work on several aspects of X-ray binaries, both observational and theoretical like the X-ray binaries timing properties (power spectra), their energy spectra, their binary evolution properties and the expected emission of gravitational waves.
Intermediate Mass Black Holes & Ultra-Luminous X-Ray Sources
As a side dish of my research I work on this topic once in a while and I produced a few theoretical papers that explore the different binary and stellar evolution between binaries with a stellar mass black hole and an intermediate mass black hole. I also explored the possible existence of intermediate mass black holes with a radio pulsar around them and which are the chances of detecting such exotic system.
Quantum Optics applied to Astronomy
This is the funniest of all topics I’ve worked on in my career. This topic was inspired by several scientists at the University of Padua, where I got my M.Sc. in 2004. I am now trying to build an instrument (together with a large number of people spread across the globe) with special optics to try to observe some obscure quantum property of light: the orbital angular momentum. I am also developing theoretical models to predict which astrophysical objects are best for a first test of our instrument.