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Extreme compact objects in the transient multi-band sky

  • Illustration magnetic fields exoplanets and radioastronomy.

Studying the most extreme transient events

The group's activities focus mainly on the study of the multi-band emission of magnetars, the newly discovered long period radio transients, accreting millisecond pulsars, transitional pulsars and X-ray binaries in general.


Multi-band observations and theoretical modelling of compact objects

The study of these objects transcends the traditional astrophysical approach and requires a multidisciplinary effort that spans from particle and nuclear physics to astrophysics, from experiment to theory, from gravitational waves to the electromagnetic domain. The group’s activities focus on the multi-band observations and theoretical modelling of compact objects. We are interested in transient high-energy emission from a large variety of systems, i.e. magnetars, the newly discovered long period radio transients, accreting and transitional millisecond pulsars and X-ray binaries in general. Recently, we got involved in multi-messenger follow-ups of gravitational wave sources, and in the connection between magnetars and gamma-ray bursts. The group is also deeply involved in the newly launched Einstein Probe mission, and in the future NewAthena large X-ray observatory.

  • Observed and simulated Pulsar proper motions. Credits: Magnesia Group.
  • NewAthena (left) and Einstein Probe (Right). Credits: ESA.

Focus

  • Multi-Wavelength Observations of Neutron Stars and stellar-mass Black Holes: We perform observations via pre-approved programmes at facilities encompassing the radio, mm, infrared, optical, ultraviolet, X-ray, and gamma-rays bands. Using approved programmes for ground-based telescopes (e.g. Parkes, GBT, ATCA, MeerKAT, FAST, GTC, etc.) and orbiting high-energy satellites (e.g. NICER, Swift, XMM-Newton, NuSTAR, Chandra, etc), observations of many different systems are performed as soon as they experience so-called X-ray outbursts. These episodes manifest as increases in the persistent X-ray emission flux by several orders of magnitude and are typically pinpointed by all-sky monitors. In all cases, these outbursts provide a unique test bed to investigate the emission physics of these sources. Observational studies focus in particular on their temporal and spectral properties and their evolution as they recover the quiescent state. Such studies make it possible to map the thermal emission on the stellar surface, understand the contribution of magnetospheric processes to the observed emission, establish the physical conditions that allow the multi-band emission to be triggered (bursts, jets, winds...), gain insights into the interaction between matter and magnetic fields and the coupling between matter accretion and ejection and ultimately estimate key source properties.
  • Population Synthesis of all classes of neutron stars with machine learning: The neutron star population is dominated by radio pulsars. However, in the last decades, several extreme and puzzling subclasses of neutron stars have been discovered: magnetars, Rotating Radio Transients, X-ray Dim Isolated Neutron stars, Central Compact Objects as well as the more common neutron stars in X-ray binary systems accreting material from a low-mass or high-mass companion star. In particular, despite being governed by a single equation of state, the neutron star zoo manifests itself as a multifaceted population. One of the main objectives of population synthesis studies is to reconstruct, from a statistical point of view, the unknown distributions of physical properties that characterise the initial population. The general approach is to use the current observed properties of pulsars, incorporating assumptions and theoretical models about their time evolution, to infer the birth distribution of physical properties such as their natal kick velocities, spin periods and magnetic field strengths. In our population-synthesis framework, different neutron star populations can be simulated starting from various initial conditions in terms of birth positions, velocities, spin periods and magnetic field strengths.  Finally, in order to perform a comparison with observed data, we need to account for observational limitations in all observing bands. The comparison with observations is challenging, since our parameter space is highly multi-dimensional. However, this comparison is necessary to constrain theoretical models used in the simulations and distributions of birth properties of the population. For this purpose, we take advantage of the power of machine learning, especially convolutional neural networks. These neural networks are able to learn complex patterns and extract information from the multi-dimensional data provided by our simulations and have the power of generalising unseen data samples, such as the true observed neutron star population.
  • Magnetic field evolution in neutron stars: We developed a 3D magneto-thermal evolution model to study the interplay between the thermal and magnetic evolution in highly magnetic neutron stars. For realistic predictions, 3D models are necessary to properly model current flows over the entire neutron star surface and incorporate realistic, non-axisymmetric field geometries. This new code allows us to predict magnetar flaring rates as a function of age and magnetic strength/geometry at birth, and determine cooling curves to be constrained observationally. This is crucial since (i) magnetic field evolution plays a critical role for pulsar population synthesis, as changes in the magnetic field affect the stars' rotational properties. These properties are key to extract pulsar properties at birth from observations, (ii) the burst activity of magnetars is driven by magnetic field evolution and the release of accumulated crustal stresses. Modelling the magneto-thermal evolution is thus needed to predict flaring rates.  
  • Pulsar based navigation systems: We are involved in the study of a pulsar-based unit for space navigation in close collaboration with SENER Aerospace and Deimos Aerospace. This project concerns a spacecraft navigation unit, designed with the purpose of providing interplanetary missions with autonomous position and velocity estimations. The unit will make use of pulsar X-ray observations to measure the distance and changes in the distance of the host spacecraft from the solar system barycenter. These measurements will then be used by an onboard orbit determination function to provide the complete orbital information to the spacecraft. The concept of X-ray pulsar navigation has been theoretically addressed in the past and demonstrated in practice by NASA's SEXTANT/NICER mission aboard the International Space Station. The aim of our action is to define a preliminary design for the unit, tackle the overall unit architecture, the optical and thermomechanical design, the unit avionics and software, as well as its function, performance, and operation. This new project is designed for a drastic reduction in the unit size, weight and a better positional accuracy.
  • Involvement in future missions Athena and Einstein Probe: The group is deeply involved in the design and scientific definition of the: 1) ESA Large Mission NewAthena, aimed at observing the hot and energetic Universe with an unprecedented sensitivity and resolution (to be launched in late 2030); 2) the newly launched CAS/ESA/MPE/CNES mission Einstein Probe, a revolutionary X-ray transient machine.

Senior institute members involved

Meet the senior researchers who participate in this research line.

  • Nanda Rea

    Nanda Rea

  • Francesco Coti Zelati

    Francesco Coti Zelati