Compact objects

Precise measurement of pulsar signals from millisecond pulsars (MSPs) , or pulsar timing, provides a powerful tool to test  the equation of state of super-dense matter, strong-field gravitational physics, the effects of cosmic strings that are predicted by gravitational string theories and more.

Pulsar timing can also be used to directly detect gravitational waves offering a complementary tool to existing gravitational wave detectors such as LIGO/Virgo.
Our group is part of the European Pulsar Timing Array and the International Pulsar Timing Array (PTA). These large-scale collaborations share resources and facilities to create a global array that can be used to detect and characterize gravitational wave created from the mergers of supermassive black holes in the early universe.

Members of our group are also involved in MeerTime, one of the key science programs on pulsar timing with MeerKAT array, which is a pathfinder facilities for the Square Kilometre Array located in South Africa. We also contribute  to the definition and scientific planning of the pulsar science program for the Square Kilometre Array.

In order to precisely measure the arrival of pulses from a pulsar, astronomers should account for and remove any distortion in their signals. The biggest contributor to such distortions is the ionised interstellar medium (it causes dispersive delays, scattering, faraday rotation, etc). The pulsar group at ASTRON collaborates closely with our colleagues  working on Solar physics and Space Weather to mitigate propagation effects on pulsar timing measurements.

We use LOFAR to conduct an all-sky pulsar survey in search of new and exotic pulsars that can provide further insight into the physics of neutron stars. A discovery of one of the slowest-spinning radio pulsar know to date ( more than 10 times slower than a typical pulsar) sheds light on how various sub-classes of neutron stars are linked and evolve during their lives.

Millisecond pulsars shine brightest in gamma-rays and they can be detected with telescopes such as NASA’s Fermi gamma-ray space telescope. Our group uses LOFAR to look at sources discovered by Fermi that may be associated with pulsars.   The main goal is to discover a neutron star that spins so rapidly that we can tightly constrain its size and hence the properties of its hyper-dense interior.  Such studies are of great interest to astrophysicists and nuclear physicists alike. So far, we have  discovered the fastest-spinning radio pulsar in the Galactic field.

It’s been more than 50 years since the discovery of pulsars by Jocelyn Bell in 1967. Despite significant progress, astronomers do not yet have a complete understanding of how these fascinating objects work.  Using low-frequency observations across a wide band of frequencies, we can learn much about  pulsar radio emission and its interaction with magnetospheric plasma and fields.

Our group at ASTRON collaborates with international colleagues on a number of pulsar studies at low frequencies including the characterisation of known millisecond and slow pulsar populations, providing reliable estimates on pulsar dispersion measures, flux densities and spectra. The group is currently working to improve the polarisation calibration of LOFAR especially at the very low frequencies below 100 MHz in order to get reliable estimates of polarisation properties of pulsars and their evolution across the LOFAR frequency band.