Compact Objects and Transient Phenomena

(Illustration: Dana Berry / SkyWorks Digital, Inc)

Members of this research group are:

Jérome Chenevez, Associate Professor
Giorgos Leloudas, Senior Researcher
Søren Brandt, Senior Researcher
Gaurava Kumar Jaisawal, Researcher
Miika Pursiainen, Postdoc
Niels Lund, Emeritus Associate Professor

We investigate the physical conditions about supernova explosions and their compact outcomes, neutron stars and black holes, as well as transient phenomena in the vicinity of  supermassive black holes at the centre of galaxies, so as to better understand the most extreme objects in the Universe. 

Extreme supernovae and other exotic transients 

supernovae. (Illustration: ESO)
This photo shows a three colour composite of the well-known Crab Nebula (also known as Messier 1). (Illustration: ESO.org)

Supernovae are the terminal explosions of stars after they have exhausted their nuclear fuel. While we have expertise in all types of supernovae, including the most usual thermonuclear and core-collapse explosions, our current research focuses on the most rare and exotic phenomena. For example, we try to uncover the explosion mechanism behind Superluminous Supernovae, a class of explosions that can reach immense luminosities and that cannot be explained by the traditional radioactive decay paradigm. We are also interested in a recently discovered class of transients that evolve radiply, in timescales much shorter than regular supernovae, as they also present a challenge for many physical models.

Compact Objects

Compact Objects. Illustration: (credits David A. Hardy & PPARC)
Explosions from White Dwarf Star RS Oph. (Illustration: David A. Hardy & PPARC)

The corpse remnants of core-collapse explosions are neutron stars or black holes, generically called “compact objects”. Our research is to understand the physics of the ultra-dense matter contained in compact objects through observations of their interactions with their immediate environment. We know hundreds X-ray binaries in our Galaxy that are powered by the gravitational interaction between a compact object and a companion star. Indeed, accretion of stellar matter towards the compact object produces X-rays that provide us information about the physical processes at work, and from which we may infer the size, the mass, and the spin rate of the accretor. We are particularly interested in the study of thermonuclear explosions on the surface of old accreting neutron stars. Such explosions are observable as X-ray bursts with facilities in orbit like INTEGRAL, NuSTAR and NICER that we use for their complementing fields of view and energy coverages. X-ray bursts are also sites of nucleosynthesis of heavy elements.


Tidal Disruption Events

TDEs. Illustration (credits ESO)
Tidal Disruption Event (TDE). (Illustration: ESO.org)

Occasionally, a star can be disrupted by the gravitational field of a supermassive black hole, resulting in a luminous flare, called a tidal disruption event (TDE). Several candidate TDEs have been discovered by transient surveys during the last few years and they offer a unique tool to probe monstrous black holes that would otherwise remain invisible. Our team plays a leading role in studying these extreme phenomena: we use various investigation tools, including spectroscopy, polarimetry (often through the ePESSTO collaboration and the ESO facilities) and comparison with models in order to uncover the origin of radiation in TDEs. 

Gravitational Wave sources

GWs. Illustration: (credits ESO)
Impression of merging neutron stars (Illustration: ESO/L. Calçada/M. Kornmesser)

The current generation of gravitational wave detectors are sensitive to the mergers of compact objects such as neutron stars and black holes. Our team is part of a larger effort to search for and study the transient electromagnetic radiation emitted by these sources. This effort has so far yielded the detection of the gamma-ray burst coincident with GW 170817 and the exciting discovery of the famous kilonova associated with it. To this end, we use both the INTEGRAL satellite in the gamma-rays, and participate in the ENGRAVE collaboration, where we lead the polarimetry Working Group, and the BlackGEM telescope array. We are also involved in the future LISA mission, which will be a gravitational wave detector in space, providing a scale consistent with supermassive black holes (https://lisa.nasa.gov).