ACCRETION STATES

Multiwavelength spectral timing of black holes and neutron stars: a new step in our understanding of accretion processes

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 Obiettivo del progetto (Objective)

'Accretion is a common physical process in the Universe, playing a crucial role in galactic and extra-galactic astronomy. Accretion can be studied in detail by looking at black holes and neutron stars in low-mass X-ray binaries (LMXBs), where changes occur on timescales favorable for human studies. X-ray observations of these systems during the last decade have revealed that accretion proceeds via various accretion states. Optical and near-infrared (OIR) emission was thought to originate from thermal reprocessing of the X-ray emission by the accretion disc. Recent spectral and timing multiwavelength studies are, however, challenging this classical scenario. The new studies show that a relativistic outflow in the form of a jet plays an important role in the accretion process. Key to these new works are fast, high signal-to-noise OIR observations. However, these new studies are not systematic in the sense that only a few accretion states and sources have been covered. The proposed project would provide the systematic study by determining the origin of the multiwavelength emission via a spectral and fast variability study covering all the accretion states observed in both neutron stars and black holes. To this end I have gathered an extensive database of simultaneous, high time-resolution X-ray, optical and near-infrared observations, and the host is a leading institute in new radio facilities like LOFAR and MeerKAT, which will provide guaranteed data to the project. Both spectral and timing analysis tools will be applied to the data set. For the interpretation I will use two numerical codes that will allow me to interpret the simultaneous high time resolution X-ray and OIR data.'

Introduzione (Teaser)

Neutron stars and black holes are unique laboratories to study matter in its most extreme conditions. To better understand their properties, EU-funded astrophysics addressed multi-wavelength observations with fundamental physics in conditions not reproducible in terrestrial experiments.

Descrizione progetto (Article)

Strong gravity and extreme magnetic fields turn neutron stars and black holes into unrivalled test beds of particle physics, general relativity and magneto-hydrodynamical phenomena. The EU-funded project ACCRETION STATES (Multiwavelength spectral timing of black holes and neutron stars: A new step in our understanding of accretion processes) focused on a process that powers these astrophysical sources: accretion.

Accreting neutron stars and black holes are among the brightest sources in the X-ray sky. X-ray observations of neutron stars allowed astrophysicists to look into the behaviour of matter under the most extreme conditions of density and magnetic field strength. On the other hand, accreting black holes were the only astrophysical objects where they could study phenomena occurring within a few gravitational radii from their source.

The study of ACCRETION STATES scientists was based on observations of more than 10 black holes and 50 neutron stars. This is the largest available sample of observations that is expected to provide a reference point to classify their behaviour into distinct accretion 'states'. Distinguished by different spectral and temporal properties, these states reflect the geometry and radiative efficiency of accretion flows.

During the so-called hard state, observed in the initial rise of accretion, the X-ray spectrum is dominated by Compton emission and shows high variability. It precedes a soft state of roughly constant luminosity. As thermal accretion dominates, the luminosity of the source slowly decays until a reverse transition to the hard state occurs. This behaviour, known as hysteresis, was found to be common among both neutron star systems and systems harbouring black holes.

These findings have been described in numerous publications in peer-reviewed journals and presented at international scientific conferences. The ACCRETION STATES project has advanced our empirical understanding of changes in the X-ray spectra of the end points of stellar evolution as well as some of the underlying theories for this origin.