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Final Report Summary - BELLA (Binary Evolution and Low-Level Accretion)

Accretion is a fundamental physical process in which an astronomical body gravitationally attracts material from its surrounding. Accretion plays an important role at all scales encountered in the universe, ranging from the formation of stars and planets on the one end, to the formation of large-scale structures through accretion by galaxy clusters at the other. Understanding how accretion proceeds under a wide range of physical conditions is therefore a central theme in modern astrophysics.

X-ray binaries allow us to study accretion onto some of the most extreme objects in our universe: black holes and neutron stars. These are the remnants of once massive stars that ended their life in a supernova explosion. Black holes and neutron stars are often found in binary star systems, in which they are accompanied by a normal (Sun-like or smaller) star. In such a configuration, the immense gravity exerted by black holes and neutron stars allows them to strip off and accrete the outer gaseous layers of their companion star. This process results in the liberation of enormous amounts of gravitational energy that is converted into electromagnetic radiation, most prominently X-rays.Black holes and neutron stars are messy eaters, and some of the gas stripped from their companion is not consumed but rather spit into space via outflows that occur in the form of winds and collimated jets. The radiation of X-ray binaries can be observed and analysed with ground-based telescopes and space-based astronomical satellites to gain insight into the accretion process. Apart from gaining more knowledge about this fundamental physical process, understanding how accretion proceeds in X-ray binaries is also of utmost importance for understanding how these binary star systems are formed and evolve.

The brightness of X-ray binaries is proportional to the rate at which gas is accreted onto the neutron star or black hole. The brightest objects are most easily observed, and therefore most of our knowledge of accretion in X-ray binaries has been inferred from studying high mass accretion rates. At lower accretion rates, however, the accretion geometry, physical emission mechanisms, and outflow properties may be very different. Recent technological developments now finally allow a detailed study the low accretion regime in X-ray binaries. The aim of this 2-year Marie Curie project was to target faint X-ray binaries using dedicated observing strategies, to further our knowledge of low-level accretion onto (stellar-mass) black holes and neutron stars.

During this project, various techniques were employed to study the accretion flow in a number of different X-ray binaries using state of the art X-ray satellites. Firstly, X-ray spectroscopy and long-term (10-yr baseline) X-ray light curves obtained with NASA’s Swift observatory were used to quantify the overall energy output in X-ray binaries accreting at low rates (Degenaar et al. 2015, JHEAp 7, 137). Secondly, X-rays reflected of the inner accretion disk and observed with the satellite NuSTAR were used to study the (changing) accretion geometry (Degenaar et al. 2015, MNRAS Letters 451, L85; Degenaar et al. 2016, MNRAS 456, 4256; Degenaar et al. 2016, MNRAS 461, 4049; Degenaar et al. 2017, MNRAS 464, 398). Thirdly, high-resolution X-ray spectroscopy performed with the Chandra X-ray observatory was used to study the presence of winds driven of the accretion disk (Degenaar et al. 2016, MNRAS 461, 4049; Degenaar et al. 2017, MNRAS 464, 398). Finally, whether accretion stops at the lowest X-ray brightness states was studied by performing X-ray spectroscopy of data obtained with the Chandra satellite (Degenaar et al. 2015, MNRAS 451, 2071; Degenaar et al. 2017, MNRAS Letters 465, L10).

The general outcome of this Marie Curie project is a showcase of how different (X-ray) analysis techniques and dedicated observing strategies can yield important information on the accretion flow properties in faint X-ray binaries. Most of the results of the project were obtained by using these X-ray spectroscopy techniques, which yield information on the inner part of the accretion flow (nearest to the black hole or neutron star), but ground work for multi-wavelength observations (extending from X-rays to UV, optical, infrared and radio) to study the entire accretion flow and associated outflows was also laid. Apart from gaining more insight into the accretion process itself, this is a very important step to achieve a better understanding of the formation and evolution of X-ray binaries.

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United Kingdom
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