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Research activities

by Laurence Boirin - 11 September 2008

I am interested in X-ray binaries. I use X-ray observations such as performed by XMM-Newton to try and better understand these enigmatic systems.

- X-ray binaries

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Artist impression of an X-ray binary
The companion star feeds the accretion disk formed around the compact object. The obscuration of the central X-ray source by a vertical structure located at the outer edge of the accretion disk will cause dips in the X-ray light curve of systems viewed close to edge-on (at high inclination).

In an X-ray binary, a compact object (either a neutron star or a black hole) accretes matter from a companion star via an accretion disk. This is a powerful way to convert gravitational energy into X-rays.

X-ray binaries are of significance to Astrophysics because they contain products of extreme stellar evolution. They provide some of the best opportunities to observe neutron stars and black-holes in which issues such as dense matter and strong gravity come to the fore. Extracting consistent information about these issues requires a good understanding of an X-ray binary’s geometry, of the emitting and absorbing processes occuring there, and of accretion physics. My research activities and projects aim to improve our understanding of these points through observations of accreting compact objects in the X-rays where the bulk of the energy is radiated.

- Highly-inclined systems to better probe the accretion disk

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Light curve of a dipper
XMM-Newton Light curve of the dipper XB1916-053 (from Boirin et al. 2004). Dipping activity occurs at the orbital period of the system.

I pay particular attention to the members of the class of X-ray binaries called dippers. A dipper shows dips in its light curve occurring at the orbital period of the system. Dips are due to the obscuration of the central X-ray source by a vertical structure located at the outer edge of the accretion disk. The structure is thought to be caused by the impact of the accretion flow from the companion star into the disk. Dippers are thus systems viewed at high-inclination (60—80 degrees), ie. almost edge-on, close to the disk plane. Dippers are of particular interest because this viewing angle makes the observation of interactions between radiation and mater (mostly occurring in or close to the disk) easier a priori.

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