Galactic jets and their engines
In X-ray binaries, mass is normally accreted from the star rotating around the black hole or neutron star in the form of a disk. The innermost regions of this accretion disk are so hot that the temperatures exceed one million degrees and emit X-rays. Close to the accreting compact object, a significant fraction of the in-falling matter is ejected in collimated jets that stretch for thousands of light years. Using high-resolution data from interferometric arrays of radio telescopes, EU-funded research shed new light on how black holes produce these powerful jets. In the GALACTIC JETS (Studies of jets from galactic black holes and neutron stars) project, scientists used the Very Large Array and the Very Large Baseline Array in the USA as well as the European VLBI Network to look at the radiation emitted by X-ray binaries at all wavelengths. The aim was to obtain observations when hard X-rays dominated their luminosity and subsequently, monitor the radiation sources over a period ranging from several days to a few weeks. This strategy allowed the scientists to catch the radio and X-ray outburst of several black holes and neutron stars. The X-ray binary known as XTE J1908+904 detected at the end of 2013 was found to eject two knots of plasma. Measurements of the apparent opening angle and expansion rate placed constraints on the maximum velocity of relativistic particles spewed into space. The scientists also detected an accreting neutron star near the massive black hole Sagittarius A, AX J1745.6-2901. During periods dominated by soft X-ray emission, powerful winds were observed that disappeared with the development of jets that predominantly emitted hard X-rays. The observations accumulated during the GALACTIC JETS project gave scientists a valuable opportunity to delve into the mystery of galactic jets. They are still not well understood and further high-quality data will be needed to conclusively confirm scientific hypotheses on how they form and what powers them.
Keywords
Galactic jets, massive black holes, X-ray binaries, neutron star, Sagittarius A
