X-ray binaries are astrophysical systems within our Galaxy composed of a star slowly ripped apart by its black hole companion. That process, known as accretion, makes the star’s material flow out of its outer layers and form a plasma disc around the black hole. This disc is heated up to tens of thousands of degrees Celsius, producing ultraviolet light and X-rays, making it shine even brighter than the star itself. These sources can be so luminous that an X-ray binary was the first X-ray source ever detected outside the Solar System! Yet X-ray binaries have another interesting property: they can produce powerful jets (collimated outflows of plasma), whose development is intimately linked to the disc. If jets are present in an X-ray binary, astrophysicists use the term microquasar to define it.

After its launch in 2008, NASA’s satellite Fermi discovered that those jets could also act as particle accelerators [1]. With its Large Area Telescope detector, Fermi measured gamma rays from an X-ray binary, a radiation naturally produced if particles were accelerated in the jet. However, theoretical models could only match the data if these particles were electrons. In regions with large densities of light particles (called photons), relativistic electrons can interact with them, transferring their energy and converting them into gamma rays. Massive stars are much brighter than those with low mass, and therefore X-ray binaries containing massive components were thought to be the only ones capable of emitting gamma rays. Indeed, the only three microquasars detected previously by Fermi contain stars at least ten times more massive than the Sun.

Consequently, it was generally believed that low-mass X-ray binaries were not powerful enough to produce gamma-rays. However, a recent study [2] now suggests otherwise: after 16 years of operations, Fermi has detected a faint gamma-ray signal consistent with the position of GRS 1915+105, a black hole microquasar with a star smaller than the Sun! GRS 1915+105 was discovered in 1992 as an X-ray source by the GRANAT observatory, and is nowadays one of the most studied systems in our Galaxy. But… How is this possible? How are those gamma rays produced? Can electrons be the particles responsible for the signal? No. A first theoretical estimate suggests that the gamma rays observed are incompatible with the standard scenario for microquasars. But additional data from the Nobeyama 45-meter radio telescope in Japan offer an alternative: the presence of other, more massive particles called protons. GRS 1915+105 has so much gas around it that, if it accelerates protons in its jet, then those can collide with the nearby gas and produce gamma rays consistent with the signal detected by Fermi! Showing, after all these years, that low-mass X-ray binaries can truly be particle accelerators.

For more information:

Guillem Martí-Devesa
Dipartimento di Fisica, Università di Trieste and INFN, sezione di Trieste
e-mail: guillem.marti-devesa@ts.infn.it

Laura Olivera-Nieto
Max Planck Institute for Nuclear Physics (MPIK), Heidelberg
email: laura.olivera-nieto@mpi-hd.mpg.de

[1] https://www.nasa.gov/universe/fermi-telescope-peers-deep-into-microquasar/
[2] https://doi.org/10.3847/2041-8213/ada14f

Image credit: Credit: Science Communication Lab for MPIK/H.E.S.S.