title: Positron annihilation signatures associated with the outburst of the microquasar V404 Cygni.
abstract: Microquasars are stellar-mass black holes accreting matter from a companion star and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 10(6) to 10(10) solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electron-positron plasma. Transient high-energy spectral features have been reported in two objects, but their positron interpretation remains contentious. Here we report observations of γ-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electron-positron plasma responsible for the bright diffuse emission of annihilation γ-rays in the bulge region of our Galaxy. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy.
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Siegert
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Diehl
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.
authors.forename: Jochen
authors.initials: J
authors.lastname: Greiner
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.|Universitäts-Sternwarte München, Ludwig-Maximilians-Universität, Scheinerstraße 1, 81679 München, Germany.
authors.forename: Martin G H
authors.initials: MG
authors.lastname: Krause
authors.affiliation: Physics Department and Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, New York 10027, USA.
authors.forename: Andrei M
authors.initials: AM
authors.lastname: Beloborodov
authors.affiliation: Max Planck Computing and Data Facility, Gießenbachstraße 2, 85748 Garching, Germany.
authors.forename: Marion Cadolle
authors.initials: MC
authors.lastname: Bel
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.
authors.forename: Fabrizia
authors.initials: F
authors.lastname: Guglielmetti
authors.affiliation: Laboratoire Astrophysique Instrumentation Modélisation, UMR 7158, CEA/CNRS/Université Paris Diderot, CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France.
authors.forename: Jerome
authors.initials: J
authors.lastname: Rodriguez
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Strong
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany.
authors.forename: Xiaoling
authors.initials: X
authors.lastname: Zhang