title: Relativistic baryonic jets from an ultraluminous supersoft X-ray source.
abstract: The formation of relativistic jets by an accreting compact object is one of the fundamental mysteries of astrophysics. Although the theory is poorly understood, observations of relativistic jets from systems known as microquasars (compact binary stars) have led to a well established phenomenology. Relativistic jets are not expected to be produced by sources with soft or supersoft X-ray spectra, although two such systems are known to produce relatively low-velocity bipolar outflows. Here we report the optical spectra of an ultraluminous supersoft X-ray source (ULS) in the nearby galaxy M81 (M81 ULS-1; refs 9, 10). Unexpectedly, the spectra show blueshifted, broad Hα emission lines, characteristic of baryonic jets with relativistic speeds. These time-variable emission lines have projected velocities of about 17 per cent of the speed of light, and seem to be similar to those from the prototype microquasar SS 433 (refs 11, 12). Such relativistic jets are not expected to be launched from white dwarfs, and an origin from a black hole or a neutron star is hard to reconcile with the persistence of M81 ULS-1's soft X-rays. Thus the unexpected presence of relativistic jets in a ULS challenges canonical theories of jet formation, but might be explained by a long-speculated, supercritically accreting black hole with optically thick outflows.
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.|College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China.
authors.forename: Ji-Feng
authors.initials: JF
authors.lastname: Liu
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.
authors.forename: Yu
authors.initials: Y
authors.lastname: Bai
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.
authors.forename: Song
authors.initials: S
authors.lastname: Wang
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.|College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China.
authors.forename: Stephen
authors.initials: S
authors.lastname: Justham
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.|College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China.
authors.forename: You-Jun
authors.initials: YJ
authors.lastname: Lu
authors.affiliation: Department of Astronomy, Xiamen University, Xiamen, Fujian Province 361005, China.
authors.forename: Wei-Min
authors.initials: WM
authors.lastname: Gu
authors.affiliation: Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, Jiangsu Province 210008, China.
authors.forename: Qing-Zhong
authors.initials: QZ
authors.lastname: Liu
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
authors.forename: Rosanne
authors.initials: R
authors.lastname: Di Stefano
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.
authors.forename: Jin-Cheng
authors.initials: JC
authors.lastname: Guo
authors.affiliation: Instituto de Astrofísica de Canarias, c/Vía Láctea s/n, E-38200 La Laguna, Tenerife, Spain.|Departamento de Astrofísica, Universidad de la Laguna, E-38205 La Laguna, Tenerife, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: Cabrera-Lavers
authors.affiliation: Instituto de Astrofísica de Canarias, c/Vía Láctea s/n, E-38200 La Laguna, Tenerife, Spain.|Departamento de Astrofísica, Universidad de la Laguna, E-38205 La Laguna, Tenerife, Spain.
authors.forename: Pedro
authors.initials: P
authors.lastname: Álvarez
authors.affiliation: Department of Astronomy, Caltech, Pasadena, California 91125, USA.
authors.forename: Yi
authors.initials: Y
authors.lastname: Cao
authors.affiliation: Department of Astronomy, Caltech, Pasadena, California 91125, USA.
authors.forename: Shri
authors.initials: S
authors.lastname: Kulkarni