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 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.  

  We report the optical spectra of an ultraluminous supersoft X-ray source (ULS) in the nearby galaxy M81 (M81 ULS-1). 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 0.17 c, and seem to be similar to those from the prototype microquasar SS 433. 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. 

By with LIU Qingzhong 

Figure 1. Keck and GTC spectra for the optical counterpart of M81 ULS-1. (a) The Keck/LRIS spectrum taken on 2010/04/13 (black) and the GTC/OSIRIS spectrum taken on 2015/04/08 (blue) for M81 ULS-1. Labeled over the lines are the broad Balmer lines (Ha and Hb), the very broad blue-shifted Ha? at 5530°A and 5648A° . 

  The work by Liu, Ji-Feng; Bai, Yu; Wang, Song; Justham, Stephen; Lu, You-Jun; Gu, Wei-Min; Liu, Qing-Zhong; di Stefano, Rosanne; Guo, Jin-Cheng; Cabrera-Lavers, Antonio; álvarez, Pedro; Cao, Yi; Kulkarni, Shri has been published in Nature. Please see Nature, Volume 528, Issue 7580, pp. 108-110 (2015) for more details.http://www.nature.com/nature/journal/v528/n7580/pdf/nature15751.pdf