title: Radio emission from an ultraluminous x-ray source.
abstract: The physical nature of ultraluminous x-ray sources is uncertain. Stellar-mass black holes with beamed radiation and intermediate black holes with isotropic radiation are two plausible explanations. We discovered radio emission from an ultraluminous x-ray source in the dwarf irregular galaxy NGC 5408. The x-ray, radio, and optical fluxes as well as the x-ray spectral shape are consistent with beamed relativistic jet emission from an accreting stellar black hole. If confirmed, this would suggest that the ultraluminous x-ray sources may be stellar-mass rather than intermediate-mass black holes. However, interpretation of the source as a jet-producing intermediate-mass black hole cannot be ruled out at this time.
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. pkaaret@cfa.harvard.edu
authors.forename: Philip
authors.initials: P
authors.lastname: Kaaret
authors.forename: Stephane
authors.initials: S
authors.lastname: Corbel
authors.forename: Andrea H
authors.initials: AH
authors.lastname: Prestwich
authors.forename: Andreas
authors.initials: A
authors.lastname: Zezas