Low-level radio flares from Cygnus X-3

Abstract

Cygnus X-3 is a poorly understood but interesting galactic X-ray source. It is unusual in that it shows variable low-state (S_v<1 Jy) radio emission¹ as well as spectacular radio flares (S_v∼20 Jy). The spectacular flares have long been interpreted as synchrotron emission from an expanding source²; more recent work³ implies that Cyg X-3 may be one of the few identified radio jets in the Galaxy. Flux-density variations with a period of 4.8 h have been discovered at X-ray⁴ and IR⁵ wavelengths, and possibly at γ-ray energies^6,7 from 10⁸ eV to 10¹⁶ eV. The X-ray emission is probably produced in a binary system composed of a low-mass star orbiting a compact object with an accretion disk and an accretion disk corona⁸. As interstellar absorption⁹ (A_v = 19) precludes study of the optical emission, the applicability of this model to Cyg X-3 rests heavily on analogy with other sources with similar X-ray properties (such as, 4U1822-37 and 4U2129+47). We report here observations with the Very Large Array (VLA) of Cyg X-3 at several radio wavelengths which show that the radio emission in its low state may be interpreted as arising entirely from the superposition of a series of flares that are wavelength dependent with an apparent period near the 4.8-h X-ray period¹⁰. A model of the source, in which each flare represents the injection of relativistic particles into a small volume which subsequently expands at constant velocity, is qualitatively consistent with the data.