A search for optical counterparts of the complex Vela X system

Abstract

The pulsar wind nebula (PWN) associated with the Vela pulsar, Vela X, is a complex system visible in radio, X-ray and y-ray wavelengths, but not in the optical wavelength. Knowledge of the brightness and structure of the PWN in optical is important in order to constrain the morphology as well as the underlying particle spectrum, the B-field properties, and particle energy losses associated with this extended source. Optical observations of the Vela X system at Sutherland with the SAAO 1.0 m telescope did not yield any significant detection. Similar observations with HST, NTT, and VLT could only give upper limits to the surface brightness in the vicinity of this system (Mignani et al., 2003). Since we find no visible optical counterparts to the PWN radio lobes within the field of view of our observations, we are inhibited from probing the change in source morphology as a function of energy. Using all these observations mentioned above, we investigate whether or not the radio synchrotron component can be smoothly extrapolated to the optical band, which would have implications for the particle injection spectrum. We derive v Fv upper limits of ~ 10-7 erg s-1 cm-2 for the B and V bands from Sutherland observations. We find that our upper limits do not constrain any broadband spectral model for Vela X. The question of whether the underlying particle injection spectrum consists of a single component or multiple components could therefore not be fully addressed. Deeper optical observations, specifically targeting the cocoon region southwest of the pulsar, may yet detect a very faint optical source and therefore may advance our understanding of this complex multiwavelength object.