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.