2009A&A...508..779M

A handful of young (a few thousand years) supernova remnants (SNRs) host point-like X-ray sources, dubbed central compact objects (CCOs), which are thought to be radio-silent isolated neutron stars formed by the supernova explosion. So far, no CCO has been firmly detected at other wavelengths. However, ground-based observation in the H_α band detected a nebula around CXOJ085201.4-461753, the CCO in the Vela Jr. SNR. The nebula has also been detected in deep R-band observations performed with the Very Large Telescope (VLT). Interestingly, both its extension and its flux in the R band are consistent with those measured in H_α, suggesting that the nebula spectrum is dominated by line emission, possibly produced by a velocity-driven bow-shock in the interstellar medium (ISM) or by its photo-ionisation from the neutron star. The aim of this work is to resolve the morphology of the H_α nebula around the CCO to verify the proposed interpretations. We performed high-resolution imaging observations of the nebula with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) through the 656N filter, almost exactly centred on the rest wavelength of the H_α line. Surprisingly enough, we did not detect the nebula in our WFPC2 image down to a 3σ flux limit of ∼3x10^–15erg/cm2/s. This limit is a factor of 10 fainter than the nebula flux measured in the discovery ground-based observations which were, however, performed with redder and broader H_α filters. The non-detection of the nebula in the narrower and bluer WFPC2 656N filter suggests that the peak of the emission might actually be at longer wavelengths. One possibility, compatible with the bow-shock scenario only, is that the H_α line is red-shifted by ∼10-60Å due to the neutron star motion with a radial velocity 450≲V_r≲2700km/s. The other possibility is that the nebula is a knot of [NII] emission (λ= 6583.6Å) unrelated to CXOJ085201.4-461753 and possibly associated with the nearby star Wray 16-30. High-resolution spectroscopy observations will help to settle the nature of the nebula.