4 Conclusions

We have carried out the first extensive survey of H  I in a 5 square degree field around the SNR SN 1006 (resolution of $\sim$4$^\prime$) in order to investigate the characteristics of the surrounding neutral interstellar medium. We have also carried out a search for cold, compact clumps of molecular gas, which may provide localized targets to accelerate electrons to TeV energies.

These new observations lead to the following conclusions: (a) The surroundings of SN 1006 are quite homogeneous, as expected at this high Galactic latitude. No obvious large-scale features are detected that can explain the SNR bilateral morphology based on external factors; (b) the existence of an H  I concentration projected on the center of SN 1006 suggests an upper limit of $\sim$-20 km s^-1 on the systemic velocity of the SNR. Under this assumption we obtain a limit for the distance to he SNR of about 1.7 kpc, in good agreement with previous estimates; (c) an average interstellar density of about 0.3 cm^-3 is estimated for the environs of SN 1006; (d) an extensive H  I concentration towards the NW, matching the location of bright Balmer filaments, is detected in the -7 to -4 km s^-1 velocity interval. An atomic volume density of $\sim$0.5 cm^-3 has been derived for this feature, in agreement with previous estimates based on optical and X-rays observations; (e) the distribution of the absorbing interstellar hydrogen between the observer and the SNR is quite uniform. The N  H towards SN 1006 is $\sim$ $6.8 \times 10^{20}$ cm^-2, with a slight increase of the column density to the center of the SNR; (f) the search for compact molecular clumps which may be the sites of acceleration of relativistic nuclei and electrons to TeV energies, produced only marginal detections at a 2$\sigma$ level between $V_{\rm LSR} \sim {{-}25}$ and -15 km s^-1. More sensitive observations of CO and of other high excitation molecular lines are required to confirm these detections.

Acknowledgements

We thank M. Arnal for providing the HI observation from the IAR Survey for use in this paper prior to publication. We are grateful to P. F. Winkler for providing the optical image. We acknowledge the staff of ATCA for technical support during the observations. G.D., E.G., and A.J.G acknowledge the support and hospitality from NRAO during their stay at the VLA. This research was partially funded through a Cooperative Science Program between CONICET (Argentina) and the National Science Foundation (USA) and through CONICET grant 4203/96. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility, managed by CSIRO. The MOST is owned and operated by the University of Sydney with support from the Australian Research Council and the University of Sydney. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.