1 Introduction

The Galactic supernova remnant (SNR) SN 1006 (G327.6+14.6) is the result of a Type Ia supernova (SN) (Schaefer 1996), probably the brightest SN observed from Earth in recorded history. SN 1006 emits throughout the whole electromagnetic spectrum, from radio wavelengths to TeV $\gamma$-rays. In the radio and X-ray regimes, SN 1006 has a bilateral appearance with two symmetric, bright limbs (towards the NE and SW) and almost no emission at the sites where the axis of symmetry intersects the SN shell (to the SE and NW). In X-rays, synchrotron emission is observed from the rims, while from the interior of the SNR thermal X-ray emission is detected (Willingale et al. 1996; Winkler & Long 1997; Dyer et al. 2001). At optical wavelengths, SN 1006 is one of the small number of Balmer-dominated SNRs, with essentially no detections of the forbidden lines typically associated with SNRs (Fesen et al. 1985). The brightest optical filaments are located to the NW of the SNR where both the radio and X-ray emission are quite faint. On the southern rim of the remnant, there is a faint thin filament extending along the boundary of the radio shell. More diffuse H${\alpha}$ appears to fill much of the SE portion of the SNR (Winkler & Long 1997).

SN 1006 was the first Galactic SNR where TeV $\gamma$-rays were detected at the 5.3 $\sigma$ and 7.7 $\sigma$ level (Tanimori et al. 1998). The emission is localized in the northeast (NE) limb of SN 1006. High energy $\gamma$-rays detected from SNRs are generally considered to originate from $\pi^0$ decays induced by collisions between swept-up matter and accelerated protons in SNRs. However, in the case of SN 1006 the $\gamma$-rays detected towards the NE limb are thought to be Inverse Compton (IC) radiation caused by the collision of high energy electrons with low energy photons of the 2.7 K cosmic microwave background (Aharonian & Atoyan 1999; Berezhko et al. 2000; Dyer et al. 2001; Tanimori et al. 2001). An IC origin is likely since the matter density in H  I outside the shock front in SN 1006 is too low to produce $\gamma$-rays from $\pi^0$ decay (predicted densities in H  I are in the range $\sim$0.05 to 0.4 cm^-3, Willingale et al. 1996; Laming et al. 1996). It is possible to accelerate electrons and nuclei to $\gamma$-rays energies through the interaction of SNRs with molecular clouds (Bykov et al. 2000). The forward and reverse shocks, if accompanied by magnetohydrodynamic turbulence, can result in high-energy $\gamma$-ray radiation.

We have investigated the neutral interstellar gas around SN 1006, looking for inhomogeneities and/or anisotropies that may explain the observed characteristics. The survey in H  I is part of an ongoing project to observe the environs of bilateral SNRs. The physical characteristics of the surrounding ISM may determine whether the origin of the peculiar bilateral morphology is caused by intrinsic factors (like biconical outflows from central compact sources), or if the SNR has been shaped by the interaction with dense "walls'' of interstellar gas aligned parallel to the SNR limbs (Gaensler 1998; Giacani et al. 2000; Dubner et al. 2002). In the case of SN 1006, investigation of the properties of the surrounding ISM may give clues as to the origin of the TeV $\gamma-$radiation, localized in only one of the two symmetrical synchrotron lobes. Furthermore, the H  I observations may disclose the existence of neutral gas concentrations towards the NW, which could explain the origin of the thin Balmer filaments. From H  I emission observations the absorbing H  I column density required to model the X-ray emission, can be directly determined.

Our H  I observations represent the first high resolution, high sensitivity HI emission study of an extensive region surrounding this large SNR (diameter about $0\hbox{$.\!\!^\circ$ }5$). We have also explored the surroundings of SN 1006 in the CO molecular lines. The molecular observations (in the ¹²CO J:1-0 and J:2-1 transitions) were carried out with high angular resolution towards the NE limb and at several points along the SNR shell, looking for cold, compact clumps of molecular gas that could provide localized targets to accelerate electrons to very high energies.

In the next sections we describe the observations, present the images and discuss the results.