Nebular-phase spectra of SN 2006aj, discovered in coincidence with XRF 060218, were obtained with Keck in 2006 July and VLT in 2006 September. At the latter epoch spectropolarimetry was also attempted, yielding a polarization upper limit of åisebox-0.5ex 2%. The spectra show strong [O I] and Mg I] emission lines, as expected in Type Ic supernovae, but weak Ca II lines. The [Fe II] lines that were strong in SN 1998bw are much weaker in SN 2006aj, consistent with its lower luminosity. The outer velocity of the line-emitting region is i̊sebox-0.5ex 8000 km s$^-1$ in July and rs̊ebox-0.5ex 7400 km s$^-1$ in September, consistent with the relatively low expansion kinetic energy of SN 2006aj. All lines have similar width, and their profiles indicate that no major asymmetries are present in the ejecta at velocities below v<8000 km s$^-1$, except perhaps in the innermost part. The spectra were modeled with a non-LTE code. The mass of $^56$Ni required to power the emission is rae̊box-0.5ex 0.20 M$_solar$, confirming earlier results based on the light curve. The oxygen mass is raib̊ox-0.5ex 1.5 M$_solar$, again much less than in SN 1998bw, but raiso̊x-0.5ex 0.7 M$_solar$ larger than the value derived from the early-time modeling. The ejected mass below 8000 km s$^-1$ is raisex̊-0.5ex 2 M$_solar$, confirming that SN 2006aj was only about twice as massive and energetic as the normal Type Ic SN 1994I. The presence of a dense inner core, containing raiseb̊-0.5ex 1 M$_solar$ of mostly oxygen and carbon, is inferred, as in all broad-lined SNe Ic. This core, which may be disklike, is too deep to influence the early light curve and too small to affect the late polarization spectrum.
Based on observations made with ESO telescopes at the Paranal and La Silla Observatories under program 277.D-5039(A).