  Detonation initiation in a reactive medium can be achieved by an externally
created shock wave. Supersonic flow onto a gravitating center, known as
Bondi-Hoyle-Lyttleton (BHL) accretion, is a natural shock wave creating
process, but, to our knowledge, a reactive medium has never been considered in
the literature. Here, we conduct an order of magnitude analysis to investigate
under which conditions the shock-induced reaction zone recouples to the shock
front. We derive three semi-analytical criteria for self-sustained detonation
ignition. We apply these criteria to the special situation where a primordial
black hole (PBH) of asteroid mass traverses a carbon-oxygen white dwarf (WD).
Since detonations in carbon-oxygen WDs are supposed to produce normal
thermonuclear supernovae (SNe Ia), the observed SN Ia rate constrains the
fraction of dark matter (DM) in the form of PBHs as $\log_{10}(f_{\rm pbh})<
0.8 \log_{10}(m_{\rm bh}/{\rm g})-18$ in the range $10^{21}-10^{22}$g
($10^{20}-10^{22}$g) from a conservative (optimistic) analysis. Most
importantly, these encounters can account for both the rate and the median
explosion mass of normal sub-Chandrasekhar SNe Ia if a significant fraction of
DM is in the form of PBHs with mass $10^{23}$g.
