Is the mysterious {\it Punctum} a primordial black hole?

Abstract

A recent study has discovered a highly polarized millimeter continuum source called {\it Punctum} located at the central region of NGC 4945 which doesn't fit any known category. Apart from the radio detection at $\approx 100$ GHz, there is no identified counterpart to {\it Punctum} in other radio, infra-red, optical, and X-ray bands. All available potential models, including magnetar, supernova remnant and microquasar, cannot provide satisfactory explanation for the exceptionally high polarization of $\sim 50$\% and large luminosity in mm band $L_{\mathrm{mm}}\approx 2\times 10^{35}$ erg/s within a small size $<2$ pc shown in {\it Punctum}. In this article, we discuss the possibility that {\it Punctum} is indeed a primordial black hole (PBH) with a dark matter density spike. We show that the synchrotron radiation due to the electrons and positrons produced from dark matter annihilation can satisfactorily explain the observed properties of {\it Punctum} and we can effectively obtain the annihilation cross section $⟨\sigma v⟩\sim 10^{-33}$ cm${}^3$/s using our developed analytic framework. Furthermore, if the {\it Punctum} PBH mass is $\sim 10-100M_{\odot }$, the preferred dark matter mass range is $m_{\mathrm{DM}}\sim 10-1000$ MeV.