Is the mysterious Punctum a primordial black hole?

A recent study has discovered a highly polarized millimeter continuum source called Punctum located at the central region of NGC 4945 which doesn't fit any known category. Apart from the radio detection at about 100 GHz, there is no identified counterpart to 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 about 50 % and large luminosity in mm band L mm about 2 times 10 35 erg/s within a small size <2 pc shown in Punctum. In this article, we discuss the possibility that 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 Punctum and we can effectively obtain the annihilation cross section σv about 10-33 cm cubed /s using our developed analytic framework. Furthermore, if the Punctum PBH mass is about 10-100M, the preferred dark matter mass range is m DM about 10-1000 MeV.