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Primordial black holes (PBHs) with a mass from $10^{-16}$ to$10^{-11}\,M_\odot$ may comprise 100% of dark matter. Due to a combination ofwave and finite source size effects, the traditional microlensing of stars doesnot probe this mass range. In this paper, we point out that X-ray pulsars withhigher photon energies and smaller source sizes are good candidate sources formicrolensing for this mass window. Among the existing X-ray pulsars, the SmallMagellanic Cloud (SMC) X-1 source is found to be the best candidate because ofits apparent brightness and long distance from Earth. We have analyzed theexisting observation data of SMC X-1 by the RXTE telescope (around 10 days) andfound that PBH as 100% of dark matter is close to but not yet excluded. Futurelonger observation of this source by X-ray telescopes with larger effectiveareas such as AstroSat, Athena, Lynx, and eXTP can potentially close the lastmass window where PBHs can make up all of dark matter.

Microlensing of X-ray Pulsars: a Method to Detect Primordial Black Hole Dark Matter