Discovery of Three Candidate Magnetar-powered Fast X-ray Transients from
  Chandra Archival Data
  It was proposed that a remnant stable magnetar could be formed in a binary
neutron-star merger, leading to a fast X-ray transient (FXT) that can last for
thousands of seconds. Recently, Xue et al. suggested that CDF-S XT2 was exactly
such a kind of source. If confirmed, such emission can be used to search for
electromagnetic counterparts to gravitational wave events from binary
neutron-star mergers that have short gamma-ray bursts and the corresponding
afterglows seen off-axis and thus too weak to be detected. Here we report the
discovery of three new FXTs, XRT 170901, XRT 030511, and XRT 110919, from
preliminary search over Chandra archival data. Similar to CDF-S XT2, these new
FXTs had a very fast rise (less than a few ten seconds) and a plateau of X-ray
flux $\sim$$1.0\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$ lasting for 1-2 ks,
followed by a steep decay. Their optical/IR counterparts, if present, are very
weak, arguing against a stellar flare origin for these FXTs. For XRT 170901, we
identified a faint host galaxy with the source at the outskirts, very similar
to CDF-S XT2. Therefore, our newly discovered FXTs are also strong candidates
for magnetar-powered X-ray transients resulting from binary neutron star
mergers.
