Discovery of Three Candidate Magnetar-powered Fast X-ray Transients from Chandra Archival Data

Abstract: 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.