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Classification of X-ray Sources in the XMM-Newton Serendipitous Source Catalog: Objects of Special Interest

We analyze 18 sources that were found to show interesting properties of periodicity, very soft spectra and/or large long-term variability in X-rays in our project of classification of sources from the 2XMMi-DR3 catalog but were poorly studied in the literature, in order to investigate their nature. Two hard sources show X-ray periodicities of ~1.62 hr (2XMM J165334.4-414423) and ~2.1 hr (2XMM J133135.2-315541) and are probably magnetic cataclysmic variables. The source 2XMM J123103.2+110648 is an active galactic nucleus (AGN) candidate showing very soft X-ray spectra (kT~0.1 keV) and exhibiting an intermittent ~3.8 hr quasi-periodic oscillation. There are six other very soft sources (with kT<0.2 keV), which might be in other galaxies with luminosities between ~10^{38}-10^{42} erg/s. They probably represent a diverse group that might include objects such as ultrasoft AGNs and cool thermal disk emission from accreting intermediate-mass black holes. Six highly variable sources with harder spectra are probably in nearby galaxies with luminosities above 10^{37} erg/s and thus are great candidates for extragalactic X-ray binaries. One of them (2XMMi J004211.2+410429, in M 31) is probably a new-born persistent source, having been X-ray bright and hard in 0.3--10 keV for at least four years since it was discovered to enter an outburst in 2007. Three highly variable hard sources appear at low galactic latitudes and have maximum luminosities below ~10^{34} erg/s if they are in our Galaxy, thus great candidates for cataclysmic variables or very faint X-ray transients harboring a black hole or neutron star. Our interpretations of these sources can be tested with future long-term X-ray monitoring and multi-wavelength observations.