We carry out classification of 4330 X-ray sources in the 2XMMi-DR3 catalog .
They are selected under the requirement of being a point source with multiple XMM-Newton observations and at least one detection with the signal-to-noise ratio larger than 20 .
For about one third of them we are able to obtain reliable source types from the literature .
They mostly correspond to various types of stars ( 611 ) , active galactic nuclei ( AGN , 753 ) and compact object systems ( 138 ) containing white dwarfs , neutron stars , and stellar-mass black holes .
We find that about 99 % of stars can be separated from other source types based on their low X-ray-to-IR flux ratios and frequent X-ray flares .
AGN have remarkably similar X-ray spectra , with the power-law photon index centered around 1.91 \pm 0.31 , and their 0.2–4.5 keV flux long-term variation factors have a median of 1.48 and 98.5 % less than 10 .
In contrast , 70 % of compact object systems can be very soft or hard , highly variable in X-rays , and/or have very large X-ray-to-IR flux ratios , separating them from AGN .
Using these results , we derive a source type classification scheme to classify the other sources and find 644 candidate stars , 1376 candidate AGN and 202 candidate compact object systems , whose false identification probabilities are estimated to be about 1 % , 3 % and 18 % , respectively .
There are still 320 associated with nearby galaxies and 151 in the Galactic plane , which we expect to be mostly compact object systems or background AGN .
We also have 100 candidate ultra-luminous X-ray sources .
They are found to be much less variable than other accreting compact objects .