We present the results of X-ray variability and spectral analysis of a sample of 15 new candidates for active galactic nuclei with relatively low-mass black holes ( BHs ) .
They are selected from the Second XMM-Newton Serendipitous Source Catalogue based on strong variability quantified by normalized excess variances .
Their BH masses are estimated to be ( 1.1 - 6.6 ) \times 10 ^ { 6 } M _ { \odot } by using a correlation between excess variance and BH mass .
Seven sources have estimated BH masses smaller than 2 \times 10 ^ { 6 } M _ { \odot } , which are in the range for intermediate-mass black holes .
Eddington ratios of sources with known redshifts range from 0.07 to 0.46 and the mean Eddington ratio is 0.24 .
These results imply that some of our sources are growing supermassive black holes , which are expected to have relatively low masses with high Eddington ratios .
X-ray photon indices of the 15 sources are in the range of \approx 0.57 - 2.57 , and 5 among them have steep ( > 2 ) photon indices , which are the range for narrow-line Seyfert 1s .
Soft X-ray excess is seen in 12 sources , and is expressed by a blackbody model with kT \approx 83 - 294 eV .
We derive a correlation between X-ray photon indices and Eddington ratios , and find that the X-ray photon indices of about a half of our sources are flatter than the positive correlation suggested previously .