The X − ray flux and spectral variability of EMSS AGN

Fifteen ROSAT PSPC observations available in the public archive are analyzed in order to study time and spectral variability of the 12 EMSS AGN detected by ROSAT with more than 2000 net counts. Time variability was investigated on 13 different time scales, ranging from 400 s to 3.15×10 6 s (1 year). Of the 12 sources analyzed, only two do not show a significant variability on any time scale. On short time scale ≤ 20 percent of AGN are seen as variable sources while on time scale of ≥100.000 s the fraction becomes ≥50 percent. However one should bare in mind that the visibility function for variability is far from being uniform and that small amplitude variations can be detected more often on long time scale than on short time scale. Spectral variability was detected in only two sources. MS1158.6−0323 shows an hardening of the spectrum with increasing intensity while MS2254.9−3712 shows a softening of the spectrum with increasing intensity. Finally, for one source (MS1416.3−1257), the observed variability is not due to an intrinsic flux variation but, instead, to a variation in the column density along the line of sight. Since this variability has been observed on a time scale of ∼ 3.9 days, it is probably associated to the broad line clouds.