1997/09/17 by P. Ciliegi, Paolo Ciliegi, Ciliegi, Paolo +3
Engineering · Mathematics · Physics and Astronomy · #Amplitude #Astronomical Observations and Instrumentation #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics (astro-ph) #Astrophysics and Cosmic Phenomena #Atmospheric sciences #FOS: Physical sciences #Flux (metallurgy) #Galaxy #Line-of-sight #Materials science #Mathematics #Optics #Physics #ROSAT #Scale (ratio) #Spatial variability #Spectral line #Statistics #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9709169
11 pages, 1 ps file, accepted for publication in MNRAS
arxiv created 1997/09/17 · openalex publication_date 1997/09/17 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
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 1 year. Of the 12 sources analyzed, only two do not show a significant variability on any time scale. On short time scale about 20 percent of AGN are seen as variable sources while on time scale greater than 100.000 s the fraction becomes greater than 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 about 3.9 days, it is probably associated to the broad line clouds.