The X-ray spectral properties of X-ray-selected AGN: ROSAT spectra of EMSS AGN

Using a sample of 63 AGNs extracted from the $Einstein$ Extended MediumSensitivity Survey (EMSS), we study the X-ray spectral properties of X-rayselected AGN in the 0.1$-$2.4 keV ROSAT band. These objects are all the EMSSAGN detected with more than 300 net counts in ROSAT PSPC images available fromthe public archive (as of May 31, 1995). A Maximum-Likelihood analysis is usedto find the mean power-law spectral index $<\alpha_p>$ and the intrinsicdispersion $\sigma_p$. We find $<\alpha_p>$=1.42 with $\sigma_p$=0.44. Thisvalue is significantly steeper ($\Delta \alpha \sim$0.4) than the mean$Einstein$/IPC spectral index obtained applying the ML analysis on the wholesample of EMSS AGN. This result shows that the soft excess already noted inoptically selected AGN is present also in X-ray selected AGN. The relativelyhigh value obtained for the intrinsic dispersion confirms that in the soft bandAGN are characterized by a variety of spectral indices and the increase withrespect to results obtained from the analysis of Einstein data ($\Delta\sigma_p \sim$0.16) suggests a further broadening of the spectral indexdistribution as one moves to softer energies. A comparison between the meanspectral index of Radio-quiet and Radio-loud subsamples shows that the meanindex of the RL sample is flatter than that of RQ, both in the IPC ($\Delta\alpha \sim$0.3) and in the PSPC ($\Delta \alpha \sim$0.4) data. This suggeststhat the additional X-ray component in RL AGN dominates the X-ray emission ofRL AGN over almost two decades of energy ($\sim$0.1$-$10 keV).Comment: 8 pages LaTex file; mn.sty macro (enclosed), 5 LaTex Tables, 12 Postscript figures; accepted for publication in MNRA