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Compared with numerous X-ray dominant active galactic nuclei (AGNs) withoutemission-line signatures in their optical spectra, the X-ray selected AGNs withoptical emission lines are probably still in the high-accretion phase of blackhole growth. This paper presents an investigation on the fraction of theseX-ray detected AGNs with optical emission-line spectra in 198 galaxy groups at$z<1$ in a rest frame 0.1-2.4 keV luminosity range 41.3 <log(L_X/erg s-1) <44.1 within the COSMOS field, as well as its variations with redshift and grouprichness. For various selection criteria of member galaxies, the numbers ofgalaxies and the AGNs with optical emission lines in each galaxy group areobtained. It is found that, in total 198 X-ray groups, there are 27 AGNsdetected in 26 groups. AGN fraction is on everage less than $4.6 (\pm 1.2)\%$for individual groups hosting at least one AGN. The corrected overall AGNfraction for whole group sample is less than $0.98 (\pm 0.11) \%$. Thenormalized locations of group AGNs show that 15 AGNs are found to be located ingroup centers, including all 6 low-luminosity group AGNs. A week risingtendency with $z$ are found: overall AGN fraction is 0.30-0.43% for the groupsat $z<0.5$, and 0.55-0.64% at 0.5 < z < 1.0. For the X-ray groups at $z>0.5$,most member AGNs are X-ray bright, optically dull, which results in a lower AGNfractions at higher redshifts. The AGN fraction in isolated fields alsoexhibits a rising trend with redshift, and the slope is consistent with that ingroups. The environment of galaxy groups seems to make no difference indetection probability of the AGNs with emission lines. Additionally, a largerAGN fractions are found in poorer groups, which implies that the AGNs in poorergroups might still be in the high-accretion phase, whereas the AGN populationin rich clusters is mostly in the low-accretion, X-ray dominant phase.

Fraction of the X-ray selected AGNs with optical emission lines in galaxy groups