Quasar X-ray spectra revisited

,

for radio-loud quasars, suggesting that a component of the X-ray emission is relativistically beamed; (3) for the RQQs the soft X-ray slopes, with a mean of ,_ 1.0, are consistent with the slopes found at higher energies (2-10 keV) although steeper than those observed for Seyfert 1 galaxies (also 2-10 keV) where the reflection model gives a good fit to the data; (4) the correlation of FeII emission line strength with X-ray energy index is confirmed for radio-quiet quasars using a subset of 18 quasars." v-"'), in the soft X-ray region (0.2-3.5 keV) for a sample of 33 quasars that were observed with sufficient signal-tonoise with the Einstein !PC.They reported that the best-fit power-law energy slopes have a wide range (-0.2 to 1.8), and that radio-loudness correlated with the soft X-ray energy index in the sense that radio-loud quasars (RLQs) had flatter X-ray slopes than radio-quiet quasars (RQQs).
They argued for a two-component model for the X-ray emission: one linked to the radio-emission and of somewhat flatter slope (--.0.5); and the second of steeper slope ('-.1) that was ubiquitous in quasars.Wilkes and Elvis (1987) explain the difference between these soft slopes and the mean slope of _0.7 observed at higher energies in terms of a mixing of the two components.when log(fR/fB) >2.In Tables 1 and 2 et al. 1990et al. , Williams et al. 1992)).

7' Conclusions
Using updated soft X-ray slopes for a sample of 45 quasars, we conclude the following: • The earlier trend that radio-loud quasars have systematically flatter power-law fits to the soft X-ray spectrum than radio-quiet quasars is confirmed.
• The lack of a significant trend when each class is considered separately implies that they belong to two separate populations.
• For the radio-quiet quasars, soft (0.2-3.5 keV) X-ray slopes are consistent with those at higher (2-10 keV) energies for radio-quiet quasars but steeper than the high energy slopes of Seyfert 1 galaxies where reflection of the X2rays gives a good fit to the data.

Within
energy indices are shown in Fig. lb.
Figure Ia

in good agreement with the mostly overlapping sample of Wilkes and Elvis (1987), and with the higher energy (2-10
A simple mean of the X-ray slopes for the RQQs is 1.0+0.1,

of the radio flux density from the nucleus to that from the radio lobes) is then a statistical measure of the angle of inclination of the nuclear jet to the line of sight (Kapahi and Saikia 1982; Orr and Browne 1982). Interpreting correlations of X-ray luminosity with nuclear and lobe radio luminosities in this framework, Browne and Murphy (1987) suggested that a component of the X-ray emission in RLQs is orientation dependent. The soft X-ray slopes have been found to be systematically flatter for core-dominated quasars than for lobe-dominated quasars; (Canizares and White 1989; Boroson 1989
; Shastri 1991).Based on this trend, Shastri (1991) suggested that the "radio-linked" component of the X-ray emission (Zamorani et al. 1981; Kembhavi et al.

1986; Wilkes and Elvis 1987)
(3C 48:0134+329) from Fig. 3, because, in these objects the cores often show complex radio structure (Fanti et al. 1990) and interaction of the radio jet with the interstellar medium is believed to be significant (van Breugel et al. 1984).The brightness of the nuclear components must reflect this interaction, in which case the parameter R is not Shastri el al. 4 a good measure of orientation.equivalentwidthdatafromZhengandO'Brien(1990)showsaverysimilar trend (Fig.4b).There is no significant correlation for the sample as a whole (as indeed is asserted by the authors), nor for the RLQs, but the RQQs show a trend similar to Fig.4a(significance >95%).In the case of the RQQs the equivalent widths of the FeII lines are entirely independent of the X-ray measurement so the correlation of Fig.4acan exist only if the soft X-ray slopes have a real intrinsic range of slopes from -._0 to ---1.5.This truly is a large range.