An X-ray spectral survey of radio-loud active galactic nuclei with ASCA

We present a uniform and systematic analysis of the 0.6-10 keV X-ray spectr a of radio-loud active galactic nuclei (AGNs) observed by ASCA. The sample, which is not statistically complete, includes 10 broad-line radio galaxies (BLRGs), five radio-loud quasars (QSRs), nine narrow-line radio galaxies ( NLRGs), and 10 radio galaxies (RGs) of mixed FR I and FR II types. For seve ral sources the ASCA data are presented here for the first time. The exposu re times of the observations and the fluxes of the objects vary over a wide range; as a result, so does the signal-to-noise ratio of the individual X- ray spectra. At soft X-rays, about 50% of NLRGs and 100% of RGs exhibit the rmal plasma emission components, with bimodal distributions of temperatures and luminosities. This indicates that the emission in such an object arise s in hot gas either in a surrounding cluster or loose group or in a hot cor ona, consistent with previous ROSAT and optical results. At energies above 2 keV, a hard power-law component (photon index Gamma similar to 1.7-1.8) i s detected in 90% of cases. The power-law photon indices and luminosities i n BLRGs, QSRs, and NLRGs are similar. This is consistent with simple orient ation-based unification schemes for lobe-dominated radio-loud sources in wh ich BLRGs, QSRs, and NLRGs harbor the same type of central engine. Moreover , excess cold absorption in the range 10(21)-10(24) cm(-2) is detected in m ost (but not all) NLRGs, consistent with absorption by obscuring tori, as p ostulated by unification scenarios. The ASCA data provide initial evidence that the immediate gaseous environment of the X-ray source of BLRGs may be different than in Seyfert 1 galaxies: absorption edges of ionized oxygen, c ommon in the latter, are detected in only one BLRG. Instead we detect large columns of cold gas in a fraction (similar to 44%-60%) of BLRGs and QSRs, comparable to the columns detected in NLRGs, which is puzzling. This differ ence hints at different physical and/or geometrical properties of the mediu m around the X-ray source in radio-loud AGNs compared to their radio-quiet counterparts, properties that can be explored further with future X-ray obs ervations. For the full sample, the nuclear X-ray luminosity is correlated with the luminosity of the [O III] emission line, the FIR emission at 12 mu m, and the lobe radio power at 5 GHz. The Fe K alpha line is detected in 5 0% of BLRGs and in one QSR, with a large range of intrinsic widths and equi valent widths. In the handful of NLRGs where it is detected, the line is ge nerally unresolved. Comparing the average power-law photon indices of the v arious classes of radio-loud AGNs to their radio-quiet counterparts from th e literature, we find only a weak indication that the ASCA 2-10 keV spectra of BLRGs are flatter than those of Seyfert 1 galaxies of comparable X-ray luminosity. This result is at odds with evidence from samples studied by ot her authors suggesting that radio-loud AGNs have flatter spectra than radio -quiet ones. Rather, it supports the idea that a beamed synchrotron self-Co mpton component related to the radio source (jet) is responsible for the fl atter slopes in those radio-loud AGNs. We argue that, because of the way th ose samples were constructed, beamed X-ray emission from the radio jets pro bably contributed to the observed X-ray spectra. The sample studied here in cludes six weak-line radio galaxies (WLRGs), powerful radio galaxies charac terized by [O III] 4569 and 5007 Angstrom of unusually low luminosity and b y unusually high [O II]/[O III] line ratios. The ASCA spectra of WLRGs can be generally decomposed into a soft thermal c omponent with kT similar to keV, plus a hard component, described either by a flat ([Gamma]= 1.5) absorbed power law or by a very hot (kT similar to 1 00 keV) thermal bremsstrahlung model. Their intrinsic luminosities are in t he range L2-10 keV similar to 10(40)-10(42) ergs s(-1), 2 orders of magnitu de lower than in other sources in our sample. If the hard X-ray emission is attributed to a low-luminosity AGN, an interesting possibility is that WLR Gs represent an extreme population of radio galaxies in which the central b lack hole is accreting at a rate well below the Eddington rate.