Person:

Birkinshaw, Mark

Chandra X-ray observations of the high redshift (z = 1.532) radio-loud quasar 3C 270.1 in 2008 February show the nucleus to have a power-law spectrum, Γ = 1.66 ± 0.08, typical of a radio-loud quasar, and a marginally detected Fe Kα emission line. The data also reveal extended X-ray emission, about half of which is associated with the radio emission from this source. The southern emission is co-spatial with the radio lobe and peaks at the position of the double radio hot spot. Modeling this hot spot, including Spitzer upper limits, rules out synchrotron emission from a single power-law population of electrons, favoring inverse Compton emission with a field of ∼11 nT, roughly a third of the equipartition value. The northern emission is concentrated close to the location of a 40◦ bend where the radio jet is presumed to encounter an external medium. It can be explained by inverse Compton emission involving cosmic microwave background photons with a field of ∼3 nT, a factor of 7–10 below the equipartition value. The remaining, more diffuse X-ray emission is harder (HR = −0.09 ± 0.22). With only 22.8 ± 5.6 counts, the spectral form cannot be constrained. Assuming thermal emission with a temperature of 4 keV yields an estimate for the luminosity of 1.8×1044 erg s−1, consistent with the luminosity–temperature relation of lower-redshift clusters. However, deeper Chandra X-ray observations are required to delineate the spatial distribution and better constrain the spectrum of the diffuse emission to verify that we have detected X-ray emission from a high-redshift cluster.

Chandra observations of a complete, flux-limited sample of 38 high-redshift (1 <z< 2), low-frequency-selected (and so unbiased in orientation) 3CRR radio sources are reported. The sample includes 21 quasars (=broad-line radio galaxies) and 17 narrow-line radio galaxies (NLRGs) with matched 178 MHz radio luminosity (log LR(5 GHz) ∼ 44–45). The quasars have high radio core fraction, high X-ray luminosities (log LX ∼ 45–46), and soft X-ray hardness ratios (HR ∼ −0.5) indicating low obscuration. The NLRGs have lower core fraction, lower apparent X-ray luminosities (log LX ∼ 43–45), and mostly hard X-ray hardness ratios (HR > 0) indicating obscuration (NH ∼ 1022–1024 cm−2). These properties and the correlation between obscuration and radio core fraction are consistent with orientation-dependent obscuration as in unification models. About half the NLRGs have soft X-ray hardness ratios and/or a high [O iii] emission line to X-ray luminosity ratio suggesting obscuration by Compton thick (CT) material so that scattered nuclear or extended X-ray emission dominates (as in NGC 1068). The ratios of unobscured to Compton-thin (1022 cm−2 < NH(int) < 1.5 × 1024 cm−2) to CT (NH(int) > 1.5 × 1024 cm−2) is 2.5:1.4:1 in this high-luminosity, radio-selected sample. The obscured fraction is 0.5, higher than is typically reported for active galactic nuclei at comparable luminosities from multi-wavelength surveys (0.1–0.3). Assuming random nuclear orientation, the unobscured half-opening angle of the disk/wind/torus structure is ∼60◦ and the obscuring material covers 30◦, ∼12◦ of which is CT. The multi-wavelength properties reveal that many NLRGs have intrinsic absorption 10–1000× higher than indicated by their X-ray hardness ratios, and their true LX values are ∼10–100× larger than the hardness-ratio absorption corrections would indicate.