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Register a new userSwift observations of high-redshift radio-loud quasars
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R.M. Sambruna
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We report on Swift observations of four z>2 radio-loud quasars (0212+735, 0537-286, 0836+710, and 2149-307), classified as blazars. The sources, well-known emitters at soft-medium X-rays, were detected at >5sigma with the BAT experiment in 15-150 keV. No flux variability was detected within the XRT and BAT exposures, with the exception of 0836+710 which shows an increase of a factor 4 of the 15-150 keV flux on a timescale of one month. The 0.3-10 keV spectra are well fitted by power law models, with rather hard continua (photon indices Gamma_XRT ~1.3-1.5); similarly, the 15-150 keV spectra are described by power laws with Gamma_BAT ~1.3-1.8. The XRT data exhibit spectral curvature, which can be modeled either in terms of excess absorption along the line of sight, or a downward-curved broken power law. In the former case, if the excess N_H is at the rest-frame of the source, columns of N_H^z=(0.3-6)x10^22 cm^-2 are measured. Modeling of the SEDs of the four quasars shows that the emission at the higher frequencies, >~ 10^16 Hz, is dominated by the jet, while the steep optical-to-UV continua, observed with the UVOT, can be attributed to thermal emission from the accretion disk. The disk luminosity is between 1% and 10% the jet power, similar to other powerful blazars.
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We report spectral, imaging, and variability results from four new XMM-Newton observations and two new Chandra observations of high-redshift (z > 4) radio-loud quasars (RLQs). Our targets span lower, and more representative, values of radio loudness than those of past samples of high-redshift RLQs studied in the X-ray regime. Our spectral analyses show power-law X-ray continua with a mean photon index, Gamma =1.74 +/- 0.11, that is consistent with measurements of lower redshift RLQs. These continua are likely dominated by jet-linked X-ray emission, and they follow the expected anti-correlation between photon index and radio loudness. We find no evidence of iron Kalpha ~ emission lines or Compton-reflection continua. Our data also constrain intrinsic X-ray absorption in these RLQs. We find evidence for significant absorption (N_H ~ 10^22 cm^-2) in one RLQ of our sample (SDSS J0011+1446); the incidence of X-ray absorption in our sample appears plausibly consistent with that for high-redshift RLQs that have higher values of radio loudness. In the Chandra observation of PMN J221-2719 we detect apparent extended (~ 14 kpc) X-ray emission that is most likely due to a jet; the X-ray luminosity of this putative jet is ~2% that of the core. The analysis of a 4.9 GHz VLA image of PMN J221-2719 reveals a structure that matches the X-ray extension found in this source. We also find evidence for long-term (450-460 days) X-ray variability by 80-100% in two of our targets.
(abridged) We firstly present the results of X-ray spectroscopic observations with XMM-Newton for four high-redshift radio-loud quasars at z>4. These observations more than double the number of z>4 radio-loud quasars having X-ray spectroscopic data to seven, which compose a significant subset of a flux-limited sample of z>4 radio-loud quasars. Based on this subset we show some preliminary results on the overall X-ray spectral properties of the sample. Soft X-ray spectral flattening, which is thought to arise from intrinsic X-ray absorption, was found in about half of the sample. We give a preliminary distribution of the absorption column density NH. For those with detected X-ray absorption, the derived NH values fall into a very narrow range (around a few times 10^(22)cm^(-2) for `cold absorption), suggesting a possible common origin of the absorber. Compared to lower-redshift samples at z<2, there is an extension, or a systematic shift, toward higher values in the intrinsic NH distribution at z>4, and an increase of the fraction of radio-loud quasars showing X-ray absorption toward high redshifts. These results indicate a cosmic evolution effect, which seems to be the strongest at redshifts around 2. The rest frame 1-50keV continua have photon indices with a mean of 1.64 and a standard deviation of 0.11. Variability appears to be common on timescales from a few months to years in the quasar rest-frame, sometimes in both fluxes and spectral slopes.
We have examined the occurrence of Extremely Red Objects (EROs) in the fields of 13 luminous quasars (11 radio-loud and two radio-quiet) at 1.8 < z < 3.0. The average surface density of K_s<=19 mag EROs is two-three times higher than in large, random-field surveys, and the excess is significant at the $approx 3$ sigma level even after taking into account that the ERO distribution is highly inhomogeneous. This is the first systematic investigation of the surface density of EROs in the fields of radio-loud quasars above z=2, and shows that a large number of the fields contain clumps of EROs, similar to what is seen only in the densest areas in random-field surveys. The high surface densities and angular distribution of EROs suggest that the excess originates in high-z galaxy concentrations, possibly young clusters of galaxies. The fainter EROs at K_s>19 mag show some evidence of being more clustered in the immediate 20 arcsec region around the quasars, suggesting an association with the quasars.Comparing with predictions from spectral synthesis models, we find that if the $K_sapprox19$ mag ERO excess is associated with the quasars at $zapprox2$, their magnitudes are typical of >~ L* passively evolving galaxies formed at z~3.5 (Omega_m=0.3, Omega_l=0.7, and H0=70 km/s/Mpc). Another interpretation of our results is that the excess originates in concentrations of galaxies at $zapprox1$ lying along the line of sight to the quasars. If this is the case, the EROs may be tracing massive structures responsible for a magnification bias of the quasars.
This paper reports on the extreme ultraviolet (EUV) spectrum of three low redshift ($z sim 0.6$) radio loud quasars, 3C 95, 3C 57 and PKS 0405-123. The spectra were obtained with the Cosmic Origins Spectrograph (COS) of the Hubble Space Telescope. The bolometric thermal emission, $L_{bol}$, associated with the accretion flow is a large fraction of the Eddington limit for all of these sources. We estimate the long term time averaged jet power, $overline{Q}$, for the three sources. $overline{Q}/L_{bol}$, is shown to lie along the correlation of $overline{Q}/L_{bol}$ and $alpha_{EUV}$ found in previous studies of the EUV continuum of intermediate and high redshift quasars, where the EUV continuum flux density between 1100 AA, and 700 AA, is defined by $F_{ u} sim u^{-alpha_{EUV}}$. The high Eddington ratios of the three quasars extends the analysis into a wider parameter space. Selecting quasars with high Eddington ratios has accentuated the statistical significance of the partial correlation analysis of the data. Namely. the correlation of $overline{Q}/L_{mathrm{bol}}$ and $alpha_{EUV}$ is fundamental and the correlation of $overline{Q}$ and $alpha_{EUV}$ is spurious at a very high statistical significance level (99.8%). This supports the regulating role of ram pressure of the accretion flow in magnetically arrested accretion models of jet production. In the process of this study, we use multi-frequency and multi-resolution Very Large Array radio observations to determine that one of the bipolar jets in 3C 57 is likely frustrated by galactic gas that keeps the jet from propagating outside the host galaxy.
ASCA observations of 4 high redshift radio--loud quasars with 1.44$<$z$<$3.21 are presented. The spectral analysis for three of them (PKS 0332-403, PKS 0537-286, PKS 2149-306) reveals that their X-ray continuum emission is well represented by a simple power--law model plus absorption with photon indices of $Gamma =1.92^{+0.30}_{-0.20}$ (PKS 0332-403), $Gamma =1.63^{+0.14}_{-0.12}$ (PKS 0537-286) and $Gamma =1.57pm 0.05$ (PKS 2149-306). The fourth and most distant object, PKS 1614+051 at z=3.21, was detected, but a detailed spectral analysis is impossible due to the small number of photons. We find evidence for excess absorption above the Galactic $N_H$--value in the ASCA data of PKS 2149-306, which is not confirmed by the ROSAT All-Sky Survey PSPC spectrum of this source. This could probably be due to variable absorption. The ROSAT spectrum of PKS 0537-286, deduced from a 10 ksec pointed PSPC observation, is consistent with the ASCA results. Thermal bremsstrahlung models also give acceptable fits to the ASCA data with best fit (rest frame) temperatures of 10.4, 33.5 and 45.8 keV for PKS 0332-403, PKS 0537-286 and PKS 2149-306, respectively. More complicated models for the X-ray continuum are not required, in particular, tight upper limits on the strength of the Fe-K emission line are given. The broad band spectral energy distributions from the radio to the $gamma$-rays are presented and discussed.
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