Person:

Elvis, Martin

We present the results of the VLA radio observations at 1.475 GHz (20 cm) of the active galactic nuclei (AGN) in the Cambridge-Cambridge ROSAT Serendipity Survey (CRSS), a sample of 123 faint X-ray sources with fx(0.5−2.0keV)≥2×10−14ergs−1cm−2 fx(0.5−2.0keV)≥2×10−14ergs−1cm−2 . Of the 80 AGN in the sample, seven show radio emission at the 5 σ level and only two ( 2.5+4.0–1.7 2.5–1.7+4.0 per cent) qualify as radio-loud (RL) objects (αro ≥ 0.35). This result, compared with 13 per cent RL in the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) sample of AGN [flux limit fx(0.3−3.5keV)∼2×10−13ergs−1cm−2 fx(0.3−3.5keV)∼2×10−13ergs−1cm−2 ], confirms that the fraction of X-ray-selected RLAGN drops rapidly as the X-ray flux limit is lowered. Combining the CRSS AGN sample with that extracted from the EMSS we study the X-ray luminosity function (XLF) and evolutionary properties for radio-quiet (RQ) and RLAGN separately. We find that the RQ and RLAGN populations show the same cosmological evolution within the errors. In fact, when the luminosity evolution is parametrized with a power law of the form L∗x(z)=L∗x(0)(1+z)k Lx∗(z)=Lx∗(0)(1+z)k , we find k = 2.43 ± 0.26 and 2.71 ± 0.10 for RL and RQ AGN populations respectively. In addition, the shapes of the de-evolved XLFs of the two classes appear to be different at both the low-luminosity (Lx < 1044 erg s–1) and high-luminosity ends. These results are robust for different cosmological models (using q0 = 0.0 and 0.5) and for different values of the threshold αro used to distinguish between RQ and RL objects. Finally, we find that the differences in the shapes of the XLFs of RQ and RLAGN can be explained by introducing an X-ray beaming model to separate the observed X-ray luminosity of radio quasars into relativistically beamed and isotropic contributions.

Steep soft X-ray (0.1-2 keV) quasars share several unusual properties: narrow Balmer lines, strong FeII emission, large and fast X-ray variability, rather steep 2-10 keV spectrum. These intriguing objects have been suggested to be the analogs of Galactic black hole candidates in the high, soft state. We present here results from ASCA observations for two of these quasars: NAB0205+024 and PG1244+026. Both objects show similar variations (factor of about 2 in 10 ks), despite a factor of about ten difference in the 0.5-10 keV luminosity (7.3E43 erg/s for PG1244+026 and 6.4E44 erg/s for NAB0205+024, assuming isotropic emission, H_0 = 50.0 and q_0 = 0.0). The X-ray continuum of the two quasars flattens by 0.5-1 going from the 0.1-2 keV band toward higher energies, strengthening recent results on another half dozen steep soft X-ray AGN. PG1244+026 shows a significant feature in the `1 keV' region, which can be described by either as a broad emission line centered at 0.95 keV (quasar frame) or as edge or line absorption at 1.17 (1.22) keV. The line emission could be due to reflection from an highly ionized accretion disk, in line with the view that steep soft X-ray quasars are emitting close to the Eddington luminosity. Photoelectric edge absorption or resonant line absorption could be produced by gas outflowing at a large velocity (0.3-0.6 c).

We present results from a Chandra X-Ray Observatory study of the field X-ray source populations in four different observations: two high-redshift (z ~ 0.5) clusters of galaxies 3C 295 and RX J003033.2+261819; and two noncluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (~900-1200 sources deg-2 at a flux limit of 1.5 × 10-15 ergs cm-2 s-1) measured in an ~8' × 8' area surrounding each cluster exceed by a factor of ~2 the value expected on the basis of the ROSAT and Chandra log N- log S, with a significance of ~2 σ each, or ~3.5 σ when the two fields are combined (i.e., a probability to be a statistical fluctuation of <1% and <0.04%, respectively). The same analysis performed on the noncluster fields and on the outer chips of the cluster fields does not show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power law with Γ ~ 1.7 similar to active galactic nuclei (AGNs) and shows no sign of intrinsic absorption. The few (~10 of 35) optical identifications available to date confirm that most of them are, as expected, AGNs, but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of a statistical variation of cosmic background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and gravitational lensing of background QSOs by the galaxy clusters. All explanations, however, are difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.

Hard X-ray selection is, arguably, the optimal method for defining a representative sample of active galactic nuclei (AGNs). Hard X-rays are unbiased by the effects of obscuration and reprocessing along the line of sight intrinsic/external to the AGN, which result in unknown fractions of the population being missed from traditional optical/soft X-ray samples. We present the far-infrared (far-IR) observations of 21 hard X-ray-selected AGNs from the HEAO 1 A2 sample observed with Infrared Space Observatory (ISO). We characterize the far-IR continua of these X-ray-selected AGNs and compare them with those of various radio and optically selected AGN samples and with models for an AGN-heated, dusty disk. The X-ray-selected AGNs show broad, warm IR continua covering a wide temperature range (~20-1000 K in a thermal emission scenario). Where a far-IR turnover is clearly observed, the slopes are less than 2.5 in all but three cases so that nonthermal emission remains a possibility, although the presence of cooler dust resulting in a turnover at wavelengths longward of the ISO range is considered more likely. The sample also shows a wider range of optical/UV shapes than the optical/radio-selected samples, extending to redder near-IR colors. The bluer objects are type 1 Seyfert galaxies, while the redder AGNs are mostly intermediate or type 2 Seyfert galaxies. This is consistent with a modified unification model in which obscuration increases as we move from a face-on toward a more edge-on line of sight. However, this relation does not extend to the mid-infrared as the 25/60 μm ratios are similar in Seyfert galaxies with differing type and optical/UV reddening. The resulting limits on the column density of obscuring material through which we are viewing the redder AGNs (NH ~ 1022 cm-2) are inconsistent with standard optically thick torus models (NH ~ 1024 cm-2) and simple unification models. Instead our results support more complex models in which the amount of obscuring material increases with viewing angle and may be clumpy. Such a scenario, already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications, allows for different amounts of obscuration of the continuum emission in different wave bands and of the broad emission line region, which, in turn, results in a mixture of behaviors for AGNs with similar optical emission-line classifications. The resulting decrease in the optical depth of the obscuring material also allows the AGN to heat more dust at larger radial distances. We show that an AGN-heated, flared, dusty disk with mass of ~109 M☉ and size of approximately a few hundred parsecs is able to generate optical-far-IR spectral energy distributions (SEDs) that reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength, cooler part of the IR continuum.

We present seven sequential weekly observations of NGC 5548 conducted in 2007 with the Suzaku X-ray Imaging Spectrometer (XIS) in the 0.2–12 keV band and Hard X-ray Detector (HXD) in the 10–600 keV band. The iron Kα line is well detected in all seven observations and Kβ line is also detected in four observations. In this paper, we investigate the origin of the Fe K lines using both the width of the line and the reverberation mapping method. With the co-added XIS and HXD spectra, we identify Fe Kα and Kβ line at 6.396+0.009 −0.007 keV and 7.08+0.05 −0.05 keV, respectively. The width of line obtained from the co-added spectra is 38+16 −18 eV (FWHM = 4200+1800 −2000 km s−1) which corresponds to a radius of 20+50 −10 light days, for the virial production of 1.220 × 107 M in NGC 5548. To quantitatively investigate the origin of the narrow Fe line by the reverberation mapping method, we compare the observed light curves of Fe Kα line with the predicted ones, which are obtained by convolving the continuum light curve with the transfer functions in a thin shell and an inclined disk. The best-fit result is given by the disk case with i = 30◦ which is better than a fit to a constant flux of the Fe K line at the 92.7% level (F-test). However, the results with other geometries are also acceptable (P > 50%). We find that the emitting radius obtained from the light curve is 25–37 light days, which is consistent with the radius derived from the Fe K line width. Combining the results of the line width and variation, the most likely site for the origin of the narrow iron lines is 20–40 light days away from the central engine, though other possibilities are not completely ruled out. This radius is larger than the Hβ emitting parts of the broad-line region at 6–10 light days (obtained by the simultaneous optical observation), and smaller than the inner radius of the hot dust in NGC 5548 (at about 50 light days).

We present the final results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low-z quasars. This sample includes all 23 quasars from the Bright Quasar Survey with z ≤ 0.400 and N^{{\rm Gal}}_{{\rm H},{\rm I}}<1.9×1020 cm-2. Pointed ROSAT PSPC observations were made for all quasars, yielding high signal-to-noise (S/N) spectra for most objects, which allowed an accurate determination of the spectral shape. The following main results were obtained:

1. The spectra of 22 of the 23 quasars are consistent, to within ~30%, with a single power-law model at rest-frame 0.2-2 keV. There is no evidence for significant soft excess emission with respect to the best-fit power law. We place a limit (95% confidence) of ~5 × 1019 cm-2 on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are ~1 × 1019 cm-2 in the two highest S/N spectra.

2. The mean 0.2-2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050-350 Å continuum recently determined by Zheng et al. (1996) for z > 0.33 quasars. This suggests that there is no steep soft component below 0.2 keV.

3. Significant X-ray absorption (τ > 0.3) by partially ionized gas ("warm absorber") in quasars is rather rare, occurring for lesssim5% of the population, which is in sharp contrast to lower luminosity active galactic nuclei (AGNs), where significant absorption probably occurs for ~50% of the population.

4. Extensive correlation analysis of the X-ray continuum emission parameters with optical emission-line parameters indicates that the strongest correlation is between the spectral slope αx and the Hβ FWHM. A possible explanation for this remarkably strong correlation is a dependence of αx on L/LEdd, as seen in Galactic black hole candidates.

5. The strong correlations between αx and L[O III], Fe II/Hβ, and the peak [O III] to Hβ flux ratio are verified. The physical origin of these correlations is still not understood.

6. There appears to be a distinct class of "X-ray-weak" quasars, which form ~10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10-30, than expected based on their luminosity at other bands and on their Hβ luminosity. These may be quasars in which the direct X-ray source is obscured and only scattered X-rays are observed.

7. Thin accretion disk models cannot reproduce the observed 0.2-2 keV spectral shape, and they also cannot reproduce the tight correlation between the optical and soft X-ray emission. An as yet unknown physical mechanism must be maintaining a strong correlation between the optical and soft X-ray emission.

8. The H I/He I ratio in the high Galactic latitude ISM must be within 20%, and possibly within 5%, of the total H/He ratio of 10, which indicates that He in the diffuse H II gas component of the interstellar medium is mostly ionized to He II or He III.

We finally note the intriguing possibility that although langαxrang in radio-loud quasars (-1.15 ± 0.14) is significantly flatter than in radio-quiet quasars (-1.72 ± 0.09) the X-ray emission may not be related to the presence of radio emission. The difference in langαxrang may result from the strong αx versus Hβ FWHM correlation and the tendency of radio-loud quasars to have broader Hβ.

In this paper we present the analysis of two broadband (0.1-150 keV) BeppoSAX observations of the Seyfert 1 galaxy NGC 3516. The two observations were taken 4 months apart, on 1996 November 8 and 1997 March 12. We report a dramatic change in the degree of obscuration of the central source between the two observations and propose, as possible explanations, transient absorption by either a stationary-state cloud of cold gas crossing the line of sight or a varying-state, initially neutral and dense amount of expanding gas with decreasing density and therefore decreasing opacity. We also report the detection of a second highly ionized absorber/emitter, which causes deep Fe XVII-XXII K edges at ~7.8 keV to appear in both of the BeppoSAX spectra of NGC 3516 and possibly produces the soft X-ray continuum emission in the 1 keV blend of Fe L recombination lines detected during the epoch of heavy nuclear obscuration.

The quasar PKS 0637-752, the first celestial X-ray target of the Chandra X-Ray Observatory, has revealed asymmetric X-ray structure extending from 3'' to 12'' west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z = 0.651, the jet is the largest (gsim100 kpc in the plane of the sky) and most luminous (~1044.6 ergs s-1) of the few so far detected in X-rays. This Letter presents a high-resolution X-ray image of the jet, from 42 ks of data when PKS 0637-752 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new Australian Telescope Compact Array radio images at 4.8 and 8.6 GHz. Observations of the parsec-scale core using the very long baseline interferometry space observatory program mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. Hubble Space Telescope images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet and a radio component to the east show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extranuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton model, but this would imply extreme departures from the conventional minimum energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.

We present seven sequential weekly observations of NGC 5548 conducted in 2007 with the Suzaku X-ray Imaging Spectrometer (XIS) in the 0.2–12 keV band and Hard X-ray Detector (HXD) in the 10–600 keV band. The iron Kα line is well detected in all seven observations and Kβ line is also detected in four observations. In this paper, we investigate the origin of the Fe K lines using both the width of the line and the reverberation mapping method. With the co-added XIS and HXD spectra, we identify Fe Kα and Kβ line at 6.396+0.009 −0.007 keV and 7.08+0.05 −0.05 keV, respectively. The width of line obtained from the co-added spectra is 38+16 −18 eV (FWHM = 4200+1800 −2000 km s−1) which corresponds to a radius of 20+50 −10 light days, for the virial production of 1.220 × 107 M in NGC 5548. To quantitatively investigate the origin of the narrow Fe line by the reverberation mapping method, we compare the observed light curves of Fe Kα line with the predicted ones, which are obtained by convolving the continuum light curve with the transfer functions in a thin shell and an inclined disk. The best-fit result is given by the disk case with i = 30◦ which is better than a fit to a constant flux of the Fe K line at the 92.7% level (F-test). However, the results with other geometries are also acceptable (P > 50%). We find that the emitting radius obtained from the light curve is 25–37 light days, which is consistent with the radius derived from the Fe K line width. Combining the results of the line width and variation, the most likely site for the origin of the narrow iron lines is 20–40 light days away from the central engine, though other possibilities are not completely ruled out. This radius is larger than the Hβ emitting parts of the broad-line region at 6–10 light days (obtained by the simultaneous optical observation), and smaller than the inner radius of the hot dust in NGC 5548 (at about 50 light days).

We present a detailed X-ray spectral analysis in the 0.1–2.4 keV ROSAT band of a complete sample of X-ray-selected AGN using the 80 AGNs in the Cambridge-Cambridge ROSAT Serendipity Survey (68 QSOs and 12 narrow-emission-line galaxies, NLXGs). We also make a comparison between the X-ray spectral properties of QSOs and NLXGs. For 36 objects we have enough net counts to allow an X-ray spectral fit, while for the other sources we characterize the spectrum using the hardness-ratio technique. A maximum-likelihood analysis is used to find the mean power-law energy spectral index ‹αx› and the standard deviation σ for QSOs and NLXGs, assuming the intrinsic distribution to be Gaussian. We find no difference between QSOs and NLXGs: ‹αx› =1.32 with dispersion σ= 0.33 for the QSOs, and ‹αx› = 1.30 with σ = 0.49 for the NLXGs. A single power law with a Galactic absorbing column density yields a good representation of the X-ray spectra for the majority of the sources. Only three objects show a significant deviation from this model. There is evidence in the NLXG sample for a flattening of the spectral slope αx with increasing redshift, and for a steepening of αx with increasing (L2500 ÅLx)- For the QSO sample we found no significant correlation. The lack of correlation between αx and z suggests that for the CRSS QSOs the power-law spectrum in the QSO rest-frame extends from the soft ( ∼ 0.1−2.4 keV) into the harder X-ray band ( ∼ 0.3−7.3 keV) with the same slope.