Person:

McDowell, Jonathan

An SNR-limited subsample of 14 quasars from the Wilkes and Elvis (1987) sample is presently investigated for low-energy excess above a high-energy power law in the X-ray spectra obtained by the Einstein Imaging Proportional Counter. A significant excess that is 1-6 times as strong as the high-energy component at 0.2 keV is noted in eight of the 14 objects. In the case of 3C273, multiple observations show the excess to be variable.

The present study examines two Einstein and three Exosat observations of PG 1211 + 143 over a 6-yr baseline which show that strong steep-spectrum low-energy X-ray emission is a persistent feature of this quasar. Exosat observations of PG 1211 + 143 detected an increase of a factor of 2.3 in its steep soft X-ray flux during 18 days. It is concluded that the bulk of the soft X-ray emission of PG 1211 + 143 comes from a region less than 5 x 10 to the 16th cm across. In another interval of 193 days, the soft X-rays decreased by a factor of 3.7. In the same interval the hard X-rays decreased by a factor of 1.6 + or - 0.05, which suggests a connection between the two energy regimes and argues against variable absorption causing the soft X-ray variations. It is contended that in order to decrease in luminosity so rapidly, a thermal source in PG 1211 + 143 would have to be optically thick to both electron scattering and free-bound absorption.

A sample of 45 quasars observed by the IPC on the Einstein satellite is used to re-examine the relationship between the soft (0.2-3.5 keV) X-ray energy index and radio-loudness. We find that, (1) the tendency for radio-loud quasars to have systematically flatter X-ray slopes than radio-quiet quasars is confirmed with the soft X-ray excess having negligible effect; (2) there is a tendency for the flatness of the X-ray slope to correlate with radio core-dominance 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. The radio-loud quasars show no evidence for a correlation. This relation suggests a connection between the ionizing continuum and the line emission from the broad emission line region (BELR) of radio-quiet quasars, but in the opposite sense to that predicted by current photoionization models; (5) the correlations of X-ray slope with radio core dominance and FeII equivalent width within the radio-loud and radio-quiet sub-classes respectively imply that the observed wide range of X-ray spectral slopes is real rather than due to the large measuring uncertainties for individual objects.

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.

PG 1407+265, discovered in the Palomar-Green Survey (Schmidt & Green 1983), was identified as a z ˜ 1 radio-quiet quasar on the basis of a single weak line. Further observations over a wide wavelength range confirm the identification but reveal the object to have unusual emission-line properties. Broad Hα is the only strong emission line with Hβ and Lyα almost undetectably weak. The emission lines show a range in redshift of over 10,000 km s-1, systematically decreasing with ionization potential or, almost equivalently, increasing to longer wavelengths. A value of z = 0.94±0.02 is a reasonable statement of our knowledge of the quasar's redshift However, the object's continuum properties are those of a normal radio-quiet quasar. We discuss a number of possible models for the object, but its nature remains puzzling.

We present an atlas of the spectral energy distributions (SEDs) of normal, nonblazar, quasars over the whole available range (radio to 10 keV X-rays) of the electromagnetic spectrum. The primary (UVSX) sample includes 47 quasars for which the spectral energy distributions include X-ray spectral indices and UV data. Of these, 29 are radio quiet, and 18 are radio loud. The SEDs are presented both in figures and in tabular form, with additional tabular material published on CD-ROM. Previously unpublished observational data for a second set of quasars excluded from the primary sample are also tabulated. The effects of host galaxy starlight contamination and foreground extinction on the UVSX sample are considered and the sample is used to investigate the range of SED properties. Of course, the properties we derive are influenced strongly by the selection effects induced by quasar discovery techniques. We derive the mean energy distribution (MED) for radio-loud and radio-quiet objects and present the bolometric corrections derived from it. We note, however, that the dispersion about this mean is large (approximately one decade for both the infrared and ultraviolet components when the MED is normalized at the near-infrared inflection). At least part of the dispersion in the ultraviolet may be due to time variability, but this is unlikely to be important in the infrared. The existence of such a large dispersion indicates that the MED reflects only some of the properties of quasars and so should be used only with caution.

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.

We present infrared (IR) to X-ray spectral energy distributions (SEDs) for 44 red active galactic nuclei (AGNs) selected from the Two Micron All Sky Survey (2MASS) survey on the basis of their red J − KS color (> 2 mag) and later observed by Chandra. In comparison with optically-, radio-, and X-ray-selected AGNs, their median SEDs are red in the optical and near-IR (NIR) with little/no blue bump. Comparison of the various broadband luminosity ratios shows that the main differences lie at the blue end of the optical and in the NIR to far-IR ratios (when available), with the red 2MASS AGNs being redder than the other samples. It thus seems that NIR color selection isolates the reddest subset of AGNs that can be classified optically. The shape of the SEDs is generally consistent with modest absorption by gas (in the X-ray) and dust (in the optical–IR), as demonstrated by comparing the optical and NIR colors with a reddened median SED and observed optical+NIR to intrinsic X-ray ratios. The levels of obscuration, estimated from X-rays, far-IR, and our detailed optical/NIR color modeling, are all consistent implying NH few × 1022 cm−2. We present SED models that show how the AGN optical/NIR colors change due to differing amounts of reddening, AGN to host galaxy ratio, redshift, and scattered light emission, and apply them to the sources in the sample. We find that the 2MASS AGN optical color, B − R, and to a lesser extent the NIR color, J − KS, are strongly affected by reddening, host galaxy emission, redshift, and in few, highly polarized objects also by scattered AGN light (<2% of intrinsic AGN light in the R band is scattered; this contribution becomes significant as the direct AGN light is absorbed). The lack of low equivalent widths in the distribution of the [O iii] λ5007 emission line implies a predominance of inclined objects in the red 2MASS sample. The obscuration/ inclination of the AGN allows us to see weaker emission components which are generally swamped by the AGN.

We investigate the relations between the observed emission-line strengths, widths, and continuum properties of a sample of 41 low-redshift (z<1) quasars for which contemporaneous IR/soft X-ray spectral energy distributions are available. This includes investigating correlations between optical and UV lines with both the luminosity and the shape of the quasars' continuum, as well as correlations between the various lines. The sample is heterogeneous, primarily selected on the existence of good-quality Einstein X-ray data, and includes 18 radio-loud and 23 radio-quiet quasars. We find anticorrelations between the equivalent width and various UV luminosities (the Baldwin effect) for the Lyα and Hβ lines and a marginal anticorrelation for C III]. Exclusion of narrow-line, low-luminosity active galactic nuclei reveals a significant Baldwin effect for the C IV and C III] lines. A significant anticorrelation of EW(C IV) with αox is also present. We find no correlations between any lines and the X-ray luminosity or X-ray slope. The Fe II optical multiplet shows no simple relationship with luminosity or continuum slope; however, there is a tendency for objects with flat X-ray spectra and/or strong X-ray luminosities to have weak Fe II.

Our data do not favor a model in which changes in continuum shape (due to, e.g., a decreasing ionization parameter) cause the Baldwin effect. The data can instead be explained by an accretion disk (AD) model in which limb darkening and the projected surface area of an optically thick, geometrically thin disk combine to cause a viewing-angle-dependent UV luminosity and a more isotropic X-ray luminosity. The scatter in our correlations is larger than that expected from this AD model, suggesting the presence of dust, which reddens both the continuum and the broad emission lines. The C IV and C III] lines show flatter slopes and larger scatter in the line-continuum relations than predicted by the AD+dusty torus model. This may be due to a significant contribution from collisional excitation that is not directly related to the ionizing continuum.

We compare the optical to soft X-ray spectral energy distributions (SEDs) of a sample of bright low-redshift (0.048 < z < 0.155), radio-quiet quasars, with a range of thermal models which have been proposed to explain the optical/UV/soft X-ray quasar emission: (1) optically thin emission from an ionized plasma, (2) optically thick emission from the innermost regions of an accretion disk in Schwarzschild and Kerr geometries. We presented ROSAT PSPC observations of these quasars in an earlier paper. Here our goals are to search for the signature of thermal emission in the quasar SEDs, and to investigate whether a single component is dominating at different frequencies. We find that isothermal optically thin plasma models can explain the observed soft X-ray color and the mean optical-ultraviolet (OUV) color. However, they predict an ultraviolet (1325 Å) luminosity a factor of 3 to 10 times lower than observed. Pure disk models, even in a Kerr geometry, do not have the necessary flexibility to account for the observed OUV and soft X-ray luminosities. Additional components are needed both in the optical and in the soft X-rays (e.g., a hot corona can explain the soft X-ray color). The most constrained modification of pure disk models, is the assumption of an underlying power-law component extending from the infrared (3 μm) to the X-ray. This can explain both the OUV and soft X-ray colors and luminosities and does not exceed the 3 microns luminosity, where a contribution from hot dust is likely to be important. We also discuss the possibility that the observed soft X-ray color and luminosity are dominated by reflection from the ionized surface of the accretion disk.

While modifications of both optically thin plasma models and pure disk models might account for the observed SED, we do not find any strong evidence that the OUV bump and soft X-ray emission are one and the same component. Likewise, we do not find any strong argument which definitely argues in favor of thermal models.