Person:

Siemiginowska, Aneta

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.

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.

BeppoSAX observed the Seyfert 1.9 galaxy NGC 4258 in 1998 December, when its 2-10 keV luminosity was about 1041 ergs s-1. Large amplitude (100%) variability is observed in the 3-10 keV band on timescales of a few tens of thousands of seconds, while variability of ~20% is observed on timescales as short as 1 hr. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5 ± 1.2) × 1022 cm-2; this component is detected at up to 70 keV with a signal-to-noise ratio of gsim3 and with a steep power-law energy spectral index of αE = 1.11 ± 0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in advection-dominated accretion flow models with strong winds, is ruled out by the data. The ratio between the nuclear radio (22 GHz) luminosity and the X-ray (5 keV) luminosity is consistent with that of radio-quiet quasars and Seyfert galaxies. X-ray variability, spectral shape, and radio/X-ray and near-IR/X-ray luminosity ratios suggest that the nucleus of NGC 4258 could be a scaled down version of a Seyfert nucleus and that the X-ray nuclear luminosity can be explained in terms of Comptonization in a hot corona. The soft (E lesssim 2 keV) X-ray emission is complex. There are at least two thermal-like components with temperatures of 0.6 ± 0.1 keV and gsim1.3 keV. The cooler (L0.1-2.4 keV~ 1040 ergs s-1) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI (Cecil et al. 1994). The luminosity of the second component, which can be modeled equally well by an unobscured power-law model with αE = 0.2img1.gif, is L0.1-2.4 keV~ 7 × 1039 ergs s-1, consistent with that expected from discrete X-ray sources (binaries and supernova remnants) in the host galaxy. Observations of NGC 4258 and other maser active galactic nuclei (AGNs) show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of water maser emission from X-ray illumination. So a large column density absorbing gas may be a necessary property of masing AGNs.

Eight high signal-to-noise ROSAT Position Sensitive Proportional Counter (PSPC) observations of six low-redshift (o.048 less than z less than 0.155) radio-quiet quasars have been analyzed to study ant soft excess. All the spectra can, at least roughly, be described int eh 0.1-2.5 keV band by simple power laws reduced at low energies by Galactic absorption. The strong oxygen edges seen in the PSPC spectra of several Seyfert galaxies and quasars are not observed in this sample. The limits implied for the abount of absorbing gas intrinsic to the quasars are particularly tight: of the order of approximately 1020/sq cm. THe range of energy indices is broad: 1.3 less than alphaE less than 2.3. The energy indices are systematically steeper than those found in the same sources at higher energies (by DELTA alphaE approximately 0.5-1 with respect to Ginga or EXOSAT (2-10 keV) measurements, and by DELTA alphaE approximately 0.5 with respect to IPC (0.2-3.5 keV) measurements). This suggests a break between the hard and soft components in the keV region and, therefore, that the PSPC spectra are strongly dominated by the soft compnents. In fact, a fit tot he composite, high signal-to-noise spectrum reveals a significant excess above approximately 1 keV withrespect to the simple power-law model. No evidence for strong emission lines is found in any of the quasars. This argues against emission from an ionized plasma as the main contributor to the soft X-ray compnentunless there is a distribution of te mperatures. If the soft X-ray spectrum of thee quasars is dominated by radiation reflected by the photoinonized surface of an accretion disk, the absence of strong emissionlines suggests high ionization parameters and therefore high accretion rates. We include in two Appendices a comarison of the two official PSPC resolution matrices, those released on1992 March and on 1993 January, a discussion of the amplitude of the residual systematic uncertainties in 1993 January matrix, and a compaison between the PSPC and IPC spectra of a sample of sources.

We have observed 14 quasars with z greater than 2.8 with the ROSAT-PSPC, and detected 12 of them, including the z=4.11 quasar 0000-263. We present the first x-ray spectrum of a radio quiet quasar with z greater than 3, 1946+768. Its x-ray spectrum is consistent with a power law with spectral index alphaE=1.8+2.1,-1.4 and no evidence for absorption in excess of the galactic column (alphaE=1.00+0.28,-0.32 assuming NH=NH(Gal)). A Position Sensitive Proportional Counter (PSPC) hardness ratio is used to constrain the x-ray spectral properties of the quasars for which there were less than 100 photons detected. For the radio quiet quasars, (alphaE) approximately equals 1.2, if one assumes that there is no absorption in excess of the galactic column. We combine the x-ray data with new ground based optical and near-IR spectrophotometry obtained at the Steward 2.3 m and Multiple Mirror Telescope, and data from the literature. The spectral energy distributions are compared to those of low redshift objects. For the radio quiet quasars with z greater than 2.5, the mean (alphaox) is approximately 1.8. This is larger than the mean for quasars with z less than 2.5, but consistent with the expected value for quasars with the high optical luminosities of the objects in this sample. For the radio-loud quasars, (alphaox) is approximately 1.4, independent of redshift. This is smaller than the expected value for the optically luminous, high redshift objects in this sample, if they are mostly GHz peaked radio sources and hence comparable to steep-spectrum, compact radio sources at lower redshift. Finally, we compare the spectral energy distributions of two representative objects to the predicted spectrum of a thin accretion disk in the Kerr geometry, and discuss the uncertainties in deriving black hole masses and mass accretion rates.

We discuss several models of quasar big blue bump emission in color-color and color-luminosity diagrams. We define several broad passbands: IR (0.8-1.6 μm), VIS (4000-8000 Å), UV (1000-2000 Å), UV1 (1400-2000 Å) and UV2 (1000-1400 Å), and SX (0.2-0.4 keV). The colors have been chosen to investigate characteristics of the big blue bump: (1) IR/VIS color represents the importance of the IR component and shows the contribution around ˜1 μm; (2) UV/VIS color shows the slope of the big blue bump (in a region where it dominates, a higher value means the bump gets steeper); (3) the combination of IR/VIS/UV colors shows the relative strength of the big blue bump and the IR component; (4) UV1/UV2 color is important as an indicator of a flattening of the spectrum in this region and the presence of the far-UV turnover, (5) UV/SX tests the relationship between the big blue bump and the soft X-ray component. All colors are needed to investigate the range of model parameters. We describe the colors for several models: accretion disk models in Schwarzschild and Kerr geometries, single-temperature optically thin emission, combination of the main emission model and nonthermal power law or dust, and irradiation of the disk surface. We test models against the sample of 47 low-redshift quasars from Elvis et al. We find that (1) modified blackbody emission from an accretion disk in a Kerr geometry can successfully reproduce both the luminosities and colors of the quasars except for the soft X-ray emission; (2) no additional components (hot dust or power-law) are needed to fit the optical-UV colors when the irradiation of the surface of the disk is included in the model; (3) even modest (10%) irradiation of the surface of the disk modifies significantly the optical colors; (4) the simplest, single-temperature, free-free models need either an additional component or a range of temperatures to explain the observations.

Tables of predicted colors for each model family are provided on the AAS CD-ROM. A part of the tables is listed in the Appendix.

At z approx. equals 3, the x-ray spectra of radio-loud and radio-quiet quasars are different. High-redshift radio-quiet quasars either have large absorbing columns, NH, and steeper power law spectral indices, alphaepsilon, than low redshift quasars, or no absorption and similar alphaepsilon's. In contrast, the radio-loud quasars at high redshift have substantial absorption and similar alphaepsilon's to low redshift quasars. Implications for the interpretation of the evolution of the luminosity function of quasars are discussed. If the absorption arises outside the central engine for both radio-loud and radio-quiet quasars, then radio-quiet quasars differ from the radio-loud quasars in that their emitted power law spectrum has evolved with redshift. We argue that this favors models where quasars are numerous and short-lived, rather than rare and long-lived.

Characteristic variability timescales in blazar γ-ray light curves can provide insights into the physical processes responsible for γ-ray variability. The power spectral density (PSD) is capable of revealing such timescales, which may appear as breaks or periodicities. Continuous-time autoregressive moving-average (CARMA) models can be used to accurately estimate a light curve’s PSD. Through a light-curve simulation study, we develop a methodology to identify PSD breaks using CARMA models. Using this methodology, we study the γ-ray light curves of 13 bright blazars observed with the Fermi Large Area Telescope in the 0.1-300 GeV band over 9.5 yr. We present the blazar γ-ray PSDs, which provide evidence for low-frequency breaks on timescales ̃1 yr in four sources, and an additional high-frequency break on a timescale ̃9 days in one source.