Hard X-ray Component Research Articles

Hard X-Ray–emitting Black Hole Fed by Accretion of Low Angular Momentum Matter

Observed spectra of Active Galactic Nuclei (AGN) and luminous X-ray binaries in our Galaxy suggest that both hot (~10^9 K) and cold (~10^6 K) plasma components exist close to the central accreting black hole. Hard X-ray component of the spectra is usually explained by Compton upscattering of optical/UV photons from optically thick cold plasma by hot electrons. Observations also indicate that some of these objects are quite efficient in converting gravitational energy of accretion matter into radiation. Existing theoretical models have difficulties in explaining the two plasma components and high intensity of hard X-rays. Most of the models assume that the hot component emerges from the cold one due to some kind of instability, but no one offers a satisfactory physical explanation for this. Here we propose a solution to these difficulties that reverses what was imagined previously: in our model the hot component forms first and afterward it cools down to form the cold component. In our model, accretion flow has initially a small angular momentum, and thus it has a quasi-spherical geometry at large radii. Close to the black hole, the accreting matter is heated up in shocks that form due to the action of the centrifugal force. The hot post-shock matter is very efficiently cooled down by Comptonization of low energy photons and condensates into a thin and cold accretion disk. The thin disk emits the low energy photons which cool the hot component.

How Saturated Are Absorption Lines in the Broad Absorption Line Quasar PG 1411+442?

Recently, convincing evidence was found for extremely large X-ray absorption by column densities greater than 1023 cm-2 in broad absorption line quasars. One consequence of this is that any soft X-ray emission from these QSOs would be the scattered light or leaked light from partially covering absorbing material. A detection of the unabsorbed soft X-ray and absorbed hard X-ray component will allow us to determine the total column density as well as the effective covering factor of the absorbing material, which are difficult to obtain from the UV absorption lines. Brinkmann et al. recently showed that both the unabsorbed and absorbed components are detected in the nearby very bright broad absorption line quasar PG 1411+442. In this Letter, we make a further analysis of the broadband X-ray spectrum and the UV spectrum from the Hubble Space Telescope and demonstrate that broad absorption lines are completely saturated at the bottom of absorption troughs.

BeppoSAX observations of 2-Jy lobe-dominated broad-line sources -- I. The discovery of a hard X-ray component

We present new BeppoSAX LEGS, MECS and PDS observations of five lobe-dominated, broad-line active galactic nuclei selected from the 2-Jy sample of southern radio sources. These include three radio quasars and two broad-line radio galaxies. ROSAT PSPC data, available for all the objects, are also used to constrain the spectral shape better in the soft X-ray band. The collected data cover the energy range 0.1-10 keV, reaching similar to 50 keV for one source (Pictor A), The main result from the spectral fits is that all sources have a hard X-ray spectrum with energy index alpha(x) similar to 0.75 in the 2-10 keV range. This is at variance with the situation at lower energies where these sources exhibit steeper spectra. Spectral breaks Delta alpha(x) similar to 0.5 at 1-2 keV characterize in fact the overall X-ray spectra of our objects. The flat, high-energy slope is very similar to that displayed by flat-spectrum/core-dominated quasars, which suggests that the same emission mechanism (most likely inverse Compton) produces the hard X-ray spectra in both classes. Finally, a tweak) thermal component is also present at low energies in the two broad-line radio galaxies included in our study.

Multicomponent X‐Ray Emissions from Regions near or on the Pulsar Surface

We present a model of X-ray emission from rotation-powered pulsars, which in general consist of one nonthermal component, two hard thermal components, and one soft thermal component. The nonthermal X-rays come from synchrotron radiation of e ± pairs created in the strong magnetic field near the neutron star surface by curvature photons emitted by charged particles on their way from the outer gap to the neutron star surface. The first hard thermal X-ray component results from polar-cap heating by the return current in the polar gap. The second hard thermal X-ray component results from polar-cap heating by the return particles from the outer gap. Because of cyclotron resonance scattering, most of the hard thermal X-rays will be effectively reflected back to the stellar surface and eventually reemitted as soft thermal X-rays. However, some of the hard thermal X-rays can still escape along the open magnetic field lines, where the e+/e− pair density is low. Furthermore, the characteristic blackbody temperatures of the two hard X-ray components emitted from the polar-cap area inside the polar gap and the polar-cap area defined by the footprints of the outer-gap magnetic field lines are strongly affected by the surface magnetic field, which can be much larger than the dipolar field. In fact, the strong surface magnetic field can explain why the effective blackbody radiation area is nearly 2 orders of magnitude larger than that deduced from the dipolar field for young pulsars (2 orders of magnitude less for old pulsars). Our model indicates how several possible X-ray components may be observed, depending on the magnetic inclination angle and viewing angle. Using the expected X-ray luminosity and spectra, we explain the observed X-ray spectra from pulsars such as Geminga, PSR B1055-52, PSR B0656+14, and PSR B1929+10.

ASCA Observations of the Supernova Remnant G156.2+5.7

We report on the ASCA X-ray spectra from the north rim and the center of the supernova remnant (SNR) G156.2+5.7. The spectra are found to comprise soft and hard X-ray components. No clear shell structure has been found, either in the soft or in the hard X-ray bands. The soft X-rays are attributable to a non-equilibrium ionization plasma with a temperature of 0.4 keV and an ionization timescale of 1011.0−11.2 cm−3s. A Sedov solution with an explosion energy of 1051 erg leads to an age of ∼ 1.5 × 104 yr and a density of ambient gas of ∼ 0.2 cm−3. The distance to G156.2+5.7 is estimated to be ∼ 1.3 kpc. The abundances of silicon and sulfur vary between the center and the north rim of the SNR, while the abundances of all other elements do not. The hard component can be modeled with either thermal emission of a temperature higher than a few keV or a non-thermal process with a power-law spectrum of photon index 1.5–1.8. Since the former model requires an uncomfortably high temperature, we favor a non-thermal model. We also discuss the nature of a point source found at the edge of the SNR.

BeppoSAX observations of 2 Jy lobe-dominated broad-line sources: the discovery of a hard x-ray component

We present new BeppoSAX LECS, MECS, and PDS observations of five lobe-dominated, broad-line active galactic nuclei selected from the 2 Jy sample of southern radio sources. These include three radio quasars and two broad-line radio galaxies. ROSAT PSPC data, available for all the objects, are also used to better constrain the spectral shape in the soft X-ray band. The collected data cover the 0.1 –10 keV energy range, reaching 40 keV for one source. Detailed spectral fitting shows that all sources have a flat hart X-ray spectrum with energy index α x ∼ 0.75 in the 2 – 10 keV energy range. This is a new result, which is at variance with the situation at lower energies where these sources exhibit steeper spectra. Spectral breaks ∼ 0.5 at 1 – 2 keV characterize the overall X-ray spectra of our objects. The flat, high-energy slope is very similar to that displayed by flat-spectrum/coredominated quasars, which suggests that the same emission mechanism (most likely inverse Compton) produces the hard X-ray spectra in both classes. Contrary to the optical evidence for some of our sources, no absorption above the Galactic values is found in our sample. Finally, a (weak) thermal component is also present at low energies in the two broad-line radio galaxies included in our study.

Deconvolution of Directly Precipitating and Trap‐Precipitating Electrons in Solar Flare Hard X‐Rays. I. Method and Tests

We develop and test a numerical code that provides a self-consistent deconvolution of energy-dependent hard X-ray (HXR) time profiles I(e, t) into two HXR-producing electron components, i.e., directly precipitating and trap-precipitating electrons. These two HXR components can be physically distinguished because their energy-dependent time delays have an opposite sign. The deconvolution is based on the following model assumptions: (1) nonthermal electrons are injected from the acceleration site into coronal flare loops by an injection function f(E, α, t) that consists of synchronized pulses in energy E and pitch angle α, (2) electrons with initially small pitch angles (α ≤ α0) precipitate directly to the HXR emission site, (3) electrons with initially large pitch angles (α ≥ α0) are temporarily trapped and precipitate after the collisional deflection time, and (4) nonthermal electrons lose their energy by Coulomb collisions and emit thick-target HXR bremsstrahlung in a high-density (fully collisional) site (near or inside the chromosphere). The numerical deconvolution provides a self-consistent determination of three physical parameters: (1) the electron time-of-flight distance lTOF between the acceleration/injection site and the HXR emission site, (2) the electron density ne in the trap region, and (3) the fraction of HXR-emitting electrons that precipitate directly, qprec, which relates to the loss cone angle by qprec(α0) = (1 - cos α0) for isotropic pitch angle distributions. This yields the magnetic mirror ratio R = Bloss/Binj = 1/sin2 (α0) between the injection and loss cone site. With this method, we measure for the first time magnetic field ratios in coronal loops by means of HXR data. Based on this ratio, together with the knowledge of the photospheric field at the footpoint, a direct measurement of the magnetic field in the coronal acceleration region can be obtained. We simulate energy-dependent HXR data I(e, t) with typical solar flare parameters (lTOF = 15,000 km, ne = 1011 cm-3, qprec = 0.5) and test the accuracy of the inversion code. We perform the inversion in 30 different simulations over the entire physically plausible parameter space and demonstrate that a satisfactory inversion of all three physical parameters lTOF, ne, and qprec is achieved in a density range of ne = 1010-1012 cm-3 for precipitation ratios of qprec = 0.1-0.9 and for signal-to-noise ratios of 100 (requiring HXR count rates of 104 counts s-1). Applications of this inversion method to solar flare observations in hard X-rays (CGRO/BATSE, Yohkoh/Hard X-Ray Telescope) and microwaves (Nobeyama) will be presented in subsequent papers.

The X‐Ray Spectral Evolution of Classical Nova V1974 Cygni 1992: A Reanalysis of theROSATData

We present a spectral analysis of the archival X-ray data of classical Nova V1974 Cygni 1992 (Nova Cygni 1992) obtained by the ROSAT Position Sensitive Proportional Counter (PSPC). The X-ray spectrum is fitted with a two-component model. The first component is a white dwarf atmosphere emission model developed for the remnants of classical novae near the Eddington luminosity. The model is used to fit the soft X-ray data in the ~0.1-1.0 keV range, where the bulk of emission is below 0.7 keV. The second component is a Raymond-Smith model of thermal plasma applied to the hard X-ray emission above ~1.0 keV. The postoutburst X-ray spectrum of the remnant white dwarf is examined in the context of evolution on the Hertzsprung-Russell diagram using an O-Ne- and a C-O-enhanced atmosphere emission model. A constant bolometric luminosity evolution is detected with increasing effective temperature and decreasing photospheric radius using the O-Ne-enhanced model. The unabsorbed soft X-ray flux for the constant bolometric luminosity phase is found to be in the range (1.7-2.2) × 10-7 ergs s-1 cm-2. A peak effective temperature of 51 eV (5.9 ×105 K) is detected 511 days after outburst. We also present the spectral development of the hard X-ray component. It is found to evolve independently of the soft one. The maximum of the hard X-ray emission is reached at ~150 days after outburst with an unabsorbed flux of ~2.0 × 10-11 ergs s-1 cm-2 corresponding to a luminosity of (0.8-2.0) × 1034 ergs s-1 at a 2-3 kpc source distance. The time evolution of the hard X-ray flux and the plasma temperatures decreasing from 10 keV to 1 keV suggest emission from shock-heated gas as the origin of the hard X-ray component.

ASCAObservations of the Symbiotic System CH Cygni

The symbiotic system CH Cyg was observed using ASCA on 1994 October. The energy spectrum clearly consists of two different components that are separated at the energy 2 keV. The soft component is represented by a superposition of the two optically thin thermal plasma emissions, the temperatures of which are 0.2 and 0.7 keV. The hard component can also be represented by the optically thin thermal plasma emission but with an order of magnitude higher temperature of 7.3 keV, undergoing a strong photoelectric absorption with NH ~ 1023 cm-2. This hard component shows a time variation as fast as ~100 s. These characteristics of the hard component strongly indicate that CH Cyg system contains an accreting white dwarf. From the temperature of the hard X-ray component, a firm lower limit of the mass of the white dwarf is obtained to be 0.44 M☉. The lower limit can hardly be reconciled with the idea that CH Cyg is the triple system with the inner binary period of 756 days, because the cool giant mass becomes greater than 7.1 M☉, although it is well established to be the SRb variable. If the cool giant mass is 1 M☉, the mass function should be greater than 0.04 M☉ for i = 90°.

A Broadband X‐Ray Study of the Geminga Pulsar

We present a comprehensive study of the pulsar at energies 0.1-10 keV using data from the ASCA, ROSAT, and EUVE satellites. The bulk of the soft X-ray flux can be parameterized as a blackbody of T = (5.6 +/- 0.6) x 10(exp 5) K, occupying a fraction 0. 10 - 0.64 of the surface area of the neutron star at the parallax distance of 160 pc. The ASCA detection of resolves the nature of the harder X-ray component previously discovered by ROSAT in favor of nonthermal emission, rather than thermal emission from a heated polar cap. The hard X-ray spectrum can be fitted by a power-law of energy index 1.0 +/- 0.5. The hard X-ray light curve has a strong main peak and a weak secondary peak; its total pulsed fraction is = 55%. Three ROSAT PSPC observations show significant variability of Geminga's light curve. In particular, a peculiar energy dependence of the modulation in the soft X-ray component, dubbed the Geminga effect in the original PSPC data, is not present in later observations. In addition, fine structure in the soft X-ray light curve, interpreted as eclipses due to cyclotron resonance scattering by a plasma screen on the closed magnetic field lines, almost disappeared in the most recent observations. All of the variable properties of can probably be associated with the nonthermal process that supplies e(sup +, sup -) pairs to its inner magnetosphere.

The Soft X‐Ray Properties of a Complete Sample of Optically Selected Quasars. II. Final Results

We present the results of ROSAT position sensitive proportional counter (PSPC) observations of 10 quasars. These objects are part of our ROSAT program to observe a complete sample of optically selected quasars. This sample includes all 23 quasars from the bright quasar survey with a redshift z less than or = 0.400 and a Galactic H I column density N(sup Gal sub H I) less than 1.9 x 10(exp 20)/sq cm. These selection criteria, combined with the high sensitivity and improved energy resolution of the PSPC, allow us to determine the soft (approximately 0.2-2 keV) X-ray spectra of quasars with about an order of magnitude higher precision compared with earlier soft X-ray observations. The following main results are obtained: Strong correlations are suggested between the soft X-ray spectral slope alpha(sub x) and the following emission line parameters: H beta Full Width at Half Maximum (FWHM), L(sub O III), and the Fe II/H beta flux ratio. These correlations imply the following: (1) The quasar's environment is likely to be optically thin down to approximately 0.2 keV. (2) In most objects alpha(sub x) varies by less than approximately 10% on timescales shorter than a few years. (3) alpha(sub x) might be a useful absolute luminosity indicator in quasars. (4) The Galactic He I and H I column densities are well correlated. Most spectra are well characterized by a simple power law, with no evidence for either significant absorption excess or emission excess at low energies, to within approximately 30%. We find mean value of alpha(sub x) = -1.50 +/- 0.40, which is consistent with other ROSAT observations of quasars. However, this average is significantly steeper than suggested by earlier soft X-ray observations of the Einstein IPC. The 0.3 keV flux in our sample can be predicted to better than a factor of 2 once the 1.69 micrometer(s) flux is given. This implies that the X-ray variability power spectra of quasars flattens out between f approximately 10(exp -5) and f approximately 10(exp -8) Hz. A steep alpha(sub x) is mostly associated with a weak hard X-ray component, relative to the near-IR and optical emission, rather than a strong soft excess, and the scatter in the normalized 0.3 keV flux is significantly smaller than the scatter in the normalized 2 keV flux. This argues against either thin or thick accretion disks as the origin of the soft X-ray emission. Further possible implications of the results found here are briefly discussed.

ASCA PV observations of the Seyfert 2 galaxy NGC 4388: the obscured nucleus and its X-ray emission

We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster observed with ASCA during its PV phase. The 0.5-10 keV X-ray spectrum consists of multiple components; (1) a continuum component heavily absorbed by a column density NH = 4E23 cm-2 above 3 keV; (2) a strong 6.4 keV line (EW = 500 eV); (3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray emission below 1 keV. The detection of strong absorption for the hard X-ray component is firm evidence for an obscured active nucleus in this Seyfert 2 galaxy. The absorption corrected X-ray luminosity is about 2E42 erg/s. This is the first time that the fluorescent iron-K line has been detected in this object. The flat spectrum in the intermediate energy range may be a scattered continuum from the central source. The soft X-ray emission below 1 keV can be thermal emission from a temperature kT = 0.5 keV, consistent with the spatially extended emission observed by ROSAT HRI. However, the low abundance (0.05 Zs) and high mass flow rate required for the thermal model and an iron-K line stronger than expected from the obscuring torus model are puzzling. An alternative consistent solution can be obtained if the central source was a hundred times more luminous over than a thousand years ago. All the X-ray emission below 3 keV is then scattered radiation.

ASCA Observation of NGC 1808

The Japanese X-ray satellite ASCA observed NGC 1808, which is known to exhibit both Seyfert and starburst activities. In the soft X-ray band, ASCA detected emission lines from highly ionized Mg and Si, which confirmed the presence of a thin thermal plasma, due possibly to starburst activity. The thin thermal plasma consists of two temperature components of kT ∼ 0.78 keV and kT ∼ 0.35 keV. We found that the intrinsic absorption toward the 0.78 keV plasma is NH ∼ 7 × 1021 cm−2, which is greater than the galactic foreground value of 2.8 × 1020 cm−2 (Dickey, Lockman 1990, AAA 53.155.163). This result implies that most of the X-ray emission from this plasma comes from the central region. The absorption toward the 0.35 keV plasma, on the other hand, shows a low value compared to the galactic absorption. This indicates that the plasma is more extended like an X-ray halo. The metal abundance of about 1 solar value found in the thin thermal plasma is larger than that of the archetypical starburst galaxy M82. In addition to the thin hot plasma, ASCA detected a hard X-ray component from NGC 1808, with a luminosity of 2×1040 erg s−1 (0.5–10 keV band), obscured by an absorbing column of 1.0×1022 cm−2. The hard X-ray flux and spectrum may also be attributable to starburst activity. However, the hard X-rays exhibited a long-term variability from the Ginga and ASCA observations, suggesting the presence of a low-luminosity AGN in NGC 1808.

The ROSAT observations of classical novae

We observed a number of classical and recurrent novae in the Galaxy and the LMC with the ROSAT X-ray telescope and searched the archival data for other serendipitous observations. Preliminary results show that only 9 out of 37 observed objects were bright enough in X-rays to be detected with ROSAT, either in outburst or at quiescence.Three basic mechanisms can cause X-ray emission from classical or re-current novae. The first is hot hydrogen burning in a thin shell of the remnant envelope left on the white dwarf after the nova explosion. Hydrogen burning post-novae should be blackbody like emitters at nearly Eddington luminosity (as per the ‘supersoft’ X-ray sources). In our sample, only GQ Mus (Nova Mus 1983, see Ögelman et al. 1993; Shanley et al. 1995) and V1974 Cyg 1992 (Krautter et al. 1996) had these characteristics. Remarkably, among 10 LMC novae that had an outburst in the last 47 yr none was detected as a ‘supersoft’ X-ray source. The 3 σ upper limits for the black-body temperatures of the post-nova white dwarfs are mainly in the range 20… 30 eV. A post-nova can also emit X-rays because of shocks occurring in the ejected shell (e.g. O’Brien et al. 1994). Three out of four classical novae that were observed in outburst displayed a hard X-ray component in the ROSAT energy band, which might be due to a shocked shell. Finally, X-ray emission is expected from quiescent nearby novae because of accretion. Only four nearby accreting sources were detected; the ROSAT upper limits for the non-detected quiescent novae are Lx < 1031… 1032 ergs−1, assuming a thermal plasma at kT = a few keV.

ASCA observations of two SGRs SGR1806-20 and SGR0526-66

ASCA has observed two Soft Gamma-ray burst Repeaters (SGRs) among three known. This is a short report of the two SGRs, 1806–20 and 0526–66(N49). The observed X-ray characteristics favor a spin-powered system for SGR1806-20 and no ASCA detection of the hard X-ray component from SGR0526–66 suggests a very soft spectrum of the ROSAT HRI X-ray point source.

ROSAT observations of NGC 2146: Evidence for a starburst-driven superwind

We have imaged the edge-on starburst galaxy NGC 2146 with the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI) on board ROSAT and have compared these data to optical images and long-slit spectra. NGC 2146 possesses a very large X-ray nebula with a half-light radius of 1 min (4 kpc) and a maximum diameter of approximately 4 min, or 17 kpc. The X-ray emission is resolved by the PSPC and preferentially oriented along the minor axis, with a total flux of 1.1 x 10(exp -12) ergs/sq cm/s over 0.2 - 2.4 keV and a luminosity of approximately 3 x 10(exp 40) ergs/s. The inner X-ray nebula is resolved by the HRI into at least four bright knots together with strong diffuse emission responsible for at least 50% of the flux within a radius of 0.5 min (approximately 2 kpc). The brightest knot has a luminosity of (2 - 3) x 10(exp 39) ergs/s. The X-ray nebula has a spatial extent much larger than the starburst ridge seen at centimeter wavelengths by Kronberg & Biermann (1981) and is oriented in a `X-like' pattern along the galaxy minor axis at a position angle of approximately 30 degrees. This minor-axis X-ray emission is associated with a region of H alpha and dust filaments seen in optical images. Optical spectra show that the emission-line gas along the minor axis is characterized by relatively broad lines (approximately 250 km/s full width half-maximum (FWHM)) and by `shocklike' emission-line flux ratios. Together with the blue-asymmetric nuclear emission-line and NaD interstellar absorption-line profiles, these optical data strongly suggest the presence of a starburst-driven superwind. The X-ray spectrum extracted from the central 5 min contains a strong Fe L emission-line complex at 0.6 - 1.0 keV and a hard excess above 1.0 keV. The spectrum is best described with a two-component model, containing a soft (kT approximately 400 - 500 eV) Raymond-Smith thermal plasma together with either a Gamma = 1.7 power-law or a kT greater than 2.2 keV bremsstrahlung component. The soft thermal component provides approximately 30% of the total luminosity over 0.2 - 2.4 keV, or approximately 10(exp 40) ergs/s. The pressure derived from the soft component of the X-ray spectrum is consistent with that predicted from a starburst-driven superwind if the filling factor of the warm gas is approximately 1% - 10 %. If the hard X-ray component is thermal gas associated with the galactic outflow, the filling factor must be close to unity. Predictions of the luminosity, temperature, and size of an adiabatic starburst-generated windblown bubble are consistent with those measured for the soft thermal X-ray emission in NGC 2146. The hard X-ray component, however, has a luminosity much larger than predicted by the superwind model if this component is thermal emission from gas heated by an internal shock in the expanding bubble. We briefly review various possibilities as to the nature of the hard X-ray component in NGC 2146.

X-ray spectrum of the black hole candidate X1755-338

We report the first detection of a hard power-law tail in the X-ray spectrum of the black hole candidate (BHC) binary X1755-338, which was observed in 1989 March-September during the TTM Galactic Centre survey. In addition, an ultrasoft thermal component with a temperature of ~1.1-1.4 keV was also detected. We demonstrate that the soft and hard X-ray components of X1755-338 vary independently, as in the spectra of the well known BHCs LMC X-1, LMC X-3 and GX339-4 in their high (intensity) state. If the hard tail observed from X1755-338 is generated near the black hole by energetic electrons up-scattering low energy photons, the un-correlated variations imply that the soft X-rays from accretion disc may not be the main photon seeds needed for inverse Comptonization. The TTM observations strongly suggest that X1755-338 does indeed belong to the family of BHCs.

ROSAT and Ginga observations of the magnetic cataclysmic variable QQ Vul: evidence for two-pole accretion

We report observations made in 1991 April of QQ Vul using the ROSAT and Ginga satellites. We find a spectral variation in soft X-rays that can be accounted for by an increase in the blackbody temperature by 3-5 eV, or by an increase in the intrinsic absorbing column density of |$(1.8-3.6)\times10^{19}\text{cm}^{-2}$|⁠, between phases 0.5 and 1.0. Together with evidence for separated hard and soft X-ray emission regions, this suggests the presence of two-pole accretion. The emission region furthest from the secondary has a soft X-ray excess ≳2.5, suggesting buried shocks arising from the accretion of dense filaments. The other emission region, closer to the secondary, is the site of the hard X-ray component. The hard X-ray data show a 6.7-keV iron K line of large equivalent width.

Active Galactic Nuclei in the Extreme Ultraviolet

The extreme ultraviolet (EUV) wavelength interval (commonly defined to constitute the range 68 — 912 Å, or 13.6 — 182 eV) hes between the intensively studied FUV (912 - 3000 Å) and X-ray (1 - 68 Å) spectral regimes, but until recently has not been subjected to systematic investigations. This lack of information has been particularly acute in the case of active galactic nuclei (AGNs), where the very ability to carry out successful EUV observations has been seriously questioned. This has stemmed from the fact that, for a Galactic H I column density NG(H I) = 1 × 1020 cm−2, the interstellar transmission factor is 0.42 at 60 Å, but a factor of 10 smaller at 100 Å. In turn, there are no known regions of the interstellar medium (ISM) that have [NG(H I)/1020 cm−2] < 0.6 (resp., 0.8) for positive (resp., negative) latitudes (e.g., Heiles 1991).There are, however, several reasons why EUV observations of AGNs could provide important information about the nature of these objects. In the case of radio-quiet QSOs, the peak power is known to be emitted in the EUV range (several hundred A). Furthermore, in these sources there is evidence for a distinct soft X-ray component (representing a “soft excess” over an extrapolation of the ISM-absorbed hard X-ray component), whose properties are at present controversial. In particular, the first results of ROSAT PSPC observations of a complete sample of optically selected QSOs (Laor et al. 1994) have pointed to a spectral index αx = 1.50 ± 0.40 in the 0.2 − 2 keV range that flattens by Δαx ≈ 0.5 above ∽ 2 keV. This study has concluded that steep-αx sources are characterized by a week hard component. A similar distribution of αx has been inferred for soft X-ray-bright QSOs and Seyfert 1 galaxies observed in the ROSAT all-sky survey. On the other hand, previous observations (enumerated in Laor et al. 1994) by the Einstein and EXOSAT satellites yielded different soft X-ray spectral slopes and led to an alternative explanation of the steep-αχ objects. These questions could in principle be clarified by EUV measurements, which would then also help determine the soft X-ray emission mechanism.

SOFT X-RAY ANALYSIS AND MULTIWAVELENGTH MODELING OF X-RAY SELECTED ACTIVE GALACTIC NUCLEI

ABSTRACT We survey the broadband spectral properties of ~500 X-ray-selected Active Galactic Nuclei (AGN) observed with the Einstein X-ray Observatory in order to better understand the X-ray emission characteristics of this population as compared to previously studied optically- and radio-selected populations. We further select a subsample of X-ray-selected AGN which possess strong components of soft X-ray emission below ~0.5 keV. This sample is used to investigate in greater detail this ultrasoft emission and to search for possible correlations between it and emission at other wavelengths. These ultrasoft sources are used for multiwavelength accretion disk modeling in a test of the current black hole plus accretion disk paradigm of AGN energy generation. We find the mean spectral index of the AGN between 0.1 and 0.6 keV to be softer, and the distributions of indices wider, than previous estimates based on analyses of the X-ray spectra of optically-selected AGN. We confirm a correlation between radio and hard X-ray luminosity, but find that the data do not support a correlation between radio and soft X-ray spectral slope. We report evidence for physically distinct soft and hard X-ray components. Imaging photometry in the J, H, and K infrared bands is carried out for a sample of eight AGN. The infrared images show the AGN to be primarily point sources, with little extended emission from the host galaxy. The photometric measurements represent part of a multiwavelength database to be used for accretion disk modeling. We report observations of six AGN in the ultraviolet region with the International Ultraviolet Explorer satellite. The ultraviolet spectra from these observations are used for measurement of the strengths of the emission lines present (e.g., Ly-alpha, CIV, and CIII]). The data also allow a measurement of the shape of the ultraviolet continua of these AGN to be used further in multiwavelength accretion disk modeling. Observations of a sample of six optically faint, X-ray-selected AGN with the ROSAT Position Sensitive Proportional Counter are reported. The spectra of the AGN are all fit satisfactorily with single power-law models and their spectral indices (with one exception) agree with those estimated from their Einstein IPC observations. Tests of short term variability are carried out, with all sources showing constant count rates over the period of their ROSAT observations. Some sources do show significant changes in count rate when compared to previous Einstein data taken a decade earlier. The measurements of the soft X-ray spectra of these objects are combined with the infrared and ultraviolet data discussed above to carry out multiwavelength accretion disk modeling. Accretion disk modeling is carried out on a sample of five Seyfert I galaxies possessing strong optical-ultraviolet-soft X-ray emission components. It is found that while bare accretion disk models cannot account for the soft X-ray emission observed in these objects, the addition of a hot (kTe~120 keV), Comptonizing component enables the soft X-ray emission to be accounted for within the black hole plus accretion disk paradigm.