A. E. Glassgold

X-Ray Ionization of the Disks of Young Stellar Objects

We have developed a Monte Carlo code for the transport of stellar X-rays in an axially symmetric disk. The code treats Compton scattering and photoelectric absorption and follows the X-rays until they are completely absorbed. We confirm that hard X-rays from a low-mass young stellar object (YSO) penetrate the associated accretion disk. Even without the low-energy photons that are strongly attenuated by the YSO wind, the ionization rate in the inner region of the accretion disk (<1 AU) is many orders of magnitude larger than the standard ionization rate due to Galactic cosmic rays. At a fixed radius from the source, the X-ray ionization rate is a universal function of the vertical column density, independent of the structural details of the disk. The ionization rate scales with the X-ray luminosity and depends only mildly on the X-ray temperature, at least for the temperatures relevant for low-mass YSOs. Thus X-rays from a YSO can ionize regions of an accretion disk from which low-energy cosmic rays are excluded, e.g., by the action of stellar winds. Using a simple theory for the electron fraction we estimate that, for a minimum solar nebula, X-rays ionize the disk beyond 5 AU at a level sufficient to couple magnetic fields and neutral disk material. Inside this radius, the X-rays are ineffective for vertical column densities much larger than ~1025 cm-2, and thus an interior region of the disk will be uncoupled from magnetic fields. If disk accretion is mediated by MHD turbulence, as proposed by Balbus & Hawley, then our results suggest that layered accretion occurs in the inner regions of a disk ionized by X-rays, in accord with Gammies suggestion based on cosmic-ray ionization.

Multifrequency observations of BL Lacertae

We present 20 years of optical, infrared, and radio monitoring data for BL Lac as well as four simultaneous multifrequency spectra covering the 10^9-10^(18) Hz range. Although there is no time delay between the optical and infrared variability, the high-frequency radio variations precede lower frequency variations, but only by weeks. The optical variability precedes the radio variability by a few years. The structure function for the radio variations is nearly that of shot noise for time scales less than 600 days, in contrast to the optical variation, which is similar to flicker noise. These results indicate that, although there are fundamental differences between the optical and radio emitting regions, they are related, possibly by the propagation of shocks between regions. The multifrequency spectra show that the power per logarithmic bandwidth has a well-defined peak in the near-infrared and a sharp cutoff in the optical-ultraviolet region. This cutoff is like those seen in a few other blazars and is attributed to synchrotron losses that prevent particle acceleration from exceeding a critical energy. The X-ray continuum is not smoothly connected to the optical-ultraviolet emission and has a flatter slope, similar to that of the infrared-millimeter region. In addition, the X-ray emission varied in the same sense as the infrared-millimeter emission but opposite that of the optical-ultraviolet emission. These X-ray properties are those expected from the synchrotron-self-Compton process. The best model suggests that the plasma radiating at ~ 10^(11.5) Hz has a size of ~ 10^(-2·5) pc, a Doppler parameter δ>= 2-3, and a magnetic field of B = 2-40 G.

Multifrequency observations of the BL Lacertae object 0735+178

In each of the present four simultaneous spectra covering the radio-through-X-ray regimes, the IR-UV synchrotron continuum dominates the total observed power and presumably becomes opague between 10 to the 11th and 10 to the 13th Hz. Nonsimultaneous observations were also conducted, over a longer time period, in order to study long- and short-term variability at X-ray, optical, and radio frequencies. These data indicate that the rapid and dramatic variations evident at IR and optical wavelengths are absent at radio and X-ray frequencies, supporting a view of IR-UV flux emanation from a small region, while the X-rays are produced by the inverse Compton process in the radio-emitting region. Particles, photons and magnetic field may not be far from equipartition in this region. Theoretical suggestions are developed regarding the radial behavior of the electron density and magnetic field.

X-ray emission from red quasars

A dozen red quasars were observed with the Einstein Observatory in order to determine their X-ray properties. The observations show that for all these sources, the infrared-optical continuum is so steep that when extrapolated to higher frequencies, it passes orders of magnitude below the measured X-ray flux. The X-ray emission is better correlated with the radio than with the infrared flux, suggesting a connection between the two. By applying the synchrotron-self-Compton model to the data, it is found that the infrared-optical region has a size of 0.01 pc or more and a magnetic field more than 0.1 G, values considerably different than are found in the radio region. Unlike other quasars, the ionizing continuum is dominated by the X-ray emission. The peculiar line ratios seen in these objects can be understood with a photoionization model, provided that the photon to gas density ratio (ionization parameter) is an order of magnitude less than in typical quasars. 25 references.

Simultaneous observations of the BL Lacertae object I Zw 187

Two sets of simultaneous spectra consisting of data obtained with radio, IR, optical, UV and X-ray telescopes were obtained 10 months apart for the X-ray bright BL Lac object IZw 187. In addition, nonsimultaneous observations were made in several of the aforementioned observing bands in order to detect flux variations and galactic light contamination was removed from all observations. The BL Lac component is found to have a weak 3000 A bump superposed on an IR-optical-UV spectrum of slope 0.9. Consistent with the arising of the IR-through-X-ray continuum from a single synchrotron source, the X-ray data fall on or near an extrapolation of this power law. No flux variations have been detected in the flat radio spectrum. Optical and X-ray fluxes are observed to vary by a factor not greater than three, and the shortest variability time scales in these bands are comparable at about one week.

Multifrequency observation of the optically violent variable quasar 3C 446

Extensive optical and radio monitoring data and seven multifrequency spectra were obtained of the violently variable quasar 3C 446. The monitoring data suggest a correlation between the radio and optical outbursts, with the optical flare preceding the radio activity by 400-600 days. A difference in the statistical behavior of the optical and radio variability indicates that considerable processing occurs to the optical emitting plasma before it becomes radio emitting plasma. Within the radio band, outbursts proceed from high to low frequencies. An outburst in 1983 showed greater and more rapid variation in the optical than in the near-IR region. The 10-100 μm fluxes did not follow the higher frequency variation, suggesting a time delay between these spectral domains. During another time, the X-ray emission varied on a time scale of days and more rapidly than the UV or optical emission. On a time scale of weeks-months, the X-ray fluxes are well correlated with the UV-IR fluxes but not with the radio fluxes. The multifrequency data show that the flat radio spectrum turns over at 3-10 x 10^(11) Hz and the continuum steepens with frequency; ɑ(IR) = 1.1, ɑ(opt-UV) = -2 to -3. The X-ray emission lies an order of magnitude above an extrapolation of the optical-UV spectrum and has a harder spectrum. The power is primarily concentrated in the submillimeter and infrared region. When the source is faint, a blue bump may be present. The flux in the Lycx line is proportional to the UV continuum flux density when the source is bright but is independent of the continuum level when the source is faint. The data suggest that the X-rays are produced by the inverse Compton process from an emitting region (10^(16) cm) smaller than but related to the synchrotron emitting UV-IR region. The characteristic size of the emitting region increases with decreasing frequency from 10^(16) (X-ray region) to 1-3 x 10^(17) cm (far IR-submillimeter region) to 10^(19)-10^(20) cm (radio region). Plasma conditions are best constrained at the frequency when the source becomes transparent, the far IR-submillimeter band, where B ≈ 3-100 G, n ≈ 40-100 cm^(-3); and the Doppler boosting factor δ ≈ 1-5.

Multifrequency observations of the flaring quasar 1156+295

A report is presented on the optically violent variable quasar 1156+295, known also as 4C 29.45 and Ton 599. A large outburst of this quasar was discovered in April 1981 in the course of a program to obtain simultaneous multifrequency spectra of variable quasars. Ultraviolet observations taken with the International Ultraviolet Explorer satellite were coordinated with ground-based observations at radio, infrared, and optical wavelengths. Measurements were made at four epochs starting immediately after the outburst was discovered, when the B-magnitude was 14.0, and at intervals of 4 days, 60 days and 1 year. The luminosity integrated only over observed wavelength bands was approximately 3 x 10 to the 48th ergs/sec on the first epoch of observation. Modeling of the source with a synchrotron self-Compton model suggests that the core of the source has a linear dimension of 0.01 pc, a magnetic field strength in the range 0.1-30 gauss, and a bulk relativistic motion in the quasar rest frame characterized by a Lorentz factor in the range 2-8.

Variability of Lyman-alpha and the ultraviolet continuum of 3C 446

IUE observations have been conducted over the 1230-3175 A range for the violently variable quasar 3C 446, beginning in June 1980, at intervals of 1.2, 2.2, 0.5, and 0.4 yr. Strong absorption of the continuum was found below 1830 A, probably corresponding to a Lyman edge at z of 1.00 + or - 0.01. The absence of Mg II 2798 A absorption implies that the column density is in the lower end of the range, unless the gas is metal-poor. The Lyman-alpha emission line was detected in five spectra; relative to the number of ionizing protons, the line strengths are the same as in normal quasars, and line equivalent widths are small due to the continuums rise redward of 912 A, which is much steeper than in normal quasars. The Lyman-alpha line and the nearby continuum vary so as to maintain constant equivalent width.

The ultraviolet spectra of intermediate-redshift quasars

This paper completes the analysis of an IUE survey of intermediate-redshift QSOs for which the results on Ly-alpha and continuum properties were given previously. The weaker lines were measured and are discussed in the context of an extended sample including Seyfert galaxies and high-redshift QSOs. The EW(C IV) anticorrelates with the luminosity of the continuum at 1450 A, showing the well-known Baldwin effect. The slopes of the two relations differ slightly, which can be explained by a decrease in the ionization parameter with increasing luminosity. Weak lines of Ly-beta + O VI, N V, and semiforbidden Si IV + O IV were measured in improved spectra for two of the sample objects, 1100 + 772 and 2201 + 315. The Ly-alpha line in both objects is highly symmetrical, with N V contribution of 5 percent or less. The line shapes were fitted with a two-component profile; 1100 + 772 has a distinct narrow component which contributes 20-25 percent of the Ly-alpha flux. 39 references.

Detection of Lyman continuum absorption in the BL Lacertae object PKS 0735+178

The detection of the Lyman edge in the BL Lac object PKS 0735+178 at the absorption red shift determined by optical measurements leads to a lower limit for the column density of atomic hydrogen, N(H I) not less than 4(17)/sq cm. The Lyman-alpha absorption line appears to have been detected, but only an approximate upper limit can be obtained from the data, of the order of 2(19)/sq cm. This amount of atomic hydrogen is less than that for a line of sight through the disk of a normal spiral galaxy. It is suggested that the absorbing material exists either in the halo of a galaxy or in the tenuous, extended, gaseous disk of a galaxy.