The Nature of the Energy Source in LINER's

LINER's (low-ionization nuclear emission-line regions) are found in about 30% of all bright galaxies, including luminous infrared galaxies. They form a heterogeneous class powered by a variety of ionizing mechanisms such as low-luminosity AGNs (active galactic nuclei), starbursts, shocks, or any combination of these. In early-type spirals, LINER's are powered by a low-luminosity AGN, or by an AGN surrounded by circumnuclear star-forming regions. In luminous infrared galaxies, LINER's are powered by starbursts with associated wind-related extended shocks, and an AGN may play a minor role, if any. LINER's in some FR I radio galaxies show strong evidence for the presence of a massive central black hole, and there are indications for the existence of shocks in the nuclear disks of these galaxies. Yet, the dominant ionizing mechanism for LINER's in radio-quiet ellipticals and FR I host galaxies is still unclear. Multifrequency high spatial resolution imaging and spectroscopy are essential to discriminate among the different ionizing mechanisms present in LINER's

Phase-Dependent Observations of Intermediate Polars and The Broad Emission and Absorption Line Region in NGC 3516

The Intermediate Polars (IP's) constitute a class of Cataclysmic Variables (CV's), which are binary star systems in which mass is transferred from a late-type main-sequence star to a white dwarf via Roche lobe overflow. In the IP's, the inner accretion disk is evidently disrupted by the magnetic field of the white dwarf. High-temperature shocks at the white dwarf's magnetic poles (where accretion occurs) produce X-rays, which are reprocessed into photons over a broad energy band across the electromagnetic spectrum. Because the white dwarf typically spins rapidly (rotation periods a few 10s to a few 100s of seconds), the signals due to both the X-rays and reprocessed photons are pulsed. The shape of the spectrum of pulse amplitude as a function of wavelength yields information about both the temperature and size of the pulse-emitting region. It has been noted by several investigators that the optical pulsation amplitudes rise steeply toward short wavelengths. It is therefore a fair surmise that pulsation amplitudes peak in the ultraviolet. The scientific goal of this project was to observe a representative sample of IP's, using IUE, in search of the expected strong UV pulsations, and hence to further our understanding of pulse-producing mechanisms

A Detailed Comparison of Hubble Space TelescopeFaint Object Spectrograph andIUEUltraviolet Spectra of Selected Seyfert Nuclei

Despite the contributions of the Hubble Space Telescope Faint Object Spectrograph (FOS) to the archive of UV observations of active galactic nuclei, the vast majority of UV reference data were obtained using the International Ultraviolet Explorer (IUE) satellite. These data remain important since they provide historical information about the intensities of the UV continua and emission lines that is needed to constrain models of the active nucleus. A detailed comparison of the FOS and IUE data is critical to understanding how the measurable quantities depend on the individual instrumental calibrations, and how any conclusions derived from modeling the observations may vary depending on the source of the UV data. Rigorous comparison of FOS and IUE spectra have so far been performed only for spectrophotometric standard star observations that are acquired accurately and have high signal-to-noise ratios. We compare typical FOS spectra that were not acquired and observed with the strict regimen that is used for standard-star observations, especially in the pre-COSTAR era. All nonproprietary UV FOS spectrophotometric archival data for the Seyfert 1 galaxies Mrk 509, NGC 3783, and NGC 5548 that have near-simultaneous (within 24 hr) IUE observations are used in the analysis. These data demonstrate that the absolute photometric calibrations of the FOS and IUE agree within ~5% in absolute flux for two of the objects. For NGC 5548, the FOS and IUE flux data disagree by ~50% in the 1200-2000 Å region. In this object there may be evidence for flux nonlinearity of the IUE detector and a contribution from the host galaxy redward of 2800 Å. Cross-correlation of the FOS and IUE spectra reveals no zero-point wavelength shift larger than the IUE wavelength calibration errors. Comparison of line flux measurements from both the FOS and IUE spectra show that for strong emission lines (e.g., Lyα, C IV, and Mg II) the measured intensities always agree within 15%, while for moderately strong lines (e.g., N V, Si IV/O IV, He II, and C III]) the agreement is ~30% (1 σ). Weak lines (e.g., O I, C II, N IV], O III], and N III]) may not even be detected in the IUE spectra, and when they are detected the disagreement between the measured fluxes can be very large

BL Lac X-ray Spectra: simpler than we thought

We report results from {\it XMM-Newton} observations of thirteen X-ray bright BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The spectra are generally well fit by power-law models, with four objects having hard ($\alpha<1; F_\nu \propto \nu^{-\alpha}$) spectra that indicates synchrotron peaks at $>5$ keV. None of our spectra show line features, indicating that soft X-ray absorption ``notches'' must be rare amongst BL Lacs, rather than common or ubiquitous as had previously been asserted. We find significant curvature in most of the spectra. This curvature is almost certainly intrinsic, as it appears nearly constant from 0.5 to 6 keV, an observation which is inconsistent with the small columns seen in these sources.Comment: 4 pages, 1 figure; to be published in proceedings of the Cozumel meeting on "Multiwavelength Surveys for AGN", Cozumel 200

Below the Lyman Edge: UV Polarimetry of Quasars

The Lyman edge at 912 \AA is an important diagnostic region for studying quasi-stellar objects (QSOs). In particular, it reveals a great deal about the physical conditions within the atmospheres of accretion disks, a ubiquitous component of QSO theories. A robust prediction of accretion disk models is a significant polarization due to electron scattering just longward (in wavelength) of the Lyman edge because of the wavelength dependence of the Hydrogen absorption opacity. Observations of the Lyman edge regions of QSOs have shown scant evidence for the predicted features-few QSOs show the broad, partial Lyman edges expected to be common according to most theories, and none show the high polarizations expected longward of the Lyman edge. Still, polarization spectra of a small number of QSOs have shown a rising polarization (up to 20%) at wavelengths shortward of the Lyman edge. We have now doubled our sample of intermediate-redshift QSOs observed with the HST/FOS spectropolarimeter to determine the amount of polarization on both sides of the Lyman limit. For this new sample of six objects, polarizations are low and mostly consistent with zero below the Lyman edge. Another important result of the new data is that it strengthens the conclusion that quasars are generally not polarized significantly just longward of the Lyman edge at $\sim 1000 \AA. There is no significant statistical wavelength dependence to the polarization longward of the Lyman edge indicating that simple plane-parallel atmospheres with scattering-dominated opacity are not significant sources of UV flux in quasars.Comment: Accepted to Ap.J., 30 pages, 8 figure

X-ray Properties of the Weak Seyfert 1 Nucleus in NGC 4639

We obtained observations of NGC 4639 with ASCA in order to investigate its mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. (1995) have previously shown that the nucleus is a pointlike source in the ROSAT soft X-ray band. We detected in the 2-10 keV band a compact central source with a luminosity of 8.3E+40 erg/s. Comparison of the ASCA data with archival data taken with the Einstein and ROSAT satellites shows that the nucleus varies on timescales of months to years. The variability could be intrinsic, or it could be caused by variable absorption. More rapid variability, on a timescale of \~10^4 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is well described by a model consisting of a lightly absorbed (N_H = 7.3E+20 cm^-2) power law with a photon index of 1.68. We find no evidence for significant emission from a thermal plasma; if present, it can account for no more than 25% of the flux in the 0.5-2.0 keV band. The limited photon statistics of our data do not allow us to place significant limits on the presence of iron K emission. (abridged)Comment: To appear in The Astrophysical Journal. LaTex, 18 pages including embedded figures and table

The Geometry and Kinematics of the Broad-Line Region in NGC 5548 from HST and IUE Observations

The spatial and radial velocity distribution of broad-line-emitting gas in the Seyfert 1 galaxy NGC 5548 is examined through the process of reverberation mapping, which is done by detailed comparison of continuum and emission-line variations. Recent spectroscopic monitoring of NGC 5548 with HST and IUE allows us to resolve the transfer function (TF) that relates the continuum and emission-line variability. We also examine the radial velocity-resolved TFs, and confirm that predominantly radial motions of the line-emitting clouds can be excluded. We find that a broad-line region comprised of clouds that are orbiting a central source of mass ~108 M☉ along randomly inclined Keplerian orbits and irradiated by a beamed continuum source yields a TF and line profile that are qualitatively consistent with the observations. In this model, the clouds that produce the variable C IV emission lie within 12 lt-days of the central source, and the continuum radiation is confined to a wide biconical beam (semi-opening angle 35°-60°) with the observer viewing into the cone

Intensive HST, RXTE and ASCA Monitoring of NGC 3516: Evidence Against Thermal Reprocessing

During 1998 April 13-16, NGC 3516 was monitored almost continuously with HST for 10.3 hr in the UV and 2.8 d in the optical, and simultaneous RXTE and ASCA monitoring covered the same period. The X-rays were strongly variable with the soft (0.5-2 keV) showing stronger variations (~65% peak-to-peak) than the hard (2-10 keV; ~50% peak-to-peak). The optical continuum showed much smaller but highly significant variations: a slow ~2.5% rise followed by a faster ~3.5% decline. The short UV observation did not show significant variability. The soft and hard X-ray light curves were strongly correlated with no significant lag. Likewise, the optical continuum bands (3590 and 5510 A) were also strongly correlated with no measurable lag above limits of <0.15 d. However no significant correlation or simple relationship could be found for the optical and X-ray light curves. These results appear difficult to reconcile with previous reports of correlations between X-ray and optical variations and of measurable lags within the optical band for some other Seyfert 1s. These results also present serious problems for "reprocessing" models in which the X-ray source heats a stratified accretion disk which then reemits in the optical/ultraviolet: the synchronous variations within the optical would suggest that the emitting region is <0.3 lt-d across, while the lack of correlation between X-ray and optical variations would indicate, in the context of this model, that any reprocessing region must be >1 lt-d in size. It may be possible to resolve this conflict by invoking anisotropic emission or special geometry, but the most natural explanation appears to be that the bulk of the optical luminosity is generated by some other mechanism than reprocessing.Comment: 23 pages including 6 figures, accepted for publication in Ap