Luis C. Ho

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

The Type IC supernova 1994I in M51: detection of helium and spectral evolution

We present a series of spectra of SN 1994I in M51, starting 1 week prior to maximum brightness. The nebular phase began about 2 months after the explosion; together with the rapid decline of the optical light, this suggests that the ejected mass was small. Although lines of He I in the optical region are weak or absent, consistent with the Type Ic classification, we detect strong He I λ10830 absorption during the first month past maximum. Thus, if SN 1994I is a typical Type Ic supernova, the atmospheres of these objects cannot be completely devoid of helium. The emission-line widths are smaller than predicted by the model of Nomoto and coworkers, in which the iron core of a low-mass carbon-oxygen star collapses. They are, however, larger than in Type Ib supernovae.

Steps toward determination of the size and structure of the broad-line region in active nuclei. 7: Variability of the optical spectrum of NGC 5548 over years

We report on the results of a continuation of a large monitoring program of optical spectroscopy of the Seyfert 1 galaxy NGC 5548. The new observations presented here were obtained between 1990 December and 1992 October, and extend the existing database to nearly 1400 days, dating back to 1988 December. The continuum variations are generally smooth and well-resolved, except during the third year of this 4 year project, when the variations were apparently more rapid and of lower amplitude than observed at other times. The broad H(beta) emission line is found to vary in response to the continuum variations with a lag of about 18 days, but with some changes from year to year. The H(beta) transfer functions for each of the 4 yr and for the entire 4 yr database are derived by using a maximum entropy method.

NGC 4395: Evidence Against the Starburst Hypothesis for Seyfert 1 Nuclei and QSOsa

In a very provocative series of papers, Terlevich et al. (1992, and references therein) argue that the properties of active galactic nuclei (AGNs) can be explained by vigorous bursts of star formation in the high-metallicity nuclei of galaxies, especially those of early Hubble type. Specifically, Terlevich & Melnick show that the ultraviolet spectrum of a cluster of massive stars can resemble a power law of index ɑ = -1.5 (where f_v ∝ v^ɑ), as in classical AGNs. The ionizing continuum will therefore cause nearby gas to produce an emission-line spectrum similar to that of Type 2 Seyfert nuclei and low-ionization nuclear emission-line regions (LINERs). The starburst model is extended to include Seyfert 1 galaxies and radio-quiet QSOs by considering the effect of Type II (hydrogen-rich) supernovae evolving in a dense (n ≈ 10^7 cm-^3) circumstellar medium. These objects become strongly radiative while still expanding at velocities of several thousand km s^(-1). Empirical evidence for this hypothesis is provided by the observed spectra of some Type II supernovae, which do indeed superficially resemble those of Seyfert 1 nuclei Only one supernova per year suffices for an AGN with M_B ≈ -21 mag. A range of stellar masses and evolutionary phases is required, however, to explain the multiwavelength spectra of AGNs. For example, the high-excitation spectrum of the narrow-line region is produced by clouds of gas irradiated by hot, evolved Wolf-Rayet stars in the cluster. These stars could account for the smooth ultraviolet/optical continuum, and the associated red supergiants would produce the near-infrared Ca II absorption triplet often observed at least in Seyfert 2 nuclei. Heckman and Filippenko discuss the strengths and weaknesses of the starburst hypothesis for AGNs. Their general conclusion is that the scenario has severe problems for AGNs that have radio jets, show rapid X-ray variability, or emit a substantial fraction of their power at hard X-ray and gamma-ray energies. On the other hand, it might explain some Seyfert 2 nuclei and a subset of LINERs and perhaps those QSOs and Seyfert 1 nuclei lacking the above properties.