6 research outputs found
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
Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. 5: Variability of the ultraviolet continuum and emission lines of NGC 3783
We report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of 7 months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line fluxes. The light curves for the continuum fluxes exhibited two well-defined local minima or 'dips,' the first lasting is less than or approximately 20 days and the second is less than or approximately 4 days, with additional episodes of relatively rapid flickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became 'harder' when brighter. The variations in the continuum occurred simultaneously at all wavelengths (delta(t) is less than 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N V (which is relatively weak and badly blended with Ly(alpha), the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or 'lag.' As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are considerably shorter than those obtained for NGC 5548, with values of (formally) approximately 0 days for He II + O III), and approximately 4 days for Ly(alpha) and C IV. The data further suggest lags of approximately 4 days for Si IV + O IV) and 8-30 days for Si III + C III). Mg II lagged the 1460 A continuum by approximately 9 days, although this result depends on the method of measuring the line flux and may in fact be due to variability of the underlying Fe II lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over timescales of is less than or approximately 2 days, and that the character of the continuum variability may change with time
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