53 research outputs found
A Spectropolarimetric Atlas of Seyfert 1 Galaxies
We present optical spectropolarimetry of the nuclei of 36 Seyfert 1 galaxies,
obtained with the William Herschel and the Anglo-Australian Telescopes from
1996 to 1999. In 20 of these, the optical emission from the active nucleus is
intrinsically polarized. We have measured a significant level of polarization
in a further 7 objects but these may be heavily contaminated by Galactic
interstellar polarization. The intrinsically polarized Seyfert 1s exhibit a
variety of characteristics, with the average polarization ranging from < 0.5 to
5 per cent and many showing variations in both the degree and position angle of
polarization across the broad H alpha emission line. We identify a small group
of Seyfert 1s that exhibit polarization properties similar to those of Seyfert
2 galaxies in which polarized broad-lines have been discovered. These objects
represent direct observational evidence that a Seyfert 2-like far-field polar
scattering region is also present in Seyfert 1s. Several other objects have
features that can be explained in terms of equatorial scattering of line
emission from a rotating disk. We propose that much of the diversity in the
polarization properties of Seyfert galaxies can be understood in terms of a
model involving both equatorial and polar scattering, the relative importance
of the two geometries as sources of polarized light being determined
principally by the inclination of the system axis to the line-of-sight.Comment: Accepted for publication in MNRAS (28 pages, 25 figures
Variability of Fe II Emission Features in the Seyfert 1 Galaxy NGC 5548
We study the low-contrast Fe II emission blends in the ultraviolet
(1250--2200A) and optical (4000--6000A) spectra of the Seyfert 1 galaxy NGC
5548 and show that these features vary in flux and that these variations are
correlated with those of the optical continuum. The amplitude of variability of
the optical Fe II emission is 50% - 75% that of Hbeta and the ultraviolet Fe II
emission varies with an even larger amplitude than Hbeta. However, accurate
measurement of the flux in these blends proves to be very difficult even using
excellent Fe II templates to fit the spectra. We are able to constrain only
weakly the optical Fe II emission-line response timescale to a value less than
several weeks; this upper limit exceeds all the reliably measured emission-line
lags in this source so it is not particularly meaningful. Nevertheless, the
fact that the optical Fe II and continuum flux variations are correlated
indicates that line fluorescence in a photoionized plasma, rather than
collisional excitation, is responsible for the Fe II emission. The iron
emission templates are available upon request.Comment: 34 pages including 12 figures and 2 tables. Accepted for publication
by ApJ (tentatively in vol. 626 June 10, 2005
Optical Variability of Narrow-Line Seyfert 1 Galaxies
We present results of a broad-band photometric study of the optical
variability of six Narrow-Line Seyfert 1 (NLS1) galaxies observed at 172
epochs. We searched for microvariability on 33 nights. Strong evidence for
microvariability was found only for our lowest luminosity object, NGC 4051, on
one night. Weaker evidence suggests such variability on a few other nights for
two other objects, but the data are not as convincing. Intra-night variability
in NLS1s is thus concluded to be rare and of low amplitude. We give
illustrations of how variable image quality can produce spurious variability.
We find that for well-studied non-NLS1s there is a spread in the amplitude of
seasonal variability (i.e., in some years an AGN is more variable than in
others). We find that the means of the variability amplitudes of non-NLS1s over
several seasons vary from object to object (i.e., some AGNs are, on average,
more variable than others). NLS1s also show a spread in seasonal variabilities.
The best-studied NLS1, Ark 564, shows a range of amplitudes of variability from
season to season that is comparable to the range found in BLS1s, and in one
season Ark 564 was as variable as the most variable non-NLS1. The seasonal
amplitudes of variability for NLS1s are mostly in the lower half of the range
of non-BLS1 seasonal amplitudes, but the absence of a suitable control sample
makes a precise comparison difficult. However, on long timescales (weeks to
years), NLS1s as a class are not more variable than non-NLS1s. The extreme
variability seen in the X-rays was not seen in the optical. This has
consequences for the models of AGNs in general as well as NLS1s in particular.Comment: Astrophysical Journal in press (tentatively scheduled for Vol. 608,
June 2004 issue);47 pages; 19 figures. Electronic tables not included
(contact first author if needed
The OI Line Emission in Active Galactic Nuclei Revisited
UV, visible, and near-infrared spectroscopy is used to study the transitions
of neutral oxygen leading to the emission of broad OI 8446,
11287 and 1304 in Active Galactic Nuclei. From the strength
of the former two lines, contrary to the general belief, we found that in six
out of seven galaxies, L-beta fluorescence is not the only mechanism
responsible for the formation of these three lines. Because OI 13165
is almost reduced to noise level, continuum fluorescence is ruled out as an
additional excitation mechanism, but the presence of OI 7774 in one of
the objects suggests that collisional ionization may have an important role in
the formation of OI 8446. The usefulness of the OI lines as a reliable
reddening indicator for the broad line region is discussed. The values of
E(B-V) derived from the OI 8446 ratio agree with those
obtained using other reddening indicators. The observations point toward a
break in the one-to-one photon relation between OI 8446 and OI
1304, attributable to several destruction mechanisms that may affect
the latter line.Comment: 26 pages, 5 figures. Accepted for publication in Ap
An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zw 1
We present an empirical template spectrum suitable for fitting/subtracting
and studying the FeII and FeIII line emission in the restframe UV spectra of
active galatic nuclei (AGNs), the first empirical UV iron template to cover the
full 1250 - 3090 A range. Iron emission is often a severe contaminant in
optical--UV spectra of AGNs. Its presence complicates and limits the accuracy
of measurements of both strong and weak emission lines and the continuum
emission, affecting studies of line and continuum interrelations, the
ionization structure, and elemental abundances in AGNs. Despite the wealth of
work on modeling the AGN FeII emission and the need to account for it in
observed AGN spectra, there is no UV template electronically available to aid
this process. The iron template we present is based on HST spectra of the
Narrow Line Seyfert 1, IZw1. Its intrinsic narrow lines (~900 km/s) and rich
iron spectrum make the template particularly suitable for use with most AGN
spectra. The iron emission spectrum and the line identifications and
measurements are presented and compared with the work of Laor et al. We
illustrate the application of the derived FeII and FeIII templates by fitting
and subtracting the iron emission from UV spectra of four high-z quasars and of
the nearby quasar, 3C273. We briefly discuss the small discrepancies between
this observed iron emission and the UV template, and compare the template with
previously published ones. We discuss the advantages and limitations of the
templates and of the template fitting method. We conclude that the templates
work sufficiently well to be a valuable and important tool for eliminating and
studying the iron emission in AGNs, at least until accurate theoretical iron
emission models are developed. (Abridged)Comment: 73 pages including 7 figures, 6 tables. To appear in ApJS. Preprint
is also available at
http://www.astronomy.ohio-state.edu/~vester/IronEmission
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. II. Ultraviolet Continuum and Emission-line Variability
We present results of an intensive two-month campaign of approximately daily
spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with
HST. The fractional variability amplitude of the continuum variations between
1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1
galaxies over a similar period of time. However, large amplitude, short
time-scale flaring behavior is evident, with trough-to-peak flux changes of
about 18% in approximately 3 days. We present evidence for wavelength-dependent
continuum time delays, with the variations at 3000 A lagging behind those at
1365 A by about 1 day. These delays may be interpreted as evidence for a
stratified continuum reprocessing region, possibly an accretion-disk structure.
The Lyman-alpha 1216 emission-line exhibits flux variations of about 1%
amplitude.Comment: 27 pages, 14 figures. Accepted by Astrophysical Journa
Spectral Optical Monitoring of the Narrow Line Seyfert 1 galaxy Ark 564
We present the results of a long-term (1999--2010) spectral optical
monitoring campaign of the active galactic nucleus (AGN) Ark 564, which shows a
strong Fe II line emission in the optical. This AGN is a narrow line Seyfert 1
(NLS1) galaxies, a group of AGNs with specific spectral characteristics. We
analyze the light curves of the permitted Ha, Hb, optical Fe II line fluxes,
and the continuum flux in order to search for a time lag between them.
Additionally, in order to estimate the contribution of iron lines from
different multiplets, we fit the Hb and Fe II lines with a sum of Gaussian
components. We found that during the monitoring period the spectral variation
(F_max/F_min) of Ark 564 was between 1.5 for Ha to 1.8 for the Fe II lines. The
correlation between the Fe II and Hb flux variations is of higher significance
than that of Ha and Hb (whose correlation is almost absent). The permitted-line
profiles are Lorentzian-like, and did not change shape during the monitoring
period. We investigated, in detail, the optical Fe II emission and found
different degrees of correlation between the Fe II emission arising from
different spectral multiplets and the continuum flux. The relatively weak and
different degrees of correlations between permitted lines and continuum fluxes
indicate a rather complex source of ionization of the broad line emission
region.Comment: Accepted for publication in ApJ
Steps Toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XV. Long-Term Optical Monitoring of NGC 5548
We present the results of three years of ground-based observations of the
Seyfert 1 galaxy NGC 5548, which combined with previously reported data, yield
optical continuum and broad-line H-beta light curves for a total of eight
years. The light curves consist of over 800 points, with a typical spacing of a
few days between observations. During this eight-year period, the nuclear
continuum has varied by more than a factor of seven, and the H-beta emission
line has varied by a factor of nearly six. The H-beta emission line responds to
continuum variations with a time delay or lag of 10-20 days, the precise value
varying somewhat from year to year. We find some indications that the lag
varies with continuum flux in the sense that the lag is larger when the source
is brighter.Comment: 29 pages, 6 figures. Accepted for publication in ApJ (1999 Jan 10
Probing Spectroscopic Variability of Galaxies & Narrow-Line Active Galactic Nuclei in the Sloan Digital Sky Survey
Under the unified model for active galactic nuclei (AGNs), narrow-line (Type
2) AGNs are, in fact, broad-line (Type 1) AGNs but each with a heavily obscured
accretion disk. We would therefore expect the optical continuum emission from
Type 2 AGN to be composed mainly of stellar light and non-variable on the
time-scales of months to years. In this work we probe the spectroscopic
variability of galaxies and narrow-line AGNs using the multi-epoch data in the
Sloan Digital Sky Survey (SDSS) Data Release 6. The sample contains 18,435
sources for which there exist pairs of spectroscopic observations (with a
maximum separation in time of ~700 days) covering a wavelength range of
3900-8900 angstrom. To obtain a reliable repeatability measurement between each
spectral pair, we consider a number of techniques for spectrophotometric
calibration resulting in an improved spectrophotometric calibration of a factor
of two. From these data we find no obvious continuum and emission-line
variability in the narrow-line AGNs on average -- the spectroscopic variability
of the continuum is 0.07+/-0.26 mag in the g band and, for the emission-line
ratios log10([NII]/Halpha) and log10([OIII]/Hbeta), the variability is
0.02+/-0.03 dex and 0.06+/-0.08 dex, respectively. From the continuum
variability measurement we set an upper limit on the ratio between the flux of
varying spectral component, presumably related to AGN activities, and that of
host galaxy to be ~30%. We provide the corresponding upper limits for other
spectral classes, including those from the BPT diagram, eClass galaxy
classification, stars and quasars.Comment: AJ accepte
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