8,927 research outputs found
On characterising the variability properties of X-ray light curves from active galaxies
We review some practical aspects of measuring the amplitude of variability in
`red noise' light curves typical of those from Active Galactic Nuclei (AGN).
The quantities commonly used to estimate the variability amplitude in AGN light
curves, such as the fractional rms variability amplitude, F_var, and excess
variance, sigma_XS^2, are examined. Their statistical properties, relationship
to the power spectrum, and uses for investigating the nature of the variability
processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var)
shows large changes from one part of the light curve to the next, even when the
variability is produced by a stationary process. This limits the usefulness of
these estimators for quantifying differences in variability amplitude between
different sources or from epoch to epoch in one source. Some examples of the
expected scatter in the variance are tabulated for various typical power
spectral shapes, based on Monte Carlo simulations. The excess variance can be
useful for comparing the variability amplitudes of light curves in different
energy bands from the same observation. Monte Carlo simulations are used to
derive a description of the uncertainty in the amplitude expected between
different energy bands (due to measurement errors). Finally, these estimators
are used to demonstrate some variability properties of the bright Seyfert 1
galaxy Markarian 766. The source is found to show a strong, linear correlation
between rms amplitude and flux, and to show significant spectral variability.Comment: 14 pages. 12 figures. Accepted for publication in MNRA
X-ray spectral complexity in narrow-line Seyfert 1 galaxies
We present a systematic analysis of the X-ray spectral properties of a sample
of 22 ``narrow-line'' Seyfert 1 galaxies for which data are available from the
ASCA public archive. Many of these sources, which were selected on the basis of
their relatively narrow H-beta line width (FWHM <= 2000 km/s), show significant
spectral complexity in the X-ray band. Their measured hard power-law continua
have photon indices spanning the range 1.6 - 2.5 with a mean of 2.1, which is
only slightly steeper than the norm for ``broad-line'' Seyfert 1s. All but four
of the sources exhibit a soft excess, which can be modelled as blackbody
emission (T_{bb} ~ 100 - 300 eV) superposed on the underlying power-law. This
soft component is often so strong that, even in the relatively hard bandpass of
ASCA, it contains a significant fraction, if not the bulk, of the X-ray
luminosity, apparently ruling out models in which the soft excess is produced
entirely through reprocessing of the hard continuum.
Most notably, 6 of the 22 objects show evidence for a broad absorption
feature centred in the energy range 1.1 - 1.4 keV, which could be the signature
of resonance absorption in highly ionized material. A further 3 sources exhibit
``warm absorption'' edges in the 0.7 - 0.9 keV bandpass. Remarkably, all 9
``absorbed'' sources have H-beta line widths below 1000 km/s, which is less
than the median value for the sample taken as a whole. This tendency for very
narrow line widths to correlate with the presence of ionized absorption
features in the soft X-ray spectra of NLS1s, if confirmed in larger samples,
may provide a further clue in the puzzle of active galactic nuclei.Comment: 13 pages, 5 figures, accepted for publication in MNRA
A complete sample of Seyfert galaxies selected at 1/4 keV
We have used the ROSAT Bright Source Catalogue to extract a complete sample
of sources selected in the band from 0.1-0.4 keV. This 1/4 keV-selected sample
is comprised of 54 Seyfert galaxies, 25 BL Lacertae objects, 4 clusters and 27
Galactic stars or binaries. Seyfert-type galaxies with ``ultrasoft'' X-ray
spectra can very often be classed optically as Narrow-line Seyfert 1s (NLS1s).
Such objects are readily detected in 1/4 keV surveys; the sample reported here
contains 20 NLS1s, corresponding to a 40% fraction of the Seyferts. Optical
spectra of the Seyfert galaxies were gathered for correlative analysis, which
confirmed the well-known relations between X-ray slope and optical spectral
properties (e.g., [O III]/H-beta ratio; Fe II strength, H-beta width). The
various intercorrelations are most likely driven, fundamentally, by the shape
of the photoionising continuum in Seyfert nuclei. We argue that a steep X-ray
spectrum is a better indicator of an ``extreme'' set of physical properties in
Seyfert galaxies than is the narrowness of the optical H-beta line. (Abridged)Comment: 17 pages, 4 figures, accepted for publication in MNRA
Decameter: Wave radio observations of Jupiter during the 1970 apparition
Observations of Jupiter's sporadic decameter wavelength radio emissions were obtained between November 1978 and March 1979. A multistation, global network of monitoring instruments were utilized in order to obtain nearly continuous, synoptic observations of the planet. Observations were obtained daily at frequencies of 16.7 and 22.2 MHz using five element Yagi antennas at each end of a two element interferometer
Locked oscillator phase modulator, appendix d final report
Design parameters for linear phase modulation of locked oscillato
Nuclear Magnetic Resonance Spectrum of the Triphenylcarbonium Ion
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71188/2/JCPSA6-34-4-1460-1.pd
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