1,475 research outputs found
A direct view of the AGN powering IRAS12393+3520
We report the first direct X-ray evidence that an AGN is hidden in the center
of IRAS12393+3520. An ASCA observation of this target unveiled a bright (0.5-10
keV luminosity 3.9 x 10^42 erg/s) and variable source, with minimum observed
doubling/halving time scale comprised in the range 30-75 ks. A model composed
by a simple power-law, with photon index ~1.8 and an absorption edge, whose
threshold energy is consistent with K-shell photoionization of OVII, provides
an adequate fit of the spectrum. This suggests that we are observing the
emission from the nuclear region through a warm absorber of N_H a few
10^{21}/cm/cm. If it has internal dust with Galactic gas-to-dust ratio, it
could explain the lack of broad Hbeta emission, even in the episodic presence
of a broad Halpha emission line. Optical spectra obtained over several years
show indeed variations in the strength of this broad Halpha component. A
distribution of dusty, optically thick matter on spatial scales a few hundreds
parsec, which does not intercept the line of sight towards the nucleus, is
probably required to account simultaneously for the relative [OIII] luminosity
deficit in comparison to the X-rays. The high IR to X-ray luminosity ratio is
most likely due to intense star formation in the circumnuclear region.
IRAS12393+3520 might thus exhibit simultaneously nuclear activity and
remarkable star formation.Comment: 9 Latex pages, 8 figures, Accepted for publication in Astronomy &
Astrophysic
ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217: The most distant matter yet probed with X-ray spectroscopy
We report on a ROSAT PSPC observation of the highly-luminous z = 4.72
radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the
final days of the ROSAT satellite. The low-energy sensitivity of the PSPC
detector was employed to constrain the intrinsic X-ray absorption of the
currently most distant X-ray detected object. Here we present the detection of
significant soft X-ray absorption towards GB 1428+4217, making the absorbing
material the most distant matter yet probed with X-ray spectroscopy. X-ray
variability by 25+-8 per cent is detected on a timescale of 6500 s in the rest
frame. The X-ray variation requires an unusually high radiative efficiency of
at least 4.2, further supporting the blazar nature of the source.Comment: 6 pages incl. 6 figures, accepted for publication in Monthly Notice
Ray-tracing in pseudo-complex General Relativity
Motivated by possible observations of the black hole candidate in the center
of our galaxy and the galaxy M87, ray-tracing methods are applied to both
standard General Relativity (GR) and a recently proposed extension, the
pseudo-complex General Relativity (pc-GR). The correction terms due to the
investigated pc-GR model lead to slower orbital motions close to massive
objects. Also the concept of an innermost stable circular orbit (ISCO) is
modified for the pc-GR model, allowing particles to get closer to the central
object for most values of the spin parameter than in GR. Thus, the
accretion disk, surrounding a massive object, is brighter in pc-GR than in GR.
Iron K emission line profiles are also calculated as those are good
observables for regions of strong gravity. Differences between the two theories
are pointed out.Comment: revised versio
Spectropolarimetry of the Luminous Narrow-Line Seyfert Galaxies IRAS 20181-2244 and IRAS 13224-3809
We observed the narrow-line Seyfert 1 galaxies IRAS 20181-2244 and IRAS
13324-3809 with a new spectropolarimeter on the RC spectrograph at the CTIO 4m
telescope. Previously it had been suggested that IRAS 20181-2244 was a Type 2
QSO and thus might contain an obscured broad-line region which could be
detected by the presence of broad Balmer lines in the polarized flux. We found
the object to be polarized at about 2%, and constant with wavelength, (unlike
most narrow-line Seyfert 1s), but with no evidence of broad Balmer lines in
polarized flux. The spectropolarimetry indicates that the scattering material
is inside the BLR. IRAS 13224-3809, notable for its high variability in X-ray
and UV wavelengths, has a low polarization consistent with a Galactic
interstellar origin.Comment: 19 pages using (AASTEX) aaspp4.sty and 5 postscript figures To be
published in the Astrophysical Journa
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
Ultraviolet and optical properties of Narrow-Line Seyfert 1 galaxies
Narrow Line Seyfert 1 (NLS1) galaxies are remarkable for their extreme
continuum and emission line properties which are not well understood. New
results bearing on the spectroscopic characteristics of these objects are
presented here, with the aim of establishing their typical ultraviolet and
optical spectral behavior. We employ HST observations of 22 NLS1s, which
represent a substantial improvement over previous work in terms of data quality
and sample size. High signal-to-noise NLS1 composite spectra are constructed,
allowing accurate measurements of the continuum shape and the strengths,
ratios, and widths for lines, including weak features which are barely
identifiable in other Active Galactic Nuclei (AGN) composites. We find that the
NLS1 sources have redder UV-blue continua than those typically measured in
other quasars and Seyferts. Objects with UV line absorption show redder
spectra, suggesting that dust is important in modifying the continuum shapes.
The data also permit a detailed investigation of the previously proposed link
between NLS1s and z >~ 4 quasars. Direct comparison of their composite spectra,
as well as a Principal Component Analysis, suggest that high-z QSOs do not show
a strong preference toward NLS1 behavior.Comment: 23 pages (incl. 9 figures, 4 tables), to appear in The Publications
of the Astronomical Society of the Pacifi
Is There a Metallicity--Luminosity Relationship in AGN? The Case of Narrow-Line Seyfert 1 Galaxies
The well known relationship between metallicity and luminosity in AGN is
addressed by introducing new metallicity measurements (based on the method of
Hamann & Ferland 1993; HF93) for a sample of narrow-line Seyfert 1 (NLS1)
galaxies. Our new results, based on a sample of 162 AGN, including 9 NLS1s,
indicate that, while broad-line AGN trace a metallicity--luminosity power law
with an index of ~0.2, NLS1s deviate significantly from this relationship at
low luminosities. Adopting the HF93 method based on the N V/C IV line ratio, we
find that NLS1 metallicities are similar to those of some high-redshift
high-luminosity quasars. We also examined the N IV]/C IV line ratio and
compared it with N V/C IV in a sample of 30 sources including several NLS1s. We
find that the two do not give a consistent answer regarding the N/C abundance
ratio. This result is marginal because of the quality of the data. We suggest
two alternative explanations to these results: 1) The HF93
metallicity--luminosity dependence is not a simple two-parameter dependence and
there is an additional hidden variable in this relationship that has not yet
been discovered. The additional parameter may be the accretion rate, the age of
the central stellar cluster or, perhaps, something else. 2) The strong line
ratios involving N V 1240A suggested by HF93 are not adequate metallicity
indicators for NLS1s and perhaps also other AGN for reasons that are not yet
fully understood.Comment: 4 pages, 2 figures; Figure 2 and Tables 1 and 2 were revised. This
did not affect any of the main results of the Letter (see also the Erratum:
2002, ApJ, 569, L59
HST STIS Ultraviolet Spectral Evidence for Outflow in Extreme Narrow-line Seyfert 1 Galaxies: I. Data and Analysis
We present HST STIS observations of two extreme NLS1s, IRAS 13224-3809 and 1H
0707-495. The spectra are characterized by very blue continua, broad, strongly
blueshifted high-ionization lines (including \ion{C}{4} and \ion{N}{5}), and
narrow, symmetric intermediate- (including \ion{C}{3}], \ion{Si}{3}],
\ion{Al}{3}) and low-ionization (e.g., \ion{Mg}{2}) lines centered at their
rest wavelengths. The emission-line profiles suggest that the high-ionization
lines are produced in a wind, and the intermediate- and low-ionization lines
are produced in low-velocity gas associated with the accretion disk or base of
the wind. In this paper, we present the analysis of the spectra from these two
objects; in a companion paper we present photoionization analysis and a toy
dynamical model for the wind. The highly asymmetric profile of \ion{C}{4}
suggests that it is dominated by emission from the wind, so we develop a
template for the wind from the \ion{C}{4} line. We model the bright emission
lines in the spectra using a combination of this template, and a narrow,
symmetric line centered at the rest wavelength. We also analyzed a comparison
sample of HST spectra from 14 additional NLS1s, and construct a correlation
matrix of emission line and continuum properties. A number of strong
correlations were observed, including several involving the asymmetry of the
\ion{C}{4} line.Comment: 26 pages, 10 figures, accepted for publication in ApJ with no change
A gain-coefficient switched Alexandrite laser
We report on a gain-coefficient switched Alexandrite laser. An electro-optic
modulator is used to switch between high and low gain states by making use of
the polarization dependent gain of Alexandrite. In gain-coefficient switched
mode, the laser produces 85 ns pulses with a pulse energy of 240 mJ at a
repetition rate of 5 Hz.Comment: 8 pages, 5 figure
X-ray Observations of the Seyfert galaxy LB 1727 (1H 0419-577)
We discuss the properties of the Seyfert 1.5 galaxy LB 1727, also known as 1H
0419-577, from X-ray observations obtained by ASCA and ROSAT along with optical
observations from earlier epochs. ASCA shows only modest (< 20%) variations in
X-ray flux within or between the observations. In contrast, a daily monitoring
campaign over 1996 Jun - Sept by the ROSAT HRI instrument reveals the soft
X-ray (0.1-2 keV) flux to have increased by a factor ~3.
The 2 - 10 keV continuum can be parameterized as a power-law with a photon
index Gamma ~ 1.45-1.68 across ~0.7 - 11 keV in the rest-frame. We also report
the first detection of iron Kalpha line emission in this source. Simultaneous
ASCA and ROSAT data show the X-ray spectrum to steepen sharply at a rest-energy
\~0.75 keV, the spectrum below this energy can be parameterized as a power-law
of slope Gamma ~3.6. We show that LB 1727 is one of the few Seyferts for which
we can rule out the possibility that the presence of a warm absorber is solely
responsible for the spectral steepening in the soft X-ray regime. Consideration
of the overall spectral-energy-distribution for this source indicates the
presence of a pronounced XUV-bump visible in optical, ultraviolet and soft
X-ray data.Comment: 27 pages, 7 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journa
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