655 research outputs found
XMM Follow-Up Observations of Three Swift BAT-Selected Active Galactic Nuclei
We present XMM-Newton observations of three AGN taken as part of a hunt to
find very heavily obscured Compton-thick AGN. For obscuring columns greater
than 10^25 cm^-2, AGN are only visible at energies below 10 keV via
reflected/scattered radiation, characterized by a flat power-law. We therefore
selected three objects (ESO 417-G006, IRAS 05218-1212, and MCG -01-05-047) from
the Swift BAT hard X-ray survey catalog with Swift X-ray Telescope XRT 0.5-10
keV spectra with flat power-law indices as candidate Compton-thick sources for
follow-up observations with the more sensitive instruments on XMM-Newton. The
XMM spectra, however, rule out reflection-dominated models based on the
weakness of the observed Fe K-alpha lines. Instead, the spectra are well-fit by
a model of a power-law continuum obscured by a Compton-thin absorber, plus a
soft excess. This result is consistent with previous follow-up observations of
two other flat-spectrum BAT-detected AGN. Thus, out of the six AGN in the
22-month BAT catalog with apparently flat Swift XRT spectra, all five that have
had follow-up observations are not likely Compton-thick. We also present new
optical spectra of two of these objects, IRAS 05218-1212 and MCG -01-05-047.
Interestingly, though both these AGN have similar X-ray spectra, their optical
spectra are completely different, adding evidence against the simplest form of
the geometric unified model of AGN. IRAS 05218-1212 appears in the optical as a
Seyfert 1, despite the ~8.5x10^22 cm^-2 line-of-sight absorbing column
indicated by its X-ray spectrum. MCG -01-05-047's optical spectrum shows no
sign of AGN activity; it appears as a normal galaxy.Comment: 18 pages including 4 figures, accepted by Ap
A Multi-Wavelength Study of the Nature of Type 1.8/1.9 Seyfert Galaxies
We focus on determining the underlying physical cause of a Seyfert galaxy's
appearance as type a 1.8 or 1.9. Are these "intermediate" Seyfert types typical
Seyfert 1 nuclei with reddened broad-line regions? Or are they objects with
intrinsically weak continua and broad emission lines? We compare measurements
of the optical reddening of the narrow and broad-line regions with each other
and with the X-ray column derived from XMM-Newton 0.5-10 keV spectra to
determine the presence and location of dust in the line of sight. We also
searched the literature to see if the objects showed evidence for broad-line
variability, and determined if the changes were consistent with a change in
reddening or a change in the intrinsic ionizing continuum flux. We find that 10
of 19 objects previously classified as Seyfert 1.8/1.9s received this
designation due to their low continuum flux. In four objects the classification
was due to BLR reddening, either by the torus or dust structures in the
vicinity of the NLR; in the remaining five objects there is not sufficient
evidence to favor one scenario over the other. These findings imply that, in
general, samples of 1.8/1.9s are not suitable for use in studies of the gas and
dust in the central torus.Comment: 85 pages, accepted by Ap
Long-Term Variability in the Optical Spectrum of the Seyfert Galaxy NGC 2992
New spectra of NGC 2992 from the Cerro Tololo Inter-American Observatory show
that this nearby AGN has changed its type classification to a Seyfert 2 in
2006. It was originally classified as a Seyfert 1.9, and has been previously
seen as a Seyfert 1.5 with strong broad Halpha emission. A comparison of the
reddening and equivalent hydrogen column density derived for the narrow-line
region from these new data with those previously calculated for different
regions closer to the nucleus shows them to be very similar, and suggests that
these different regions are all being absorbed by the same opacity source, a
large 100-pc scale dust lane running across the nucleus. However, obscuration
by dust in this lane is probably not responsible for classification changes
which occur in only a few years. It is more likely that NGC 2992's observed
variations are due to a highly variable ionizing continuum. We therefore
conclude that, although NGC 2992 was originally identified as a Seyfert 1.9,
this was not because of an oblique viewing angle through the atmosphere of a
central dusty torus, but because its active nucleus was identified when it was
in a low continuum state.Comment: To be published in the Astronomical Journal. 19 pages, with 5 figure
The First IRAM/PdBI Polarimetric Millimeter Survey of Active Galactic Nuclei. I. Global Properties of the Sample
We have studied the linear polarization of 86 active galactic nuclei (AGN) in
the observed frequency range 80-267 GHz (3.7-1.1mm in wavelength),
corresponding to rest-frame frequencies 82-738 GHz, with the IRAM Plateau de
Bure Interferometer (PdBI). The large number of measurements, 441, makes our
analysis the largest polarimetric AGN survey in this frequency range to date.
We extracted polarization parameters via earth rotation polarimetry with
unprecedented median precisions of ~0.1% in polarization fractions and ~1.2
degrees in polarization angles. For 73 of 86 sources we detect polarization at
least once. The degrees of polarization are as high as ~19%, with the median
over all sources being ~4%. Source fluxes and polarizations are typically
highly variable, with fractional variabilities up to ~60%. We find that BLLac
sources have on average the highest level of polarization. There appears to be
no correlation between degree of polarization and redshift, indicating that
there has been no substantial change of polarization properties since z~2.4.
Our polarization and spectral index distributions are in good agreement with
results found from various samples observed at cm/radio wavelengths; thus our
frequency range is likely tracing the signature of synchrotron radiation
without noticeable contributions from other emission mechanisms. The
"millimeter-break" located at frequencies >1 THz appears to be not detectable
in the frequency range covered by our survey.Comment: 19 pages, 9 figures, 2 long tables (p. 12-19). Accepted by A&A
Evidence for a Long-Standing Top-Heavy IMF in the Central Parsec of the Galaxy
We classify 329 late-type giants within 1 parsec of Sgr A*, using the
adaptive optics integral field spectrometer SINFONI on the VLT. These
observations represent the deepest spectroscopic data set so far obtained for
the Galactic Center, reaching a 50% completeness threshold at the approximate
magnitude of the helium-burning red clump (Ks ~ 15.5 mag.). Combining our
spectroscopic results with NaCo H and Ks photometry, we construct an observed
Hertzsprung-Russell diagram, which we quantitatively compare to theoretical
distributions of various star formation histories of the inner Galaxy, using a
chi-squared analysis. Our best-fit model corresponds to continuous star
formation over the last 12 Gyr with a top-heavy initial mass function (IMF).
The similarity of this IMF to the IMF observed for the most recent epoch of
star formation is intriguing and perhaps suggests a connection between recent
star formation and the stars formed throughout the history of the Galactic
Center.Comment: 18 pages, 10 figures, Accepted to ApJ: 15 July 200
On the orientation of the Sagittarius A* system
The near-infrared emission from the black hole at the Galactic center (Sgr
A*) has unique properties. The most striking feature is a suggestive periodic
sub-structure that has been observed in a couple of flares so far. Using
near-infrared polarimetric observations and modelling the quasi-periodicity in
terms of an orbiting blob, we try to constrain the three dimensional
orientation of the Sgr A* system. We report on so far unpublished polarimetric
data from 2003. They support the observations of a roughly constant mean
polarization angle of 60 degr \pm 20 degr from 2004-2006. Prior investigations
of the 2006 data are deepened. In particular, the blob model fits are evaluated
such that constraints on the position angle of Sgr A* can be derived.
Confidence contours in the position-inclination angle plane are derived. On a
3sigma level the position angle of the equatorial plane normal is in the range
60 degr - 108 degr (east of north) in combination with a large inclination
angle. This agrees well with recent independent work in which radio
spectral/morphological properties of Sgr A* and X-ray observations,
respectively, have been used. However, the quality of the presently available
data and the uncertainties in our model bring some ambiguity to our
conclusions.Comment: 10 pages, 4 figures; Research Note accepted by A&A for publicatio
Thermodynamic Properties of the One-Dimensional Extended Quantum Compass Model in the Presence of a Transverse Field
The presence of a quantum critical point can significantly affect the
thermodynamic properties of a material at finite temperatures. This is
reflected, e.g., in the entropy landscape S(T; c) in the vicinity of a quantum
critical point, yielding particularly strong variations for varying the tuning
parameter c such as magnetic field. In this work we have studied the
thermodynamic properties of the quantum compass model in the presence of a
transverse field. The specific heat, entropy and cooling rate under an
adiabatic demagnetization process have been calculated. During an adiabatic
(de)magnetization process temperature drops in the vicinity of a field-induced
zero-temperature quantum phase transitions. However close to field-induced
quantum phase transitions we observe a large magnetocaloric effect
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