499 research outputs found
When two become one: an apparent QSO pair turns out to be a single quasar
We report on our serendipitous discovery that the objects Q 01323-4037 and Q
0132-4037, listed in the V\'eron-Cetty & V\'eron catalog (2006) as two
different quasars, are actually a quasar and a star. We briefly discuss the
origin of the misidentification, and provide a refined measurement of the
quasar redshift.Comment: 3 pages, 2 figures; accepted for publication in A&
VLBI observations of weak extragalactic radio sources for the alignment of the future GAIA frame with the ICRF
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between the optical and radio
positions, it will be important to align the GAIA frame and the International
Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is
found that only 10% of the ICRF sources are suitable to establish this link,
either because they are not bright enough at optical wavelengths or because
they have significant extended radio emission which precludes reaching the
highest astrometric accuracy. In order to improve the situation, we have
initiated a VLBI survey dedicated to finding additional high-quality radio
sources for aligning the two frames. The sample consists of about 450 sources,
typically 20 times weaker than the current ICRF sources, which have been
selected by cross-correlating optical and radio catalogues. This paper presents
the observing strategy and includes preliminary results of observation of 224
of these sources with the European VLBI Network in June 2007
Uncertainties of the masses of black holes and Eddington ratios in AGN
Black hole masses in Active Galactic Nuclei have been determined in 35
objects through reverberation mapping of the emission line region. I mention
some uncertainties of the method, such as the ``scale factor'' relating the
Virial Product to the mass, which depends on the unknown structure and dynamics
of the Broad Line Region.
When the black hole masses are estimated indirectly using the empirical
size-luminosity relation deduced from this method, the uncertainties can be
larger, especially when the relation is extrapolated to high and low masses
and/or luminosities. In particular they lead to Eddington ratios of the order
of unity in samples of Narrow Line Seyfert 1. As the optical-UV luminosity is
provided by the accretion disk, the accretion rates can be determined and are
found to be much larger than the Eddington rates. So, accretion must be
performed at a super-critical rate through a slim disk, resulting in rapid
growth of the black holes. The alternative is that the mass determination is
wrong at this limit.Comment: 5 pages, 2 figures. To appear in the proceedings of "Black Holes:
from Stars to Galaxies", IAU Symp. No. 238, V. Karas & G. Matt (eds.),
Cambridge University Pres
Radio Loud AGN in the 2XMMi catalogue
We are carrying out a search for all radio loud Active Galactic Nuclei
observed with XMM-Newton, including targeted and field sources to perform a
multi-wavelength study of these objects. We have cross-correlated the
Veron-Cetty & Veron (2010) catalogue with the XMM-Newton Serendipitous Source
Catalogue (2XMMi) and found about 4000 matched sources. A literature search
provided radio, optical, and X-ray data for 403 sources. Here we summarize the
first results of our study.Comment: Proceedings og the IAU275 Symposium: Jets at all scale
The emission line spectrum of the UV deficient quasar Ton 34: evidence of shock excitation?
Emission lines in quasars are believed to originate from a photoionized
plasma. There are, however, some emission features which appear to be
collisionally excited, such as the FeII multiplet bands. Shortward of Ly_alpha,
there also are a few permitted lines of species from low to intermediate
ionization. Ton 34 (z=1.928) exhibits the steepest far-UV continuum decline
known (Fnu propto nu^{-5.3}) shortward of 1050A. This object also emits
unusually strong low to intermediate excitation permitted lines shortward of
the Lyman limit. Using archive spectra of Ton 34 from HST, IUE and Palomar, we
measure the fluxes of all the lines present in the spectra and compare their
relative intensities with those observed in composite quasar spectra. Our
analysis reveals unusual strengths with respect to Ly_alpha of the following
low to intermediate excitation permitted lines: OII+OIII (835A), NIII+OIII
(686-703A) and NIII+NIV (765A). We compare the observed line spectrum with both
photoionization and shock models. Photoionization cannot reproduce the
strengths of these far-UV lines. Shocks with Vs ~ 100 km/s turn out to be
extremely efficient emitters of these lines and are favored as excitation
mechanism.Comment: 5 figures, accepted for publication in A&
The optical emission line spectrum of Mark 110
We analyse in detail the rich emission line spectrum of Mark 110 to determine
the physical conditions in the nucleus of this object, a peculiar NLS1 without
any detectable Fe II emission associated with the broad line region and with a
line ratio unusually large for a NLS1. We use 24 spectra
obtained with the Marcario Low Resolution Spectrograph attached at the prime
focus of the 9.2 m Hobby-Eberly telescope at the McDonald observatory. We
fitted the spectrum by identifying all the emission lines (about 220) detected
in the wavelength range 4200-6900 \AA (at rest). The narrow emission lines are
probably produced in a region with a density gradient in the range
10 cm with a rather high column density (5
cm). In addition to a narrow line system, three major broad line systems
with different line velocity and width are required. We confirm the absence of
broad Fe II emission lines. We speculate that Mark 110 is in fact a BLS1 with
relatively "narrow" broad lines but with a BH mass large enough compared to its
luminosity to have a lower than Eddington luminosity.Comment: 13 pages, 5 figures, accepted by A&
Multi-wavelength study of the gravitational lens system RXS J1131-1231: III. Long slit spectroscopy: micro-lensing probes the QSO structure
(ABRIDGED)
Aims: We discuss and characterize micro-lensing among the 3 brightest lensed
images (A-B-C) of the gravitational lens system RXS J1131-1231 (a quadruply
imaged AGN) by means of long slit optical and NIR spectroscopy. Qualitative
constraints on the size of different emission regions are derived.
Methods: We decompose the spectra into their individual emission components
using a multi-component fitting approach. A complementary decomposition of the
spectra enables us to isolate the macro-lensed fraction of the spectra
independently of any spectral modelling.
Results: -1. The data support micro-lensing de-amplification of images A and
C. Not only is the continuum emission microlensed in those images but also a
fraction of the Broad Line emitting Region (BLR).-2. Micro-lensing of a very
broad component of MgII emission line suggests that the corresponding emission
occurs in a region more compact than the other components of the emission line.
-3. We find evidence that a large fraction of the FeII emission arises in the
outer parts of the BLR. We also find very compact emitting region in the ranges
3080-3540 A and 4630-4800 A that is likely associated with FeII. -4. The OIII
narrow emission line regions are partly spatially resolved. This enables us to
put a lower limit of 110h^{-1} pc on their intrinsic size. -5. Analysis of MgII
absorption found in the spectra indicates that the absorbing medium is
intrinsic to the quasar, has a covering factor of 20%, and is constituted of
small clouds homogeneously distributed in front of the continuum and BLRs. -6.
Two neighbour galaxies are detected at redshifts z=0.10 and z=0.289. These
galaxies are possible members of galaxy groups reported at those redshifts.Comment: Accepted by Astronomy and Astrophysics. Small modifications to match
the final versio
RX J1643.7+3402: a new bright cataclysmic variable
We report the discovery of a new bright (V12.6) cataclysmic variable star identified with the ROSAT X-ray source RX J1643.7+3402. Spectroscopic and photometric observations show it to be a novalike variable sharing some of the characteristics of the SW Sex sub-class of novalike CVs. The spectroscopic period may be either 2\fh575 or 2\fh885, within the period "gap." A photometric modulation with a probable period of 2\fh595 and an amplitude of 0.1 mag in V is present on most nights and could be either a "positive" or a "negative" superhump modulation (depending on the exact spectroscopic period), indicating the presence of a precessing accretion disk in this system. Rapid variations of 0.1 to 0.2 mag amplitude in V repeat with a time scale of 15 min
Decision table for classifying point sources based on FIRST and 2MASS databases
With the availability of multiwavelength, multiscale and multiepoch
astronomical catalogues, the number of features to describe astronomical
objects has increases. The better features we select to classify objects, the
higher the classification accuracy is. In this paper, we have used data sets of
stars and quasars from near infrared band and radio band. Then best-first
search method was applied to select features. For the data with selected
features, the algorithm of decision table was implemented. The classification
accuracy is more than 95.9%. As a result, the feature selection method improves
the effectiveness and efficiency of the classification method. Moreover the
result shows that decision table is robust and effective for discrimination of
celestial objects and used for preselecting quasar candidates for large survey
projects.Comment: 10 pages. accepted by Advances in Space Researc
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