2,735 research outputs found
The First Spectroscopically Resolved Sub-parsec Orbit of a Supermassive Binary Black Hole
One of the most intriguing scenarios proposed to explain how active galactic
nuclei are triggered involves the existence of a supermassive binary black hole
system in their cores. Here we present an observational evidence for the first
spectroscopically resolved sub-parsec orbit of a such system in the core of
Seyfert galaxy NGC 4151. Using a method similar to those typically applied for
spectroscopic binary stars we obtained radial velocity curves of the
supermassive binary system, from which we calculated orbital elements and made
estimates about the masses of components. Our analysis shows that periodic
variations in the light and radial velocity curves can be accounted for an
eccentric, sub-parsec Keplerian orbit of a 15.9-year period. The flux maximum
in the lightcurve correspond to the approaching phase of a secondary component
towards the observer. According to the obtained results we speculate that the
periodic variations in the observed H{\alpha} line shape and flux are due to
shock waves generated by the supersonic motion of the components through the
surrounding medium. Given the large observational effort needed to reveal this
spectroscopically resolved binary orbital motion we suggest that many such
systems may exist in similar objects even if they are hard to find. Detecting
more of them will provide us with insight into black hole mass growth process.Comment: 29 pages, 10 figures, published in ApJ, 759, 11
The sensing and perception subsystem of the NASA research telerobot
A useful space telerobot for on-orbit assembly, maintenance, and repair tasks must have a sensing and perception subsystem which can provide the locations, orientations, and velocities of all relevant objects in the work environment. This function must be accomplished with sufficient speed and accuracy to permit effective grappling and manipulation. Appropriate symbolic names must be attached to each object for use by higher-level planning algorithms. Sensor data and inferences must be presented to the remote human operator in a way that is both comprehensible in ensuring safe autonomous operation and useful for direct teleoperation. Research at JPL toward these objectives is described
Black hole mass estimates in quasars - A comparative analysis of high- and low-ionization lines
The inter-line comparison between high- and low-ionization emission lines has
yielded a wealth of information on the quasar broad line region (BLR) structure
and dynamics, including perhaps the earliest unambiguous evidence in favor of a
disk + wind structure in radio-quiet quasars. We carried out an analysis of the
CIV 1549 and Hbeta line profiles of 28 Hamburg-ESO high luminosity quasars and
of 48 low-z, low luminosity sources in order to test whether the
high-ionization line CIV 1549 width could be correlated with Hbeta and be used
as a virial broadening estimator. We analyze intermediate- to high-S/N,
moderate resolution optical and NIR spectra covering the redshifted CIV and
H over a broad range of luminosity log L ~ 44 - 48.5 [erg/s] and
redshift (0 - 3), following an approach based on the quasar main sequence. The
present analysis indicates that the line width of CIV 1549 is not immediately
offering a virial broadening estimator equivalent to H. At the same time
a virialized part of the BLR appears to be preserved even at the highest
luminosities. We suggest a correction to FWHM(CIV) for Eddington ratio (using
the CIV blueshift as a proxy) and luminosity effects that can be applied over
more than four dex in luminosity. Great care should be used in estimating
high-L black hole masses from CIV 1549 line width. However, once corrected
FWHM(CIV) values are used, a CIV-based scaling law can yield unbiased MBH
values with respect to the ones based on H with sample standard
deviation ~ 0.3 dex.Comment: 43 pages, 15 Figures, submitted to A&
Quasars: from the Physics of Line Formation to Cosmology
Quasars accreting matter at very high rates (known as extreme Population A
[xA] or super-Eddington accreting massive black holes) provide a new class of
distance indicators covering cosmic epochs from the present-day Universe up to
less than 1 Gyr from the Big Bang. The very high accretion rate makes it
possible that massive black holes hosted in xA quasars radiate at a stable,
extreme luminosity-to-mass ratio. This in turns translates into stable physical
and dynamical conditions of the mildly ionized gas in the quasar low-ionization
line emitting region. In this contribution, we analyze the main optical and UV
spectral properties of extreme Population A quasars that make them easily
identifiable in large spectroscopic surveys at low-z (z < 1) and intermediate-z
(2 < z < 2.6), and the physical conditions that are derived for the formation
of their emission lines. Ultimately, the analysis supports the possibility of
identifying a virial broadening estimator from low-ionization line widths, and
the conceptual validity of the redshift-independent luminosity estimates based
on virial broadening for a known luminosity-to-mass ratio.Comment: 13 pages, 5 figures. Invited lecture at SPIG 2018, Belgrade. To
appear in Ato
Detailed Analysis of Balmer Lines in a Sloan Digital Sky Survey Sample of 90 Broad Line Active Galactic Nuclei
In order to contribute to the general effort aiming at the improvement of our
knowledge about the physical conditions within the Broad Line Region (BLR) of
Active Galactic Nuclei (AGN), here we present the results achieved by our
analysis of the spectral properties of a sample of 90 broad line emitting
sources, collected at the Sloan Digital Sky Survey (SDSS) database. By focusing
our attention mainly onto the Balmer series of hydrogen emission lines, which
is the dominant feature in the optical wavelength range of many BLR spectra, we
extracted several flux and profile measurements, which we related to other
source properties, such as optical continuum luminosities, inferred black hole
masses, and accretion rates. Using the Boltzmann Plot method to investigate the
Balmer line flux ratios as a function of the line profiles, we found that
broader line emitting AGN typically have larger H_alpha / H_beta and smaller
H_gamma / H_beta and H_delta / H_beta line ratios. With the help of some recent
investigations, we model the structure of the BLR and we study the influence of
the accretion process on the properties of the BLR plasma.Comment: 14 pages, 11 figures, fixes the wrong names of 4 objects; published
on Ap
An ASCA Study of the W51 Complex
We present the analysis of ASCA archival data from the Galactic source W51.
The ASCA spectra show that the soft (kT<= 2.5 keV) X-rays are of thermal origin
and are compatible with W51C being a single, isothermal (kT~0.3 keV) supernova
remnant at the far-side of the Sagittarius arm. The ASCA images reveal hard
(kT>=2.5 keV) X-ray sources which were not seen in previous X-ray observations.
Some of these sources are coincident with massive star-forming regions and the
spectra are used to derive X-ray parameters. By comparing the X-ray absorbing
column density with atomic hydrogen column density, we infer the location of
star-forming regions relative to molecular clouds. There are unidentified hard
X-ray sources superposed on the supernova remnant and we discuss the
possibility of their association.Comment: 13 pages, 11 figures, to be published in Astronomical Journa
The Usefulness of the Source Images of Magnetic Resonance Angiogram in the Carotid Cavernous Fistula
THE VISIBILITY AND INVISIBILITY OF HERDERS' KRAALS IN SOUTHERN AFRICA, WITH REFERENCE TO A POSSIBLE EARLY CONTACT PERIOD KHOEKHOE KRAAL AT KFS 5, WESTERN CAPE
The Europeans who landed on the shores of the South African Cape from the late 15th century onwards encountered local herders whom they later referred to as the Hottentots (now known as the Khoekhoe). There are written references to the settlements and livestock of these pastoralists, but archaeologists have not had much success in discovering any such sites. This absence of archaeological evidence for recent Khoekhoe kraals has been interpreted by some scholars as an indication for a general archaeological invisibility of nomadic pastoralist sites. This article reports on the archaeology of an extensive, low density surface spread of artefacts, KFS 5 (Western Cape), which possibly represents a Khoekhoe kraal dating to the time of the first contact with Europeans. Data are compared to other archaeological evidence of cattle pens in southern Africa and the issues of the visibility of prehistoric and historic kraals are re-addresse
Narrow-line Seyfert 1s: what is wrong in a name?
Narrow-line Seyfert 1s (NLSy1s) are an ill-defined class. Work done over the
past 20 years as well as recent analyses show a continuity in properties (e.g.,
Balmer line profiles, blueshifts of high-ionization lines) between sources with
FWHM above and below 2000 km/s, the defining boundary of NLSy1s. This finding
alone suggests that comparisons between samples of NLSy1s and rest of
broad-line AGNs are most likely biased. NLSy1s can be properly contextualized
by their location on the quasar main sequence originally defined by Sulentic et
al 2000. At one end, NLSy1s encompass sources with strong FeII emission and
associated with high Eddington ratio that hold the promise of becoming useful
distance indicators; at the other end, at least some of them are sources with
broad profiles seen face-on. Any rigid FWHM limit gives rise to some physical
ambiguity, as the FWHM of low-ionization lines depends in a complex way on
mass, Eddington ratio, orientation, and luminosity. In addition, if the scaling
derived from luminosity and virial dynamics applies to the broad line regions,
NLSy1s at luminosity higher than 1E47 erg/s become physically impossible.
Therefore, in a broader context, a proper subdivision of two distinct classes
of AGNs and quasars may be achieved by the distinction between Pop. A and B
with boundary at = 4000 km/s in samples at z < 1, or on the basis of
spectrophotometric properties which may ultimately be related to differences in
accretion modes if high-luminosity quasars are considered.Comment: Contribution presented at the meeting Revisiting narrow-line Seyfert
1 galaxies and their place in the Universe, held at the Padova Botanical
Garden, Italy, on 9-13 April 201
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