3,215 research outputs found
Wavelet transforms for non-uniform speech recognition
An algorithm for nonuniform speech segmentation and its application in speech recognition systems is presented. A method based on the Modulated Gaussian Wavelet Transform based Speech Analyser (MGWTSA) and the subsequent parametrization block is used to transform a uniform signal into a set of nonuniformly separated frames, with the accurate information being fed into a speech recognition system. The algorithm needs a frame characterizing the signal where necessary, trying to reduce the number of frames per signal as much as possible, without an appreciable reduction in the recognition rate of the system.Peer ReviewedPostprint (published version
Optical spectroscopy of the microquasar GRS 1758-258: a possible intermediate mass system?
Context. GRS 1758-258 is one of two prototypical microquasars towards the
Galactic Center direction discovered almost a quarter of a century ago. The
system remains poorly studied in the optical domain due to its counterpart
being a very faint and absorbed target in a crowded region of the sky. Aims.
Our aim is to investigate GRS 1758-258 in order to shed light on the nature of
the stellar binary components. In particular, the main physical parameters of
the donor star, such as the mass or the spectral type, are not yet well
constrained. Methods. GRS 1758-258 has remained so far elusive to optical
spectroscopy owing to its observational difficulties. Here, we use this
traditional tool of stellar astronomy at low spectral resolution with a 10 m
class telescope and a long slit spectrograph. Results. An improved spectrum is
obtained as compared to previous work. The quality of the data does not allow
the detection of emission or absorption features but, nevertheless, we manage
to partially achieve our aims comparing the de-reddened continuum with the
spectral energy distribution expected from an irradiated disc model and
different donor star templates. Conclusions. We tentatively propose that GRS
1758-258 does not host a giant star companion. Instead, a main sequence star
with mid-A spectral type appears to better agree with our data. The main
impacts of this finding are the possibility that we are dealing with an
intermediate mass system and, in this case, the prediction of an orbital period
significantly shorter than previously proposed.Comment: 5 pages, 6 figures, accepted for publication in A&
Excited electron-bubble states in superfluid helium-4: a time-dependent density functional approach
We present a systematic study on the excited electron-bubble states in
superfluid helium-4 using a time-dependent density functional approach. For the
evolution of the 1P bubble state, two different functionals accompanied with
two different time-development schemes are used, namely an accurate
finite-range functional for helium with an adiabatic approximation for electron
versus an efficient zero-range functional for helium with a real-time evolution
for electron. We make a detailed comparison between the quantitative results
obtained from the two methods, which allows us to employ with confidence the
optimal method for suitable problems. Based on this knowledge, we use the
finite-range functional to calculate the time-resolved absorption spectrum of
the 1P bubble, which in principle can be experimentally determined, and we use
the zero-range functional to real-time evolve the 2P bubble for several
hundreds of picoseconds, which is theoretically interesting due to the break
down of adiabaticity for this state. Our results discard the physical
realization of relaxed, metastable 2P electron-bubblesComment: 16 pages, 12 figure
Identification of the optical and near-infrared counterpart of GRS 1758-258
Context. Understood to be a microquasar in the Galactic center region, GRS
1758-258 has not yet been unambiguously identified to have an
optical/near-infrared counterpart, mainly because of the high absorption and
the historic lack of suitable astrometric stars, which led to the use of
secondary astrometric solutions. Although it is considered with 1E 1740.7-2942
as the prototypical microquasar in the Galactic center region, the Galactic
origin of both sources has not yet been confirmed. Aims. We attempt to improve
previous astrometry to identify a candidate counterpart to GRS 1758-258. We
present observations with the Gran Telescopio de Canarias (GTC), in which we
try to detect any powerful emission lines that would infer an extragalactic
origin of this source. Methods. We use modern star catalogues to reanalyze
archival images of the GRS 1758-258 field in the optical and near-infrared
wavelengths, and compute a new astrometric solution. We also reanalyzed
archival radio data of GRS 1758-258 to determine a new and more accurate radio
position. Results. Our improved astrometric solution for the GRS 1758-258 field
represents a significant advancement on previous works and allows us to
identify a single optical/near-infrared source, which we propose as the
counterpart of GRS 1758-258. The GTC spectrum of this source is however of low
signal-to-noise ratio and does not rule out a Galactic origin. Hence, new
spectral observations are required to confirm or discard a Galactic nature.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic
Real-time evolution of a large-scale relativistic jet
Context. Astrophysical jets are ubiquitous in the Universe on all scales, but
their large-scale dynamics and evolution in time are hard to observe since they
usually develop at a very slow pace.
Aims. We aim to obtain the first observational proof of the expected
large-scale evolution and interaction with the environment in an astrophysical
jet. Only jets from microquasars offer a chance to witness the real-time,
full-jet evolution within a human lifetime, since they combine a 'short', few
parsec length with relativistic velocities.
Methods. The methodology of this work is based on a systematic recalibraton
of interferometric radio observations of microquasars available in public
archives. In particular, radio observations of the microquasar GRS 1758-258
over less than two decades have provided the most striking results.
Results. Significant morphological variations in the extended jet structure
of GRS 1758-258 are reported here that were previously missed. Its northern
radio lobe underwent a major morphological variation that rendered the hotspot
undetectable in 2001 and reappeared again in the following years. The reported
changes confirm the Galactic nature of the source. We tentatively interpret
them in terms of the growth of instabilities in the jet flow. There is also
evidence of surrounding cocoon. These results can provide a testbed for models
accounting for the evolution of jets and their interaction with the
environment.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics Letter
The radio jets of SS 433 at millimetre wavelengths
Context. SS 433 is historically a well-known microquasar in the Galaxy that
has been deeply studied during the four decades elapsed since its discovery.
However, observations at very high radio frequencies with good angular
resolution are still very scarce in the literature. The present paper tries to
partially fill this gap using archival data of the source obtained with the
Atacama Large Millimeter Array (ALMA). Aims. We aim to study the SS 433 jet
properties at radio frequencies corresponding to millimetre wavelengths where
the synchrotron emitting particles are expected to lose their energy much
faster than at lower frequencies of centimetre wavelengths. Results. A resolved
view of the SS 433 radio core and jets is presented. In addition to spectral
index and magnetic field measurements, we are able to estimate the age of the
oldest visible ejecta still radiating significantly at millimetre wavelengths.
By combining our findings with those of previous authors at lower frequencies,
we confirm that the energy loss of the radiating electrons is dominated by
adiabatic expansion instead of synchrotron radiative losses. In addition, we
find suggestive evidence for the previously proposed period of slowed expansion
within the first months of the ejecta flow, needed to simultaneously match the
radiative lifetime observed in the centimetre domain. Our results argue for the
need for future coordinated millimetre and centimetre interferometric
observations with good time sampling throughout the SS 433 precessional cycle
to better understand energetic processes in stellar relativistic jets.Comment: 14 pages, 7 figures, Accepted for publication in Astronomy &
Astrophysic
The low-mass diskless population of Corona Australis
We combine published optical and near-infrared photometry to identify new low-mass candidate members in an area of about 0.64 deg^2 in Corona Australis with the S-parameter method. Five new candidate members of the region are selected. They have estimated ages between 3 and 15 Myr and masses between 0.05 and 0.15 M_⊙. With Spitzer photometry we confirm that these objects are not surrounded by optically thick disks. However, one of them is found to display excess at 24 μm, thus suggesting it harbors a disk with an inner hole. With an estimated mass of 0.07 M_⊙ according to the SED fitting, this is one of the lowest-mass objects reported to possess a transitional disk. Including these new members, the fraction of disks is about 50% among the total Corona Australis population selected by the same criteria, lower than the 70% fraction reported earlier for this region. Even so, we find a ratio of transitional to primordial disks (45%) very similar to the value derived by previous authors. This ratio is higher than for solar-type stars (5–10%), suggesting that disk evolution is faster in the latter, and/or that the “transitional disk” stage is not such a short-lived step for very low-mass objects. However, this impression needs to be confirmed with better statistics
Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: implications for its evolution and recent reactivation
We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on 'growing' the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years
A sensitive study of the peculiar jet structure HST-1 in M87
To obtain a better understanding of the location and mechanisms for the
production of the gamma-ray emission in jets of AGN we present a detailed study
of the HST-1 structure, 0.8 arcsec downstream the jet of M87, previously
identified as a possible candidate for TeV emission. HST-1 shows a very
peculiar structure with superluminal as well as possible stationary
sub-components, and appears to be located in the transition from a parabolic to
a conical jet shape, presumably leading to the formation of a recollimation
shock. This scenario is supported by our new RHD simulations in which the
interaction of a moving component with a recollimation shock leads to the
appearance of a new superluminal component. To discern whether HST-1 is
produced by a recollimation shock or some other MHD instability, we present new
polarimetric 2.2 and 5 GHz VLBA, as well as 15, 22 and 43 GHz JVLA observations
obtained between November 2012 and March 2013.Comment: 6 page
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