arXiv subjects

D. L. Jones

Publications and source records attributed to D. L. Jones.

At least 19 recordsLinked to original sources

arXiv2019

A two-step system for sound event localization and detection

Sound event detection and sound event localization requires different features from audio input signals. While sound event detection mainly relies on time-frequency patterns to distinguish different event classes, sound event localization uses magnitude or phase differences between microphones to estimate source directions. Therefore, we propose a two-step system to do sound event localization and detection. In the first step, we detect the sound events and estimate the directions-of-arrival separately. In the second step, we combine the results of the event detector and direction-of-arrival estimator together. The obtained results show a significant improvement over the baseline solution for sound event localization and detection in DCASE 2019 task 3 challenge. Using the evaluation dataset, the proposed system achieved an F1 score of 93.4% for sound event detection and an error of 5.4 degrees for direction-of-arrival estimation, while the winning solution achieved an F1 score of 94.7% and an angle error of 3.7 degrees respectively.

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arXiv2011

A Radio Observatory on the Lunar Surface for Solar studies (ROLSS)

By volume, more than 99% of the solar system has not been imaged at radio frequencies. Almost all of this space (the solar wind) can be traversed by fast electrons producing radio emissions at frequencies lower than the terrestrial ionospheric cutoff, which prevents observation from the ground. To date, radio astronomy-capable space missions consist of one or a few satellites, typically far from each other, which measure total power from the radio sources, but cannot produce images with useful angular resolution. To produce such images, we require arrays of antennas distributed over many wavelengths (hundreds of meters to kilometers) to permit aperture synthesis imaging. Such arrays could be free-flying arrays of microsatellites or antennas laid out on the lunar surface. In this white paper, we present the lunar option. If such an array were in place by 2020, it would provide context for observations during Solar Probe Plus perihelion passes. Studies of the lunar ionosphere's density and time variability are also important goals. This white paper applies to the Solar and Heliospheric Physics study panel.

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arXiv2010

VLBA Astrometric Observations of the Cassini Spacecraft at Saturn

The planetary ephemeris is an essential tool for interplanetary spacecraft navigation, studies of solar system dynamics (including, for example, barycenter corrections for pulsar timing ephemeredes), the prediction of occultations, and tests of general relativity. We are carrying out a series of astrometric VLBI observations of the Cassini spacecraft currently in orbit around Saturn, using the Very Long Baseline Array (VLBA). These observations provide positions for the center of mass of Saturn in the International Celestial Reference Frame (ICRF) with accuracies ~0.3 milli-arcsecond (1.5 nrad), or about 2 km at the average distance of Saturn. This paper reports results from eight observing epochs between 2006 October and 2009 April. These data are combined with two VLBA observations by other investigators in 2004 and a Cassini-based gravitational deflection measurement by Fomalont et al. in 2009 to constrain a new ephemeris (DE 422). The DE 422 post-fit residuals for Saturn with respect to the VLBA data are generally 0.2 mas, but additional observations are needed to improve the positions of all of our phase reference sources to this level. Over time we expect to be able to improve the accuracy of all three coordinates in the Saturn ephemeris (latitude, longitude, and range) by a factor of at least three. This will represent a significant improvement not just in the Saturn ephemeris but also in the link between the inner and outer solar system ephemeredes and in the link to the inertial ICRF.

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arXiv2010

The Commensal Real-time ASKAP Fast Transients (CRAFT) survey

We are developing a purely commensal survey experiment for fast (<5s) transient radio sources. Short-timescale transients are associated with the most energetic and brightest single events in the Universe. Our objective is to cover the enormous volume of transients parameter space made available by ASKAP, with an unprecedented combination of sensitivity and field of view. Fast timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields. In addition, the detection of extragalactic objects affords us an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. We outline here our approach to the considerable challenge involved in detecting fast transients, particularly the development of hardware fast enough to dedisperse and search the ASKAP data stream at or near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of the key technologies and survey modes proposed for high time resolution science with the SKA.

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arXiv2007

Radio Wavelength Observatories within the Exploration Architecture

Observations at radio wavelengths address key problems in astrophysics, astrobiology, and lunar structure including the first light in the Universe (the Epoch of Reionization), the presence of magnetic fields around extrasolar planets, particle acceleration mechanisms, and the structure of the lunar ionosphere. Moreover, achieving the performance needed to address these scientific questions demands observations at wavelengths longer than those that penetrate the Earth's ionosphere, observations in extremely "radio quiet" locations such as the Moon's far side, or both. We describe a series of lunar-based radio wavelength interferometers of increasing capability. The Radio Observatory for Lunar Sortie Science (ROLSS) is an array designed to be deployed during the first lunar sorties (or even before via robotic rovers) and addressing particle acceleration and the lunar ionosphere. Future arrays would be larger, more capable, and deployed as experience is gained in working on the lunar surface.

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arXiv2005

The Fastest Relativistic Jets: VLBA Observations of Blazars with Apparent Speeds Exceeding 25c

We have measured peak apparent speeds of 25.6 +/- 7.0c, 25.6 +/- 4.4c, and 28.2 +/- 6.6c in the jets of 0235+164, 0827+243, and 1406-076, respectively, based on six epochs of high-sensitivity VLBA observations at 22 and 43 GHz during 2002 and 2003 (H_{0}=71 km/(s Mpc), Omega_{m}=0.27, and Omega_{Lambda}=0.73). These blazars had been identified as potentially having apparent speeds exceeding 40c in an earlier VLBA survey of EGRET blazars by Jorstad et al. We therefore confirm (with high confidence in 0827+243, and lower confidence in 0235+164 and 1406-076) the presence of highly relativistic pattern speeds in these three jets, although not at the >40c levels reported by Jorstad et al. The lower limit to the bulk Lorentz factor implied by the observed apparent speeds is Gamma>25-30 in these three sources, if the pattern speeds are equal to or slower than the bulk flow speed.

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arXiv2005

On the dynamics of the ABDoradus system

We present an astrometric analysis of the binary systems ABDorA /ABDorC and ABDorBa / ABDorBb. These two systems of well-known late-type stars are gravitationally associated and they constitute the quadruple ABDoradus system. From the astrometric data available at different wavelengths, we report: (i) a determination of the orbit of ABDorC, the very low mass companion to ABDorA, which confirms the mass estimate of 0.090Msun reported in previous works; (ii) a measurement of the parallax of ABDorBa, which unambiguously confirms the long-suspected physical association between this star and ABDorA; and (iii) evidence of orbital motion of ABDorBa around ABDorA, which places an upper bound of 0.4Msun on the mass of the pair ABDorBa / ABDorBb (50% probability). Further astrometric monitoring of the system at all possible wavelengths would determine with extraordinary precision the dynamical mass of its four components.

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arXiv2004

Disks, Tori, and Cocoons: Emission and Absorption Diagnostics of AGN Environments

One of the most important problems in the study of active galaxies is understanding the detailed geometry, physics, and evolution of the central engines and their environments. The leading models involve an accretion disk and torus structure around a central dense object, thought to be a supermassive black hole. Gas found in the environment of AGN is associated with different structures: molecular accretion disks, larger scale atomic tori, ionized and neutral "cocoons" in which the nuclear regions can be embedded. All of them can be studied at radio wavelengths by various means. Here, we summarize the work that has been done to date in the radio band to characterize these structures. Much has been learned about the central few parsecs of AGN in the last few decades with contemporary instruments but the picture remains incomplete. In order to be able to define a more accurate model of this region, significant advances in sensitivity, spectral and angular resolution, and bandpass stability are required. The necessary advances will only be provided by the Square Kilometer Array and we discuss the possibilities that these dramatic improvements will open for the study of the gas in the central region of AGN.

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arXiv2004

Spacecraft Tracking with the SKA

The possibility of using the SKA for occasional support of scientific space missions should be considered, along with all other science goals, in the design of the array. The benefits of higher data rates from distant spacecraft during high priority, short duration mission phases can be dramatic, while the technical requirements on the SKA to allow this capability are very few. The most fundamental requirement is coverage of the primary downlink frequencies for deep space missions. Additional benefits can result from real-time high precision angular position measurements of spacecraft. Such measurements allow spacecraft navigation with reduced errors and reduced risk, but require at least some long (thousands of km) baselines.

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arXiv2002

Searching for low mass objects around nearby dMe radio stars

Nearby M-dwarfs are best suited for searches of low mass companions. VLBI phase-referencing observations with sensitive telescopes are able to detect radio star flux-densities of tenths of mJy as well as to position the star on the sky with submilliarcsecond precision. We have initiated a long-term observational program, using EVN telescopes in combination with NASA DSN dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of the astrometry allows us to search for possible companions with masses down to 1 Jupiter mass. In this contribution we report preliminary results of the first observation epochs, in which we could detect some of the radio stars included in our program.

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arXiv2001

Space-VLBI phase-reference mapping and astrometry

We present 5 GHz space-VLBI observations of the quasar pair B1342+662/B1342+663 that demonstrate the feasibility of phase-reference techniques using an antenna in space. The space-based data were taken by the satellite HALCA, of the space-VLBI mission VSOP. From residual (referenced) phases we derive an upper bound of 10 meters to the uncertainty of the spacecraft orbit reconstruction. An analysis of the phase-reference maps of the sources additionally suggests that the above mentioned uncertaintyis likely not larger than 3 meters. With errors of this magnitude, HALCA is a useful tool for astrometric studies of close pairs of radio sources.

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arXiv2001

The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. I. The Images

We present images from a space-VLBI survey using the facilities of the VLBI Space Observatory Programme (VSOP), drawing our sample from the well-studied Pearson-Readhead survey of extragalactic radio sources. Our survey has taken advantage of long space-VLBI baselines and large arrays of ground antennas, such as the Very Long Baseline Array and European VLBI Network, to obtain high resolution images of 27 active galactic nuclei, and to measure the core brightness temperatures of these sources more accurately than is possible from the ground. A detailed analysis of the source properties is given in accompanying papers. We have also performed an extensive series of simulations to investigate the errors in VSOP images caused by the relatively large holes in the (u,v) plane when sources are observed near the orbit normal direction. We find that while the nominal dynamic range (defined as the ratio of map peak to off-source error) often exceeds 1000:1, the true dynamic range (map peak to on-source error) is only about 30:1 for relatively complex core-jet sources. For sources dominated by a strong point source, this value rises to approximately 100:1. We find the true dynamic range to be a relatively weak function of the difference in position angle (PA) between the jet PA and (u,v) coverage major axis PA. For low signal-to-noise regions typically located down the jet away from the core, large errors can occur, causing spurious features in VSOP images that should be interpreted with caution.

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arXiv2000

VSOP Observations of a Sub-Parsec Accretion Disk

The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption of background radio emission by ionized gas in the disk. We report high angular resolution observations of the nearby galaxy NGC 4261 which show evidence for free-free absorption by a thin, nearly edge-on disk at several frequencies. The angular width, and probably the depth, of the absorption appears to increase with decreasing frequency, as expected. Because free-free absorption is much larger at lower frequencies, the longest possible baselines are needed to provide adequate angular resolution; observing at higher frequencies to improve resolution will not help.

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arXiv1999

The Radio Jets and Accretion Disk in NGC 4261

The structure of AGN accretion disks on sub-parsec scales can be probed through free-free absorption of synchrotron emission from the base of symmetric radio jets. We present new VLBA observations of the nearby FR-I radio galaxy NGC 4261 at 22 and 43 GHz, and combine these with previous VLBA observations at 1.6 and 8.4 GHz to map absorption caused by an inner accretion disk. Assuming the disk is geometrically and optically thin and composed of a uniform 10^4 K plasma, the average electron density in the inner 0.1 pc is 10^3 - 10^8 cm^-3. Equating thermal gas pressure and magnetic field strength gives a disk magnetic field of 10^-4 - 10^-2 Gauss at 0.1 pc. The jet opening angle is between 0.3 and 20 degrees during the first 0.2 pc of the jet, and must be less than 5 degrees during the first 0.8 pc. We include an appendix containing expressions for a simple, optically thin, gas pressure dominated accretion disk model which may be applicable to other galaxies in addition to NGC 4261.

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arXiv1997

Deceleration in the Expansion of SN1993J

A rarity among supernova, SN 1993J in M81 can be studied with high spatial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structure was previously revealed to be shell-like and to be undergoing a self-similar expansion at a constant rate. From VLBI observations at the wavelengths of 3.6 and 6 cm in the period 6 to 42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the supernova ejecta and circumstellar material in standard supernova explosion models.

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arXiv1996

PKS 1830-211: A Possible Compound Gravitational Lens

Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211 (Jauncey et al., 1991), whilst obscured by our Galaxy at optical wavelengths, has recently been shown to contain absorption at the millimetre waveband at a redshift of 0.89 (Wiklind and Combes, 1996a). We report the detection of a new absorption feature, most likely due to neutral hydrogen in a second redshift system at z = 0.19. Follow-up VLBI observations have spatially resolved the absorption and reveal it to cover the NE compact component and part of the lower surface brightness ring. This new information, together with existing evidence of the unusual VLBI radio structure and difficulties in modeling the lensing system, points to the existence of a second lensing galaxy along our line of sight and implies that PKS 1830-211 may be a compound gravitational lens.

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arXiv1996

Size of the Vela Pulsar's Radio Emission Region: 500 km

We use interstellar scattering of the Vela pulsar to determine the size of its emission region. From interferometric phase variations on short baselines, we find that radio-wave scattering broadens the source by 3.4+/-0.3 milliarcseconds along the major axis at position angle 81+/-3 degrees. The ratio of minor axis to major axis is 0.51+/-0.03. Comparison of angular and temporal broadening indicates that the scattering material lies in the Vela-X supernova remnant surrounding the pulsar. From the modulation of the pulsar's scintillation on very short baselines, we infer a size of 500 km for the pulsar's emission region. We suggest that radio-wave refraction within the pulsar's magnetosphere may plausibly explain this size.

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arXiv1996

Interstellar Broadening of Images in the Gravitational Lens PKS 1830-211

The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 10E10 K, less than is typical for compact synchrotron radio sources and less than is implied by the short timescales of flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis. At 22 GHz the measured brightness temperature is at least 10E11 K, and the deconvolved size of the core in the southwest compact component is proportional to the wavelength squared between 1.3 cm (22 GHz) and 18 cm (1.7 GHz). VLBI observations at still higher frequencies should be unaffected by interstellar scattering.

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