693 research outputs found
All Transients, All the Time: Real-Time Radio Transient Detection with Interferometric Closure Quantities
We demonstrate a new technique for detecting radio transients based on
interferometric closure quantities. The technique uses the bispectrum, the
product of visibilities around a closed-loop of baselines of an interferometer.
The bispectrum is calibration independent, resistant to interference, and
computationally efficient, so it can be built into correlators for real-time
transient detection. Our technique could find celestial transients anywhere in
the field of view and localize them to arcsecond precision. At the Karl G.
Jansky Very Large Array (VLA), such a system would have a high survey speed and
a 5-sigma sensitivity of 38 mJy on 10 ms timescales with 1 GHz of bandwidth.
The ability to localize dispersed millisecond pulses to arcsecond precision in
large volumes of interferometer data has several unique science applications.
Localizing individual pulses from Galactic pulsars will help find X-ray
counterparts that define their physical properties, while finding host galaxies
of extragalactic transients will measure the electron density of the
intergalactic medium with a single dispersed pulse. Exoplanets and active stars
have distinct millisecond variability that can be used to identify them and
probe their magnetospheres. We use millisecond time scale visibilities from the
Allen Telescope Array (ATA) and VLA to show that the bispectrum can detect
dispersed pulses and reject local interference. The computational and data
efficiency of the bispectrum will help find transients on a range of time
scales with next-generation radio interferometers.Comment: Accepted to ApJ. 8 pages, 5 figures, 2 tables. Revised to include
discussion of non-Gaussian statistics of techniqu
The dynamics of critical Kauffman networks under asynchronous stochastic update
We show that the mean number of attractors in a critical Boolean network
under asynchronous stochastic update grows like a power law and that the mean
size of the attractors increases as a stretched exponential with the system
size. This is in strong contrast to the synchronous case, where the number of
attractors grows faster than any power law.Comment: submitted to PR
AMPS definition study on Optical Band Imager and Photometer System (OBIPS)
A study was conducted to define the characteristics of a modular optical diagnostic system (OBIPS) for AMPS, to provide input to Phase B studies, and to give information useful for experiment planning and design of other instrumentation. The system described consists of visual and UV-band imagers and visual and UV-band photometers; of these the imagers are most important because of their ability to measure intensity as a function of two spatial dimensions and time with high resolution. The various subsystems of OBIPS are in themselves modular with modules having a high degree of interchangeability for versatility, economy, and redundancy
Sporadic Long-term Variability in Radio Activity from a Brown Dwarf
Radio activity has been observed in a large variety of stellar objects,
including in the last few years, ultra-cool dwarfs. To explore the extent of
long-term radio activity in ultra-cool dwarfs, we use data taken over an
extended period of 9 hr from the Very Large Array of the source 2MASS
J05233822-1403022 in September 2006, plus data taken in 2004. The observation
taken in September 2006 failed to detect any radio activity at 8.46 GHz. A
closer inspection of earlier data reveals that the source varied from a null
detection on 3 May 2004, to 95 Jy on 17 May 2004, to 230 Jy
on 18 June 2004. The lack of detection in September 2006 suggests at least a
factor of ten flux variability at 8.46 GHz. Three short photometric runs did
not reveal any optical variability. In addition to the observed pulsing nature
of the radio flux from another ultra-cool source, the present observations
suggests that ultra-cool dwarfs may not just be pulsing but can also display
long-term sporadic variability in their levels of quiescent radio emission. The
lack of optical photometric variability suggests an absence of large-scale
spots at the time of the latest VLA observations, although small very high
latitude spots combined with a low inclination could cause very low amplitude
rotational modulation which may not be measurable. We discuss this large
variability in the radio emission within the context of both gyrosynchrotron
emission and the electron-cyclotron maser, favoring the latter mechanism.Comment: 7 pages, 2 figures, 1 table, accepted for publication in A&A Letter
The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science case, survey design and initial results
The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution (â2.â5), sensitivity (a 1Ï goal of 70 ÎŒJy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2â4 GHz). The first observations began in 2017 September, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hr of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (decl. > â40°), a total of 33 885 degÂČ. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations
We Could, but Should We? Ethical Considerations for Providing Access to GeoCities and Other Historical Digital Collections
We live in an era in which the ways that we can make sense of our past are evolving as more artifacts from that past become digital. At the same time, the responsibilities of traditional gatekeepers who have negotiated the ethics of historical data collection and use, such as librarians and archivists, are increasingly being sidelined by the system builders who decide whether and how to provide access to historical digital collections, often without sufficient reflection on the ethical issues at hand. It is our aim to better prepare system builders to grapple with these issues. This paper focuses discussions around one such digital collection from the dawn of the web, asking what sorts of analyses can and should be conducted on archival copies of the GeoCities web hosting platform that dates to 1994.This research was supported by the Natural Sciences and Engineering Research Council of Canada, the Social Sciences and Humanities Research Council of Canada, the US National Science Foundation (grants 1618695 and 1704369), the Andrew W. Mellon Foundation, Start Smart Labs, and Compute Canada
Rotational Velocities of Individual Components in Very Low Mass Binaries
We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s^(â1)), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349â25B and HD 130948C, are rotating at ~30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes âŸ3.5 AU, have component v sin i values that differ by greater than 3Ï. To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948AâBC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569AâBab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349â25AB and 2MASS 0746+20AB, are also known radio sources
No bursts detected from FRB121102 in two 5-hour observing campaigns with the Robert C. Byrd Green Bank Telescope
Here, we report non-detection of radio bursts from Fast Radio Burst FRB
121102 during two 5-hour observation sessions on the Robert C. Byrd 100-m Green
Bank Telescope in West Virginia, USA, on December 11, 2017, and January 12,
2018. In addition, we report non-detection during an abutting 10-hour
observation with the Kunming 40-m telescope in China, which commenced UTC 10:00
January 12, 2018. These are among the longest published contiguous observations
of FRB 121102, and support the notion that FRB 121102 bursts are episodic.
These observations were part of a simultaneous optical and radio monitoring
campaign with the the Caltech HIgh- speed Multi-color CamERA (CHIMERA)
instrument on the Hale 5.1-m telescope.Comment: 1 table, Submitted to RN of AA
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