DARIS : a low-frequency distributed aperture array for radio astronomy in space

The frequency band below 30 MHz is one of the last unexplored bands in radio astronomy. This band is well suited for studying the early cosmos at high hydrogen redshifts, the so-called dark ages, extragalactic surveys, (extra) solar planetary bursts, and high energy particle physics. In addition, space research such as space weather tomography, are also areas of scientific interest. \ud \ud Due to ionospheric scintillation (below 30MHz) and its opaqueness (below 15MHz), earth-bound radio astronomy observations in these bands are either severely limited in sensitivity and spatial resolution or entirely impossible. A radio telescope in space obviously would not be hampered by the Earth's ionosphere. In the past, several (limited) studies have been conducted to explore possibilities for such an array in space. These studies considered aperture synthesis arrays in space, at the back-side of the Moon, or a satellite constellation operating in a coherent mode. \u

Using a satellite swarm for building a space-based radio telescope for low frequencies

In radio astronomy, as in astronomy in general, a wide range of frequencies is observed as each spectral band o_ers a unique window to study astrophysical phenomena. In the recent years, new observatories have been designed and built at the extreme limits of the radio spectrum. For the low frequencies several Earth-based radio telescopes are constructed at this moment. In the Netherlands, the Low Frequency Array (LOFAR) is being constructed at this moment and will be operational later this year. LOFAR observes the sky between 30 and 240 MHz. Observing at even lower frequencies is very interesting, but, due to the inuence of the Earth's ionosphere this is not possible from Earth. Thus, the only option to observe low frequencies is a telescope in spac

OLFAR - orbiting low frequency array; using a satellite swarm for building a space-based radio telescope for low frequencies

In radio astronomy, as in astronomy in general, a wide range of frequencies is observed as each spectral band offers a unique window to study astrophysical phenomena. In the recent years, new observatories have been designed and built at the extreme limits of the radio spectrum. For the low frequencies several Earth-based radio telescopes are constructed at this moment. In the Netherlands, the Low Frequency Array (LOFAR) is being constructed at this moment and will be operational later this year. LOFAR observes the sky between 30 and 240 MHz. Observing at even lower frequencies is very interesting, but, due to the influence of the Earth’s ionosphere this is not possible from Earth. Thus, the only option to observe low frequencies is a telescope in space

Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}

For describing the first-order isostructural valence phase transition in mixed valence compounds we develop a new approach based on the lattice Anderson model. We take into account the Coulomb interaction between localized f and conduction band electrons and two mechanisms of electron-lattice coupling. One is related to the volume dependence of the hybridization. The other is related to local deformations produced by f- shell size fluctuations accompanying valence fluctuations. The large f -state degeneracy allows us to use the 1/N expansion method. Within the model we develop a mean-field theory for the first-order valence phase transition in YbInCu_{4}. It is shown that the Coulomb interaction enhances the exchange interaction between f and conduction band electron spins and is the driving force of the phase transition. A comparison between the theoretical calculations and experimental measurements of the valence change, susceptibility, specific heat, entropy, elastic constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a good quantitative agreement is found. On the basis of the model we describe the evolution from the first-order valence phase transition to the continuous transition into the heavy-fermion ground state in the series of compounds YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.

Multiband GPI imaging of the HR 4796A debris disk

Stars and planetary system

Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background

The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58×10-8, Ω0V<6.35×10-8, and Ω0S<1.08×10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society

Erratum: "A Gravitational-wave Measurement of the Hubble Constant Following the Second Observing Run of Advanced LIGO and Virgo" (2021, ApJ, 909, 218)

[no abstract available

Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model

We present results from a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the J-statistic, and by analyzing data from Advanced LIGO's second observing run. In the frequency range searched, from 60 to 650 Hz, we find no evidence of gravitational radiation. At 194.6 Hz, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95% confidence) of h095%=3.47×10-25 when marginalizing over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering. © 2019 American Physical Society

Search for Gravitational Waves Associated with Gamma-Ray Bursts Detected by Fermi and Swift during the LIGO-Virgo Run O3b

We search for gravitational-wave signals associated with gamma-ray bursts (GRBs) detected by the Fermi and Swift satellites during the second half of the third observing run of Advanced LIGO and Advanced Virgo (2019 November 1 15:00 UTC-2020 March 27 17:00 UTC). We conduct two independent searches: A generic gravitational-wave transients search to analyze 86 GRBs and an analysis to target binary mergers with at least one neutron star as short GRB progenitors for 17 events. We find no significant evidence for gravitational-wave signals associated with any of these GRBs. A weighted binomial test of the combined results finds no evidence for subthreshold gravitational-wave signals associated with this GRB ensemble either. We use several source types and signal morphologies during the searches, resulting in lower bounds on the estimated distance to each GRB. Finally, we constrain the population of low-luminosity short GRBs using results from the first to the third observing runs of Advanced LIGO and Advanced Virgo. The resulting population is in accordance with the local binary neutron star merger rate. © 2022. The Author(s). Published by the American Astronomical Society