skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: The excess radio background and fast radio transients

Abstract

In the last few years ARCADE 2, combined with older experiments, has detected an additional radio background, measured as a temperature and ranging in frequency from 22 MHz to 10 GHz, not accounted for by known radio sources and the cosmic microwave background. One type of source which has not been considered in the radio background is that of fast transients (those with event times much less than the observing time). We present a simple estimate, and a more detailed calculation, for the contribution of radio transients to the diffuse background. As a timely example, we estimate the contribution from the recently-discovered fast radio bursts (FRBs). Although their contribution is likely 6 or 7 orders of magnitude too small (though there are large uncertainties in FRB parameters) to account for the ARCADE 2 excess, our development is general and so can be applied to any fast transient sources, discovered or yet to be discovered. We estimate parameter values necessary for transient sources to noticeably contribute to the radio background.

Authors:
; ;  [1]
  1. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 (United States)
Publication Date:
OSTI Identifier:
22525227
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Cosmology and Astroparticle Physics; Journal Volume: 2015; Journal Issue: 10; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; COSMIC RADIO SOURCES; GHZ RANGE; MHZ RANGE; RELICT RADIATION; STELLAR ACTIVITY; TRANSIENTS

Citation Formats

Kehayias, John, Kephart, Thomas W., and Weiler, Thomas J., E-mail: john.kehayias@vanderbilt.edu, E-mail: thomas.w.kephart@vanderbilt.edu, E-mail: tom.weiler@vanderbilt.edu. The excess radio background and fast radio transients. United States: N. p., 2015. Web. doi:10.1088/1475-7516/2015/10/053.
Kehayias, John, Kephart, Thomas W., & Weiler, Thomas J., E-mail: john.kehayias@vanderbilt.edu, E-mail: thomas.w.kephart@vanderbilt.edu, E-mail: tom.weiler@vanderbilt.edu. The excess radio background and fast radio transients. United States. doi:10.1088/1475-7516/2015/10/053.
Kehayias, John, Kephart, Thomas W., and Weiler, Thomas J., E-mail: john.kehayias@vanderbilt.edu, E-mail: thomas.w.kephart@vanderbilt.edu, E-mail: tom.weiler@vanderbilt.edu. Thu . "The excess radio background and fast radio transients". United States. doi:10.1088/1475-7516/2015/10/053.
@article{osti_22525227,
title = {The excess radio background and fast radio transients},
author = {Kehayias, John and Kephart, Thomas W. and Weiler, Thomas J., E-mail: john.kehayias@vanderbilt.edu, E-mail: thomas.w.kephart@vanderbilt.edu, E-mail: tom.weiler@vanderbilt.edu},
abstractNote = {In the last few years ARCADE 2, combined with older experiments, has detected an additional radio background, measured as a temperature and ranging in frequency from 22 MHz to 10 GHz, not accounted for by known radio sources and the cosmic microwave background. One type of source which has not been considered in the radio background is that of fast transients (those with event times much less than the observing time). We present a simple estimate, and a more detailed calculation, for the contribution of radio transients to the diffuse background. As a timely example, we estimate the contribution from the recently-discovered fast radio bursts (FRBs). Although their contribution is likely 6 or 7 orders of magnitude too small (though there are large uncertainties in FRB parameters) to account for the ARCADE 2 excess, our development is general and so can be applied to any fast transient sources, discovered or yet to be discovered. We estimate parameter values necessary for transient sources to noticeably contribute to the radio background.},
doi = {10.1088/1475-7516/2015/10/053},
journal = {Journal of Cosmology and Astroparticle Physics},
number = 10,
volume = 2015,
place = {United States},
year = {Thu Oct 01 00:00:00 EDT 2015},
month = {Thu Oct 01 00:00:00 EDT 2015}
}
  • Recently, the ARCADE 2 experiment measured the cosmic radio background (CRB) and found the brightness temperature of the CRB at 1.4 GHz to be {approx}480 mK. Integrating the flux density from the observed 1.4 GHz radio source count produces a brightness temperature of {approx}100 mK-less than a quarter of the observed CRB at 1.4 GHz. Radio-quiet active galactic nuclei (AGNs) are a large fraction of the 1.4 GHz {mu}Jy sources and typically host significant star formation. Thus, it is possible that AGN and host star formation could be responsible for some fraction of the excess CRB at 1.4 GHz. Here,more » an X-ray background population synthesis model is used in conjunction with empirical radio to X-ray luminosity ratios to calculate the AGN contribution to the CRB at 1.4 GHz, including the emission from host star formation. It is found that AGN and host star formation contribute {approx}<9% of the CRB at 1.4 GHz. When all known 1.4 GHz radio source classes are considered, {approx}<60% of the CRB at 1.4 GHz is accounted for; therefore, it is necessary that either known radio sources evolve significantly at flux densities below current survey sensitivity limits or a new population of low flux density radio sources exist.« less
  • Analyses of the distribution of absolute brightness temperature over the radio sky have recently led to suggestions that there exists a substantial unexplained extragalactic radio background. Consequently, there have been numerous attempts to place constraints on plausible origins of this 'excess'. We suggest here that this expectation of a large extragalactic background, over and above that contributed by the sources observed in the surveys, is based on an extremely simple geometry adopted to model the Galactic emission and the procedure adopted in the estimation of the extragalactic contribution. In this paper, we derive the extragalactic radio background from wide-field radiomore » images using a more realistic modeling of the Galactic emission and decompose the sky maps at 150, 408, and 1420 MHz into anisotropic Galactic and isotropic extragalactic components. The anisotropic Galactic component is assumed to arise from a highly flattened spheroid representing the thick disk, embedded in a spherical halo, both centered at the Galactic center, along with Galactic sources, filamentary structures, and Galactic loops and spurs. All components are constrained to be positive and the optimization scheme minimizes the sky area occupied by the complex filaments. We show that in contrast with simple modeling of Galactic emission as a plane parallel slab, the more realistic modeling yields estimates for the uniform extragalactic brightness that are consistent with expectations from known extragalactic radio source populations.« less
  • Most black holes (BHs) will absorb a neutron star (NS) companion fully intact without tidal disruption, suggesting the pair will remain dark to telescopes. Even without tidal disruption, electromagnetic (EM) luminosity is generated from the battery phase of the binary when the BH interacts with the NS magnetic field. Originally, the luminosity was expected to be in high-energy X-rays or gamma-rays, however, we conjecture that some of the battery power is emitted in the radio bandwidth. While the luminosity and timescale are suggestive of fast radio bursts (FRBs; millisecond-scale radio transients) NS–BH coalescence rates are too low to make thesemore » a primary FRB source. Instead, we propose that the transients form a FRB sub-population, distinguishable by a double peak with a precursor. The rapid ramp-up in luminosity manifests as a precursor to the burst which is 20%–80% as luminous given 0.5 ms timing resolution. The main burst arises from the peak luminosity before the merger. The post-merger burst follows from the NS magnetic field migration to the BH, causing a shock. NS–BH pairs are especially desirable for ground-based gravitational wave (GW) observatories since the pair might not otherwise be detected, with EM counterparts greatly augmenting the scientific leverage beyond the GW signal. The EM signal’s ability to break degeneracies in the parameters encoded in the GW and probe the NS magnetic field strength is quite valuable, yielding insights into open problems in NS magnetic field decay.« less
  • We relate the underlying properties of a population of fast radio-emitting transient events to its expected detection rate in a survey of finite sensitivity. The distribution of the distances of the detected events is determined in terms of the population luminosity distribution and survey parameters, for both extragalactic and Galactic populations. The detection rate as a function of Galactic position is examined to identify regions that optimize survey efficiency in a survey whose field of view is limited. The impact of temporal smearing caused by scattering in the interstellar medium has a large and direction-dependent bearing on the detection ofmore » impulsive signals, and we present a model for the effects of scattering on the detection rate. We show that the detection rate scales as {Omega}S{sup -3/2+{delta}}{sub 0}, where {Omega} is the field of view, S{sub 0} is the minimum detectable flux density, and 0 < {delta} {<=} 3/2 for a survey of Galactic transients in which interstellar scattering or the finite volume of the Galaxy is important. We derive formal conditions on the optimal survey strategy to adopt under different circumstances for fast transient surveys on next generation large-element, wide-field arrays, such as ASKAP, LOFAR, the MWA, and the SKA, and show how interstellar scattering and the finite spatial extent of a Galactic population modify the choice of optimal strategy.« less
  • Recent discoveries of dispersed, non-periodic impulsive radio signals with single-dish radio telescopes have sparked significant interest in exploring the relatively uncharted space of fast transient radio signals. Here we describe V-FASTR, an experiment to perform a blind search for fast transient radio signals using the Very Long Baseline Array (VLBA). The experiment runs entirely in a commensal mode, alongside normal VLBA observations and operations. It is made possible by the features and flexibility of the DiFX software correlator that is used to process VLBA data. Using the VLBA for this type of experiment offers significant advantages over single-dish experiments, includingmore » a larger field of view, the ability to easily distinguish local radio-frequency interference from real signals, and the possibility to localize detected events on the sky to milliarcsecond accuracy. We describe our software pipeline, which accepts short integration ({approx} ms) spectrometer data from each antenna in real time during correlation and performs an incoherent dedispersion separately for each antenna, over a range of trial dispersion measures. The dedispersed data are processed by a sophisticated detector and candidate events are recorded. At the end of the correlation, small snippets of the raw data at the time of the events are stored for further analysis. We present the results of our event detection pipeline from some test observations of the pulsars B0329+54 and B0531+21 (the Crab pulsar).« less