Eight-meter-wavelength Transient Array (ETA)

Science Primer

The ETA is a radio telescope designed to observe the short duration radio pulse --- the radio transient --- that is expected to be produced by a number of high-energy astrophysical phenomena (exploding primordial black holes, gamma ray bursts, supernovae, and compact object mergers). The ETA will search for these transients during continuous observation of nearly all of the northern hemisphere of the sky. Each of these astrophysical phenomena are discussed below. Of course, new classes of objects may also be discovered.

This page is aimed at readers with an "intermediate" level of understanding of the concepts. "Beginners" may find it helpful to consult the glossary of terms for unfamiliar concepts and objects. Links to the glossary are scattered throughout this document. "Advanced" readers may find this document a useful summary of the science.

Table of Contents

Giant Pulses --- The Crab Pulsar

Pulsars typically emit pulses of roughly constant magnitude. However, a handful of pulsars are known to emit an occasional "giant pulse" (GP). The Crab Pulsar (PSR B0531+21) is the best known of these pulsars, with giant pulses of 10 to 1000 times the mean pulse intensity. Indeed, the Crab Pulsar was accidentally discovered, in the late 1960s, through the detection of its giant pulses at 81 MHz. Some of these giant pulses have been observed to have subpulses that last no longer than a few nanoseconds, indicating the emission region is no larger than a few nano-lightseconds in size (a few feet in size). At radio wavelengths, only the Sun appears brighter than the Crab pulsar during one of these "nano-giant pulses." We expect the ETA will be able to detect the Crab's giant pulses, providing a useful diagnostic for the system. Of course, we may also detect giant pulses from other pulsars. These extreme emission events currently defy explanation; any additional observations of giant pulses could be useful in pinning down possible models.

Exploding Primordial Black Holes

"Primordial" black holes of a range of sizes may be produced as a by-product of the density fluctuations in the Big Bang. If black holes evaporate as suggested by specific combinations of general relativity and quantum mechanics, then those primordial black holes with masses below about 10^14 g will evaporate in about 10^10 years. There may be as many as 10^23 of these black holes in our Galaxy alone. The evaporation process quickens as the mass of the black hole decreases and the process terminates in an explosion releasing 10^30 erg or more of energy in much less than 1 second. The resulting relativistic expansion of charged particles could interact with the ambient interstellar magnetic field to create an electromagnetic pulse of length approximately 1s that could be detected in the radio spectrum. Given a reasonable value of about 10^5 for the Lorentz factor of the relativistically expanding fireball, the resulting radio pulse could be detectable at wavelengths on the order of 1 m by a single nondirectional antenna up to a distance of about 10^4 parsecs (10^4 pc). The 12-antenna ETA may be able to detect such a pulse from an exploding primordial black hole in our Galaxy. A discovery would be enormously significant as a test of our current understanding of quantum and gravitational physics. However, even a nondetection with improved limits established on the rate of explosions would be useful in further constraining the spectrum of density fluctuations in the early universe.

Gamma-Ray Bursts

Gamma-ray bursts have been observed, but these enigmatic objects are yet unexplained. These short duration events are undoubtedly due to high-energy events. Fading optical emission and even radio emission has been observed from such events, but prompt radio emission from these events would be very useful in pinning down the physics of the bursts, the nature of the progenitor object, and possibly the medium in which it occurs. If these phenomena occur at large redshifts, there is the possibility that the observations could probe the Epoch of Reionization, or the intergalactic medium. A number of models have been proposed to explain the gamma-ray bursts, ranging from compact object mergers (see below), to maser-like coherent emission. These models are not well constrained by current observations.


As for primordial black holes, the violent expansion of charged particles, from a supernova, into the ambient magnitude field would produce a pulse that might be detectable by the ETA. Approximately one supernova event per century is expected in a galaxy.

Compact Object Mergers

Binary star systems consisting of closely separated compact objects such as neutron stars and/or black holes will eventually merge as a result of the emission of gravitational radiation (and due to any other energy loss). Such mergers will potentially produce a burst of emission that could be detected in the radio spectrum. Several mergers involving a pair of neutron stars are expected per year within 5 Megaparsecs. Mergers of a neutron star and black hole are expected to be more common, but the resulting pulses are expected to be weaker. The various model parameters assumed in these calculations are very poorly constrained, and that any number of surprises could result in pulses which are significantly stronger than projected.

Sensitivity of the ETA

(An "advanced" section)

The 12 dual-polarization dipole antennas have a total effective collecting area of A = 476 square-meters at the zenith. The system temperature T is dominated by galactic emission (T is about 9000K, at the observing frequency of 38 MHz). For the full bandpass of df = 18MHz sampled at time intervals of dt = 66 microseconds, and adding both polarizations, the standard radiometer equation yields an rms response of

rms = T / sqrt (2*12*df*dt) = 9000K / sqrt(2*12*18MHz*66microseconds) = 53K,

where "sqrt" means "take the square root."

We may, therefore, estimate the expected signal-to-noise ratio for the Crab giant pulses. For one such estimate, we take the flux density of about S = 100 Jy measured for the Crab at 74 MHz by Rickett and Seiradakis (1982 Astrophysical Journal, 256, 612). The flux density may be higher at 38 MHz, or lower, depending on where the turnover in the Crab spectrum occurs. For a giant pulse of 10 times this flux density, the antenna temperature Ta would be given by

Ta = 10 S A / k = 350 K,

where k is Boltzmann's constant, and both polarizations are being added. Thus the expected signal-to-noise ratio would be Ta/rms = 7. Flux densities up to 1000 times larger than the average flux would yield a signal-to-noise ratio of up to 700.

In reality, the pulse will be spread out over a time width of the order of a second by interstellar scattering, which means smoothing and sampling at dt=1s would be more appropriate, with a reduced rms and larger signal-to-noise ratio. The ETA design allows for reconfigurable approaches to sampling, etc.

NSF Acknowledgment and Disclaimer

This material is based upon work supported by the National Science Foundation under Grant No. AST-0504677. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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