A Search for Astrophysical Radio Transients at Meter Wavelengths
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Astrophysical phenomena such as exploding primordial black holes (PBHs), gamma-ray bursts (GRBs), compact object mergers, and supernovae, are expected to produce a single pulse of electromagnetic radiation detectable at the low-frequency end of the radio spectrum. Detection of any of these pulses would be significant for the study of the objects themselves, their host environments, and the interstellar/intergalactic medium. Furthermore, a detection of a radio transient from an exploding PBH could be a signature of an extra spatial dimension, which would drastically alter our perception of spacetime. However, even upper limits on the existence of PBHs, from transient searches, would be important to discussions of cosmology. We describe a method to carry out an agnostic single-dispersed-pulse search, and apply it to data collected with the Eight-meter-wavelength Transient Array (ETA). ETA is a radio telescope dedicated to searching for transient pulses. It consists of 12 crossed-dipole antenna stands with Galactic-noise-limited performance from 29-47 MHz. There is a vast amount of data collected from an ETA observation. It is therefore greatly beneficial to use a computer cluster, which works in parallel on different parts of a data set, in order to carry out a single-pulse search quickly and efficiently. Each spectrogram in a data set needs to be analyzed individually, without reference to the rest, in order to utilize a computer cluster's capabilities. The data reduction software has been developed for single-dispersed-pulse searches, and is described in this thesis. The data reduction involves sweeping through the collected data with a dedispersion routine assuming a range of dispersion measures. The resulting time series are searched with multiple matched filters for signals above a signal-to-noise threshold. Applying the single pulse search to ~ 30 hours of ETA data did not yield a compelling detection of an astrophysical signal. However, from ~ 5 hours of interference-free data we find an observational upper limit to the rate of exploding PBHs of r – 4.8 Ã 10⁻⁷ pc⁻³ y⁻¹ for a PBH with a fireball Lorentz-factor f = 104.3. This limit is applicable to PBHs in the halo of the Galaxy to distances ~ 2 kpc, and dispersion measures ~ 80 pc cm⁻³ . We also find a source-agnostic rate limit ~ 0.25 events y⁻¹ deg⁻² for pulses of duration < 3 s, and having apparent energy densities â ¥ 2.6 Ã 10⁻²³ J m⁻² Hz⁻¹ at 38 MHz.
- Doctoral Dissertations