J. M. Hartman

The LWA1 Radio Telescope

LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instruments “all dipoles” modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and four independently steerable beams utilizing digital “true time delay” beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

Detection of Radio Emission from Fireballs

We present the findings from the Prototype All-Sky Imager, a back end correlator of the first station of the Long Wavelength Array, which has recorded over 11,000 hr of all-sky images at frequencies between 25 and 75 MHz. In a search of this data for radio transients, we have found 49 long-duration (10 s of seconds) transients. Ten of these transients correlate both spatially and temporally with large meteors (fireballs), and their signatures suggest that fireballs emit a previously undiscovered low frequency, non-thermal pulse. This emission provides a new probe into the physics of meteors and identifies a new form of naturally occurring radio transient foreground.

Monitoring the Sky with the Prototype All-Sky Imager on the LWA1

We present a description of the Prototype All-Sky Imager (PASI), a backend correlator and imager of the first station of the Long Wavelength Array (LWA1). PASI cross-correlates a live stream of 260 dual-polarization dipole antennas of the LWA1, creates all-sky images, and uploads them to the LWA-TV website in near real time. PASI has recorded over 13,000hr of all-sky images at frequencies between 10 and 88MHz creating opportunities for new research and discoveries. We also report rate density and pulse energy density limits on transients at 38, 52, and 74MHz, for pulse widths of 5s. We limit transients at those frequencies with pulse energy densities of >2.7×10−23, >1.1×10−23, and >2.8×10−23Jm−2Hz−1 to have rate densities <1.2×10−4, <5.6×10−4, and <7.2×10−4 year−1deg−2.

Limits on Gamma-Ray Burst Prompt Radio Emission Using the LWA1

As a backend to the first station of the Long Wavelength Array (LWA1), the Prototype All Sky Imager has been imaging the sky > –26° declination during 34 gamma-ray bursts (GRBs) between 2012 January and 2013 May. Using this data, we were able to put the most stringent limits to date on prompt low-frequency emission from GRBs. While our limits depend on the zenith angle of the observed GRB, we estimate a 1σ rms sensitivity of 68, 65, and 70 Jy for 5 s integrations at 37.9, 52.0, and 74.0 MHz at zenith. These limits are relevant for pulses ≥5 s and are limited by dispersion smearing. For 5 s pulses, we are limited to dispersion measures (DMs) ≤ 220, 570, and 1600 pc cm–3 for the frequencies above. For pulses lasting longer than 5 s, the DM limits increase linearly with the duration of the pulse. We also report two interesting transients, which are, as of yet, of unknown origin and are not coincident with any known GRBs. For general transients, we give rate density limits of ≤7.5 × 10–3, 2.9 × 10–2, and 1.4 × 10–2 yr–1 deg–2 with pulse energy densities >1.3 × 10–22, 1.1 × 10–22, and 1.4 × 10–22 J m–2 Hz–1 and pulse widths of 5 s at the frequencies given above.