We report the results of a low frequency radio variability and slow transient search using archival observations from the Very Long Array. We selected six 325 MHz radio observations from the spring of 2006, each centered on the Spitzer-Space-Telescop
e Wide-area Infrared Extragalactic Survey (SWIRE) Deep Field: 1046+59. Observations were spaced between 1 day to 3 months, with a typical single-epoch peak flux sensitivity below 0.2 mjb near the field pointing center. We describe the observation parameters, data post-processing, and search methodology used to identify variable and transient emission. Our search revealed multiple variable sources and the presence of one, day-scale transient event with no apparent astronomical counterpart. This detection implies a transient rate of 1$pm$1 event per 6.5 $deg^2$ per 72 observing hours in the direction of 1046+59 and an isotropic transient surface density $Sigma = 0.12 deg^{-2}$ at 95% confidence for sources with average peak flux density higher than 2.1 mJy over 12 hr.
We present 325 MHz (90 cm wavelength) radio observations of ultracool dwarfs TVLM 513-46546 and 2MASS J0036+1821104 using the Very Large Array (VLA) in June 2007. Ultracool dwarfs are expected to be undetectable at radio frequencies, yet observations
at 8.5 GHz (3.5 cm) and 4.9 GHz (6 cm) of have revealed sources with > 100 {mu}Jy quiescent radio flux and > 1 mJy pulses coincident with stellar rotation. The anomalous emission is likely a combination of gyrosynchrotron and cyclotron maser processes in a long-duration, large-scale magnetic field. Since the characteristic frequency for each process scales directly with the magnetic field magnitude, emission at lower frequencies may be detectable from regions with weaker field strength. We detect no significant radio emission at 325 MHz from TVLM 513-46546 or 2MASS J0036+1821104 over multiple stellar rotations, establishing 2.5{sigma} total flux limits of 795 {mu}Jy and 942 {mu}Jy respectively. Analysis of an archival VLA 1.4 GHz observation of 2MASS J0036+1821104 from January 2005 also yields a non-detection at the level of < 130 {mu}Jy . The combined radio observation history (0.3 GHz to 8.5 GHz) for these sources suggests a continuum emission spectrum for ultracool dwarfs which is either flat or inverted below 2-3 GHz. Further, if the cyclotron maser instability is responsible for the pulsed radio emission observed on some ultracool dwarfs, our low-frequency non-detections suggest that the active region responsible for the high-frequency bursts is confined within 2 stellar radii and driven by electron beams with energies less than 5 keV.
During the past decade there have been several attempts to detect cosmogenic ultra high energy (UHE) neutrinos by searching for radio Cerenkov bursts resulting from charged impact showers in terrestrial ice or the lunar regolith. So far these radio s
earches have yielded no detections, but the inferred flux upper limits have started to constrain physical models for UHE neutrino generation. For searches which use the Moon as a target, we summarize the physics of the interaction, properties of the resulting Cerenkov radio pulse, detection statistics, effective aperture scaling laws, and derivation of upper limits for isotropic and point source models. We report on initial results from the RESUN search, which uses the Expanded Very Large Array configured in multiple sub-arrays of four antennas at 1.45 GHz pointing along the lunar limb. We detected no pulses of lunar origin during 45 observing hours. This implies upper limits to the differential neutrino flux E^2 dN/dE < 0.003 EeV km^{-2} s^{-1} sr^{-1} and < 0.0003 EeV km$^{-2} s^{-1} at 90% confidence level for isotropic and sampled point sources respectively, in the neutrino energy range 10^{21.6} < E(eV) < 10^{22.6}. The isotropic flux limit is comparable to the lowest published upper limits for lunar searches. The full RESUN search, with an additional 200 hours observing time and an improved data acquisition scheme, will be be an order of magnitude more sensitive in the energy range 10^{21} < E(eV) < 10^{22} than previous lunar-target searches, and will test Z burst models of neutrino generation.
