Browsing by Author "Lazio, T. J. W."

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    Detection and flux density measurements of the millisecond pulsar j2145-0750 below 100 mhz
    Dowell, J.; Ray, P. S.; Taylor, G. B.; Blythe, J. N.; Clarke, Tracy E.; Craig, J.; Ellingson, Steven W.; Helmboldt, J. F.; Henning, P. A.; Lazio, T. J. W.; Schinzel, F.; Stovall, K.; Wolfe, C. N. (IOP Publishing, 2013-09-01)
    We present flux density measurements and pulse profiles for the millisecond pulsar PSR J2145-0750 spanning 37 to 81 MHz using data obtained from the first station of the Long Wavelength Array. These measurements represent the lowest frequency detection of pulsed emission from a millisecond pulsar to date. We find that the pulse profile is similar to that observed at 102 MHz. We also find that the flux density spectrum between approximate to 40 MHz to 5 GHz is suggestive of a break and may be better fit by a model that includes spectral curvature with a rollover around 730 MHz rather than a single power law.
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    The Extreme Scattering Event toward PKS 1741-038: VLBI Images
    Lazio, T. J. W.; Fey, A. L.; Dennison, B.; Mantovani, F.; Simonetti, John H.; Alberdi, A.; Foley, A. R.; Fiedler, R.; Garrett, M. A.; Hirabayashi, H.; Jauncey, D. L.; Johnston, K. J.; Marcaide, J.; Migenes, V.; Nicolson, G. D.; Venturi, T. (IOP PUBLISHING LTD, 2000-05)
    We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the source's diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the source's diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order I AU, the level of scattering within an ESE lens may be a factor of 10(7) larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 mu s, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 mu m emission.
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    Observations of crab giant pulses in 20-84 MHz using LWA1
    Ellingson, Steven W.; Clarke, Tracy E.; Craig, J.; Hicks, B. C.; Lazio, T. J. W.; Taylor, G. B.; Wilson, T. L.; Wolfe, C. N. (IOP Publishing, 2013-05-01)
    We report the detection and observed characteristics of giant pulses from the Crab Nebula pulsar (B0531+21) in four frequency bands covering 20-84 MHz using the recently completed Long Wavelength Array Station 1 (LWA1) radio telescope. In 10 hr of observations distributed over a 72 day period in fall of 2012, 33 giant pulses having peak flux densities between 400 Jy and 2000 Jy were detected. Twenty-two of these pulses were detected simultaneously in channels of 16 MHz bandwidth centered at 44 MHz, 60 MHz, and 76 MHz, including one pulse which was also detected in a channel centered at 28 MHz. We quantify statistics of pulse amplitude and pulse shape characteristics, including pulse broadening. Amplitude statistics are consistent with expectations based on extrapolations from previous work at higher and lower frequencies. Pulse broadening is found to be relatively high, but not significantly greater than expected. We present procedures that have been found to be effective for observing giant pulses in this frequency range.