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Fig. 1. Multiwavelength image of the field surrounding the burst. The gray scale and contours respectively show Hα and H Ι emission associated with the SMC (32, 33). Crosses mark the positions of the five known radio pulsars in the SMC and are annotated with their names and DMs in parentheses in units of cm–3 pc. The open circles show the positions of each of the 13 beams in the survey pointing of diameter equal to the half-power width. The strongest detection saturated the single-bit digitizers in the data acquisition system, indicating that its S/N ≫ 23. Its location is marked with a square at right ascension 01h 18m 06s and declination –75° 12′ 19″ (J2000 coordinates). The other two detections (with S/Ns of 14 and 21) are marked with smaller circles. The saturation makes the true position difficult to localize accurately. The positional uncertainty is nominally ±7′ on the basis of the half-power width of the multibeam system. However, the true position is probably slightly (a few arcmin) northwest of this position, given the nondetection of the burst in the other beams.
Fig. 2. Frequency evolution and integrated pulse shape of the radio burst. The survey data, collected on 24 August 2001, are shown here as a two-dimensional “waterfall plot” of intensity as a function of radio frequency versus time. The dispersion is clearly seen as a quadratic sweep across the frequency band, with broadening toward lower frequencies. From a measurement of the pulse delay across the receiver band, we used standard pulsar timing techniques and determined the DM to be 375 ± 1 cm–3 pc. The two white lines separated by 15 ms that bound the pulse show the expected behavior for the cold-plasma dispersion law assuming a DM of 375 cm–3 pc. The horizontal line at ∼1.34 GHz is an artifact in the data caused by a malfunctioning frequency channel. This plot is for one of the offset beams in which the digitizers were not saturated. By splitting the data into four frequency subbands, we have measured both the half-power pulse width and flux density spectrum over the observing bandwidth. Accounting for pulse broadening due to known instrumental effects, we determine a frequency scaling relationship for the observed width W = 4.6 ms (f/1.4 GHz)–4.8 ± 0.4, where f is the observing frequency. A power-law fit to the mean flux densities obtained in each subband yields a spectral index of –4 ± 1. The inset shows the total-power signal after a dispersive delay correction assuming a DM of 375 cm–3 pc and a reference frequency of 1.5165 GHz. The time axis on the inner figure also spans the range 0 to 500 ms.
