Wideband Spectroscopy of Two Radio Bursts on AD Leonis

We report high-time-resolution, broadband spectroscopic observations of two radio bursts on the classical flare star AD Leonis. The observations were acquired by the 305 m telescope at Arecibo Observatory on 2003 June 13-14. Using the Wideband Arecibo Pulsar Processor, these observations sampled a total bandwidth of 400 MHz, distributed over a 500 MHz frequency range, 1120--1620 MHz, with a frequency resolution of 0.78 MHz and a time resolution of 10 ms. A radio burst observed on June 13 is characterized by the presence of multitudes of short duration ($\Delta t \sim$30 ms), high brightness temperature ($T_{b}>10^{14}$K), highly circularly polarized, fast-drift radio sub-bursts, with median bandwidths $\Delta \nu/\nu \sim$5%. The inverse drift rates are small, and have a symmetric distribution (both positive and negative frequency drifts) with a Gaussian FWHM inverse drift rate of 4.5$\times10^{-4}$ s/MHz. The fast-drift sub-bursts occur at a mean rate of 13 s$^{-1}$ and show no evidence for periodic recurrence. The fast-drift radio events on AD Leo are highly reminiscent of solar decimetric spike bursts. We suggest the emission is due to fundamental plasma radiation. A second highly circularly polarized radio burst, recorded June 14, has markedly different properties: a smoothly varying intensity profile characterized by a slow drift in frequency with time (-52 MHz s$^{-1}$). Under the assumption that the source is due to a disturbance propagating through the low corona, a source size of 0.1--1 R$_{\star}$ is inferred, implying a brightness temperature range 6$\times10^{11}$--6$\times10^{13}$K: another example of a coherent radio burst.