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 \times 10 ^ { -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 \times 10 ^ { 11 } –6 \times 10 ^ { 13 } K : another example of a coherent radio burst .