arXiv subjects

T. S. Bastian

Publications and source records attributed to T. S. Bastian.

22 records · Page 2Linked to original sources

arXiv2007

Ultra-high-time Resolution Observations of Radio Bursts on AD Leonis

We report observations of a radio burst that occurred on the flare star AD Leonis over a frequency range of 1120-1620 MHz (lambda ~18-27 cm). These observations, made by the 305m telescope of the Arecibo Observatory, are unique in providing the highest time resolution (1 ms) and broadest spectral coverage (Delta nu/nu =0.36) of a stellar radio burst yet obtained. The burst was observed on 2005 April 9. It produced a peak flux density of ~500 mJy and it was essentially 100% right circularly polarized. The dynamic spectrum shows a rich variety of structure: patchy emission, diffuse bands, and narrowband, fast-drift striae. Focusing our attention on the fast-drift striae, we consider the possible role of dispersion and find that it requires rather special conditions in the source to be a significant factor. We suggest that the emission may be due to the cyclotron maser instability, a mechanism known to occur in planetary magnetospheres. We briefly explore implications of this possibility.

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arXiv2007

Synchrotron Radio Emission from a Fast Halo Coronal Mass Ejection

An interplanetary (IP) type-II-like radio burst is analyzed. It occurred on 2003 June 17-18 in association with a fast halo coronal mass ejection (CME), an M6.8 soft-X-ray (SXR) flare, and produced a solar proton event. Unlike coronal type II bursts and the ma jority of IP type II radio emissions, the IP type-II-like event associated with the fast halo CME on June 17-18 varies smoothly in time and frequency and has a frequency bandwidth that is several times larger than is typical for coronal and IP type II emissions. Moreover, the frequency change with time is inconsistent with that expected from plasma radiation associated with a CME-driven shock. I suggest that this IP type-II-like event, referred to here as an IP type II-S event, is not due to plasma radiation but, rather, incoherent synchrotron radiation from near-relativistic electrons entrained in the CME magnetic field, or in the sheath region between the shock and the CME driver. This event may be an example of a new and distinct class of interplanetary radio phenomenon.

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arXiv2007

Radio Spectral Evolution of an X-ray Poor Impulsive Solar Flare: Implications for Plasma Heating and Electron Acceleration

We present radio and X-ray observations of an impulsive solar flare that was moderately intense in microwaves, yet showed very meager EUV and X-ray emission. The flare occurred on 2001 Oct 24 and was well-observed at radio wavelengths by the Nobeyama Radioheliograph (NoRH), the Nobeyama Radio Polarimeters (NoRP), and by the Owens Valley Solar Array (OVSA). It was also observed in EUV and X-ray wavelength bands by the TRACE, GOES, and Yohkoh satellites. We find that the impulsive onset of the radio emission is progressively delayed with increasing frequency relative to the onset of hard X-ray emission. In contrast, the time of flux density maximum is progressively delayed with decreasing frequency. The decay phase is independent of radio frequency. The simple source morphology and the excellent spectral coverage at radio wavelengths allowed us to employ a nonlinear chi-squared minimization scheme to fit the time series of radio spectra to a source model that accounts for the observed radio emission in terms of gyrosynchrotron radiation from MeV-energy electrons in a relatively dense thermal plasma. We discuss plasma heating and electron acceleration in view of the parametric trends implied by the model fitting. We suggest that stochastic acceleration likely plays a role in accelerating the radio-emitting electrons.

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arXiv2005

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 ($Δt \sim$30 ms), high brightness temperature ($T_{b}>10^{14}$K), highly circularly polarized, fast-drift radio sub-bursts, with median bandwidths $Δν/ν\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.

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