Multiwavelength observations of flares and variability in the coronae of active binary systems

Details Multiwavelength observations of flares and variability in the coronae of active binary systems Multiwavelength observations of flares and variability in the coronae of active binary systems

Aug 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........3o&link_type=abstract

Thesis (PhD). UNIVERSITY OF COLORADO AT BOULDER, Source DAI-B 63/02, p. 838, Aug 2002, 398 pages.

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I investigate the multi-wavelength properties of stellar flares on active binary systems, concentrating on X- ray/extreme ultraviolet (EUV) and radio observations of thermal and nonthermal coronal plasmas. I examine the frequency of flaring events in active binary systems in the EUV bandpass, and find that they undergo large scale flares ≈40% of the time, with a dynamic range between quiescent and flare luminosities less than a factor of ten. Long duration flares lasting days appear to be common, and there is evidence for variability even when no large flares are obvious. Some coronal flares exhibit long rise times, comparable to the decay timescale. Other flare phenomena observed include a slow brightening before the impulsive phase of the flare, and two different timescales during the flare decay. Dramatic abundance and temperature changes are present in coronae during some flares, with enhancements up to a factor of four between quiescent and flaring iron abundances, and evidence for plasma temperatures >50 MK during flares. The distribution of thermal coronal plasma with temperature is bimodal with a peak around 6 8 106K and another at 20 30 106 K. The hotter temperature peak moves to higher temperatures during flares. Highly polarized, highly time-variable emission at 20 cm is due to a coherent emission mechanism, most likely emission at the plasma frequency or second harmonic, and is a common feature of low frequency radio emission on active binary systems. Gyrosynchrotron flares at higher radio frequencies show very little net polarization, in contrast with coherent phenomena which reveal large degrees of polarization. Radio and X-ray flares do show correlations; some patterns are consistent with solar flare multi-wavelength patterns while others are not. This presents a complex view of flaring. Flares demonstrate an increased contrast over the quiescent level at higher X-ray energies, and the flare evolution proceeds at a faster rate at higher energies. The question to what extent the solar-stellar connection holds with regards to the physics of flares is complicated: there is evidence for and against a connection.

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