COORDINATED GINGA, IUE, AND VLA OBSERVATIONS OF STELLAR ACTIVITY IN SIGMA(2) CORONAE BOREALIS

We observed the active cool star binary system sigma(2) Corona Borealis with the Large Area Counters (LAC) on the Ginga X-ray satellite over 2.5 days, or approximately 2 orbital periods. Throughout the Ginga observation, sigma(2) CrB exhibited X-ray variability on time scales ranging from tens of minutes to hours. The LAC count rate was 4.2 +/- 1.2 counts s-1 (1.7-11.0 keV) corresponding to L(x) approximately 10(29.6) ergs s-1 (2-10 keV), depending upon the assumed source spectrum. The X-ray light curve shows evidence of flaring during the course of the observation. The summed X-ray pulse height spectrum from flaring events compared with similar spectra from quiescent periods indicates an increase in coronal temperature. Periodogram analysis of the entire X-ray light curve shows a possible periodicity at almost-equal-to 0-d-40-0-d-44, with no strong signal at the orbital period or 1/2 the orbital period. Coordinated multiwavelength observations with IUE and the VLA indicate both ultraviolet and microwave flaring during one X-ray outburst. The X-ray flare peak may have been missed due to an Earth occultation; however, it appears at least 10 times less energetic than the largest reported RS CVn flares. The quiescent microwave spectrum is not consistent with a thermal gyrosynchrotron process: nonthermal gyrosynchrotron radiation is the most likely emission process. Microwave emission during the flare rise phase peaks earlier at 20 cm than at 6 and 3.6 cm, suggesting free-free or self-absorption at longer wavelengths. The microwave spectral index (6-3.6 cm) changes noticeably during the course of the observation, first decreasing from a slightly negative or flat slope (alpha less-than-or-equal-to 0) to alpha almost-equal-to -2.5, then gradually increasing to alpha almost-equal-to + 1 during the flare rise. Thermal gyrosynchrotron or possibly thermal bremsstrahlung radiation may thus be contributing to the flare microwave spectrum.