Coordinated Ginga, IUE, and VLA¹ observations of stellar activity in σ² Coronae Borealis

We observed the active cool star binary system σ² Corona Borealis with the Large Area Counters (LAC) on the Ginga X-ray satellite over 2.5 days, or ∼2 orbital periods. Throughout the Ginga observation, σ² 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 ∼ 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 quiesc-ent periods indicates an increase in coronal temperature. Periodogram analysis of the entire X-ray light curve shows a possible periodicity at ≈0^d.-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 (α ≤ 0) to α ≈ -2.5, then gradually increasing to α ≈ + 1 during the flare rise. Thermal gyrosynchrotron or possibly thermal bremsstrahlung radiation may thus be contributing to the flare microwave spectrum.