Rotational modulation and flares on RS CVn and BY DRA systems. VII. simultaneous X-ray, radio and optical data for the dMe star YZ CMi on 4/5 March 1985.

Broad-band photometric observations of YZ CMi show a 1.2mag U-band flare at 19:55 UT on 4 March 1985, however, simultaneous X-ray observations from EXOSAT show no detectable increase. Two possible explanations for the lack of an X-ray flare may be i) a flare occurring low in the chromosphere, where the chromosphere may not have been coupled to the transition region and therefore the corona via magnetic fields or ii) proton beam heating as opposed to electron beams. During the flare, optical spectra show excess emission in the wings of all the hydrogen Balmer lines. Interpreted in terms of mass flows would imply material moving at ∼300 km s^-1 simultaneously to the blue and red or alternatively random mass motions with a velocity of a similar magnitude. At flare maximum, all the Balmer lines show excess emission in the wings with Hγ and Hσ showing symmetrically broadened lines while higher members of the series such as Hζ and Hη show predominately red shifted material. Assuming a single loop flare, an interpretation in terms of directed mass flows would imply a loop of length ∼2-3 10⁹ cm, however this would place the material in the corona where we should have observed it in X-rays. An alternative explanation, also assuming directed mass flows could involve several small flare kernels, which brighten successively, thus producing a broadened profile. An explanation similar to this has been suggested to explain the excess emission seen in the wings of Hα during solar flares. The total optical flare energy was ∼6 10³¹ erg in this event, with the Balmer lines contributing ∼10% to the energy in the U-band during the flare. Many short-lived bursts or micro-flares were detected in both the Johnson U-band and the Balmer emission lines. The probability for a correlation between these two data-sets to have occurred by chance was less than 10^-5 . The coronal X-ray flux show only a moderate dependence on Balmer line emission and none on the U-band variations. Thus for YZ CMi not only does the large optical flare at 19:55 UT produce no significant X-ray emission but also the low level optical flares, which are seen almost continuously, are similarly absent in soft X-rays. Several X-ray flares/micro-flares were observed, however the majority of these did not show an optical counter-part. The largest of these X-ray events were at 18:00 UT and 23:30 UT, lasting an hour and 1 1/2 h respectively. Their energy in the 0.02-2 keV range were 9.1 10³¹ erg and 2.1 10³² erg respectively. No optical data was available for the first large X-ray flare, while the second event had only limited coverage. The photometric data implied an optical energy of at least 2.6 10³¹ erg. The Balmer line contribute at least 10% to the U-band energy in both the 19:55 UT and 23:30 UT flares. Radio observations at 326 MHz give an upper limit of 4 mJy for the flux.