2017A&A...602A..26L

Context. Low-mass ultrafast rotators show the typical signatures of magnetic activity and are known to produce flares, probably as a result of magnetic reconnection. As a consequence, the coronae of these stars exhibit very large X-ray luminosities and high plasma temperatures, as well as a pronounced inverse FIP effect.
Aims. To probe the relationship between the coronal properties with spectral type of ultra-fast rotators with P_rot<1d, we analyse the K3 rapid-rotator LO Peg in comparison with other low-mass rapid rotators of spectral types G9-M1.
Methods. We report the results of a 42ks long XMM-Newton observation of LO Peg and investigate the temporal evolution of coronal properties like the temperatures, emission measures, abundances, densities and the morphology of the involved coronal structures. In addition, we also use the XMM-Newton data from a sample of rapid rotators and compare their coronal properties to those of LO Peg.
Results. We find two distinguishable levels of activity in the XMM-Newton observation of LO Peg, which shows significant X-ray variability both in phase and amplitude, implying the presence of an evolving active region on the surface. The X-ray flux varies by ∼28%, possibly due to rotational modulation. During our observation a large X-ray flare with a peak X-ray luminosity of ∼2x10³⁰erg/s and a total soft X-ray energy release of 7.3x10³³erg was observed. Further, at the onset of the flare we obtain clear signatures for the occurrence of the Neupert effect. During the flare a significant emission measure increase in the hotter plasma component is observed, while the emission measure in the cooler plasma component is only marginally affected, indicating that different coronal structures are involved. The flare plasma also shows an enhancement of iron by a factor of ~=2 during the rise and peak phase of the flare. The electron densities measured using the OVII and NeIX triplets during the quiescent and flaring state are ~=6x10¹⁰cm^–3 and 9x10¹¹cm^–3, respectively, and the large errors prevent us from finding significant density differences between quiescent and flaring states. Our modeling analysis suggests that the scale size of the flaring X-ray plasma is smaller than 0.5R_*. Further, the flare loop length appears to be smaller than the pressure scale height of the flaring plasma. Our studies show that the X-ray properties of the LO Peg are very similar to those of other low-mass ultrafast rotators, i.e., the X-ray luminosity is very close to saturation, its coronal abundances follow a trend of increasing abundance with increasing first ionisation potential, the so-called inverse FIP effect.