Quasi-periodic X-ray flares on protostar YLW15 Original Research Article

We detected three intermittent X-ray flares from a protostar YLW15. Each flare occurred every ∼20 hours, and showed an exponential decay of time constant 30–60 ks. The time-sliced X-ray spectra are all explained by a thin thermal plasma emission. The plasma temperature shows a fast-rise and a slow-decay for each flare, while the emission measure of the plasma increases only at the beginning of the first flare and is almost constant during the second and third flares. Thus the X-rays are attributable to a single plasma produced by the first large flare, then the plasma is re-heated quasi-periodically. The re-heating interval is significantly shorter than the spin periods known on T Tauri stars (2–3 days at minimum), but comparable to the Keplerian rotation period of inner accretion disk at a distance of r − 5 R⊙ (M/M⊙)1.5. Assuming that the temperature decrease is due to radiative cooling, and the plasma geometry is a solar-like flare loop of an aspect ratio (=length/diameter) of 10, we estimate that the plasma loop length is about 10 R⊙. Possible origin for the quasi-periodic flaring, magnetic activity produced in a star and inner disk interacting region, is briefly discussed.