Quasi‐periodic X‐Ray Flares from the Protostar YLW 15

With ASCA , we have detected three X-ray flares from the class I protostar YLW 15. The flares occurred every ~20 hr and showed an exponential decay with time constant 30-60 ks. The X-ray spectra are explained by a thin thermal plasma emission. The plasma temperature shows a fast rise and slow decay for each flare with kT peak ~ 4-6 keV. The emission measure of the plasma shows this time profile only for the first flare, and remains almost constant during the second and third flares, at the level of the tail of the first flare. The peak flare luminosities, L X,peak , were ~5-20 × 10 31 ergs s -1 , which are among the brightest X-ray luminosities observed to date for class I protostars. The total energy released in each flare was 3-6 × 10 36 ergs. The first flare is well reproduced by the quasi-static cooling model, which is based on solar flares, and it suggests that the plasma cools mainly radiatively, confined by a semicircular magnetic loop of length ~14 R ☉ , with diameter-to-length ratio ~0.07. The two subsequent flares were consistent with the reheating of the same magnetic structure as in the first flare. The large-scale magnetic structure and the periodicity of the flares imply that the reheating events of the same magnetic loop originate in an interaction between the star and the disk due to the differential rotation.