摘要：X-ray emission is a characteristic feature of young stellar objects (YSOs) and the result of the interplay between rotation, magnetism, and accretion. For this reason high energy phenomena are key elements to understand the process of star formation, the evolution of their circumstellar disks, and eventually the formation of planets. We investigated the X-ray characteristics of the Class I YSO Elias 29 with jointXMM-NewtonandNuSTARobservations of total duration 300 ks and 450 ks, respectively. These are the first observations of a very young (<1 Myr) stellar object in a band encompassing simultaneously both soft and hard X-rays (0.3 − 10 keV inXMM-Newtonand ≈3 − 80 keV inNuSTAR). The quiescent spectrum is well described by one thermal component at ∼4.2 keV absorbed byNH ∼ 5.5 × 1022cm−2. In addition to the hot Fe complex at 6.7 keV, we observed fluorescent emission from Fe at ∼6.4 keV, confirming the previous findings. The line at 6.4 keV is detected during quiescent and flaring states and its flux is variable. The equivalent width is found varying in the range ≈0.15 − 0.5 keV. These values make unrealistic a simple model with a centrally illuminated disk and suggest a role of the cavity containing Elias 29 and possible reverberation processes that could occur in it. We observed two flares that have durations of 20 ks and 50 ks, respectively, and we observed the first flare with bothXMM-NewtonandNuSTAR. For this flare, we used its peak temperature and timing as diagnostics to infer a loop size of about 1 − 2 R⊙in length, which is about 20%–30% of the stellar radius. This implies a relatively compact structure. We systematically observed an increase inNHof a factor five during the flares. This behavior has been observed during flares previously detected in Elias 29 withXMM-Newtonand ASCA. The phenomenon suggests that the flaring regions could be buried under the accretion streams and at high stellar latitudes because the X-rays from flares pass through gas denser than the gas along the line of sight of the quiescent corona. In a different scenario, a contribution from scattered soft photons to the primary coronal emission could mimic a shallowerNHin the quiescent spectrum. In the spectrum of the fullNuSTARexposure, we detect hard X-ray emission in the band ≈20 − 80 keV which is in excess with respect to the thermal emission and that is significant at a level of ≥2σ. We speculate that the hard X-ray emission could be due to a population of energetic electrons accelerated by the magnetic field along the accretion streams. These particles, along with X-ray photons withE > 7.11 keV, could be responsible for pumping up the Fe fluorescence when hitting cold Fe in the circumstellar disk.
