Evidence for magnetic activity at starbirth: a powerful X-ray flare from the Class 0 protostar HOPS 383

Context. Class 0 protostars represent the earliest evolutionary stage of solar-type stars, during which the majority of the system mass resides in an infalling envelope of gas and dust and is not yet in the central, nascent star. Although X-rays are a key signature of magnetic activity in more evolved protostars and young stars, whether such magnetic activity is present at the Class 0 stage is still debated.Aims. We aim to detect a bona fide Class 0 protostar in X-rays.Methods. We observed HOPS 383 in 2017 December in X-rays with the Chandra X-ray Observatory (～84 ks) and in near-infrared imaging with the Southern Astrophysical Research telescope.Results. HOPS 383 was detected in X-rays during a powerful flare. This hard (E？> ？2 keV) X-ray counterpart was spatially coincident with the northwest 4 cm component of HOPS 383, which would be the base of the radio thermal jet launched by HOPS 383. The flare duration was ～3.3 h; at the peak, the X-ray luminosity reached ～4？×？10^{31 erg s？1 in the 2？8 keV energy band, a level at least an order of magnitude larger than that of the undetected quiescent emission from HOPS 383. The X-ray flare spectrum is highly absorbed (N_{H？～？7？×？1023 cm？2), and it displays a 6.4 keV emission line with an equivalent width of ～1.1 keV, arising from neutral or low-ionization iron.Conclusions. The detection of a powerful X-ray flare from HOPS 383 constitutes direct proof that magnetic activity can be present at the earliest formative stages of solar-type stars}}