The nature of the fluorescent iron line in V 1486 Orionis

The fluorescent 6.4 keV iron line provides information on cool material in the vicinity of hard X-ray sources as well as on the characteristics of the X-ray sources themselves. First discovered in the X-ray spectra of the flaring Sun, X-ray binaries and active galactic nuclei (AGN), the fluorescent line was also observed in a number of stellar X-ray sources. The young stellar object (YSO) V1486 Ori was observed in the framework of the Chandra Ultra Deep Project (COUP) as the source COUP 331. We investigate its spectrum, with emphasis on the strength and time variability of the fluorescent iron Kα line, derive and analyze the light curve of COUP 331 and proceed with a time-resolved spectral analysis of the observation. The light curve of V 1486 Ori shows two major flares, the first one lasting for ≈20 ks with a peak X-ray luminosity of 2.6 $\times$ 1032 erg/s (dereddened in the $1{-}10$ keV band) and the second one – only partially observed – for >60 ks with an average X-ray luminosity of 2.4 $\times$ 1031 erg/s (dereddened). The spectrum of the first flare is very well described by an absorbed thermal model at high temperature, with a pronounced 6.7 keV iron line complex, but without any fluorescent Kα line. The X-ray spectrum of the second flare is characterized by even higher temperatures ($\ga$10 keV) without any detectable 6.7 keV Fe XXV feature, but with a very strong fluorescent iron Kα line appearing predominantly in the 20 ks rise phase of the flare. Preliminary model calculations indicate that photoionization is unlikely to account for the entire fluorescent emission during the rise phase