Discovery of Extremely Embedded X-Ray Sources in the R Coronae Australis Star-forming Core

Abstract

With the XMM-Newton and Chandra observatories, we detected two extremely embedded X-ray sources in the R Corona Australis (R CrA) star-forming core, near IRS 7. These sources, designated as X_E and X_W, have X-ray absorption columns of ~3×10²³ cm^-2 equivalent to A_V~180 mag. They are associated with the VLA centimeter radio sources 10E and 10W, respectively; X_W is the counterpart of the near-infrared source IRS 7, whereas X_E has no K-band counterpart above 19.4 mag. This indicates that X_E is younger than typical Class I protostars, probably a Class 0 protostar, or in an intermediate phase between Class 0 and Class I. The X-ray luminosity of X_E varied between 29<logL_X<31.2 ergs s^-1 on timescales of 3-30 months; X_E also showed a monotonic increase in X-ray brightness by a factor of 2 in 30 ks during an XMM-Newton observation. The XMM-Newton spectra indicate emission from a hot plasma with kT~3-4 keV and also show fluorescent emission from cold iron. Although the X-ray spectrum from X_E is similar to flare spectra from Class I protostars in luminosity and temperature, the light curve does not resemble the light curves of magnetically generated X-ray flares, because the variability timescale of X_E is too long and variations in X-ray count rate were not accompanied by variations in spectral hardness. The short-term variation of X_E may be caused by the partial blocking of the X-ray plasma, while the month-long flux enhancement may be driven by mass accretion.