XMM-Newton Observations of the Enigmatic Long Period Eclipsing Binary Epsilon Aurigæ: Constraining the Physical Models

We report on an XMM-Newton observation of the unusual, long period (F0 supergiant + cool disk companion) eclipsing binary star epsilon Aurigæ observed in X-rays prior to the onset of the 2009-2011 eclipse phase. While it appears certain that a disk-like body is obscuring the star, it remains unclear what kind of object is at the center of the disk. The XMM-Newton observation had a duration of ~25 ks. The X-ray image does not show a source at the optical position of the star. We have determined a limiting X-ray flux of about 2.5 × 10^-15 erg cm^-2 s^-1. This flux is equivalent to a luminosity of about log L_X ~ 29.3-29.7 erg s^-1 for an unabsorbed source at the assumed distance, estimated to be between 650 and 1300 pc. The direct emission from an otherwise unobscured primary is excluded at a level above L_X ~ 4 × 10²⁹ erg s^-1. We explore the physical constraints that these limits put on the primary and several proposed models for the system secondary. For a black hole, we find that the direct absorption required by the disk is approximately N _H >= 10²⁵ cm^-2. While not unreasonable for a protoplanetary disk, the lack of secondary X-rays makes this model unlikely. For high-mass models, in which the disk harbors pre-main-sequence stars, we find a minimum disk absorption of N _H >= 10²³ cm^-2. Since this is not unlikely for a protoplanetary disk, this is not a significant constraint. For low-mass models, in which the disk harbors older stars, the known line-of-sight absorption is nearly enough to account for the non-detection. The data do not discriminate between these models.