Search for a point-source counterpart of the unidentified gamma-ray source TeV J2032+4130 in Cygnus

We have made a multiwavelength study of the overlapping error boxes of the unidentified γ-ray sources TeV J2032+4130 and 3EG J2033+4118 in the direction of the Cygnus OB2 association (d = 1.7 kpc) in order to search for a point-source counterpart of the first unidentified TeV source. Optical identifications and spectroscopic classifications for the brighter X-ray sources in ROSAT PSPC and Chandra ACIS images are obtained, without finding a compelling counterpart. The classified X-ray sources are a mix of early- and late-type stars, with one exception. The brightest source in the Chandra observation is a new, hard absorbed source that is both transient and rapidly variable. It lies 7′ from the centroid of the TeV emission, which places it outside of the claimed 2 σ location (r ≈ 4′.8). A possible eclipse or "dip" transition is seen in its light curve. With a peak 1-10 keV luminosity of ≈7 × 10³²(d/1.7 kpc)² ergs s^-1, this source could be a quiescent low-mass X-ray binary that lies beyond the Cyg OB2 association. A coincident, reddened optical object of R = 20.4, J = 15.4, H = 14.2, and K = 13.4 is observed but not yet classified as a result of the lack of obvious emission or absorption features in its spectrum. Alternatively, this Chandra and optical source might be a considered a candidate for a "proton blazar," a long hypothesized type of radio-weak γ-ray source. More detailed observations will be needed to determine the nature of this variable X-ray source and to assess the possibility of its connection with TeV J2032+4130.