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 pointsource 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 2σ 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× 10 (d/1.7 kpc) ergs s, 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 due to 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. Department of Physics & Astronomy, Barnard College, New York, NY 10027 Department of Astronomy, Columbia University, New York, NY 10027 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.