Letter to the Editor

An unidentified TeV source in the vicinity of Cygnus OB2

¹ Max-Planck-Institut für Kernphysik, Postfach 103980, 69029 Heidelberg, Germany
² Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia
³ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
⁴ Universität Kiel, Institut für Experimentelle und Angewandte Physik, Leibnizstraße 15-19, 24118 Kiel, Germany
⁵ Universität Wuppertal, Fachbereich Physik, Gaußstr. 20, 42097 Wuppertal, Germany
⁶ Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
⁷ Universidad Complutense, Facultad de Ciencias Físicas, Ciudad Universitaria, 28040 Madrid, Spain
⁸ Now at Washington University, St. Louis, MO 63130, USA
⁹ Now at IFAE, Unversitat Autònoma de Barcelona, Spain
¹⁰ On leave from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia
¹¹ Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan

Corresponding author: G. P. Rowell, D. Horns Gavin.Rowell@mpi-hd.mpg.de,Dieter.Horns@mpi-hd.mpg.de

Received: 24 July 2002
Accepted: 9 August 2002

Abstract

Deep observation (~113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and ~ north of Cyg X-3. The source centre of gravity is RA : , Dec : . The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius  at the ~ level, and has a differential power-law flux with hard photon index of . The integral flux above 1 TeV amounts ~3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage γ-ray production via a jet-driven termination shock.