Deep radio images of the HEGRA and Whipple TeV sources in the Cygnus OB2 region^*

¹ Departamento de Física, Escuela Politécnica Superior, Universidad de Jaén, Las Lagunillas s/n, 23071 Jaén, Spain e-mail: jmarti@ujaen.es
² Departament d'Astronomia i Meteorologia, Facultat de Física, Universitat de Barcelona, Martí i Franquès, 1, 08028 Barcelona, Spain e-mail: jmparedes@ub.edu
³ National Center for Radio Astrophysics, TIFR, PB No. 3, Ganeshkhind, Pune 7, India e-mail: ishwar@ncra.tifr.res.in
⁴ Max Planck Institut für Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany e-mail: vbosch@mpi-hd.mpg.de

Received: 24 April 2007
Accepted: 21 May 2007

Abstract

Context.The modern generation of Cherenkov telescopes has revealed a new population of gamma-ray sources in the Galaxy. Some of them have been identified with previously known X-ray binary systems while other remain without clear counterparts a lower energies. Our initial goal here was reporting on extensive radio observations of the first extended and yet unidentified source, namely TeV J2032+4130. This object was originally detected by the HEGRA telescope in the direction of the Cygnus OB2 region and its nature has been a matter of debate during the latest years. The situation has become more complex with the Whipple and MILAGRO telescopes new TeV detections in the same field which could be consistent with the historic HEGRA source, although a different origin cannot be ruled out.

Aims.We aim to pursue our radio exploration of the TeV J2032+4130 position that we initiated in a previous paper but taking now into account the latest results from new Whipple and MILAGRO TeV telescopes. The data presented here are an extended follow up of our previous work.

Methods.Our investigation is mostly based on interferometric radio observations with the Giant Metre Wave Radio Telescope (GMRT) close to Pune (India) and the Very Large Array (VLA) in New Mexico (USA). We also conducted near infrared observations with the 3.5 m telescope and the OMEGA2000 camera at the Centro Astronómico Hispano Alemán (CAHA) in Almería (Spain).

Results.We present deep radio maps centered on the TeV J2032+4130 position at different wavelengths. In particular, our 49 and 20 cm maps cover a field of view larger than half a degree that fully includes the Whipple position and the peak of MILAGRO emission. Our most important result here is a catalogue of 153 radio sources detected at 49 cm within the GMRT antennae primary beam with a full width half maximum (FWHM) of 43 arc-minute. Among them, peculiar sources inside the Whipple error ellipse are discussed in detail, including a likely double-double radio galaxy and a one-sided jet source of possible blazar nature. This last object adds another alternative counterpart possibility to be considered for both the HEGRA, Whipple and MILAGRO emission. Moreover, our multi-configuration VLA images reveal the non-thermal extended emission previously reported by us with improved angular resolution. Its non-thermal spectral index is also confirmed thanks to matching beam observations at the 20 and 6 cm wavelengths.