Radio detections towards unidentified variable EGRET sources^*

¹ Departament d'Astronomia i Meteorologia and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (UB/IEEC), Martí i Franquès 1, 08028 Barcelona, Spain e-mail: jmparedes@ub.edu
² Departamento de Física, EPS, Universidad de Jaén, Campus Las Lagunillas s/n, Edif. A3, 23071 Jaén, Spain e-mail: [jmarti;jcombi;ajmunoz]@ujaen.es
³ Grupo de Investigación FQM-322, Universidad de Jaén, Campus Las Lagunillas s/n, Edif. A3, 23071 Jaén, Spain e-mail: jrssutil@hotmail.com
⁴ NCRA, TIFR, Post Bag 3, Ganeshkhind, Pune-411 007, India e-mail: ishwar@ncra.tifr.res.in
⁵ ICREA & Institut de Ciencies de l'Espai, Campus UAB, Facultat de Ciencies, Universitat Autònoma de Barcelona Torre C5, parell, 2da planta, Bellaterra, 08193 Barcelona, Spain e-mail: dtorres@aliga.ieec.uab.es
⁶ Instituto Argentino de Radioastronomía, C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina e-mail: romero@iar.unlp.edu.ar
⁷ Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del Bosque, 1900 La Plata, Argentina e-mail: romero@fcaglp.unlp.edu.ar
⁸ Max Planck Institut für Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany e-mail: vbosch@mpi-hd.mpg.de

Received: 17 July 2007
Accepted: 7 February 2008

Abstract

Context. A considerable fraction of the γ-ray sources discovered with the Energetic Gamma-Ray Experiment Telescope (EGRET) remain unidentified. The EGRET sources that have been properly identified are either pulsars or variable sources at both radio and gamma-ray wavelengths. Most of the variable sources are strong radio blazars. However, some low galactic-latitude EGRET sources, with highly variable γ-ray emission, lack any evident counterpart according to the radio data available until now.

Aims. The primary goal of this paper is to identify and characterise the potential radio counterparts of four highly variable γ-ray sources in the galactic plane through mapping the radio surroundings of the EGRET confidence contours and determining the variable radio sources in the field whenever possible.

Methods. We have carried out a radio exploration of the fields of the selected EGRET sources using the Giant Metrewave Radio Telescope (GMRT) interferometer at 21 cm wavelength, with pointings being separated by months.

Results. We detected a total of 151 radio sources. Among them, we identified a few radio sources whose flux density has apparently changed on timescales of months. Despite the limitations of our search, their possible variability makes these objects a top-priority target for multiwavelength studies of the potential counterparts of highly variable, unidentified gamma-ray sources.