Low-frequency constraints on the spectra of the lobes of the microquasar GRS 1758–258

Department of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK e-mail: mjh@star.herts.ac.uk

Received: 26 July 2004
Accepted: 22 December 2004

Abstract

I present low-frequency observations of the lobes of the microquasar GRS 1758-258, which, together with earlier radio observations, provide the best constraints yet on the lobe spectra, and show that they are consistent with the electron energy indices expected from standard shock acceleration models. There is no statistically significant evidence for synchrotron ageing along the length of the lobes, which is consistent with expectation if the magnetic field strength is close to the minimum-energy value. If the environment of the lobes has typical ISM properties, the lobes will dissipate on timescales of thousands of years or less, long before synchrotron ageing can become significant.