Time variability of five strong 12 GHz methanol masers

¹ Universitá di Bologna, Dipartimento di Astronomia, via Zamboni 33, 40100, Bologna, Italy, and Istituto di Radioastronomia (C.N.R.), via Gobetti 101, 40129, Bologna, Italy
² Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy

Send offprint request to: L. Moscadelli, Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn 1, Germany

Received: 8 December 1994
Accepted: 19 September 1995

Abstract

Five intense 12 GHz methanol masers with structured profile have been monitored during the period 90 Nov.-92 Apr., in order to study the variability on time scales ranging from one week to about 2 years. The integrated flux density of W3(OH), G188.94 +0.89, NGC 7538 remains constant (within the measurement uncertainties) during our period of observation. By amplitude normalization and removal of instrumental velocity shifts, we could investigate variations of flux density as small as a few per cent. For W3(OH) and G188.94+0.89, we find that the flux density varies up to a maximum of 8% over 1.5 years; for NGC 7538, we obtain an upper limit of 10% over 1.5 years. The integrated flux density of the remaining two sources, Cep A and W48, varies up to a factor of 2-3 and both show an intensity increase which varies with velocity. We could study the dependence of the intensity variation on the velocity with a precision limited only by the noise. We interpret the small time variability presented by the first three sources as due to proper motions of the maser spots. To explain the velocity dependence of the intensity variation of the two more variable sources, we suggest, for Cep A, the presence of still unsaturated emission components and, for W48, an edge-on disk configuration.