Long-term monitoring of 6.7-GHz methanol masers

Abstract

A sample of 54 6.7-GHz methanol masers was monitored using the Hartebeesthoek 26-m telescope during the period 1999 January -2003 March. The observations were taken at 1-2 week intervals, with daily observations when possible if a source was seen to be varying rapidly. It was found that the majority of the sources display a significant level of variability. The timerange of variations range from a few days up to several years. The types of behaviour observed included non-varying, monotonic increases or decreases, as well as aperiodic, quasi-periodic and periodic variations. Seven sources show clear evidence of periodicity, with periods ranging from 132 d up to 520 d.Seven sources, viz. G188.95+0.89, 196.45-1.68, G328.237-0.548, G331.13-0.24, G338.92-0.06, G339.62-0.12 and G9.62+0.20, show strong evidence of periodicity. Rigorous analysis of the periodicity is beyond the scope of this paper and will be presented in Goedhart, Gaylard & van der Walt (in preparation). The following sections describe the variability seen in the entire source sample. Variability indexEach maser spectrum consists of a number of peaks, corresponding to emission from maser spots at different velocities. Channels with no maser emission show the maxima and minima of the noise in the spectra. Velocity channels at the maximum of each peak were selected for the following analysis. Because the relation of the spatial structure of the maser to the velocity structure is not known in many cases, it was decided not to average the channels in each peak (to C 2004 RAS, MNRAS 355, 553-584Long-term monitoring of 6.7-GHz methanol masers 555

A search for water masers associated with class II methanol masers – II. Longitude range 341° to 6°

“…This pair of methanol masers has been found to show variability in both sites (Caswell et al 1995). Goedhart et al (2004) monitored these methanol masers over more than four years and found no periodicities on these timescales. The water maser associated with 345.003-0.223 was also detected by Breen et al (2010a) in 2003 to have a peak flux density of 3.0 Jy, in 2004 at 3.7 Jy.…”

“…Det Caswell et al (1995) and monitored by Goedhart et al (2004), and no periodicities in the variability were observed. The associated water maser emission is also quite variable, in 2003 it had a peak flux density of 7 Jy, in 2004 14 Jy (Breen et al 2010a) and in our 2011 observations 80.2 Jy.…”

The shortest periodic and flaring flux variability of a methanol maser emission at 6.7 GHz in G 014.23−00.50

“…First, in the point of view of the ratio of the peak flux density in the flare and quiescence, the flares at the component G in G 014.23 can be compared to a spectral component of V lsr = −5.88 km s −1 in G 351.42+00.64 (NGC6334 F: Goedhart et al 2004) and those of V lsr = −1.32 and −0.55 km s −1 in Cepheus A (Szymczak et al 2014). The spectral components in both sources showed ratios of the peak flux density in the flare of 170 and 240 in comparison with the quiescent phase.…”

“…So far, the periodic flux variability of the methanol masers has been detected in 20 sources (including quasi-periodic ones). Their periods range from 29.5 to 668 days (Goedhart et al 2004Araya et al 2010;Szymczak et al 2011Szymczak et al , 2014Szymczak et al , 2015Szymczak et al , 2016Fujisawa et al 2014a;Maswanganye et al 2015Maswanganye et al , 2016. Patterns of the variability have been classified into two categories: the sinusoidal one, and the intermittent one with a quiescent phase.…”

“…Long-term and frequent flux monitoring was conducted toward source samples only less than 20%, which was estimated from ∼200 sources (e.g., Goedhart et al 2004;Szymczak et al 2015;Maswanganye et al 2016) in more than 1000 methanol masers samples (e.g., Pestalozzi et al 2005;Xu et al 2009;Caswell et al 2010Caswell et al , 2011Green et al 2010Green et al , 2012bOlmi et al 2014;Sun et al 2014;Breen et al 2015, and references therein). Furthermore, most of the monitored sources have been selected simply by their peak flux densities > 5 Jy in Szymczak, Wolak, and Bartkiewicz (2015).…”