1 Introduction

The source of the H₂O maser emission G43.8-0.1 (B1950 R.A. $=19 ^{\rm h}9^{\rm m}31.2^{\rm s}$, Dec. $=9\deg30\hbox{$^\prime$ }51\hbox{$^{\prime\prime}$ }$) is located in a region of active star formation. It was discovered in 1976 by Genzel & Downes (1977). The source is associated also with a compact HII region (seen in the continuum at 6 cm) and with an OH 1665 MHz maser within $10\hbox{$^{\prime\prime}$ }$ from it (Matthews et al. 1978; Evans et al. 1979). VLBI observations showed that H₂O maser condensations in G43.8-0.1 form a shell-like structure with a radius of $\sim0.2\hbox{$^{\prime\prime}$ }$ (Downes et al. 1979). For a distance to the source of 3 kpc, this corresponds to a radius of the envelope of $\sim 10^{16}$ cm.

In the catalogue of IRAS sources that are associated with a molecular outflow and/or dense molecular core G43.8-0.1 has a name of IRAS 19095+0930. Later observations of this area with a high angular resolution at 2 and 3.6 cm (Kurtz et al. 1994) showed that the HII region is shifted by $11.4\hbox{$^{\prime\prime}$ }$ relative to the source IRAS 19095+0930. According to Downes et al. (1979), the H₂O maser source does not coincide with the HII region and is displaced by $\sim5\hbox{$^{\prime\prime}$ }$ with respect to it. Most likely, this is caused by a greater error of measurements than it was stated in the work of Downes et al. (1979). All this does not permit us to estimate the evolutionary status of the source. It is only possible to infer an age of <10⁵ years (Downes et al. 1979).

Under the assumption that the positions of the maser source and ultracompact HII region do coincide, the mass and luminosity of the central star that creates the HII region G43.80-0.13 can be estimated. This can be done proceeding from the size of the envelope formed by maser spots and from the dispersion of radial velocities. In this procedure it is necessary to exclude the highest- and lowest-velocity features. According to Downes et al. (1979), in this case the estimated radius of the envelope is $\sim 0.1\hbox{$^{\prime\prime}$ }$, and the velocity dispersion is about 5 km s^-1. The limiting mass of the star in this case is about $9~M_{\odot}$.

The observations that were carried out in 1987 at 12.2 GHz (Koo et al. 1988) did not detect a methanol maser with a flux density F>5 Jy. In subsequent observations of 1992-1993 a methanol maser at 12.2 GHz was not detected with an upper limit of 0.4 Jy (Caswell et al. 1995), but the CH₃OH emission was instead revealed at another frequency, 6.6 GHz, in an interval of radial velocities of 39-44 km s^-1 with a peak flux density of 152 Jy (Menten 1991). Caswell et al. (1995) observed a methanol maser at the same frequency from February 1992 to September 1993. The spectrum consisted of four features. The main feature had a flux density of 140 Jy and velocity $V_{\rm LSR}=39.5$ km s^-1. No flux variability of all the features of the spectrum was found.

Observations of the G43.8-0.1 region were carried out also in the lines of the CO and CS molecules (Plume et al. 1992), which yielded an estimate of the kinematic distance to the source (3 kpc).

The high-velocity molecular gas in regions of formation of massive stars was studied by Shepherd & Churchwell (1996). They explored the high-velocity wings of the ¹²CO (J = 1-0) line for 94 sources (among them G43.8-0.1). However, they did not get an unambiguous criterion for the existence of a bipolar outflow, because high-velocity wings of the line may appear for other reasons. For instance, the cause may be numerous components, collapse, rotation, or impacts (Shepherd & Churchwell 1996). At present nothing definite can be said about the existence of a bipolar flow in G43.8-0.1.

The peculiarity of the H₂O maser source G43.8-0.1 is that, since the time of its discovery in 1976 (Genzel & Downes 1977) and till now, emission is constantly present at $V_{\rm LSR}=42.2$ km s^-1 in its H₂O spectrum. We have been observing the emission at this velocity all the time without interruptions (Lekht 1994, 1995). For almost 6 years (1981-1987), the intensity of this emission was growing. Then, this feature remained sufficiently stable during about 4 years. The peak flux density was within the limits of 2400-3000 Jy, and the velocity remained virtually unchanged (42.20 $\pm$ 0.05 km s^-1). Since 1991, the maser condensation has faded. In one year, the flux decreased twofold, and the emission again stabilized at this level (1200-1500 Jy) till 1994.

Figure 1: a-g) Spectra of the H2O maser emission of G43.8-0.1 from May 1994 to November 1998. The vertical bar gives the flux scale

Figure 1: continued

Strong flares at different radial velocities occurred in G43.8-0.1 more or less regularly (with intervals of 1-3 years). Flare duration was from 3 months to 1.5 years, and the flux reached 2500-4550 Jy (Lekht 1994). Most frequently, single features flared, whose intensity considerably exceeded that of blending features in the H₂O spectrum. Thanks to this, such flares could be analysed as individual components. For them, we traced the variability of the flux, radial velocity and linewidth. Moreover, for two components we found a functional relationship between the variations of the flux and linewidth (Lekht 1994). From the character of this relationship, the degree of saturation of the H₂O maser component can be estimated (Mattila et al. 1985; Strel'nitskij 1982).

This work is devoted to an investigation of the maser emission of the source G43.8-0.1 through the entire timespan of its observations with the aim to determine the activity period of the central star and to study variability of the emission of individual spectral components during the last flare. The results of investigation of individual components, obtained before 1994, are presented in our earlier works (Lekht 1994, 1995).