ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O O O O B-Wavelength I-Wavelength I-Wavelength O B-Wavelength I-Wavelength O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O
tokens: GHz transition under study belongs to the A series. Maser emission in transitions of this series was not observed or theoretically predicted. However, qualitative considerations show that this transition probably belongs to Class II. Observations show that the strongest Class II maser occurs in the A transition, while the strongest Class I maser occurs in the A transition. Class II methanol masers are therefore characterized by overpopulation of A levels of the ladder with respect to the A levels of the ladder, while for Class I masers the situation is opposite. Model calculations for Class II methanol maser conditions by Cragg et al. (2005) show that A levels of the ladder tend to be overpopulated with respect to A levels of the ladder. This means that the 349.1070 GHz transition from A series most probably belongs to the Class II family. Strong support for this conclusion comes from the observational evidence that emission in the newly discovered maser transition covers the same velocity range and shows a similar position-velocity dependence as the strongest Class II methanol maser at 6.7 GHz ( Moscadelli et al. 2017 ). The spectra and the peak locations of the new maser may not coincide with those of the 6.7 GHz masers because of the different sensitivity to the physical conditions, which is usual for Class II methanol maser transitions ( Cragg et al. 2005 ). The A line series contains transitions with frequencies starting from 303.37 GHz. Many of them can be observed with the same instrument, which allows avoiding calibration problems in determining the relative line intensities. The lower levels of these transitions cover a rather wide range of energies from 2.32 K for the A level to 243.45 K for the A lower level of the transition under study and higher. It is worth mentioning that in our SMA data obtained in December 2010, the CHOH A line at 342.729796 GHz was detected in quasi-thermal emission ( Zinchenko et al. 2015 ). This may indicate that the discovered maser is related to the accretion burst in 2015, but additional modeling and monitoring of these lines is needed for an exact answer. Observations of the methanol line series proved to be a good tool for the physical parameter diagnostics both in maser and quasi-thermal cases ( Voronkov et al. 2006 ; Menten et al. 1988 ; Salii Sobolev 2006 ). This is very important because the new maser line was detected toward positions that are spread over the disk. An inspection of our channel maps shows that the peaks of the maser emission at different velocities lie on a curve that resembles an arc (Fig. 3 c). This can be part of a ring in the disk or on the surface of the disk at a substantial distance of several hundred AU from the star. Taking into account the disk orientation, which follows from the outflow data ( Zinchenko et al. 2015 ; Burns et al. 2016 ), it becomes clear that the brightest emission comes from the part of the disk that is closer to us. For the lower brightness