The object BX Monoceros (BX Mon) is an eclipsing binary system. Its optical spectrum shows a combination of strong hydrogen and helium emission lines and TiO absorption bands of a late-type star (Iijima, 1985, A&A, 153, 35; Viotti, et al., 1986, A&A,159, 16) and indicates its symbiotic nature. The red component is a normal M4-M6 class star (Mondal, et al., 2020, MNRAS, 492, 2326), not of a Mira variable. It loses mass, and a part of lost material is accreted by the compact companion. The emission spectrum originates in the ionized envelope formed by the red giant wind. In addition, the HeII 4686A emission line and traces of [OIII] 5007A were also observed in the visible region of the spectrum (Iijima, 1985, A&A, 153, 35). Some authors consider that the hot star is a shell flash white dwarf (Dumm, et al., 1998, A&A,336, 637; Mondal, et al., 2020, MNRAS, 492, 2326).
Despite BX Mon is an eclipsing system, Influence of the hot star eclipses is negligible (Leibowitz & Formiggini, 2011, MNRAS, 414, 240). It was suggested that the origin of brightness fluctuations is in periodic episodes of mass accretion from the M giant onto the hot component of the system during the periastron passage. The brightening of BX Mon were observed in 1978, 1984, 2003 and 2013 (Dumm, et al., 1998, A&A,336, 637; Leibowitz & Formiggini, 2011, MNRAS, 414, 240). New brightening began in the early 2019.

Figure 1. Variation of the red spectrum of BX Mon. Axis X corresponds to λ(A), axis Y – absolute fluxes in erg/cm2 sec A. Panel (a)- spectrum at the previous maximum in 2018. Panel (b) – the spectrum at the beginning of brightening 2019.

At the Fesenkov Astrophysical Institute, spectral and photometric observations of this object were carried out from 2011. Our photometric results for the entire period are presented in Table 1. The orbital phases are given in the third row. They were calculated with the ephemeris
given by (Fekel, et al., 2000) JD(p) = 2449680 +1259xE; where JD(p) is the time moment of periastron passage. Active stage of BX Mon lasted from January 2019 until April 2021 (Fig 2).
After publishing our data in Astronomer’s Telegram, Munari U. shared with us his unpublished photometric data for 2005-2022. New information has revealed that the brightening of 2019-2022 are a continuation of the regular brightness fluctuations of the object.

Figure 2. Variation of the red spectrum of BX Mon. Axis X corresponds to λ(A), axis Y – absolute fluxes in erg/cm2 sec A. Panels (c) and (d) – the spectra during brightening 2019. Pane l(e)- spectrum during the minimum of brightness, and panel (f) – the spectrum in the new brightening in 2022.

Main features of the 2019 brightening are as follow:
1. Maximal values of brightness in 2019 are comparable only to those of flash 2003. The other maxima were weaker.
2. Brightening 2019 began at the zero phase (simultaneously with periastron passage), although previous flashes occurred in about 350 days after periastron passage[Leibowitz, MNRAS, 2011, 41, 2406].
3. Behavior of the spectrum of BX Mon during the 2019-2022 is of particular interest.
Emission lines of HI were always presented in the spectrum. Sometimes, emissions of HeI and HeII were observed.
Fig 1, presents fragments of the spectrum of BX Mon in the red range, obtained In 2019 – 2022. For comparison we added the spectrum of the previous maximum in 2018 with the emission lines of Hα and HeI,6678 (Fig 1a). On contrary, during brightening 2019 only Hα was observed, and it was gradually weakening (panels (b),(c),(d) ) up to the complete disappearance (Fig 1e). Moreover, during the next active stage in 2022, the emission lines were also absent (Fig 1f). Two “blue” spectra of BX Mon (Fig 2) show, that Hβ had absorption component already in the beginning of 2019, and other blue lines were also in absorption (Fig 2).

Thus, the emission spectrum of BX Mon is absent for a long time (about a year). It happens for the first time in the history of observations of the object. It looks like that in 2019 the outburst has led to fundamental changes in the stellar system. It was believed that the source of ionization in BX Mon was accretion disc around the hot star. Absence of emission lines in the recent spectra suggests that the accretion disk was destroyed in 2019, and now we observed the results of recombination of a previously ionized gas. Probably, the hot star is shielded by an envelope formed from the accretion material from the red giant.

Figure 3. Variation of the blue spectrum of BX Mon during maximal and minimal brightness. The axes are the same as in Fig 1,2.