Abstract

--- - Context. Methanol masers of Class I (collisionally pumped) and Class II (radiatively pumped) have been studied in great detail in our Galaxy in a variety of astrophysical environments such as shocks and star-forming regions and are they are helpful to analyze the properties of the dense interstellar medium. However, the study of methanol masers in external galaxies is still in its infancy. - Aims. Our main goal is to search for methanol masers in the central molecular zone (CMZ; inner 500 pc) of the nearby starburst galaxy NGC 253. - Methods. Covering a frequency range between 84 and 373 GHz (lambda = 3.6-0.8 mm) at high angular (1.('')6 similar to similar to 27 pc) and spectral (similar to 8-9 km s(-1)) resolution with ALCHEMI (ALMA Comprehensive High-resolution Extragalactic Molecular Inventory), we have probed different regions across the CMZ of NGC 253. In order to look for methanol maser candidates, we employed the rotation diagram method and a set of radiative transfer models. - Results. We detect for the first time masers above 84 GHz in NGC 253, covering an ample portion of the J(-1)->(J - 1)(0) - E line series (at 84, 132, 229, and 278 GHz) and the J(0)->(J - 1)(1) - A series (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, which was already reported, but now being detected in more than one location. For the J(-1)->(J- 1)(0) - E line series, we observe a lack of Class I maser candidates in the central star-forming disk. - Conclusions. The physical conditions for maser excitation in the J(-1)->(J - 1)(0) - E line series can be weak shocks and cloud-cloud collisions as suggested by shock tracers (SiO and HNCO) in bi-symmetric shock regions located in the outskirts of the CMZ. On the other hand, the presence of photodissociation regions due to a high star-formation rate would be needed to explain the lack of Class I masers in the very central regions.