Measurements of thermal relaxation of the OGRAN underground setup.

An upgraded version of the OGRAN -- combined optical-acoustic gravitational wave detector -- has been investigated in a long-term operation mode. This installation, located at the Baksan Neutrino Observatory (BNO) INR RAS, is designed to work under the program for detecting collapsing stars in parallel with the neutrino detector: Baksan Underground Scintillation Telescope (BUST). Such joint search corresponds to the modern trend for a development of "multi-messenger astronomy". In this work the effects of thermal relaxation OGRAN are experimentally investigated using passive and active thermal stabilization systems in the underground laboratory BNO PK-14.