Supernova neutrino burst monitor at the Baksan Underground Scintillation Telescope

Abstract

The experiment on recording neutrino bursts operates since the mid-1980. As a target, we use two parts of the facility with a total mass of 240 tons. The current status of the experiment and some results related to the investigation of background events and the stability of facility operation are presented. Over the period of June 30, 1980 to December 31, 2018, the actual observational time is 33.02 years. No candidate for the stellar core collapse has been detected during the observation period. The corresponding upper bound of the mean frequency of core collapse supernovae in our Galaxy is 0.070 year${}^{-1}$ (90% CL).

Introduction

Recording the supernova SN 1987A has made a considerable impact on both theoretical investigation of the SN phenomenon and the development of experimental facilities. Core collapse supernovae are among the most powerful sources of neutrinos in the Universe.

The detection of neutrinos from SN1987A experimentally proved the critical role of neutrinos in the explosion of massive stars, as it was suggested more than 50 years ago [1], [2], [3].

Due to their high penetration power, neutrinos deliver information on physical conditions in the core of the star during the gravitational collapse. SN1987A has become the nearest supernova in the past several hundred years, which allowed the SN formation process to be observed in unprecedented detail beginning with the earliest time of radiation. It was the first time that a possibility arose for comparing the main parameters of the existing theory – total radiated energy, neutrino temperature, and neutrino burst duration – with the experimentally measured values [4], [5].

The SN1987A event has demonstrated significant deviations from spherical symmetry. It means the SN phenomenon is a substantially multidimensional process. In recent years great progress has been achieved in two-dimensional (2D) and three-dimensional (3D) computer simulations of a SN explosion. 3D simulations of the evolution of massive stars at the final stage of their life (SN progenitors) have revealed the very important role of non-radial effects. However, further analysis would be mandatory when high-resolution 3D-simulations will become available.

Since light (and electromagnetic radiation in general) can be partially or completely absorbed by dust in the galactic plane, the most appropriate tool for finding supernovae with core collapse are large neutrino detectors. In the past decades (since 1980), the search for neutrino bursts was carried out with such detectors as the Baksan Scintillation Telescope [6], [7], Kamiokande [8] and Super-Kamiokande [9], MACRO [10], LVD [11], AMANDA [12] and SNO [13]. Over the years, our understanding of how massive stars explode and how the neutrino interacts with hot and dense matter has increased by a tremendous degree. At present the scale and sensitivity of the detectors capable of identifying neutrinos from a Galactic supernova have grown considerably so that current generation detectors [14], [15], [16], [17] are capable of detecting of order ten thousand neutrinos for a supernova at the Galactic Center.

So the neutrino flux from a next SN will be measured with several detectors providing an unprecedented reliability of obtained information.

The Baksan Underground Scintillation Telescope (BUST) [18] is a multipurpose detector intended for wide range of investigations in cosmic-ray and particle physics. One of the tasks is the search for neutrino bursts. The facility has been uninterruptedly used for this purpose since the middle of 1980. In 2001, a significant modernization of the data acquisition system was carried out. This system operate since March 6, 2001. The data processing system until 2001 is described in [6]. The total observation time of the Galaxy amounts to 90% of the calendar time.

The paper is organized as follows. Section 2 is a brief description of the facility. Section 3 is devoted to the neutrino burst detection method. Section 4 describes some characteristics of background events. In Section 5, we present the use of two parts of the BUST as two independent detectors (this allows to increase the target mass). The process of generating a warning about a neutrino burst is described in Section 6. Conclusion is presented in Section 7.