Abstract:
Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. Some of them, the short GRBs, are important sources of gravitational waves originating in mergers of neutron stars or possibly also in mergers of neutron stars with black holes. I will present the detector performance and early science results from GRBAlpha, a 1U CubeSat mission, which is a technological pathfinder to a future constellation of nanosatellites monitoring and localazing GRBs. The localization can be achieved by measuring the time difference between the arrival of the signal at different satellites (synchronized by GPS). GRBAlpha was launched in March 2021 and has been operating already about a year on a 550 km altitude sun-synchronous orbit. The onboard gamma-ray burst detector consists of a 75×75×5mm CsI(Tl) scintillator, read out by a dual-channel multi-pixel (SiPM) photon counter (MPPC) setup. It is sensitive in the ~30-900 keV range. The main goal of GRBAlpha is the in-orbit demonstration of the detector concept, verification of the detector's lifetime, and measurement of the background level on low-Earth orbit, including polar regions and in the South Atlantic Anomaly. GRBAlpha has already detected five GRBs and was even able to detect two GRBs within 8 hours, proving that nanosatellites can be used for routine detection of gamma-ray transients. For one GRB, we were able to obtain a high resolution spectrum and compare it with measurements from the Swift satellite. We find that, due to the variable background, about half of the low-Earth polar orbit is suitable for gamma-ray burst detection. One year after launch, the detector performance is good and the degradation of the SiPM photon counters remains at an acceptable level. The same detector system, but double in size, was launched in January 2022 on VZLUSAT-2 (3U CubeSat). The GRB detectors perform as expected and I will present the early measurements from this mission as well. Our ultimate aim is to develop and launch a constellation of nanosatellites monitoring GRBs and these precursor missions help us to converge to this ambitious goal.
