## 4 Observational Plan and Intended Measurements in Orbit
### Astrophysical Goals and Issues  
Detecting and observing celestial targets in space in the spectral region of hard X-rays and low-energy gamma rays remains a challenging task especially with small instrumentation suitable for nanosatellite deployment. There is a need for novel ideas regarding how  
Figure 12: Deposited energy spectra of registered particles from a 241-Am source. Results are given for the data used in Figure 11. The distributions are shown for the entire area of the pixel detector (ROI = 0, top left) and separately for the four distinct optical-collimator segments (labeled ROI = 1 thru ROI = 4). to further miniaturize astrophysical payloads to fit the limited volumes and mass limits of CubeSats [8; 15]. However, there are also expectations for additional application areas such as detection and analyses of high-energy events in the Earth atmosphere such as terrestrial gamma ray flashes (TGF) and auroral X-ray emissions. Having a CubeSat on very low orbit, even miniature instrumentation can provide valuable scientific data due to high fluency of radiaton from these phenomena. This orbit type, including necessary drives, is recently under study at the Czech Technical University in Prague, Faculty of Electrical Engineering, in collaboration with external partners. The X-ray and Gamma-ray sky in the energy range covered by the experiment described is rich in violent and variable sources such as high mass X-ray binaries (HMXB), low mass X-ray binaries (LMXB), cataclysmic variables (CV), active galactic nuclei (AGNs), and Gamma-ray bursts (GRBs) [32; 33]. The spectral energy distribution of samples of GRBs is well consistent with the energy range 5 keV - 0.5 MeV of the payload described in this paper [34]. Approximately 50% of all GRBs observed by BATSE within the 1 MeV energy range have a peak energy between 70 and 500 keV. However, the cutoff power-law model (CPL) [35] suggests that most arriving photons are of lower energy than the reported energy. Furthermore, long GRBs and afterglows are also predominantly located within the lower energy range. Thus, the effective observation of GRB favors s detector energy range from 10's keV to no less than 500 keV. The region above 700 keV is expected to provide limited impact. In the sample of more than 300 GRB detected by Suzaku wide-band all-sky monitor covering the energy range 100-10,000 keV [36], the energy fluence ranged from 2.87 \(\times\) 10\({}^{-7}\) to 3.15 \(\times\) 10\({}^{-4}\) erg cm\({}^{-2}\) for long bursts and 4.39 \(\times\) 10\({}^{-7}\) to 9.57 \(\times\) 10\({}^{-6}\)erg cm\({}^{-2}\) for short bursts. This is equal to from 1.16 to 462 photons cm\({}^{-2}\) for long bursts, and from 0.337 to 10.1 photons cm\({}^{-2}\) for short bursts, with the conversion factor from photon fluence to energy fluence amounts to approximately 4.3 \(\times\) 10\({}^{-7}\) erg cm\({}^{-2}\). The 1 sec peak flux was estimated in the sample from 1.91 \(\times\) 10\({}^{-7}\) to 7.26 \(\times\) 10\({}^{-5}\) erg cm\({}^{-2}\)s\({}^{-1}\) for long bursts [36]. The TGF represent probably the most promising target for the payload described in this paper. It is well known that both thunderclouds and lightning can emit high energy photons in the X-ray and Gamma-ray range. The duration of these high-energy triggers is from sub-microseconds (for X-ray emission associated with lightning leaders as well as flares of Gamma-rays known as terrestrial Gamma-ray flashes (TGFs)), to minutes (for glows detected on the Earth surface as well as in the atmosphere). TGFs emitted by thunderclouds, are so luminous that they can affect instruments on satellites by saturation. In addition to that, TGFs represent sources of energetic secondary electrons and positrons detectable by satellites. These X-ray and Gamma-ray emissions are believed to be generated, through Bremsstrahlung, by energetic runaway electrons accelerated by atmospheric electric fields [7].