Detection prospects of the Telescope Array hotspot by space observatories

In the present-day cosmic ray data, the strongest indication of anisotropy of the ultrahigh energy cosmic rays is the 20-degree hotspot observed by the Telescope Array with the statistical significance of $3.4\sigma$. In this work, we study the possibility of detecting such a spot by space-based all-sky observatories. We show that if the detected luminosity of the hotspot is attributed to a physical effect and not a statistical fluctuation, the KLYPVE and JEM-EUSO experiments would need to collect $\sim 300$ events with $E>57\mathrm{EeV}$ in order to detect the hotspot at the $5\sigma$ confidence level with the 68% probability. We also study the dependence of the detection prospects on the hotspot luminosity.

Figure 1

Probability distributions of the number of events $n_s$ in the TA hotspot region for the isotropic distribution (H0) and in the case of the hotspot with parameters as determined by TA [21] (H1). The total number of events is 250. The vertical line shows the value ${\overline{n}}_s$ such that 68% of realizations have the signal at least that strong.

Figure 2

The significance of the isotropy hypothesis rejection as a function of the total number of detected events $N$. The central curve (red) $f=f_0$: hotspot brightness as deduced from the five-year TA data. The shaded band: corresponding $1\sigma$ uncertainty. Horizontal dashed lines show the $3\sigma$ evidence and $5\sigma$ discovery levels.