Development of a Method for Determining the Search Window for Solar Flare Neutrinos

Abstract

Neutrinos generated during solar flares remain elusive. However, after 50 years of discussion and search, the potential knowledge unleashed by their discovery keeps the search crucial. Neutrinos associated with solar flares provide information on otherwise poorly known particle acceleration mechanisms during a solar flare. For neutrino detectors, the separation between atmospheric neutrinos and solar flare neutrinos is technically encumbered by an energy band overlap. To improve differentiation from background neutrinos, we developed a method to determine the temporal search window for neutrino production during solar flares. Our method is based on data recorded by solar satellites, such as the Geostationary Operational Environmental Satellite (GOES), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and GEOTAIL. In this study, we selected 23 solar flares above X5.0 class that occurred between 1996 and 2018. We analyzed the light curves of soft X-rays, hard X-rays, $\gamma$-rays, line $\gamma$-rays from neutron capture as well as the derivative of soft X-rays. The average search windows are determined as follows: 4178 s for soft X-ray, 700 s for the derivative of soft X-ray, 944 s for hard X-ray (100 – 800 keV), $1{,}586$ s for line $\gamma$-ray from neutron captures, and 776 s for hard X-ray (above 50 keV). This method allows neutrino detectors to improve their sensitivity to solar flare neutrinos.

Appendices

Appendix A: Line $\gamma$-ray Light Curves for Solar Flares

As mentioned in Section 2, the observation of $\gamma$-rays indicates the production of neutrinos via hadronic interactions, thus constituting the most important channel in this analysis. The present study captures four solar flares that include line $\gamma$-rays. In the main text, light curves for the solar flare occurring on November 2, 2003 are shown, because all channels are available for this event. Light curves of the other three flares are shown as follows: July 23, 2002 (Figure 10), October 29, 2003 (Figure 11), and January 20, 2005 (Figure 12).

Figure 11

Light curves of the solar flare occurring on October 29, 2003. Gray bands show the search windows as same in Figure 5. The vertical dotted lines show the peak timing for each channel. As described in the appendix text, both hard X-ray and line $\gamma$-ray are not used to determine the time window since their light curves were contaminated. The solid black line on third panel from top shows hard X-ray light curve using imaging method. The solid black line on fourth panel from top shows time profile of line $\gamma$-ray flux which is the same as Figure 6(c) in Kurt et al. (2017).

Figure 12

Light curves of the solar flare occurred on January 20, 2005. Gray bands show the search windows as same in Figure 5. The vertical dotted lines show the peak timing for each channel.

Appendix B: Comment on the Solar Flare Occurring on October 29, 2003

The time profile of the flare occurring on October 29, 2003 is not similar to those of the other flares, because signal is contaminated with non-solar hard X-rays of magnetospheric origin. For only this flare, the peak timings of both hard X-rays and line $\gamma$-rays are delayed relative to that of soft X-ray as shown in Figure 11. The light curve of hard X-rays shows several minor peaks. To confirm the contamination was caused by non-solar-flare origin signal, we used spatial information from the imaging method to see the flare region on the surface of the Sun. The light curve with imaging method is shown in the third panel of Figure 11. The line $\gamma$-ray light curves shown in Kurt et al. (2017) is overlaid in the fourth panel of Figure 11.

Appendix C: Light Curves for the Largest Solar Flare; November 4, 2003

The largest solar flare on record occurred on November 4, 2003, with class X28.0. This flare attracts significant attention because of the presumably chance of solar-flare neutrino detection. Figure 13 shows the light curves for this solar flare.

Figure 13

Light curves of the solar flare occurring on November 4, 2003. Gray bands show the search windows as same in Figure 5. The vertical dotted lines show the peak timing for each channel. Because of high intensity, the soft X-ray flux was saturated.

The GOES satellite instrument was saturated due to the high intensity of soft X-rays, and did not continue to record data for more than 15 min around 19:45–20:00. Because of this situation, we set the peak timing of soft X-ray for this flare at the middle point of the saturation phase, instead of the way outlined in the body of this paper. On the other hand, we could not set the peak timing of the derivative of soft X-rays due to the saturation. For this reason, we excluded this flare from comparison of peak timing with soft X-rays shown in Figure 8. Moreover, we used a special treatment to set the end time of the derivative of soft X-rays at the end time of the saturation of soft X-rays.

Unfortunately, the RHESSI satellite did not record data related to this solar flare because it entered the Earth’s shadow soon after the flare occurred.