On-ground detection of an electron-positron annihilation line from thunderclouds

Thunderclouds can produce bremsstrahlung gamma-ray emission, and sometimes even positrons. At 00:27:00 (UT) on 13 January 2012, an intense burst of gamma rays from a thundercloud was detected by the GROWTH experiment, located in Japan, facing the Sea of Japan. The event started with a sharp gamma-ray flash with a duration of $<300$ ms coincident with an intracloud discharge, followed by a decaying longer gamma-ray emission lasting for $\sim 60$ s. The spectrum of this prolonged emission reached $\sim 10$ MeV, and contained a distinct line emission at $508\pm 3(\mathrm{stat}.)\pm 5(\mathrm{sys}.)$ keV, to be identified with an electron-positron annihilation line. The line was narrow within the instrumental energy resolution $(\sim 80\text{keV})$, and contained $520\pm 50$ photons which amounted to $\sim 10\%$ of the total signal photons of $5340\pm 190$ detected over 0.1–10 MeV. As a result, the line equivalent width reached $280\pm 40$ keV, which implies a nontrivial result. The result suggests that a downward positron beam produced both the continuum and the line photons.

Figure 1

Count-rate histories of the present burst recorded by the NaI detector. Abscissa and ordinates represent time (second) and count rates, respectively. Error bars show statistical $1\sigma$. (a)–(c) represent the count rates in 121–2544 keV (1 s binning), 584–2544 keV (10 s binning), and 448–557 keV containing the 511 keV line (10 s binning), respectively. (d) is the 511 keV line count rate obtained via Gaussian fitting to spectra integrated every 10 s. (e) gives those in 121–420 keV (1 s binning), which includes Compton-downscattered 511 keV photons. Dashed curves represent exponentials with a decay constant of 30 s.

Figure 2

NaI hit density, plotted on the plane of time (abscissa) and energy (ordinate). The intracloud discharge happened at time $t=0$. The gray scale represents counts per pixel, with sizes of 2 s in time and $\simeq 27$ keV in energy.

Figure 3

Energy spectra of the NaI detector accumulated for 100 s before (dark gray) and after (light gray) the discharge. The red histogram shows the difference between them, which represents net signals from the present phenomenon.

Figure 4

A comparison of the signal spectrum of 20120113a (crosses) to that of another GROWTH event detected on 13 December 2007 (open circles [8]). The latter one is rescaled by a factor of 1/10, and its highest energy bin contains all the photons with energies $>10$ MeV.