Observation of a muon excess following a gamma-ray burst event detected at the International Space Station

Abstract

On April 24, 2012, at 16:47:14 UT, the Gas Slit Camera (GSC) of the Japanese Monitor of All-sky X-ray Image (MAXI) instrument on the International Space Station detected a short x-ray transient lasting about 34 seconds. The MAXI/GSC transient was most likely a gamma-ray burst (GRB), because of the high Galactic latitude, spectral hardness ratio, and the absence of known bright x-ray sources at the detected position. In addition, the MAXI/GSC transient GRB 120424A coordinates were in the field of view of the inclined Tupi muon telescope located at ground level (3 m above sea level) at (22:9 W, 43:2 S) in the South Atlantic Anomaly region. We report here that the Tupi telescope registered a muon excess with a signal significance 6:2 within the MAXI/GSC transient time period. Assuming a power law function with a spectral index of ¼ À1:54 in the tail of the primary gamma-ray energy spectrum, we can conclude that the fluence obtained from the muon excess detected by the Tupi telescope is consistent with the preliminary value obtained by the MAXI team. This result agrees with an assumption that the muons were produced in photonuclear reactions in the Earth's atmosphere. In addition, we show also that the South Atlantic Anomaly region can be a favorable place at ground for the detection of the tail of the energy spectrum (the GeV counterpart) of some GRBs.

Possible GeV counterpart at the ground level associated with Fermi LAT gamma-ray bursts

“…At least in part, this is a consequence of the very low detection energy threshold of Tupi detectors, as well as the specific physical location of the instrument in the South Atlantic Anomaly (SAA) region, where the geomagnetic field is anomalously weak ∼22,000 nT. We also noticed that the time of an excess in the previously reported cases [23,24] was near the time of the maximum of the Earth's fair weather atmospheric electric field (the Carnegie curve) [25]. This time is close to the sunset time at Tupi location.…”

“…In the past three years (since 8 June 2014) New-Tupi detector situated in the SAA, searched for temporal (within the T90 duration) and spatial (within the FoV) at the time of the occurrence) association with GRBs detected by satellites [23,24]. New-Tupi is an upgrade of the previous Tupi telescopes, the detector size becomes six times larger and the plastic scintillators thickness was doubled.…”

Biological radiation dose from secondary particles in a Milky Way gamma-ray burst

“…For GRB 120424A (Augusto et al, 2013), which is known to have produced muons on the ground, the reported observed rate was 1.78x10 -7 erg/cm 2 /sec over 34 sec, which equals 6.05x10 -9 J/m 2 fluence. Using a spectral index of -1.54 from the above reference, we computed the muon fluence on the surface to be 6x10 -4 muons/m 2 at the ground integrated over 10-100 GeV range.…”

“…We use two cases of hardspectrum GRBs and evaluate their terrestrial effects. We used the Band et al (1993) and GRB 120424A spectra (Augusto et al, 2013) for this work.…”

“…GRB 120424A was observed with the MAXI/GSC instrument abroad the ISS where the prompt emission lasted for 34 seconds (Serino et al, 2012). Muons were observed with the ground-based Tupi muon observatory with a significance of 6.2 standard deviations (Augusto et al, 2013). This suggests muon production by gamma rays of energies greater than 10 GeV.…”