SAVNET and Japanese VLF observations of ionospheric disturbances caused by giant recurrent gamma-ray flares on January 2009

An exotic neutron star 1E 1547.0-5408, which is known to be an Anomalous X-ray Pulsar or Magnetar, emitted energetic gamma-ray flares repeatedly on January 2009. Since the gamma-ray fluxes were estimated to be comparable to that of X-class large solar flares, it was expected that gamma-rays from the flares affect the Earth's ionosphere. South America VLF Network (SAVNET) is a new observing facility in Very Low Frequency range, and has been recently installed in South America. In the SAVNET data, we found sudden amplitude and phase changes of VLF signals at the corresponding times of the flares. These changes indicate that gamma-rays from the flares unusually ionized the lower ionosphere, and caused ionospheric disturbances. Since the flares mainly illuminated the Southern hemisphere of the Earth, such a sudden change was not found in the VLF data received in Japan. Most of the flares were detected by X-ray/gamma-ray detectors on satellites in space, but only the light curves were reported and the energy spectra were not presented so far due to lack of energy resolution and other instrumental problems caused by the intense gamma-ray flux and unexpected photon incident angle. Here, we deduce spectral information of each flare from the VLF data combined with model calculations. First, by using Monte Carlo method, we calculate the altitude profile of electrons produced by incident gamma-rays. Then, we simulate the VLF radio wave propagation using FDTD (Finite Difference Time Domain) method, and compare the simulation results with the observation. Thus, we derive the most probable spectral shapes and discuss possible flare mechanisms.