Observation of an energetic radiation burst from mountain-top thunderclouds

During thunderstorms on 2008 September 20, a simultaneous detection of gamma rays and electrons was made at a mountain observatory in Japan located 2770 m above sea level. Both emissions, lasting 90 seconds, were associated with thunderclouds rather than lightning. The photon spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung gamma rays arriving from a source which is 60 - 130 m in distance at 90% confidence level. The observed electrons are likely to be dominated by a primary population escaping from an acceleration region in the clouds.Comment: 12 pages, 3 figures, accepted for publication in Physical Review Letter

Broad band X-ray spectroscopy of A0535+262 with SUZAKU

The transient X-ray binary pulsar A0535+262 was observed with Suzaku on 2005 September 14 when the source was in the declining phase of the August-September minor outburst. The ~103 s X-ray pulse profile was strongly energy dependent, a double peaked profile at soft X-ray energy band (<3 keV) and a single peaked smooth profile at hard X-rays. The width of the primary dip is found to be increasing with energy. The broad-band energy spectrum of the pulsar is well described with a Negative and Positive power-law with EXponential (NPEX) continuum model along with a blackbody component for soft excess. A weak iron K_alpha emission line with an equivalent width ~25 eV was detected in the source spectrum. The blackbody component is found to be pulsating over the pulse phase implying the accretion column and/or the inner edge of the accretion disk may be the possible emission site of the soft excess in A0535+262. The higher value of the column density is believed to be the cause of the secondary dip at the soft X-ray energy band. The iron line equivalent width is found to be constant (within errors) over the pulse phase. However, a sinusoidal type of flux variation of iron emission line, in phase with the hard X-ray flux suggests that the inner accretion disk is the possible emission region of the iron fluorescence line.Comment: 21 pages, 6 figures. Accepted for publication in the Astrophysical Journal, 2008 January issu

Ambient Conditions of Winter Thunderstorms in Japan to Reproduce Observed Gamma‐Ray Glow Energy Spectra

Electric field of thunderclouds modifies components and energy spectra of the cosmic-ray air shower. In particular, thunderstorms accelerate charged particles, resulting in an enhancement of gamma-ray fluxes on the ground, known as a gamma-ray glow. This phenomenon has been observed in recent years by the Gamma-Ray Observation of Winter THunderclouds collaboration from winter thunderstorms in the Hokuriku area of Japan. The present work examines the ambient conditions required to produce spectral features of the previously detected gamma-ray glows, by using Monte Carlo simulations of particle interactions in the atmosphere. We focus on three parameters, the strength and length of the electric field, and the length of a null-field attenuation region below the electrified region. The average spectrum of observed gamma-ray glows in winter thunderstorms of Japan requires an electric field intensity close to 0.31 MV/m, slightly exceeding the Relativistic Runaway Electron Avalanche threshold of 0.284 MV/m. The vertical size of the electric field region should be comparable to 1 km. The estimated attenuation region size is 300–500 m, necessary to reduce the low-energy photon flux of the average gamma-ray glows. There is still a wide range of acceptable parameter sets with degeneracy to make a similar spectrum

Termination of Electron Acceleration in Thundercloud by Intra/Inter-cloud Discharge

An on-ground observation program for high energy atmospheric phenomena in winter thunderstorms along Japan Sea has been performed via measurements of gamma-ray radiation, atmospheric electric field and low-frequency radio band. On February 11, 2017, the radiation detectors recorded gamma-ray emission lasting for 75 sec. The gamma-ray spectrum extended up to 20 MeV and was reproduced by a cutoff power-law model with a photon index of $1.36^{+0.03}_{-0.04}$, being consistent with a Bremsstrahlung radiation from a thundercloud (as known as a gamma-ray glow and a thunderstorm ground enhancement). Then the gamma-ray glow was abruptly terminated with a nearby lightning discharge. The low-frequency radio monitors, installed $\sim$50 km away from the gamma-ray observation site recorded leader development of an intra/inter-cloud discharge spreading over $\sim$60 km area with a $\sim$300 ms duration. The timing of the gamma-ray termination coincided with the moment when the leader development of the intra/inter-cloud discharge passed 0.7 km horizontally away from the radiation monitors. The intra/inter-cloud discharge started $\sim$15 km away from the gamma-ray observation site. Therefore, the glow was terminated by the leader development, while it did not trigger the lightning discharge in the present case.Comment: 12 pages, 4 figures, accepted for publication in Geophysical Research Letter

Energy-Dependent Harmonic Ratios of the Cyclotron Features of X0331+53 in the 2004-2005 Outburst

We report on changes of the cyclotron resonance energies of the recurrent transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the 2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between 1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a clear anti-correlation to the source luminosity, and without any hysteresis effects between the rising and declining phases of the outburst. In contrast, the second harmonic energy changed from ~49 to ~54 keV, implying a weaker fractional change as a function of the luminosity. As a result, the observed resonance energy ratio between the second harmonic and the fundamental was ~2.2 when the source was most luminous, whereas the ratio decreased to the nominal value of 2.0 at the least luminous state. Although the significance of this effect is model dependent, these results suggest that the fundamental and second harmonic resonances represent different heights in the accretion column, depending on the mass accretion rate.Comment: 39 pages, 15 figures, 4 tables. Accepted for publication in Astrophysical Journa