Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 48 (2015)

Timing analysis of solar flares in hard X-ray and soft γ-ray bands
measured by the Suzaku Wide-band All-sky Monitor
W.B. Iwakiri,∗1 on behalf of Suzaku WAM teamHard X-ray and γ-ray observations have proved to bea powerful tool to for studying the physics of particle
acceleration and transfer in solar ﬂares. These highenergy photons are often observed in X class ﬂares deﬁned by the GOES satellite. The origin of emission isthought to be the thick-target bremsstrahlung of highenergy particles accelerated by magnetic reconnection.
Previous observations have revealed that the arrival
times of hard X-ray photons (20 - 200 keV) dependon their energy1) . The energy-dependent time delays
can be explained through the trap plus precipitation
model1) . First, the model suggests that electrons accelerated by magnetic reconnection are injected intoa magnetic loop and trapped (magnetic mirror trap).
Next, the trapped electrons escape the loop by pitchangle scattering and precipitate to the chromosphere.
As a result, hard X-ray photons are produced by thicktarget bremsstrahlung. If Coulomb collisions represent
the dominant pitch angle scattering for trapped electrons, the collisional deﬂection time τ can be represented as2)
τ (E) ∝ E 1.5 .

(1)In this progress report, we performed a timing analysisof the Sept 24 2011 solar ﬂare observed by the Wideband All-sky Monitor (WAM)3) onboard Japanese ﬁfth
X-ray satellite Suzaku from 80 keV to 7 MeV.
The WAM consists of four lateral walls composed ofbismuth germanium oxide Bi4 Ge3 O12 (BGO) crystals.
The event data are recorded with a 1-s time resolution
in the 55 energy band. The WAM has a large eﬀective
area that reaches 400 cm2 at 1 MeV. This is the largest
area in among currently working γ-ray spectrometers
on-board astronomical satellites. Therefore, WAM is
suitable for hard X-ray and soft γ-ray observation ofsolar ﬂares4) . Figure 1 shows the energy-resolved time
proﬁles of the solar ﬂare on Sept 24 2011 observed byWAM. The lightcurves show that the main peak has
shifted progressively later at the higher energy band.
In order to evaluate the time lag quantitatively, wemeasured the delay using a cross correlation techniquebetween the 80 - 130 keV lightcurve and the other eightenergy-resolved lightcurves (130 - 7000 keV band). Before the analysis, we used a low-pass ﬁlter for eachlightcurve by running an average of 10 bins (the timebin size is 1 s) to exclude the higher frequency component, because the observed fast structures are causedby the diﬀerence of electron time-of-ﬂight from directly
precipitating electrons1) . The derived time delays as afunction of energy are shown in Fig.2. The results show
the energy dependence of the time delays are clearlychanged around 1000 keV. In the lower energy band,the relation is roughly consistent with E1.5 , which ispredicted by the pitch angle scattering of Coulomb
collision2) . On the other hand, above 1000 keV, therelation becomes more ﬂat. If we ﬁt the results using
the broken power-law model and the power-law index
of the lower energy is ﬁxed at 1.5, the derived index
of the higher energy band is 0.253±0.008 with a cutoﬀ
energy of 598±9 keV (the errors are 90% conﬁdence).
The estimation of systematic errors caused by a pileup eﬀect is currently in progress.

Fig. 2. The measured energy-dependent time delay of theSept 24 2011 flare using the cross correlation method.
Fig. 1. Background-subtracted lightcurves of solar flare
triggered on Sept 24 2011 09:35:00 UT observed by
Suzaku-WAM in nine energy bands.
∗1RIKEN Nishina CenterReferences
1) M. J. Aschwanden et al: ApJ, 487, 936 (1997)
2) D. B. Melrose, & J. C. Brown: MNRAS, 176, 15 (1976)
3) K.Yamaoka et al: PASJ, 61, 35 (2009)4) A.Endo et al: PASJ, 62, 1341 (2010)- 173 -

完全版2014_本文.indd 173

15/10/16 17:51