Impulsive radio and hard X-ray emission from an M-class flare

¹ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
e-mail: liusm@pmo.ac.cn, zhangping@pmo.ac.cn
² University of Chinese Academy of Sciences, Yuquan Road 19, Beijing 100049, PR China
³ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
⁴ Paul Scherrer Institut, Villigen PSI 5232, Switzerland
e-mail: ping.zhang@psi.ch

Received: 30 May 2017
Accepted: 25 January 2018

Abstract

Context. Impulsive radio and hard X-ray emission from large solar flares are usually attributed to a hard distribution of high-energy electrons accelerated in the energy dissipation process of magnetic reconnection.

Aims. We report the detection of impulsive radio and hard X-ray emissions produced by a population of energetic electrons with a very soft distribution in an M-class flare: SOL2015-08-27T05:45 .

Methods. The absence of impulsive emission at 34 GHz and hard X-ray emission above 50 keV and the presence of distinct impulsive emission at 17 GHz and lower frequencies and in the 25–50 keV X-ray band imply a very soft distribution of energetic electrons producing the impulsive radio emission via the gyro-synchrotron process, and impulsive X-rays via bremsstrahlung.

Results. The spectrum of the impulsive hard X-ray emission can be fitted equally well with a power-law model with an index of ∼6.5 or a super-hot thermal model with a temperature as high as 100 MK. Imaging observations in the extreme-UV and X-ray bands and extrapolation of the magnetic field structure using a nonlinear force-free model show that energetic electrons trapped in coronal loops are responsible for these impulsive emissions.

Conclusions. Since the index of the power-law model is nearly constant during the impulsive phase, the power-law distribution or the super-hot component should be produced by a bulk energization process such as the Fermi and betatron acceleration of collapsing magnetic loops.