We report a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only ∼5 counts s-1 detector-1 observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4 1027 erg. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as "flare-like" properties.
Bibliographical noteFunding Information:
Support for this work was provided by NASA HSR grant NNX14AG07G, as well as an NSF Faculty Development Grant (AGS-1429512) to the University of Minnesota. S.K. acknowledges funding from the Swiss National Science Foundation (200021-140308 and 200020-169046). I.G.H. is supported by a Royal Society University Fellowship. The authors are grateful to the NuSTAR Science and Operation teams for promoting and supporting solar observations, to Matej Kuhar and Kathy Reeves for insightful comments on paper drafts, and to the anonymous reviewer for excellent comments and questions that undoubtedly improved the paper.
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- Sun: X-rays, gamma rays
- Sun: corona
- Sun: flares
- X-rays: stars