A Revisit of the Masuda Flare

Abstract

We revisit the flare on 1992 January 13, which is now universally termed the "Masuda flare". The revisit is motivated not only by its uniqueness despite accumulating observations of \hxr coronal emission, but also by the improvement of Yohkoh hard X-ray imaging, which was achieved after the intensive investigations on this celebrated event. Through an uncertainty analysis, we show that the hard X-ray coronal source is located much closer to the soft X-ray loop in the re-calibrated HXT images than in the original ones. Specifically, the centroid of the M1-band (23--33 keV) coronal source is above the brightest pixel of the SXT loop by ~5000+/-1000 km (~9600 km in the original data); and above the apex of the 30% brightness contour of the SXT loop by ~2000+/-1000 km (~7000 km in the original data). We suggest that this change may naturally account for the fact that the spectrum of the coronal emission was reported to be extremely hard below ~20 keV in the pre-calibration investigations, whereas it has been considerably softer in the literature since Sato's re-calibration circa 1999. Still, the coronal spectrum is flatter at lower energies than at higher energies, owing to the lack of a similar source in the L-band (14--23 keV), which remains a puzzle.