RESUMO
We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket flight on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to energies of 4-20 keV. Through spectral analysis of the microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to ~10 MK and emission measures down to ~1044 cm-3. Though nonthermal emission was not detected for the FOXSI-2 microflares, a study of the parameter space for possible hidden nonthermal components shows that there could be enough energy in nonthermal electrons to account for the thermal energy in microflare 1, indicating that this flare is plausibly consistent with the standard thick-target model. With a solar-optimized design and improvements in HXR focusing optics, FOXSI-2 offers approximately five times greater sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array for typical microflare observations and allows for the first direct imaging spectroscopy of solar HXRs with an angular resolution at scales relevant for microflares. Harnessing these improved capabilities to study small-scale events, we find evidence for spatial and temporal complexity during a sub-A class flare. This analysis, combined with contemporaneous observations by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, indicates that these microflares are more similar to large flares in their evolution than to the single burst of energy expected for a nanoflare.
RESUMO
We study the structure and dynamics of extreme flaring events on young stellar objects (YSOs) observed in hard X-rays by the Nuclear Spectroscopic Telescope Array (NuSTAR). During 2015 and 2016, NuSTAR made three observations of the star-forming region ρ Ophiuchi, each with an exposure ~50 ks. NuSTAR offers unprecedented sensitivity above ~7 keV, making this data set the first of its kind. Through improved coverage of hard X-rays, it is finally possible to directly measure the high-energy thermal continuum for hot plasmas and to sensitively search for evidence of nonthermal emission from YSO flares. During these observations, multiple flares were observed, and spectral and timing analyses were performed on three of the brightest flares. By fitting an optically thin thermal plasma model to each of these events, we found flare plasma heated to high temperatures (~40-80 MK) and determined that these events are ~1000 times brighter than the brightest flares observed on the Sun. Two of the studied flares showed excess emission at 6.4 keV, and this excess may be attributable to iron fluorescence in the circumstellar disk. No clear evidence for a nonthermal component was observed, but upper limits on nonthermal emission allow for enough nonthermal energy to account for the estimated thermal energy in the flare on protostar IRS 43, which is consistent with the standard model for solar and stellar flares.
