THE FIRST FOCUSED HARD X-RAY IMAGES OF THE SUN WITH NuSTAR

Grefenstette, Brian W.; Glesener, Lindsay; Krucker, S.; Hudson, H. S.; Hannah, I. G.; Smith, David M.; Vogel, Julia K.; White, S. M.; Madsen, Kristin K.; Marsh, Andrew; Caspi, Amir; Bin, Chen; Shih, Albert Y.; Kuhar, Matej; Boggs, Steven E.; Christensen, Finn Erland; Craig, William W.; Forster, Karl; Hailey, Charles J.; Harrison, Fiona A.; Miyasaka, Hiromasa; Stern, Daniel; Zhang, William W.

doi:10.3847/0004-637x/826/1/20

Cited by 64 publications

(86 citation statements)

References 35 publications

(41 reference statements)

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“…Since the pileup of photons arriving in quick succession could, in principle, produce a high-energy excess, we checked the pileup probability as indicated by the "non-physical" event grades; see Appendix C of Grefenstette et al (2016) for an explanation. Since no events associated with the microflare were found to have non-physical grades, we conclude that pulse pileup does not affect our spectra.…”

Section: High-energy Excess In the Impulsive Phasementioning

confidence: 99%

“…In recent years, new instruments have begun to demonstrate the dramatically increased sensitivity available via direct HXR focusing as opposed to RHESSI's indirect imaging method , with the first two flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket (Krucker et al 2014;Glesener et al 2016) and occasional solar pointings by the Nuclear Spectroscopic Telescope Array (NuSTAR) astrophysics spacecraft (Harrison et al 2013;Grefenstette et al 2016). Focusing HXR instruments, with their larger effective areas and drastically reduced detector backgrounds, can measure flares of smaller temperatures, brightnesses, and total energies than those available to indirect imagers.…”

Section: Introductionmentioning

confidence: 99%

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NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

Glesener

Krucker

Hannah

et al. 2017

ApJ

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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×10 27 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.

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Section: High-energy Excess In the Impulsive Phasementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

Glesener

Krucker

Hannah

et al. 2017

ApJ

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“…However, NuSTAR was designed for astrophysical observations and is therefore not optimized for observations of the Sun. This leads to various technical challenges (see Grefenstette et al 2016), but NuSTAR is nevertheless a unique instrument for solar observations and has pointed at the Sun several times. NuSTAR has observed several faint sources from quiescent ARs (Hannah et al 2016) and emission from an occulted flare, in the EUV late phase (Kuhar et al 2017).…”

Section: Introductionmentioning

confidence: 99%

Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA

Wright

Hannah

Grefenstette

et al. 2017

ApJ

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NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/ XRT on 2015 April 29 at ∼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed in soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single thermal model of 3.2-3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK; but during the microflare's impulsive phase, the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about2.5 10 25 erg s −1. As the NuSTAR spectrum is purely thermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of heating, a power-law spectrum of  d 7 with a low-energy cutoff  E 7 c keV is required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.

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“…Of particular interest is the ratio of pairs of emission lines characteristic of cool and hot plasma, as was recently discussed by Brosius et al (2014). When high-temperature spectral coverage is limited, information from high-energy instruments (Ishikawa et al 2014;Grefenstette et al 2016;Hannah et al 2016) would be then desirable, but the energies of interest (≈1 keV) are highly Figure 11. Emission measure ratio as a function of t N for the single-fluid (left), electron heating (center), and ion heating (right) cases for four different line pairs: Fe XIX/Fe XII (blue), Fe XIX/Fe XV (green), Fe XX/Fe XII (red), and Fe XX/Fe XV (yellow).…”

Section: 942mentioning

confidence: 99%

Inference of Heating Properties From “Hot” Non-Flaring Plasmas in Active Region Cores. Ii. Nanoflare Trains

Barnes

Cargill

Bradshaw

2016

ApJ

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Despite its prediction over two decades ago, the detection of faint, high-temperature ("hot") emission due to nanoflare heating in non-flaring active region cores has proved challenging. Using an efficient two-fluid hydrodynamic model, this paper investigates the properties of the emission expected from repeating nanoflares (a nanoflare train) of varying frequency as well as the separate heating of electrons and ions. If the emission measure distribution (EM(T)) peaks at T = T m , we find that EM(T m ) is independent of details of the nanoflare train, and EM(T) above and below T m reflects different aspects of the heating. Below T m , the main influence is the relationship of the waiting time between successive nanoflares to the nanoflare energy. Above T m , power-law nanoflare distributions lead to an extensive plasma population not present in a mono-energetic train. Furthermore, in some cases, characteristic features are present in EM(T). Such details may be detectable given adequate spectral resolution and a good knowledge of the relevant atomic physics. In the absence of such resolution we propose some metrics that can be used to infer the presence of "hot" plasma.

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THE FIRST FOCUSED HARD X-RAY IMAGES OF THE SUN WITH NuSTAR

Cited by 64 publications

References 35 publications

NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA

Inference of Heating Properties From “Hot” Non-Flaring Plasmas in Active Region Cores. Ii. Nanoflare Trains

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