skip to main content

Search for: All records

Creators/Authors contains: "Grefenstette, Brian W."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract

    Solar active regions (ARs) contain a broad range of temperatures, with the thermal plasma distribution often observed to peak in the few millions of kelvin. Differential emission measure (DEM) analysis can allow instruments with diverse temperature responses to be used in concert to estimate this distribution. Nuclear Spectroscopic Telescope ARray (NuSTAR) hard X-ray (HXR) observations are uniquely sensitive to the highest-temperature components of the corona, and thus extremely powerful for examining signatures of reconnection-driven heating. Here, we use NuSTAR diagnostics in combination with extreme-ultraviolet and soft X-ray observations (from the Solar Dynamics Observatory/Atmospheric Imaging Assembly and Hinode/X-Ray Telescope) to construct DEMs over 170 distinct time intervals during a 5 hr observation of an alternately flaring and quiet active region (NOAA designation AR 12712). This represents the first HXR study to examine the time evolution of the distribution of thermal plasma in an AR. During microflares, we find that the initial microflare-associated plasma heating is predominantly heating of material that is already relatively hot, followed later on by broader heating of initially cooler material. During quiescent times, we show that the amount of extremely hot (>10 MK) material in this region is significantly (∼2–4 orders of magnitude) less than that found in the quiescent AR observed in HXRs by FOXSI-2. This result implies there can be radically different high-temperature thermal distributions in different ARs, and strongly motivates future HXR DEM studies covering a large number of these regions.

    more » « less

    Microflares are energetically smaller versions of solar flares, demonstrating the same processes of plasma heating and particle acceleration. However, it remains unclear down to what energy scales this impulsive energy release continues, which has implications for how the solar atmosphere is heated. The heating and particle acceleration in microflares can be studied through their X-ray emission, finding predominantly thermal emission at lower energies; however, at higher energies it can be difficult to distinguish whether the emission is due to hotter plasma and/or accelerated electrons. We present the first application of nested sampling to solar flare X-ray spectra, an approach that provides a quantitative degree of confidence for one model over another. We analyse Nuclear Spectroscopic Telescope Array X-ray observations of a small active region microflare (A0.02 GOES/XRS class equivalent) that occurred on 2021 November 17, with a new python package for spectral fitting, sunkit-spex, to compute the parameter posterior distributions and the evidence of different models representing the higher energy emission as due to thermal or non-thermal sources. Calculating the Bayes factor, we show that there is significantly stronger evidence for the higher energy microflare emission to be produced by non-thermal emission from flare-accelerated electrons than by an additional hot thermal source. Qualitative confirmation of this non-thermal source is provided by the lack of hotter (10 MK) emission in Solar Dynamic Observatory’s Atmospheric Imaging Assembly’s extreme ultraviolet data. The nested sampling approach used in this paper has provided clear support for non-thermal emission at the level of 3 × 1024 erg s−1 in this tiny microflare.

    more » « less
  3. Abstract We present NuSTAR observations of the nearby SN 2023ixf in M101 ( d = 6.9 Mpc) that provide the earliest hard X-ray detection of a nonrelativistic stellar explosion to date at δ t ≈ 4 days and δ t ≈ 11 days. The spectra are well described by a hot thermal bremsstrahlung continuum with T > 25 keV shining through a thick neutral medium with a neutral hydrogen column that decreases with time (initial N Hint = 2.6 × 10 23 cm −2 ). A prominent neutral Fe K α emission line is clearly detected, similar to other strongly interacting supernovae (SNe) such as SN 2010jl. The rapidly decreasing intrinsic absorption with time suggests the presence of a dense but confined circumstellar medium (CSM). The absorbed broadband X-ray luminosity (0.3–79 keV) is L X ≈ 2.5 × 10 40 erg s −1 during both epochs, with the increase in overall X-ray flux related to the decrease in the absorbing column. Interpreting these observations in the context of thermal bremsstrahlung radiation originating from the interaction of the SN shock with a dense medium we infer large particle densities in excess of n CSM ≈ 4 × 10 8 cm −3 at r < 10 15 cm, corresponding to an enhanced progenitor mass-loss rate of M ̇ ≈ 3 × 10 − 4 M ⊙ yr −1 for an assumed wind velocity of v w = 50 km s −1 . 
    more » « less
    Free, publicly-accessible full text available July 1, 2024
  4. Abstract We present the first survey of quiet Sun features observed in hard X-rays (HXRs), using the Nuclear Spectroscopic Telescope ARray (NuSTAR), a HXR focusing optics telescope. The recent solar minimum, combined with NuSTAR’s high sensitivity, has presented a unique opportunity to perform the first HXR imaging spectroscopy on a range of features in the quiet Sun. By studying the HXR emission of these features, we can detect or constrain the presence of high temperature (> 5 MK) or non-thermal sources, to help understand how they relate to larger, more energetic solar phenomena, and determine their contribution to heating the solar atmosphere. We report on several features observed in the 28 September 2018 NuSTAR full-disk quiet Sun mosaics, the first of the NuSTAR quiet Sun observing campaigns, which mostly include steady features of X-ray bright points and an emerging flux region, which later evolved into an active region, as well as a short-lived jet. We find that the features’ HXR spectra are well fitted with isothermal models with temperatures ranging between 2.0 – 3.2 MK. Combining the NuSTAR data with softer X-ray emission from Hinode/XRT and EUV from SDO/AIA, we recover the differential emission measures, confirming little significant emission above 4 MK. The NuSTAR HXR spectra allow us to constrain the possible non-thermal emission that would still be consistent with a null HXR detection. We found that for only one of the features (the jet) was there a potential non-thermal upper limit capable of powering the heating observed. However, even here, the non-thermal electron distribution had to be very steep (effectively mono-energetic) with a low energy cut-off between 3 – 4 keV. 
    more » « less
  5. ABSTRACT We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray and the Solar Dynamic Observatory’s Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager. We find GOES sub-A class equivalent microflare energies of 1026–1028 erg reaching temperatures up to 10 MK with consistent quiescent or hot active region (AR) core plasma temperatures of 3–4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal hard X-ray emission during its impulsive phase (of non-thermal power ∼7 × 1024 erg s−1) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the extreme-ultraviolet, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the AR core (>5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of eight microflares is also observed. 
    more » « less
  6. null (Ed.)