Imaging and Spectroscopy of Six NuSTAR MicroFlares SH11D-2898 Contact: J. Duncan1, L. Glesener1, I. Hannah2, D. Smith3, B. Grefenstette4 (1Univ, of Minnesota; 2Univ. of Glasgow; 3UC Santa Cruz; 4California Institute of Technology) [email protected] Abstract MicroFlare Time Evolution Hard X-ray (HXR) emission in solar flares originates from regions of high temperature plasma, as well as from non-thermal particle populations [1]. Both of these sources of HXR radiation make solar observation in this band important for study of • NuSTAR observed 6 MicroFlares during this observation. Time evolution is shown in both raw and normalized NuSTAR flare energetics. NuSTAR is the first HXR telescope with direct focusing optics, giving it a dramatic increase in sensitivity countsLivetime acrosscorrection several applied energy ranges for each flare. Counts are livetime-corrected (NuSTAR livetime ranged from 1-14%). Livetime correction applied over previous indirect imaging methods. Here we present NuSTAR observation of six microflares from one solar active Livetime correction applied

5 5 6 5×10 4×10 2.0 10 5 × 4×10 5 region during a period of several hours on May 29th, 2018. In conjunction with simultaneous data from SDO/AIA, data 3×10 5 2-4 keV 2-4 keV 6 3×10 1.5 10 Orbit1, Flare B 4-6 keV 2×105 4-6 keV × 5

counts 2 10 2-4 keV × Orbit1, Flare A 6-8 keV counts Orbit1, Flare C 6-8 keV from this observation has been used to create flare-time images showing the spatial extent of HXR emission. Additionally, 5 5 1 10 1×10 8-10 keV × 8-10 keV 6 4-6 keV 1.0×10 (Left) Estimated GOES A5 flare, with 0 0 1.2 1.2 16:07 16:08 16:09 16:10 16:11 16:46 16:48 16:50 16:52 16:54 NuSTAR lightcurves show time evolution in four different HXR energy ranges over the course of each flare. Finally, counts 6-8 keV 1.0 1.0 5.0×105 HXR emission showing similar 0.8 2-4 keV 0.8 2-4 keV 8-10 keV 0.6 4-6 keV 0.6 4-6 keV spectral fitting of emission at each flare time shows excess high energy emission over an isothermal spectral component behavior to that generally observed 0.4 6-8 keV 0.4 6-8 keV

0 Normalized 0.2 1.2 Normalized 0.2 8-10 keV 8-10 keV 16:20 16:24 16:28 16:32 in larger flares: an impulsive rise 0.0 0.0 in all six flares. The most likely origin of this excess is discussed for each. 1.0 -0.2 -0.2 16:07 16:08 16:09 16:10 16:11 16:46 16:48 16:50 16:52 16:54 0.8 followed by a more gradual fall, as Start Time (29-May-18 16:06:00) Start Time (29-May-18 16:46:00) SDO AIA_4 94 29-May-2018 19:42:11.120 UT 2-4 keV 0.6 4-6 keV well as an earlier peak time in higher 0.4 6-8 keV energy emission. (Above) Smaller Orbit 1 flares (estimated GOES class

Normalized 0.2 1000 before the largest flare, and Flare C after. Observation Overview 0.0 8-10 keV -0.2 Livetime correction applied 16:20 16:24 16:28 16:32 NuSTAR (Nuclear Spectroscopic Telescope Array) Start Time (29-May-18 16:18:00) 1.2×106 1.0×106 • HXR instrument optimized for astrophysical sources, but also capable of solar observation. 500 (Below) Orbit 2 flares. Two brightenings in quickLivetime succession, correction a smaller applied (GOES ~A1) flare AR 12710 8.0×105 2-4 keV NuSTAR is especially suited for observation of quiescent active regions and smaller first, followed by a larger GOES ~A2 Flare with a more typical large-flare time profile. 5 4-6 keV Livetime correction applied 6.0×10 Orbit3, Flare A counts 4.0×105 6-8 keV microflare events. 6 1.4×10 5 5 2.0×10 8-10 keV 6×10 1.2×106 5 105 0 • Two grazing-incidence telescopes, 10m focal length, 12’ x 12’ FOV [2]. × 6 Orbit2, Flare B 1.2 4×105 2-4 keV 1.0×10 2-4 keV 19:44 19:48 19:52 19:56 0 AR 12712 5 1.0 5 4-6 keV 8.0×10 3×10 Orbit2, Flare A 4-6 keV 0.8

counts 5 5 2-4 keV Y (arcsec) 2×10 6-8 keV 6.0×10 counts 0.6 5 8-10 keV 5 6-8 keV 1×10 4.0×10 4-6 keV 0 0.4 1.2 5 8-10 keV 6-8 keV 17:38 17:40 17:42 17:44 17:46 2.0×10

Normalized 0.2 1.0 0 0.8 2-4 keV 1.2 0.0 8-10 keV 0.6 4-6 keV 1.0 17:48 17:50 17:52 17:54 17:56 17:58 -0.2 -500 Co-Observation 0.4 6-8 keV 0.8 2-4 keV 19:44 19:48 19:52 19:56 Normalized 0.2 8-10 keV Start Time (29-May-18 19:42:00) 0.0 0.6 4-6 keV • This NuSTAR observation was planned to coordinate with the -0.2 0.4 17:38 17:40 17:42 17:44 17:46 6-8 keV Start Time (29-May-18 17:36:00) Hi-C 2.1 sounding rocket, which flew~10min before NuSTAR’s Normalized 0.2 0.0 8-10 keV (Above) GOES class ~A1 Flare from Orbit 3, the longest Orbit 3. -0.2 duration brightening we observed. -1000 17:48 17:50 17:52 17:54 17:56 17:58 • IRIS co-observed with NuSTAR/Hi-C, and achieved high- Spectroscopy Start Time (29-May-18 17:46:00) -1000 -500 0 500 1000 resolution UV spectral coverage of two of the NuSTAR Approximate NuSTARX (arcsec) pointing locations, microflares. shown over full-disk AIA 94A image. • Hinode EIS/XRT and SDO/AIA coverage also overlapped with • Emission from both NuSTAR telescopes was simultaneously fit using XSPEC spectral fitting software over each flaring region/ NuSTAR orbits. • May 29th observation consisted of 5 ~60 min NuSTAR orbits. time. • All orbits targeted AR 12712, with a ~15 min observation of AR • Three spectral models were tested for each flare: an isothermal bremsstrahlung model (VAPEC), a combination of two VAPEC 12710 during orbit 4. components,Livetime correctionand oneapplied VAPEC component combined with a non-thermal broken power law (BKNPOWER).

• All flares 1.2showed significant high energy excess over the isothermal fits, which could either be explained by the presence of a 1.0 0.8 2-4 keV second, higher0.6 temperature4-6 keV volume of plasma, or by non-thermal emission. For some of the flares, determination between 0.4 6-8 keV NuSTAR HXR Imaging Normalized 0.2 8-10 keV these possibilities0.0 is ongoing. -0.2 16:07 16:08 16:09 16:10 16:11 Start Time (29-May-18 16:06:00) • NuSTAR has 1-2 arcmin uncertainty in its pointing Orbit 1, Flare B: Orbit 1, Flare C: when observing the sun, as its forward-facing star- Spectral fits for both double thermal and thermal + broken power law models One of the smaller (GOES

• To mitigate this, NuSTAR data is co-aligned with AIA 4 4 104 10 10 104 1 1 1 − 1 − − context data (see right). − 1000 1000 1000 1000 keV keV keV 1 keV 1 100 100 1 100 1 100 − − − − VAPEC+VAPEC VAPEC+BKNPOWER VAPEC+VAPEC • Images below are NuSTAR >2 keV contours, with 10 10 10 10 VAPEC+BKNPOWER CSTAT: 830.38 CSTAT: 666.36 counts s counts s 1 CSTAT: 413.71 1 counts s gaussian smoothing applied over a ~12’’ FWHM. counts s 1 1 CSTAT: 385.55 0.1 0.1 0.1 0.1 100 100 1 1 1

NuSTAR emission is plotted over AIA 94A context 1 − − − − 100 100 10 10 keV keV keV keV 1 1 1 1 10 − data. 10 − − − s s 1 s s 1 2 2 2 2 − − − A pre-flare-time AIA image is subtracted NuSTAR pointing is shifted within our − 1 1 0.1 0.1 from a peak-time image to isolate flare spatial uncertainty range to best align with 0.1 0.1 0.01 0.01 Photons cm Photons cm Photons cm emission, and NuSTAR contours are the differenced AIA image. The same shift 0.01 0.01 Photons cm plotted over the differenced image. can then be used for other context data. 5 5 5 5

0 0 0 0 model)/error model)/error model)/error model)/error − − − − (data (data (data (data Orbit1, Flare B Orbit2, Flare B Orbit3, Flare A −5 −5 −5 −5 5 5 5 5 Energy (keV) Energy (keV) Energy (keV) Energy (keV) Low Temp. Component High Temp. Component Thermal Component Broken Power Law Low Temp. Component High Temp. Component Thermal Component Broken Power Law 3.94 MK 10.04 MK 6.52 MK Spectral Index: 10.62 3.17 MK 7.20 MK 4.47 MK Spectral Index: 9.83 (EM 2.34*10^47 cm^-3) (EM 4.44*10^45 cm^-3) (EM 3.00*10^46 cm^-3) (Break Energy 6.33 keV) (EM 4.62*10^47 cm^-3) (EM 1.48*10^45 cm^-3) (EM 4.08*10^46 cm^-3) (Break Energy 5.60 keV)

Orbit 2, Flare B: Other microflares: We retrieve similar results for our second largest microflare, and report model For the remaining three microflares, a double thermal model best describes the parameters for both double thermal and thermal + broken power law fits emission: below: Flare ID Low Temp. Component High Temp. Component VAPEC+VAPEC CSTAT: 665.32 VAPEC+BKNPOWER CSTAT: 597.59 Orbit 1 Flare A 3.99 MK 8.83 MK Low Temp. Component High Temp. Component Thermal Component Broken Power Law (EM 1.14*10^47 cm^-3) (EM 5.19*10^44 cm^-3) 3.99 MK 10.00 MK 5.90 MK Spectral Index: 11.15 Orbit 2 Flare A 4.08 MK 9.98 MK (EM 2.01*10^47 cm^-3) (EM 2.15*10^45 cm^-3) (EM 2.99*10^46 cm^-3) (Break Energy 6.37 keV) (EM 1.00*10^47 cm^-3) (3.48*10^44 cm^-3) Orbit 3 Flare A 3.46 MK 8.02 MK (EM 2.09*10^46 cm^-3) (EM 5.45*10^44 cm^-3) (Emission integrated over 16:18-16:27 UT) Note (Emission integrated over 17:44-17:53 UT) This (Emission integrated over 19:40-19:55 UT), the that the excess emission in the bottom left of the flare was also observed by IRIS. flare from the orbit immediately following the Hi-C image is the result of a pointing jump (not a flight. physical feature). This microflare was also Future Work observed by IRIS & XRT. Moving forward, work will continue to determine the best spectral fit for each of the May 29th microflares. Additional spectroscopy will be performed for quiescent times during the observation, and the best-fit models from that analysis will be compared with flare- References Acknowledgements time model results. NuSTAR data will also be considered alongside data from each co-observing instrument- in particular there is an [1] Glesener, L., Krucker, S., Hannah, I., et al. 2017, ApJ, 845:122 Work presented here was supported under the NASA NuSTAR Guest Observer program exciting opportunity for analysis of two microflares with both NuSTAR and IRIS. Additionally, the growing population of NuSTAR- [2] Grefenstette, B., Glesener, L., Krucker, S. et al. 2016, ApJ, 826:20 (80NSSC18K1744), as well as the NSF CAREER program (NSF-AGS-1752268). observed microflares will be analyzed to determine what commonality exists across these events in terms of their spatial, temporal, and spectral properties.