24 research outputs found
Modeling Radiation from the Solar Atmosphere in the sub-mm, mm and cm Wavelength Range
Na slikama Sunca u milimetarskom području mogu se uočiti područja povećane emisije (visokotemperaturna područja, VTP) i područja povećane apsorpcije (niskotemperaturna područja, NTP). Termičko zakočno zračenje i žiromagnetsko (ciklotronsko) zračenje su procesi koji mogu objasniti temperature sjaja dobivene mjerenjima. Koristimo postupak za računanje temperature sjaja za danu valnu duljinu i model atmosfere koji integrira jednadžbu prijenosa zračenja za termičko zakočno zračenje. Do sada je ovaj postupak primijenjen samo na zračenje valne duljine od 8 mm. Cilj ovog rada je primijeniti modele za različite strukture Sunčeve atmosfere na širi spektar valnih duljina (od 0.3 mm do 1 cm), otprilike jednak onome na kojem radi Atacama Large Millimeter/submillimeter Array (ALMA) te dobivene rezultate usporediti s dostupnim mjerenjima. Zaključujemo kako je termičko zakočno zračenje dominantni mehanizam zračenja na submilimetarskim i milimetarskim valnim duljinama koji može objasniti dosadašnja mjerenja intenziteta zračenja Sunca. U bliskoj budućnosti, rezultati modela će se moći usporediti s novim mjerenjima pomoću teleskopa ALMA.Maps of the Sun in millimeter wavelength range reveal emission features (high temperature regions, HTR) and apsorptive features (low temperature regions, LTR). Thermal bremsstrahlung and gyromagnetic (cyclotron) radiation mechanism can be important for explaining the observed phenomena. We use a procedure for calculating the brightness temperature for a given wavelength and model atmosphere, which integrates the radiative transfer equation for thermal bremsstrahlung. Until now, it has been used just for the radiation with wavelength of 8 mm. The aim of the present work is to apply the models for different structures of the solar atmosphere on a broader wavelength range (from 0.3 mm to 1 cm), closely related to that of Atacama Large Millimeter/submillimeter Array (ALMA), and to compare the results with observations. We conclude that thermal bremsstrahlung is the only important radiation mechanism in the submillimeter and millimeter wavelength range, which can explain previous observations of the Sun’s intensity of radiation. In the near future, we will be able to compare the results of our models to new observations of ALMA telescope
NuSTAR detection of X-ray heating events in the quiet Sun
The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2–4.1 MK and emission measures of (0.6–15) × 1044 cm−3 describe their spectra well, resulting in thermal energies in the range (2–6) × 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares
Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
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 heating of
material to several million Kelvin, observed in Soft X-rays (SXRs) with
Hinode/XRT, and was faintly visible in Extreme Ultraviolet (EUV) with SDO/AIA.
For three of the four NuSTAR observations of this region (pre-, decay, and post
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 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 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
about erg s. 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 the
heating requires a power-law spectrum of with a low energy
cut-off keV. In summary, this first NuSTAR microflare
strongly resembles much more powerful flares.Comment: Accepted for publication in ApJ. 14 pages with 12 figures and 1 tabl
DNA Sampling: a method for probing protein binding at specific loci on bacterial chromosomes
We describe a protocol, DNA sampling, for the rapid isolation of specific segments of DNA, together with bound proteins, from Escherichia coli K-12. The DNA to be sampled is generated as a discrete fragment within cells by the yeast I-SceI meganuclease, and is purified using FLAG-tagged LacI repressor and beads carrying anti-FLAG antibody. We illustrate the method by investigating the proteins bound to the colicin K gene regulatory region, either before or after induction of the colicin K gene promoter
First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events
We present the first results of a search for transient hard X-ray (HXR)
emission in the quiet solar corona with the \textit{Nuclear Spectroscopic
Telescope Array} (\textit{NuSTAR}) satellite. While \textit{NuSTAR} was
designed as an astrophysics mission, it can observe the Sun above 2~keV with
unprecedented sensitivity due to its pioneering use of focusing optics.
\textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, although
out-of-view active regions contributed a notable amount of background in the
form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun
transient brightenings on time scales of 100 s and set upper limits on emission
in two energy bands. We set 2.5--4~keV limits on brightenings with time scales
of 100 s, expressed as the temperature T and emission measure EM of a thermal
plasma. We also set 10--20~keV limits on brightenings with time scales of 30,
60, and 100 s, expressed as model-independent photon fluxes. The limits in both
bands are well below previous HXR microflare detections, though not low enough
to detect events of equivalent T and EM as quiet Sun brightenings seen in soft
X-ray observations. We expect future observations during solar minimum to
increase the \textit{NuSTAR} sensitivity by over two orders of magnitude due to
higher instrument livetime and reduced solar background.Comment: 11 pages, 7 figures; accepted for publication in The Astrophysical
Journa
The First Focused Hard X-ray Images of the Sun with NuSTAR
We present results from the the first campaign of dedicated solar
observations undertaken by the \textit{Nuclear Spectroscopic Telescope ARray}
({\em NuSTAR}) hard X-ray telescope. Designed as an astrophysics mission, {\em
NuSTAR} nonetheless has the capability of directly imaging the Sun at hard
X-ray energies (3~keV) with an increase in sensitivity of at least two
magnitude compared to current non-focusing telescopes. In this paper we
describe the scientific areas where \textit{NuSTAR} will make major
improvements on existing solar measurements. We report on the techniques used
to observe the Sun with \textit{NuSTAR}, their limitations and complications,
and the procedures developed to optimize solar data quality derived from our
experience with the initial solar observations. These first observations are
briefly described, including the measurement of the Fe K-shell lines in a
decaying X-class flare, hard X-ray emission from high in the solar corona, and
full-disk hard X-ray images of the Sun.Comment: 11 pages, accepted to Ap
NuSTAR Detection of X-Ray Heating Events in the Quiet Sun
The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2–4.1 MK and emission measures of (0.6–15) × 10^(44) cm^(−3) describe their spectra well, resulting in thermal energies in the range (2–6) × 10^(26) erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares
Evidence of significant energy input in the late phase of a solar flare from NuSTAR x-ray observations
We present observations of the occulted active region AR 12222 during the third Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaign on 2014 December 11, with concurrent Solar Dynamics Observatory (SDO)/AIA and FOXSI-2 sounding rocket observations. The active region produced a medium-size solar flare 1 day before the observations, at ∼18 UT on 2014 December 10, with the post-flare loops still visible at the time of NuSTAR observations. The time evolution of the source emission in the SDO/AIA 335 Å channel reveals the characteristics of an extreme-ultraviolet late-phase event, caused by the continuous formation of new post-flare loops that arch higher and higher in the solar corona. The spectral fitting of NuSTAR observations yields an isothermal source, with temperature 3.8\ndash4.6 MK, emission measure (0.3\ndash1.8) × 10⁴⁶ cm‑3, and density estimated at (2.5\ndash6.0) × 10⁸ cm‑3. The observed AIA fluxes are consistent with the derived NuSTAR temperature range, favoring temperature values in the range of 4.0\ndash4.3 MK. By examining the post-flare loops\rsquo cooling times and energy content, we estimate that at least 12 sets of post-flare loops were formed and subsequently cooled between the onset of the flare and NuSTAR observations, with their total thermal energy content an order of magnitude larger than the energy content at flare peak time. This indicates that the standard approach of using only the flare peak time to derive the total thermal energy content of a flare can lead to a large underestimation of its value