Hard X-ray Intensity Research Articles

Intensity and polarization of the hard X-ray radiation of solar flares at the top and footpoints of a magnetic loop

The spatial distributions of the intensity and polarization of hard X-rays (HXR) were modeled based on a numerical solution of the nonstationary kinetic Fokker-Plank equation. Two cases of nonstationary long injections (∼10 s) of nonthermal electrons at the top of a flaring loop are considered. In the first case, the injection occurs anisotropically in a cone directed along the field to one of the loop’s footpoints. In the second case, electrons are injected isotropically.

Study of the electrode material and insulator length effect on high-energy X-rays emitted by a 4-kJ plasma focus device

In this paper, behavior of hard X-ray (HXR) anisotropy and its intensity along the anode bar of the APF plasma focus device (16 kV, 36 μF, 115 nH) with different anode tip materials is studied. The signals obtained by scintillation detectors by a special arrangement at each angular position show that the employment of higher Z anode insert materials not only increases the intensity of HXR signal, but also results in the high degree of HXR emission isotropy. The highest HXR intensity was observed with a conic W anode insert and the lowest, with a flat Al anode insert. The side-on emitted intensity along the anode bar has been studied by moving the scintillation detector in the direction of the central electrode axis. For Pyrex and quartz insulators, the lengths of 40 and 50 mm, respectively, would seem optimal to produce the maximum HXR intensity. Also the Pyrex insulator would cause a higher HXR intensity than the quartz insulator.

Dependence of Hard X-ray Emissions with the Charging Pressure in a Small Plasma Focus

The dependence with the filling pressure of the hard X-ray emission of a 250 J Plasma-Focus device is presented. The study was performed on the range of Hydrogen pressures between 5 and 23 mbar. It was found that the pressure dependence of the hard X-ray intensity presents a maximum at 16.4 mbar. A threshold in the voltage drop on the pinch was observed for the X-ray emission, which agrees with the Dreicer theory for electrons runaway.

Measurement of Hard X-Rays on KSTAR

Hard X-ray (HXR) emission from KSTAR superconducting Tokamak has been measured during discharges which includes ECH assisted start up. Since the HXR signal indicates energetic electrons we can assume that runaway electrons are being generated. On KSTAR two tangentially arranged NaI(Tl) detectors are installed to measure the HXR intensity and energy in the forward and backward directions, respectively. We investigate the HXR emission trends in KSTAR tokamak and how the ECH heating source affects the generation of runway electrons.

Influence of Pyrex Insulator Thickness and it’s Length on Hard X-Ray Intensity in APF Plasma Focus Device

In order to improve the HXR emission from APF plasma focus device we have investigated the effect of insulator sleeve outer diameter (O.D.) and its length. Fourteen different insulator sleeve geometries at three different filling gas pressures of 6, 7 and 8 Torr of neon are used in the present investigation. The average HXR yield, measured using scintillation detector, has been found to increase with the increase in insulator sleeve O.D. from 31 to 34 mm. Further increase in insulator sleeve diameter to 37 mm, however, decreased the HXR yield. The highest magnitude of 234.5 kA was achieved for 34 mm O.D. of sleeve at filling gas pressure of 7 Torr and voltage of 12 kV.

The relation between the long-term X-ray and optical activity of the polar AM Her (RX J1816.2 + 4952)

We analyze the relation between the long-term (1996–2009) activity of the polar AM Her in the optical and hard X-ray spectral regions. We investigate the mean values of the intensities in the individual high-state episodes. We made use of the ASM/ RXTE observations for a time-series analysis of the long-term variations in the 1.5–12 keV band. The optical data came from the AFOEV database. We reveal a complicated relation between the optical ( I O) and hard X-ray intensity ( I X). We argue that our observations cannot be explained by the variations of the orbital modulation in the high-state. Also, the precession of the spin axis of the white dwarf or asynchronous rotation of this object are unlikely in our case. We show that the basic properties of the emitting region (s) is (are) established in the early phase (several days long) of the high-state episode but they are not reproduced for every episode. The increase of the mass transfer rate from the donor that switches the polar from the low to the high-state also establishes a division of the emission released during the accretion process into various spectral regions that is valid only for a given episode. These results enable us a better understanding of the multifrequency behavior of polars on long time-scales.

A detection of an anti-correlated hard X-ray lag in AM Herculis

Context {Earlier cross-correlation studies for AM Her were performed in various energy range from optical to X-ray and suggested that it mostly shows a high level of correlation but on occasion it shows a low level of correlation or uncorrelation.} Aims {To investigate the degree of correlation between soft (2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of the X-ray data sets of AM Her obtained with {\it RXTE}.} Methods {We cross-correlate the background-subtracted soft and hard X-ray light curves using the XRONOS program crosscor and fit a model to the obtained cross-correlation functions.} Results {We detect a hard X-ray lag of $192\pm33$ s in a specific section of energy-dependent light curve, where the soft X-ray (2-4 keV) intensity decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral analysis, we find that the X-ray emission temperature increases during the anti-correlated intensity variation. In two other observations, the cross-correlation functions show a low level of correlation, which is consistent with the earlier results performed in a different energy range.} Conclusions {We report a detection of an anti-correlated hard X-ray lag of $\sim$190 s from the proto-type polar AM Her. The hard X-ray lag is detected for the first time in the given energy range, and it is the longest lag among those reported in magnetic cataclysmic variables. We discuss the implications of our findings regarding the origin of the hard X-ray lag and the anti-correlated intensity variation.}

Hybrid X-pinch with conical electrodes

A hybrid X-pinch configuration consisting of solid conical electrodes connected by a wire has been tested on a 45 ns risetime, 500 kA peak current pulsed power generator. Wires of different materials were loaded through holes in the cones; wire lengths were varied from 0.6 to 2 mm. Most of these hybrid X-pinches generated an intense single burst of soft x-rays and developed a single hot spot that was of micron-scale size. Hybrid X-pinches generate less hard x-ray intensity than standard X-pinches.

Effect of Quartz and Pyrex Insulators Length on Hard-X ray Signals in APF Plasma Focus Device

This paper, presents the variation of hard X-ray (HXR) intensity in the APF plasma focus device for different insulator sleeves. For Pyrex and Quartz insulators, the lengths of 40 and 50 mm seems optimal to yield maximum HXR intensity, respectively. Also using the Pyrex insulator leads to production of higher HXR intensity than the Quartz insulator. The results illustrate that the length and the material of the insulator has a considerable effect on the HXR intensity.

THE MOUSE THAT ROARED: A SUPERFLARE FROM THE dMe FLARE STAR EV LAC DETECTED BYSWIFTAND KONUS-WIND

We report on a large stellar flare from the nearby dMe flare star EV Lac observed by the Swift and Konus-Wind satellites and the Liverpool Telescope. It is the first large stellar flare from a dMe flare star to result in a Swift trigger based on its hard X-ray intensity. Its peak f_X from 0.3--100 keV of 5.3x10^-8 erg/cm2/s is nearly 7000 times larger than the star's quiescent coronal flux, and the change in magnitude in the white filter is >4.7. This flare also caused a transient increase in EV Lac's bolometric luminosity (L_bol) during the early stages of the flare, with a peak estimated L_X/L_bol ~3.1. We apply flare loop hydrodynamic modeling to the plasma parameter temporal changes to derive a loop semi-length of l/Rstar =0.37 +/-0.07. The soft X-ray spectrum of the flare reveals evidence of iron Kalpha emission at 6.4 keV. We model the Kalpha emission as fluorescence from the hot flare source irradiating the photospheric iron, and derive loop heights of h/Rstar=0.1, consistent within factors of a few with the heights inferred from hydrodynamic modeling. The Kalpha emission feature shows variability on time scales of ~200 s which is difficult to interpret using the pure fluorescence hypothesis. We examine Kalpha emission produced by collisional ionization from accelerated particles, and find parameter values for the spectrum of accelerated particles which can accommodate the increased amount of Kalpha flux and the lack of observed nonthermal emission in the 20-50 keV spectral region.

Ultraviolet radiation diagnostics of the flare transient layer

The correlation between the pulsed and smooth components of ultraviolet radiation and hard X-rays from solar flares has been interpreted on the basis of the time behavior of the energy flux penetrating from the magnetic field reconnection region to the boundary of the heated region. It has been shown that the time behavior of the primary flare energy flux is easily reconstructed from the time profile of the intensity of hard X-rays, and the time profile of the intensity of ultraviolet radiation represents the release of the flare energy in the flare transient layer. The smooth or pulsed component of the ultraviolet radiation depends on the form of the primary energy release. Information on the dynamics of the heating process and the formation and equalization of the gas pressure in the flare transient layer has been obtained from the observed temperature dependence of the emission measure. The described scheme explains the set of flare phenomena and the correlation between hard X-rays with pulsed ultraviolet bursts.

Observations of the December 6, 2006 solar flare: Electron acceleration and plasma heating

The flare plasma temperature calculated from GOES-11 (1.5–12.4 and 3.1–24.8 keV) data is compared with the solar nonthermal fluxes in various energy ranges in the December 6, 2006 event. Particle acceleration and plasma heating episodes took place in the pre-flare and impulsive phases; a hard (ACS SPI > 150 keV) X-ray emission was observed 5 min before the onset of the GOES X-ray flare and was not accompanied by a temperature rise. A close correlation has been found between the flare plasma temperature and the hard X-ray intensity. The temperature delayed by 0.4 min turned out to be directly proportional to the logarithm of the ACS SPI count rate within the first 3 min of the impulsive phase. This shows that the accelerated electrons responsible for the X-ray emission were the main plasma heating source in the pre-flare and impulsive phases. The correlation between the temperature and the hard X-ray intensity disappears after the observation of a resonance peak at a frequency of 245 MHz. Significant electron fluxes may no longer be able to effectively heat the expanding plasma when its density in the interaction region reaches ∼109 cm−3. The observations of the July 23, 2002 and December 5, 2006 events confirm the trends found.

G-BAND AND HARD X-RAY EMISSIONS OF THE 2006 DECEMBER 14 FLARE OBSERVED BYHINODE/SOT ANDRHESSI

We report on G-band emission observed by the Solar Optical Telescope onboard the Hinode satellite in association with the X1.5-class flare on 2006 December 14. The G-band enhancements originate from the footpoints of flaring coronal magnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the available 2 minute cadence, the G-band and hard X-ray intensities are furthermore well correlated in time. Assuming that the G-band enhancements are continuum emission from a blackbody, we derived the total radiative losses of the white-light flare (white-light power). If the G-band enhancements additionally have a contribution from lines, the derived values are overestimates. We compare the white-light power with the power in hard X-ray producing electrons using the thick target assumption. Independent of the cutoff energy of the accelerated electron spectrum, the white-light power and the power of accelerated electrons are roughly proportional. Using the observed upper limit of ~30 keV for the cutoff energy, the hard X-ray producing electrons provide at least a factor of 2 more power than needed to produce the white-light emission. For electrons above 40 keV, the powers roughly match for all four of the time intervals available during the impulsive phase. Hence, the flare-accelerated electrons contain enough energy to produce the white-light flare emissions. The observed correlation in time, space, and power strongly suggests that electron acceleration and white-light production in solar flares are closely related. However, the results also call attention to the inconsistency in apparent source heights of the hard X-ray (chromosphere) and white-light (upper photosphere) sources.

MEASUREMENTS OF THE CORONAL ACCELERATION REGION OF A SOLAR FLARE

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of 2007 December 31. The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of {approx}16 Mm and {approx}25 Mm, respectively. Despite the occultation, intense hard X-ray emission up to {approx}80 keV occurs during the impulsive phase from a coronal source that is also seen in microwaves. The hard X-ray and microwave source during the impulsive phase is located {approx}6 Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare. A single non-thermal electron population with a power-law distribution (with spectral index of {approx}3.7 from {approx}16 keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively. This clearly establishes the non-thermal nature of the above-the-loop-top source. The large hard X-ray intensity requires a very large number (>5 x 10{sup 35} above 16 keV for themore » derived upper limit of the ambient density of {approx}8 x 10{sup 9} cm{sup -3}) of suprathermal electrons to be present in this above-the-loop-top source. This is of the same order of magnitude as the number of ambient thermal electrons. We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (tens of keV) within seconds. Hence, the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work. Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region.« less

Experimental Study of Hard X-ray Emission with Different Anode Tips in APF Plasma Focus Device

To investigate the effect of different anode tips on the hard X-ray (HXR) emission from APF plasma focus device (16 kV, 36 μf, and 115 nH) we considered two shapes of anode tips, i.e., flat or conic, with Cu, Al and W anode tip materials. The highest HXR intensity was observed with conic W anode insert and the lowest with flat Al anode insert. In comparison with the hollow anode tube the HXR signals obtained from flat and especially conic W inserts nearly tending to have the same intensity along the axis of the device. Therefore the shape and the material of the anode tip has significant effect on the production of HXR emissions. This study show that the employment of a convenient shape of anode tip results to more isotropic emission of HXR.

Experimental Study of Current Discharge Behavior and Hard X-ray Anisotropy by APF Plasma Focus Device

Amirkabir (APF) is a new Mather-type plasma focus device (16 kV, 36 μf, and 115 nH). In this work we present some experimental results as variation of discharge current signal respect to applied voltage at the optimum pressure, focusing time of plasma versus gas pressure, and variations of current discharge with different insulator sleeve dimensions. As we prospected optimum pressure tending to increase as we tried to higher voltage levels. The time taken by the current sheath to lift-off the insulator surface and therefore quality of pinched plasma depends on the length of the insulator sleeve. The results show that the insulator diameter can influence on pinch quality. Behavior of hard X-ray (HXR) signals with the pressure and also anisotropy of HXR investigated by the use of two scintillation detectors. The distribution of HXR intensity shows a large anisotropy with a maximum intensity between 22.5° and 45° and also between −22.5° and −67.5°.

Hard X-ray emission from imploding wire arrays

Results are presented from experimental studies of the generation of hard X-ray (HXR) emission with photon energies above 20 keV during the implosion of wire arrays in the Angara-5-1 facility. An analysis of X-ray images of the Z-pinch shows that the dimensions and spatial structures of the emitting regions are different for hard and soft X rays. It is found that the HXR emission peak is delayed with respect to the soft X-ray (SXR) one. The dependence of the HXR power on the material, initial diameter, and mass (implosion time) of the wire array is determined. It is shown that the HXR intensity in the spectral range >50 keV is several orders higher than the emission intensity in the high-energy tail of the SXR spectrum (assuming that this spectrum is thermal). A comparison of the time evolution and spatial localization of the HXR and SXR sources during the implosion of wire arrays indicates the presence of a new superthermal phenomenon that differs qualitatively from the processes determining the peak power of the SXR pulse. Possible mechanisms that can be responsible for the generation of HXR pulses are considered.

Hard X-Ray Intensity Distribution along Hα Ribbons

Unusual ribbon-like hard X-ray sources were found with the RHESSI observation of a 2B/M8.0 flare on 2005 May 13. We use this unique observation to investigate the spatial distribution of the hard X-ray intensity along the ribbons and compare it with the local magnetic reconnection rate and energy release rate predicted by the standard magnetic reconnection model for two ribbon flares. In the early phase of the flare, the hard X-ray sources appear to be concentrated in strong field regions within the Hα ribbons, which is explicable by the model. At and after the flare maximum phase, the hard X-ray sources become spatially extended to resemble Hα ribbons in morphology, during which the spatial distribution of hard X-ray intensity lacks a correlation with that of the local magnetic reconnection rate and energy release rate predicted by the model. We argue that the magnetic reconnection during this event may involve the rearrangement of magnetic field along the magnetic arcade axis that is inevitably overlooked by the two-dimensional model and suggest that this type of three-dimensional reconnection will be best seen in so-called sigmoid-to-arcade transformations.

Electron cyclotron current drive experiments in LHCD plasmas using a remote steering antenna on the TRIAM-1M tokamak

A remote steering antenna was recently developed for electron cyclotron heating and current drive (ECH/ECCD) experiments on the TRIAM-1M tokamak. This is the first application of the remote steering antenna concept for ECH/ECCD experiments, which have conditions relevant to the International Thermonuclear Experimental Reactor (ITER). Fundamental ECH and ECCD experiments were conducted in the ITER frequency from the low field using this antenna system. In addition to the angles near 0°, the launcher was a symmetric direction antenna with an extended steering-angle capability of ±(8°–19°). The output beam from the antenna was a well-defined Gaussian with a proper steering angle. The Gaussian content and the steering-angle accuracy were 0.85 and −0.5°, respectively. The high power tests measured the antenna transmission efficiency at 0.90–0.94. The efficiencies obtained in the low and high power tests were consistent with the calculations using higher-order modes. In order to excite the pure O/X-modes in the oblique injection, two polarizers were used to control the elliptical polarization of the incident beam for the ECCD experiments. The fundamental O/X-mode ECH/ECCD was applied to lower hyrid current drive plasmas at the optimized incident polarization. In the X-mode experiment, at medium density (∼1 × 1019 m−3), clear differences in the plasma current and the hard x-ray intensity were observed between the co- and counter-steering injections due to the ECCD effect on the coupling of forward fast electrons.

Search for Neutron-Capture Gamma-Ray Lines from A0535+26 in Outburst

In 2005 May-June, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed the Be/X-ray binary system A0535+26 during a giant outburst, during which time its hard X-ray intensity reached several crab. This bright source presented a unique opportunity to search for redshifted neutron-capture lines from the surface of the neutron star. Such lines, if discovered, would strongly constrain the neutron star equation of state, motivating this search. An upper limit on the narrow, unredshifted line has been set at 6.5 × 10-4 photons cm-2 s-1, while width-dependent limits on a broadened, redshifted line are set in the range of (4.0-10.5) × 10-4 photons cm-2 s-1. To our knowledge, these are the first measured upper limits on redshifted 2.2 MeV emission from an accreting neutron star.