Intense Hard X-ray Research Articles

Compression and acceleration of dense electron bunches by ultraintense laser pulses with sharp rising edge

In this paper, the generation of a single ultrashort and coherent relativistic electron bunch (relativistic electron mirror) during interaction of an ultraintense femtosecond laser pulse having a sharp enough rising edge (nonadiabatic laser pulse) with a thin plasma layer is considered. It is shown that due to the action of the radiation reaction forces the Coulomb repulsion among the bunch electrons is partially compensated and the initial geometry of the bunch is supported in the acceleration process. Besides, the bunch can be compressed by many times in the longitudinal direction at the initial stage of interaction with the front of the nonadiabatic laser pulse. As a result, all of the bunch electrons can be synchronously accelerated to ultrarelativistic velocities during the first several half periods of the external electromagnetic field that can correspond to time intervals of hundreds of femtoseconds in the laboratory frame. The characteristics of the accelerated electron bunches for different laser-plasma parameters (shape of laser pulse, initial density of electrons in the target, and initial target thickness) are investigated. One possible application for relativistic electron mirrors is considered. It is shown that the reflection of a probe counter-propagating laser pulse from such a mirror can produce intense, ultrashort, and coherent hard x-ray pulse. The spectrum of the frequency up-shifted radiation is investigated.

Dynamics in a cluster under the influence of intense femtosecond hard X-ray pulses

In this paper we examine the behavior of small cluster of atoms in a short (10-50 fs) very intense hard x-ray (10 keV) pulse. We use numerical modeling based on the non-relativistic classical equation of motion. Quantum processes are taken into account by the respective cross sections. We show that there is a Coulomb explosion, which has a different dynamics than one finds in classical laser driven cluster explosions. We discuss the consequences of our results to single molecule imaging by the free electron laser pulses.

A compact laser FEL in a X-ray regime

A new Free Electron Laser (FEL) using high power laser system as a wiggler/undulater and an electron beam of transverse laser cooling in a storage ring and a combination of chicane for longitudinal compression is proposed. This scheme will open an intense FEL hard X-ray laser with very compact system.

An investigation of small GOES flares with intense hard X-ray bursts

Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments.

Optimization of micropinch plasmas produced by vacuum spark discharges

A high-current low-inductance vacuum spark was operated to produce hot micropinches in copper plasmas. In order to improve the reproducibility of the intense hard x-ray emission, different trigger modes and anode shapes were investigated. Based on the standard configuration used in many vacuum spark set-ups, the temporal jitter was reduced by a factor of eight while the spatial reproducibility of the plasma points was improved by a factor of three.

X-ray intensity fluctuation spectroscopy observations of critical dynamics in Fe3Al.

We have carried out intensity fluctuation spectroscopy measurements using coherent x rays to study the dynamics of critical fluctuations in a binary alloy at equilibrium. An intense coherent hard x-ray beam, produced from an undulator source, was scattered from a single crystal of ${\mathrm{Fe}}_{3}\mathrm{Al}$ held at temperatures near the $B2\ensuremath{-}{\mathrm{DO}}_{3}$ order-disorder transition. A speckle pattern was observed at the $(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ superlattice reflection. Below ${T}_{c}$ it was essentially static, while above ${T}_{c}$ it fluctuated in time. The behavior of the normalized time correlation function is consistent with predictions of theory.

A Yohkoh search for “black-light flares”

Calculations which predict that a phenomenon analogous to stellar negative pre-flares could also exist on the Sun were published by Henouxet al. (1990), and Aboudarhamet al., (1990), who showed that at the beginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under certain conditions. They named this event a “black light flare” (BLF). Such a BLF event should appear as diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with intense and impulsive hard X-ray bursts. The BLF location would be at (or in the vicinity of) the forthcoming bright patches. Their predicted contrast depends on the position of the flare on the solar disc and on the wavelength band of the observation. TheYohkoh satellite provided white-light data from the aspect camera of the SXT instrument (Tsunetaet al., 1991), at 431 nm and with a typical image interval of 10–12 s. We have studied nine white-light flares observed with this instrument, with X-ray class larger than M6. We have found a few interesting episodes, but no unambiguous example of the predicted BLF event. This study, although the best survey to date, was not ideal from the observational point of view. We therefore encourage further searches. Successful observations of this phenomenon on the Sun would greatly strengthen our knowledge of the lower solar atmosphere and its effects on solar luminosity variations.

Impulsive phase Fe K-alpha emission in a flare of 1989 March

view Abstract Citations (15) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Impulsive Phase Fe K alpha Emission in a Flare of 1989 March Zarro, D. M. ; Dennis, B. R. ; Slater, G. L. Abstract Evidence for enhanced Fe K-alpha line emission that is coincident (to within 15 s) of an intense hard X-ray (greater than 50 keV) burst is presented on the basis of observations of the Fe K-alpha soft X-ray line made with the SMM. Three different models for explaining the K-alpha enhancement are investigated, namely, photoexcitation by soft X-ray thermal bremsstrahlung radiation from an isothermal source, collisional excitation by nonthermal thick-target electrons, and photoexcitation by a nonthermal hard X-ray flux distribution which extends with a power-law spectrum down to the Fe K-alpha ionization threshold at 7.1 keV. It is found that, to within the limits of isothermal temperature and emission measure set by soft X-ray observations, the thermal photoexcitation model cannot reproduce satisfactorily the intensity of enhanced K-alpha emission during the hard X-ray impulsive phase. The impulsive phase K-alpha flux exceeds that predicted by the thermal photoexcitation model by about 3 sigma. The implications of these results for the nonthermal interpretation of impulsive hard X-ray bursts in solar flares are discussed. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171395 Bibcode: 1992ApJ...391..865Z Keywords: Iron; K Lines; Solar Flares; Solar X-Rays; Thermal Radiation; Bremsstrahlung; Gamma Ray Astronomy; Photoexcitation; Solar Maximum Mission; Solar Radio Bursts; Solar Physics; RADIATION MECHANISMS: THERMAL; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Observations of a Buneman double layer and associated electron heating in a high-voltage straight plasma discharge

The spatial evolution of a potential hump (electron hole) into a Buneman double layer was for the first time observed at the instant of current limitation in the initial phase of a high-voltage straight plasma discharge with a preexisting hydrogen plasma. Potential profiles along the axis numerically calculated from the real time E-field data demonstrate the formation of a strong double layer with an inverse potential jump (eφDL/kTe∼103) moving toward the center of the device with a mean velocity 1.2×108 cm/sec, nearly 0.3 times the electron thermal velocity. The strong HV double layer developed near the center then quickly grew up to an extremely strong dynamic double layer propagating toward the anode. The formation of a highly anisotropic velocity distribution of energetic electrons associated with the strong double layer was evidenced by intense hard x-ray emissions that were first detected in the direction almost parallel to the magnetic field with a delay time 250–350 nsec from the generation of the double layer.

Flare fragmentation and type III productivity in the 1980 June 27 flare

We present observations of the solar flare on 1980 June 27, 16:14–16:33 UT, which was observed by a balloon-borne 300 cm2 phoswich hard X-ray detector and by the IKARUS radio spectrometer. This flare shows intense hard X-ray (HXR) emission and an extreme productivity of (at least 754) type III bursts at 200–400 MHz. A linear correlation was found between the type III burst rate and the HXR fluence, with a coefficient of ≈ 7.6 × 1027 photons keV−1 per type III burst at 20 keV. The occurrence of ≈ 10 type III bursts per second, and also the even higher rate of millisecond spikes, suggests a high degree of fragmentation in the acceleration region. This high quantization of injected beams, assuming the thick-target model, shows up in a linear relationship between hard X-ray fluence and the type III rate, but not as fine structures in the HXR time profile. The generation of a superhot isothermal HXR component in the decay phase of the flare coincides with the fade-out of type III production.

High spectral resolution measurements of a solar flare hard X-ray burst

Observations are reported of an intense solar flare hard X-ray burst on June 27, 1980, made with a balloon-borne array of liquid nitrogen-cooled Ge detector which provided unprecedented spectral resolution (no more than 1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 0.1-1 billion K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting about 3-15 sec, which have a hard spectrum and a break energy of 30-65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 kev to at least 100 keV through the event. The double power-law shape indicates that DC electric field acceleration, similar to that occurring in the earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission.

Electron acceleration in solar flares

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (≲1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 108–109K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting ∼3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to ≳100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically ∼102 kV compared to ∼10 kV for auroral substorms.

Characteristics of gamma-ray line flares

Observations of solar gamma rays by the Solar Maximum Mission (SMM) demonstrate that energetic protons and ions are rapidly accelerated during the impulsive phase. To understand the acceleration mechanisms for these particles, the characteristics of the gamma ray line flares observed by SMM were studied. Some very intense hard X-ray flares without detectable gamma ray lines were also investigated. Gamma ray line flares are distinguished from other flares by: (1) intense hard X-ray and microwave emissions; (2) delay of high energy hard X-rays; (3) emission of type 2 and/or type 4 radio bursts; and (4) flat hard X-ray spectra (average power law index: 3.1). The majority of the gamma ray line flares shared all these characteristics, and the remainder shared at least three of them. Positive correlations were found between durations of spike bursts and spatial sizes of flare loops as well as between delay times and durations of spike bursts.

Hard X-ray imaging observations of solar hot thermal flares with the HINOTORI spacecraft

Two solar hard X-ray bursts of a new type (hot thermal flare) were observed with hard X-ray imaging telescopes and other instruments on Japanese spacecraft Hinotori. The flares have no clear impulsive phase below 40 keV and emit intense hard X-rays (10--50 keV) with extremely steep spectra from a small region with size (FWHM) of 10''--20''. This source contains a hot thermal plasma of (3--3.5) x 10/sup 7/ K with an emission measure of the order of 10/sup 49/ cm/sup -3/. One of the flares occurred just on the limb, and the centroid of the hard X-ray (14-38 keV) source was located at (6 +- 3) x 10/sup 3/ km above the photosphere. It is concluded that the energy continuously released goes into heating rather than acceleration almost throughout the flare. Typical impulsive flares may usually have a similar nature in the later phase (gradual phase) of the flare evolution. The number density of the hard X-ray emitting region was higher than (6-10) x 10/sup 10/ cm/sup -3/ even in the initial phase of the flare, probably due to its low altitude. We infer that this high density made acceleration difficult, and therefore unusually intense heating occurred from the startmore » of the flare.« less

Long time delay between the peaks of intense solar hard X-ray and microwave bursts

It is shown that the long time delays more than five seconds between the peaks of intense hard X-ray and microwave bursts are concerned with two independent phenomena. One is the energy dependent time delays in X-rays and the other is the frequency dependent time delays in microwaves. The time delays of 5 s to 10 s between the peaks of solar hard X-ray burst (≲100 keV) obtained with Hinotori spacecraft and microwave burst at 17 GHz were observed exceptionally in three intense events with a spectral maximum at about 17 GHz. It is found that the peak of harder X-rays (≳300 keV) also delays in these events by about the same amount with respect to the softer X-rays (≲100 keV), so that the peak at 17 GHz nearly coincides (≲4s) with that of the harder X-rays. This is quite reasonable because the gyro-synchrotron emissions from the electrons below about 100 keV in the solar flares are generally negligible at high microwave frequencies (≳10 GHz). The optical thickness of the radio source decreases with frequency and is unity generally at about 10–20 GHz in intense bursts as inferred from the radio spectrum. Further delay of the peaks at the lower microwave frequencies is attributed to the temporal increase in the effective size of radio source which is optically thick at the lower frequencies.

Recurrent pulse trains in the solar hard X-ray flare of 1980 June 7

This study presents a detailed examination of the solar hard X-ray and gamma-ray flare of 1980 June 7 as seen by the Hard X-Ray Burst Spectrometer on SMM. The hard X-ray profile is most unusual in that it is characterized by an initial pulse train of seven intense hard X-ray spikes. However, the event is unique among the 6300 events observed by HXRBS in that the temporal signature of this pulse train recurs twice during the flare. Examinations of the hard X-ray data in conjunction with radio and gamma-ray observations show that the 28-480 keV X-ray emission is simultaneous with the 17 GHz microwave fluxes within 128 ms and that the 3.5-6.5 MeV prompt gamma-ray line emission is coincident with secondary maxima of the microwave and X-ray fluxes. Although the absence of spatially resolved hard X-ray observations leaves other possibilities open, a parameterization of the event as a set of seven sequentially firing loops is presented that offers many satisfying explanations of the observations.

Solar proton flares with weak impulsive phases

The current picture of a proton flare includes a well-defined impulsive phase characterized by a prominent hard X-ray (or microwave) peak. Lin and Hudson have argued that the correlation between intense flare hard X-ray bursts and large proton events is evidenced that the second stage of particle acceleration, during which the protons observed at 1 AU are thought to be accelerated, is fueled by energy originally contained in flash phase 10--100 keV electrons. In our examination of large (J(>10 MeV)< or =10 protons cm/sup -2/ s/sup -1/ sr/sup -1/), prompt, proton events occurring between 1965--1979, however, we found several events that originated in flares with relatively weak (Sp(9 GHz)<100 sfu) impulsive phases. Various lines of evidence indicate that these flares were associated with mass ejection events which, given the absence of a prominent flash phase, appear to have been magnetically driven.

The high energy X-ray spectrum of 4U 1700-37 observed from OSO 8

The most intense hard X-ray source in the confused region in Scorpius has been identified as 4U 1700-37 (=HD 153919). Observations extending over three binary periods in 1978 September were carried out with the high-energy X-ray spectrometer on OSO 8. The 3.4 day modulation is seen above 20 keV with the intensity during eclipse being consistent with zero flux. The photonumber spectrum from 20 to 150 keV is well represented by a single power law with a photonumber spectral index of -2.77 + or - 0.35 or by a thermal bremsstrahlung spectrum with kT = 27 (+15, -7)keV. The counting rate above 20 keV outside of eclipse shows no evidence for the 96.8 minute X-ray modulation previously reported at lower energies. Despite the difficulties that exist in reconciling both the lack of periodic modulation in the emitted X-radiation and the orbital dynamics of the system with our currently accepted theories of the evolution and physical properties of neutron stars, the observed properties of 4U 1700-37 are all consistent with the source being a spherically accreting neutron star rather than a black hole.