Hard X-ray Bursts Research Articles

Spectral evolution of pulse structures in gamma-ray bursts

The Hard X-Ray Burst Spectrometer (HXRBS) and Gamma-Ray Spectrometer (GRS) data from the Solar Maximum Mission satellite have been searched for gamma-ray bursts with sufficient intensities and relatively simple time profiles such that their spectral behavior may be studied on a time scale of about 1 s. Ten such events were observed with the GRS experiment, and four of these were also detected within the HXRBS field of view. Details are presented for two moderately intense bursts with relatively simple structure. The spectral evolutions of the remaining events are summarized briefly. Results suggest a pattern in the spectral evolution within burst pulses: a tendency for the high-energy emission to lead the low-energy emission, in contrast to the correlation of intensity and spectral hardness reported by Golenetskii et al. (1983).

Observations at the onset of a large solar flare and their relevance to energy transfer

Many large flares are dominated at their onset by an impulsive energy release characterised by a rapidly-fluctuating hard X-ray burst. The resulting hot plasma is then largely a secondary phenomenon. Flares where most of the plasma heating occurs before most of the hard X-ray emission present a challenge to the conventional picture as the energy input to the plasma cannot come from electrons responsible for the hard X-rays. We present an analysis of the July 14 1980 flare (GOES X1) where the impulsive hard X-ray burst occurred near the end of the soft X-ray flare. The hard X-ray maximum was only 37s before the soft X-ray peak at 3.5–8.0 keV. These soft X-rays had reached 89% of their maximum intensity by the time of the strongest hard X-ray burst, which coincided with the peak microwave flux at 19.6 GHz. The e-folding decay of the soft X-rays ≃70s. The ion temperature and turbulent velocity are highest in the 5m preceeding the hard X-ray burst maximum. From examination of the X-ray spatial structure we discuss the flare evolution, together with possible energy transfer mechanisms other than electrons. The rapid decay appears to result from expansion of the flare plasma into a large overlying coronal structure.

UV observations of transition-region plasmas in solar flares: Plasma diagnostics and relationships between UV and hard X-ray bursts

The temporal and spatial relationships between UV and hard X-ray bursts in solar flares observed from SMM are presented.

A study of hard X-ray associated meter-decameter bursts observed on December 19, 1979

We present the results of a study of the relationship of a complex meter-decameter wavelength radio burst observed with the Clark Lake E-W and N-S interferometers, with a hard X-ray burst observed with the X-ray spectrometer aboard ISEE-3. The radio burst consisted of several type III's, reverse drift type III's, a U burst, and type II and type IV bursts. The X-ray emission was also complex. The radio as well as hard X-ray emissions were observed before the flash phase of the flare; they were not always associated and we conjecture that this may constitute evidence for acceleration of electrons high in the corona. On the other hand, all components of the reverse drift burst were associated with hard X-ray subpeaks, indicating multiple injection of electron beams along field lines with different density gradients. While the type II burst appeared to be related to the hard X-ray burst, a detailed correspondence between individual features of the radio and hard X-ray burst emissions could not be found. The type IV burst started after all hard X-ray emissions ceased. Its source appeared to be a magnetic arch, presumably containing energetic electrons responsible for the gyrosynchrotron radiation of type IV.

The excitation of the iron K? feature in solar flares

We evaluate the relationship between the hard X-ray photon spectrum and the flux of iron Kα emission in a thick-target electron bombardment model. Results are presented for various power-law hard X-ray spectra. We then apply these results to two events observed with the Hard X-Ray Burst Spectrometer and the Kα channel of the X-Ray Polychromator Bent Crystal Spectrometer on the Solar Maximum Mission satellite. For one of the events, on 29 March, 1980, at 09:18 UT, the Kα flux predicted for a thick-target non-thermal process is significant compared to the background fluorescent component, and the data are indeed consistent with an enhancement of the predicted amount. For the other event, on 14 October, 1980 at 06:09 UT, the hard X-ray spectrum is so steep that no significant Ka flux is predicted for this process, and no enhancement is seen. We conclude that the agreement between the predicted Kα flux and the observed magnitude of the Kα enhancement above the fluorescent background at the time of the large hard X-ray bursts lends support to a thick-target non-thermal interpretation of impulsive hard X-ray emission in solar flares.

Correlated Observations of Impulsive UV and Hard X-Ray Bursts from the Solar Maximum Mission

In the past decade, impulsive hard X-ray bursts have been extensively observed (cf. Kane et al., 1980). These observations have increased our knowledge of the energy spectrum of the accelerated electrons and their temporal evolution. However, because of the lack of spatial resolution and direct plasma diagnostics, many important questions concerning the nature of the impulsive phase are still left unanswered. Since direct imaging of hard X-rays above 30 keV with high resolution of is still beyond our present technology, we have to use other indirect means to deduce the spatial structure of the hard X-ray source. With the recent launch of the Solar Maximum Mission (SMM) satellite, we are able to obtain correlated observations of the flare impulsive phase in hard X-ray and simultaneously in the UV lines of Si IV (1402 Å) and 0 IV (1401 Å). The Si IV/0 IV intensity ratio is density sensitive and therefore provides plasma diagnostics in the emission region. Analysis of the spatially resolved UV observations with the correlated hard X-ray observations allows us to study the spatial structure and physical conditions in the UV and hard X-ray sources (Cheng et al., 1981; 1982; 1984). Descriptions of the various solar instruments on SMM can be found in Solar Physics (vol. 65, pp 5-116). In this paper, I briefly summarize the important observational results and discuss their theoretical interpretation.

White light flares seen from the spectroscopic observations and the direct images

White light flares are classified into three types from the form of the bright region and time coincidence with a hard X-ray burst: impulsive, gradual, and diffuse types. The impulsive type shows a good correlation between the brightness change and hard X-ray flux, the gradual one appears in a gradual phase of a flare. The bright region is compact in both types. The diffuse one, however, appears in an extended area at an impulsive phase.

High resolution observation of H? solar flares and temporal relation between H? and X-ray, microwave emission

We studied the evolutional characteristics of fine structures in Hα flare emitting regions and their relation to X-ray and microwave emissions for selected events observed with the 60 cm Domeless Solar Telescope at Hida Observatory, University of Kyoto. The principal conclusions of this investigation are: (1) Hα kernel consists of some finer bright points or Hαflare points whose individual size is less than 1 arc sec. (2) Impulsive brightnenings of Hα flare points occurred simultaneously with the spikes of the hard X-ray and microwave bursts within the time resolution of our Hα observations which varied from 1 to 10 s. (3) It is concluded that fast electron beams must be the principal mechanism of heating Hα flares during the impulsive phase of a flare.

Simultaneous imaging and spectral observations in microwaves and hard X-rays of the impulsive phase of a solar limb flare

We have observed the impulsive phase of a solar flare at microwave wavelengths and in hard X-rays, and deduced the strength of the magnetic field and the number of energetic electrons producing the burst. The microwave observations, using the Very Large Array at 6 cm, had spatial resolution of 8″×8″, close to the resolution of the Hard X-ray Imaging Spectrometer on SMM which also imaged this flare. The Hard X-ray Burst Spectrometer determined the spectrum of the burst in the range 25–512 keV, and several patrol telescopes recorded the microwave time profile at frequencies from 2.8 to 19.6 GHz. The combined data show that the derived number of microwave emitting electrons is at least three orders of magnitude fewer than the number of thick target electrons producing the hard X-rays. We propose that the reason for this discrepancy in numbers is that the fast electrons are highly beamed, and radiate gyrosynchrotron emission less efficiently than isotropically distributed electrons. However, we cannot rule out the alternate explanation: that the electron energy spectrum steepens at moderate energies (> 150 keV), and then flattens at higher energies (> 200 keV) where gyrosynchrotron processes become important.

The Coronal Oscillations Revealed by Solar Hard X-ray Bursts

The Coronal Oscillations Revealed by Solar Hard X-ray Bursts

Solar gradual hard X-ray bursts: Observations and an interpretation

A recent study of solar gradual hard X-ray bursts is summarized. The data are interpreted in terms of a model involving the acceleration and trapping of electrons in post flare loop systems following coronal mass ejections. A controversy about the classification of the metric continuum that typically accompanies gradual hard X-ray events is addressed.

The Ha Characteristics of Hard X-ray Bursts

The Ha Characteristics of Hard X-ray Bursts

Spectral evolution in gamma-ray bursts

The Hard X-ray Burst Spectrometer (HXRBS) and the Gamma-Ray Spectrometer (GRS) on NASA's Solar Maximum Mission satellite (SMM) have independently monitored cosmic gamma-ray bursts since launch in February 1980. Several bursts with relatively simple pulse structure and sufficient intensity have been analyzed for evidence of spectral variability on timescales shorter than the pulse durations. In many of these bursts we find pulse structures, ranging in duration from 1 to 10 seconds, which exhibit a trend of hard-to-soft spectral evolution. No significant evidence for soft-to-hard evolution has been found, although the possibility of weak, extended low-energy emission is suggested in a few bursts. The HXRBS data above 100 keV and the GRS data above 1 MeV indicate that the spectral evolution generally is not due to time-varying absorption features at energies below 100 keV.

Results of a search for a new class of GRBS in the SMM data

We present here the results of a search for the soft and short Gamma-Ray Bursts (GRBs) in the data of the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) that constitute a new event series discovered by Hurley /1/ and Laros /2/. Data for four events are presented, including their time profiles and spectral characteristics. In one case the instrument time resolution reveals a total burst duration of 55 ms with rise and decay times of ⩽ 5 ms.

Solar flare development during the impulsive phase as deduced from correlated radio and hard X-ray observations

A correlative study of metric type III/V bursts and hard X-ray events during the preflash and the impulsive phases of solar flares is presented. A total of 55 groups of type III bursts were observed with the Nancay radiospectrograph, and 32 of these were associated with hard X-ray bursts detected with instruments on ISEE 3, SMM, or both. X-ray bursts associated with type III or U bursts, and type V continuum emission lasting less than 10 s are relatively small, with peak fluxes at 30 keV of not more than 1 photon/sq cm per sec per keV. X-ray bursts associated with type III bursts and continuum emission lasting longer than 30 s are considerably more intense, with peak fluxes at 30 keV exceeding 1 photon/sq cm per sec per keV and spectra extending to not less than 100 keV. For these latter events, type III emission was observed at 169 MHz with the Nancay radioheliograph from several locations during the preflash phase. At the time of the rapid increase in the hard X-ray flux, a new source appeared at 169 MHz and proceeded to fluctuate in phase with one of the original sources that brightened at the same time. This type of behavior, observed in 13 flares, is interpreted as being caused by the rapid interaction of two or more magnetic structures giving rise to the impulsive phase of the flare.

Energy release topology in a multiple-loop solar flare

The temporal and spatial structures of the UV and X-ray emissions and the magnetic field configuration in the November 12, 1980 flare observed from SMM have been studied. The UV observations were done in the O V and Fe XXI lines with a spatial resolution of 10 arcsec. The observations show that the impulsive UV bursts, and also the hard X-ray bursts by their temporal correlation with the impulsive O V emission, occurred in small localized kernels. By comparing the O V, Fe XXI, and X-ray raster images of the flare with the magnetogram, these emission kernels were identified as footpoints of interacting magnetic flux loops. The temporal evolution of the O V/Fe XXI emission shows that there was considerable preheating in the flare plasma some 8-9 minutes prior to the onset of the main hard X-ray bursts. The results are interpreted as indicating that the primary flare energy release occurred in a highly sheared multiloop structure, which lies along a magnetic neutral line. By either beam particle propagation or convective motion, flare energy is transported via a common footpoint to another loop which brightened later. The preheating of the flare plasma is shown to create a more favorable environment for energetic particle acceleration which resulted in the main impulsive hard X-ray bursts.

Simultaneous observations of hard X-ray and microwave burst sources in a limb flare

Observations of a flare which occurred just behind the west solar limb on 1981 August 3 are reported. A hard X-ray source (20-30 keV) and a microwave source at 5 GHz were observed simultaneously using the Very Large Array and the hard X-ray telescope aboard the Hinotori spacecraft. Both sources were located in the corona, apparently near the tops of two independent coronal arcades or loops. The source may be composed of many thin filaments unresolved by the VLA observations. 18 references.

Solution of the electron continuity equation in an inhomogeneous medium via the method of characteristics

Many astrophysical problems require a knowledge of the spatial and temporal evolution of fast electron distributions as they interact (e.g., collisionally) with an ambient medium; an example is provided by hard X-ray burst modelling in solar flares. In the mean scattering-rate approximation (which neglects dispersion in the distribution function), this evolution is governed by the continuity equation in phase space. Here we present a rigorous solution of the continuity equation, derived via the method of characteristics, which confirms and generalises the results of previous authors, and gives greater physical and mathematical insight into these solutions. We illustrate the calculation of this solution for mildly relativistic electrons undergoing Coulomb collisions, and briefly indicate the extension of the method to other physical situations.

The numbers of fast electrons in solar flares as deduced from hard X-ray and microwave spectral data

The previously reported discrepancy between the number of fast solar flare electrons deduced from hard X-ray burst observations and microwave observations of the same events is reexamined. The thin and thick-target models for hard X-ray production and their consequences for gyrosynchrotron emission of microwaves are discussed. Previous work that led to the perceived discrepancy is reviewed, and it is suggested that the discrepancy can be reduced when thick-target hard X-ray emission is assumed. The number of electrons in microwaves and hard X-rays are calculated assuming a homogeneous source for a sample of flares given by Wiehl et al. (1983). The numbers are given separately under the two assumptions of thin and thick-target hard X-ray emission. It is found that when the latter emission is assumed, the number of electrons deduced from the microwaves is very similar to the number deduced from the hard X-rays. 36 references.

Solar hard X-ray bursts

Solar hard X-ray bursts