Hard X-ray and γ-ray observations of solar flares with the Phebus experiment

The Phebus experiment on board the GRANAT satellite provides temporal and spectral observations of solar and cosmic -ray bursts in the 0.1 100 MeV nominal energy range. The experiment was turned on January 8, 1990 and is still in operation. In this paper we present the main characteristics of the Phebus experiment and we describe and discuss some of the observational properties of the 18 solar hard X-ray/-ray events detected during the first semester of the Phebus operation. It is found that: (i) events of a few minutes duration, detected above 100 keV, systematically show subsecond time variations; (ii) longer duration events (>5 min) do not exhibit fast time variations and generally consist of 1-min peaks superimposed on a less intense, sometimes harder, slowly varying component. In addition to these general trends we discuss in more detail three events for which significant count-rates have been detected above 10 MeV.     

An association of CMEs with solar flares detected by Fermi γ-ray burst monitor during solar cycle 24

By performing certain spatial and temporal criteria, we obtained 492 CME events simultaneously associated with GBM solar flare events (hereafter, GBM-flare–CME) from the total number 5123 Gamma-ray Burst Monitor (GBM) solar flares and 15228 Coronal Mass Ejections (CMEs) detected during the solar cycle 24 (2008–2019). Among these 492 events, which represent about 9.6% of the total number of the detected GBM flares, there are just 381 events (77.4%) representing the CMEs associated with the flares that are detected instantly by both GBM and RHESSI detectors. We found no significant distinction in the results after applying the spatial criteria compared with those arising from applying the temporal criteria only.Actually, all CMEs are ejected within the flare's preflare and the impulsive phases only. From our results, we conclude that the GBM flares whose long duration are most frequently associated with faster and wider CMEs and vice versa. In addition, the longer the flare's duration, the more interval time between the start time of GBM solar flare and CME's ejection time through a linear correlation [Mean Interval = 0.464 × Duration (min)] with a correlation coefficient equals 0.93. We conclude also that, the highly probable, γ-ray emitting flares (detected by GBM only) have a shorter duration and time interval than X-ray flares (detected also by RHESSI). As well as the GBM - CMEs events, without RHESSI associated CMEs are faster and wider than those associated with RHESSI events.     

The characteristics of the coronal mass ejections preceding the associated X-ray and γ-ray burst solar flares

In this study, investigated 14,786 coronal mass ejection (CME) events and 5092 Gamma-ray Burst Monitor (GBM) solar flare events (called γ-ray burst solar flare) recorded during 2008–2017, by using temporal and spatial conditions criteria, we found 845 (about 16%) CME events associated with γ-ray burst solar flare events only (hereafter, CME–γ-preflare). All the 845 events are associated with solar flares that are detected in both GBM and RHESSI simultaneously. Investigating the characteristics of these events, we found that the best time interval is 0–2 h before the flare's start time. The mean time interval for these CME–γ-preflare associated events is 61 min, with the flare's duration mean value of 12 min, which is greater than non-associated γ-ray solar flare's duration. CME width of CME-γ-preflare associated events 64° is slightly wider and slightly faster (remain lower than solar wind's speed) than non-associated CME 53°.     

Evolutionary relationship between the γ-ray flares and millimeter wave outbursts in PKS 0528+134

During the period of 1991–1993 two strong high energy γ-ray flares were observed by the Compton Gamma Ray Observatory in the flat spectrum radio source PKS 0528+134. They were associated with strong mm-radio outbursts with a few months time-delays. In this paper the spectral energy distributions (SED) of the radiations in the γ-hand X-ray and the IR-optical bands are analysed. It is shown that the high energy γ-ray radiation may be due to the inverse Compton scattering of the ambient UV and soft X-ray photons by the relativistic electrons in the jet. Basing on the comparison between the properties of the synchrotron radiation of the γ-ray source and the spectral evolution of the mm-radio outbursts, the evolutional relationship between the γ-ray emitting blobs and the mm-radio emitting blobs is discussed.     

On a new class of impulsive flares with no nuclear γ-ray line emission

The new class of -ray spectra from impulsive flares without nuclear -ray lines is compared with bremsstrahlung spectra of energetic electrons undergoing stochastic acceleration, Coulomb and synchrotron losses. The remarkable agreement of both the produced -spectra from the precipitated electrons and the electron spectra measured in the interplanetary space leads to the conclusion that seed population and acceleration process are identical for both classes of electrons. A new estimate of the electron bremsstrahlung contribution in -spectra of impulsive solar flares seems to be necessary.     

Particle propagation effects on solar flare X- and γ-ray time profiles

Much of the evidence for second stage particle acceleration in solar flares lies in the temporal variation of solar X- and -ray emissions. However, the solar flare X- and -ray burst time-intensity profiles are governed not only by the production or acceleration of electrons and protons but by the propagation of these particles in the solar atmosphere. The effects of particle propagation on X-ray and -ray time profiles are illustrated and compared through the use of three models with the result that a variety of particle propagation schemes reproduce effects commonly associated with second stage acceleration. The first model is that of a closed uniform density trap. The other two models employ particle diffusion from a trap to denser regions of the solar atmosphere to produce the high energy radiation. These calculations show that delayed peaking of the photon flux with respect to particle production and reduction in the impulsiveness of the high energy emission is to be expected, effects commonly associated with second stage acceleration. Thus, well understood physical processes are capable of producing so-called time delays in the high energy emission independent of any delays produced by additional particle acceleration processes. Diagnostic differences between these models are also discussed.     

Observation of Hα line impact polarization in solar flares

We present the results of studying the impact linear polarization of 32 solar flares of X-ray classes C, M, and X (two flares) observed with the Large Solar Vacuum Telescope. It has turned out that there is evidence for impact polarization only in 13 of them. The newly obtained data have confirmed that the linear Stokes parameters are predominantly 2–7%, while the spatial sizes of flaring points with nonzero Stokes parameters are small (1″-2″). Two features of the manifestation of impact polarization in flares revealed by these studies are of greatest interest: (1) at the two foot points of a single flare loop or an arcade of loops, both the Hα intensity profiles and the Stokes profiles differ in behavior; (2) based on the Hα line, we have found for the first time that the sign of the Stokes parameters changes not only across the flare ribbon but also with depth of the chromosphere.     

High energy particle acceleration in solar flares — observational evidence

The recent gamma ray and neutron observations made by the SMM Gamma Ray Spectrometer are reviewed. The implication these observations hold for understanding particle acceleration in solar flares are discussed. The data require that both electrons and ions must be accelerated together to relativistic energies and interact with matter in a time scale of seconds.     

The Hα/Hβ ratio in solar flares

We have measured the ratio of H to H central intensities in the peak kernels of 14 flares, using simultaneous filtergrams. The ratio is typically one with some scatter. By contrast, in stellar flares the ratio is about 0.8.     

The excitation of the iron Kα feature in solar flares

We evaluate the relationship between the hard X-ray photon spectrum and the flux of iron K 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 K 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 K 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 K flux and the observed magnitude of the K 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.     

Non-thermal processes during the ‘build-up’ phase of solar flares and in absence of flares

Hard X-ray and radio observations lead to the conclusion that production of non-thermal electrons is a common phenomenon of the active Sun. A preliminary analysis of three hard X-ray bursts observed with the OGO-5 satellite and the radio observations reported in the literature indicates that non-thermal particles are present in the flare region prior to the impulsive (flash) phase and also during the gradual rise and fall (GRF) bursts which are usually explained in terms of purely thermal radiation. The principal difference between the non-thermal electrons observed before the flash phase and during the flash phase appears to be in their total number rather than in the hardness of their energy spectrum. This indicates that the basic characteristics of the two acceleration processes are probably similar although the total energy converted into non-thermal electrons is considerably larger in the flash phase than in the build-up phase. Transient absorbing H features and filament activations are discussed in terms of their ability to produce energetic particle events and magnetic energy release.Presently at the Space Sciences Laboratory, University of California, Berkeley and Institute of Plasma Research, Stanford University, Stanford, California.     

Hα profiles from electron-heated solar flares

We briefly review the status of models of optical flare heating by electron bombardment. We recompute Brown's (1973a) flare model atmospheres using considerably revised radiative loss rates, based on Canfield's (1974b) method applied to , L, and H^–. Profiles of are computed and compared with observation. The computed profiles agree satisfactorily with those observed during the large 1972 August 7 flare, if spatial and velocity inhomogeneities are assumed. The electron injection rate inferred from is one order of magnitude less than that inferred from hard X-rays, for this event. This may be due to either (1) the neglect of a mechanism that reduces the thick-target electron injection rate or (2) failure to incorporate important radiative loss terms.     

Occurrence of bidirectional type III bursts in solar flares

The 13 pairs of type III bursts with the bidirectional drift structures recorded with the spectrograph in the frequency ranges of 230–300 MHz and 625–1500 MHz at the Yunnan Observatory and 2600–3800 MHz at the Beijing National Astronomical Observatories are analyzed in this present article and the outstanding characteristics of these events are obtained. These bursts respectively reveal that the separatrix frequency between the bursts with positive and negative drifts comes between 250 MHz and 3420 MHz, with a gap being between 0.6 MHz and 110 MHz; the duration is 53 ms–1880 ms and the frequency drift rate is between 45 MHz/s and 56000 MHz/s. The drift rate at metric wavelengths is relatively low, only a few decades of MHz while it is comparatively high at microwave wavelengths, reaching 56000 MHz/s. The qualitative explanation of these events is given in this paper.     

Hα red asymmetry of solar flares

The evolutional characteristics of the red asymmetry of H flare line profiles were studied by means of a quantitative analysis of H flare spectra obtained with the Domeless Solar Telescope at Hida Observatory. Red-shifted emission streaks of H line are found at the initial phase of almost all flares which occur near the disk center, and are considered to be substantial features of the red asymmetry. It is found that a downward motion in the flare chromospheric region is the cause of the red-shifted emission streak. The downward motion abruptly increases at the onset of a flare, attains its maximum velocity of about 40 to 100 km s^-1 shortly before the impulsive peak of the microwave burst, and rapidly decreases before the intensity of H line reaches its maximum. Referring to the numerical simulations made by Livshits et al. (1981) and Somov et al. (1982), we conclude that the conspicuous red-asymmetry or the red-shifted emission streak of H line is due to the downward motion of the compressed chromospheric flare region produced by the impulsive heating by energetic electron beam or thermal conduction.Contributions from the Kwasan and Hida Observatories, University of Kyoto, No. 258.     

Irradiation phenomena in young solar-type stars and the early solar system: X-ray observations and γ-ray constraints

The high levels of X-ray flaring activity observed in young stars and protostars suggest correspondingly high levels of low-energy particle irradiation of their circumstellar disks, and hence of early solar system material like meteorites. We first briefly review the latest X-ray observational results obtained by Chandra and XMM-Newton on two ‘typical’ star-forming regions, Orion and ρ Ophiuchi. We then discuss a new ‘accretion–ejection–irradiation–transport’ model for young stars which, when scaled to the X-ray fluxes, accounts simultaneously for four extinct radioactivity ratios, in particular the purely spallogenic ¹⁰Be/⁹Be ratio, and the ²⁶Al/²⁷Al ratio. We point out the importance of the environment in which nearby star formation is taking place today, namely the Gould Belt, and the possible connection between Comptel detections of ²⁶Al γ-ray line emission from these regions, and new constraints on the origin of the solar system.     

Hβ Doppler-velocity fields within sites of flares in a solar active region

In this paper, we analyze the relationship between photospheric magnetic fields and chromospheric velocity fields in a solar active region, especially evolving features of the chromospheric velocity field at preflare sites. It seems that flares are related to unusually distributed velocity field structures, and initial bright kernels and ribbons of the flares appear in the red-shifted areas (i.e., downward flow areas) close to the inversion line of H Dopplergrams with steep gradients of the velocity fields, no matter whether the areas have simple magnetic structure or a weak magnetic field, or strong magnetic shear and complex structure of the magnetic fields. The data show that during several hours prior to the flares, while the velocity field evolves, the sites of the flare kernels (or ribbons) with red-shifted features come close to the inversion line of the velocity field. This result holds regardless of whether or not the flare sites are wholly located in blue-shifted areas (i.e., upward flow areas), or are far from the inversion line of the Doppler velocity field (V = 0 line), or are partly within red-shifted areas. There are two cases favourable for the occurrence of flares, one is that the gulf-like neutral lines of the magnetic field (B = 0 line) occur in the H red-shifted areas, the other is that the gulf-like inversion lines of the H Doppler velocity field (V = 0 line) occur in the unipolar magnetic areas. These observational facts indicate that the velocity field and magnetic field have the same effect on the process of flare energy accumulation and release.     

Electron acceleration and heating in solar flares: Interpretation of Hα signatures

Vector magnetogram, H, and hard X-ray observations of flares are reviewed which show that nonthermal electron signatures in H are never cospatial with regions of maximum current density for the small number of flares analyzed, but lie to the sides of these regions. By considering electron acceleration and transport requirements, four conditions are found that must be fulfilled to observe nonthermal electron signatures in H: (1) The plasma beta 0.3 in the acceleration region. (2) The energy flux of electrons above 20 keV is greater than 10¹⁰ erg cm^–2 s^–1. (3) The column densityN 10²⁰ cm^–2 between the electron source and the chromosphere. (4) The coronal pressure in the flux tube connecting to the H layerp 100 dyne cm^–2. Condition 2 can be most easily met in the initial stages of flares. In contrast, the only condition for a high-pressure H signature isp 1000 dyne cm^–2, which is most easily met in a region of maximum current density or heating and far enough into the flare for significant heating to have occurred. Thus, high-pressure signatures should be expected to occur more frequently than nonthermal electron signatures and to occur generally later in time.Also Guest Worker at NOAA Space Environment Laboratory Boulder Colorado U.S.A.     

The luminosity of γ-ray pulsars

We point out that in the polar cap model, the visible part is only part of the cap and this fact should be taken into account when calculating the pulsar luminosity. When this is done, the calculated luminosities are in basic agreement with the observed values. Based on our calculations we give a list of 12 possible γ-ray candidates.     

Low energy γ-ray events in imaging Čerenkov telescope arrays

One of the major goals in VHE--ray astronomy is to open the energy range below 100 GeV with earthbound detectors. This paper demonstrates a new method for analyzing erenkov light of a shower in a erenkov telescope array. This method is successful for showers in this low energy regime where previous techniques (e.g. alpha analysis) are not applicable. A Monte Carlo simulation is applied to a system of 19 Whipple type [3, Cawley 1990] Imaging Atmospheric erenkov telescopes (IAT), each built as a 10 m diameter reflector and equipped with a 109 photomultiplier tube camera. The energy threshold for a single detector of this type is given [5, Kerrick et al. 1995] as 250 GeV. Analysis of simulated coincident events of the system for those events not having enough light to apply a standard imaging analysis [4, Hillas 1985], leads to a considerably lower threshold of 85 GeV. With a new analysis method of these events it is shown that it should be possible to distinguish between -ray induced and proton induced showers. The improvement of sensitivity (Q = figure of merit) of this analysis method is found to be Q=2.9.