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.     

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°.     

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.     

Study of X-ray and γ-ray flare from Mkn-421 during February 2010 using Swift and Fermi observations

In this work, we report on the intense flaring activity from Mkn-421 in X-ray and γ-ray regimes simultaneously observed by Swift-XRT/BAT and Fermi-LAT satellite telescopes in February 2010. With the aim of understanding the underlying physics of the flaring state in Mkn-421, we have performed a detailed spectral analysis of Swift/XRT and Fermi/LAT observations of Mkn-421 during February 12–25, 2010 (MJD 55239–55252). Over this period, we study the daily light curves and spectral variability of the source in 1–10 keV, 0.1–1 GeV and 1–100 GeV energy bands. We have performed the spectral analysis of Swift-XRT and Fermi/LAT observations to study the spectral evolution in the X-ray and gamma-ray energy domains respectively. We also compute the fractional variability amplitude in both the energy bands during the above period. We study trends between spectral parameters and physical insights provided by the parameter responsible for X-ray and γ-ray emission from the source. We search for energetic features phenomenologically linked to the single zone SSC model for blazar emission. We also produce the broad band SED with a leptonic single zone SSC model for the source.     

A study of the γ-ray flux during the total solar eclipse of 1 August 2008 at Novosibirsk,Russia

In the past, there have been reports of the observation of decrease in the flux of secondary cosmic γ-rays during a total solar eclipse. We have measured the flux of secondary cosmic γ-rays during the total solar eclipse that occurred at Novosibirsk in Russia, on 1 August 2008. Highly sensitive measurements were carried out by using a detector system with built-in redundancy. The system consisted of two independent, large volume NaI(Tl) scintillator detectors for sensitive and reliable measurements. The data display significant variability in the flux of secondary γ-rays in the energy range 50–4600 keV. Just prior to the total solar eclipse a change ∼9% in the flux was observed, followed by a small but steady decrease ∼4% during the eclipse. The temporal variation in the observed flux of γ-rays were found to be nearly identical for the two detectors. The energy dependence of the variation was further studied by binning the yield in three energy ranges, namely, 100–200, 200–400 and 400–4600 keV. The nearly identical time variation observed in the two independent measurements provides confidence that the measured variation is real and not an artifact of the instrumentation. Systematic observations during the future eclipses are required to study this fascinating phenomenon which is not yet understood.     

Statistical properties of X-class flares and their relationship with super active regions during solar cycles 21–23

We investigate the statistical distribution of X-class flares and their relationship with super active regions (SARs) during solar cycles 21–23. Analysis results show that X1.0–X1.9 flares accounted for 52.71 % of all X-class flares, with X2.0–X2.9 flares at 20.59 %, X3.0–X4.9 at 13.57 %, X5–X9.9 at 8.37 % and ≥X10 at 4.75 %. All X-class flares occurred around the solar maximum during solar cycle 22, while in solar cycle 23, X-class flares were scattered in distribution. In solar cycle 21, X-class flares were distributed neither in a concentrated manner like cycle 22 nor in a scattered manner as cycle 23. During solar cycles 21–23, 32.2 % of the X1.0–X1.9 flares, 31.9 % of the X2.0–X2.9 flares, 43.3 % of the X3.0–X4.9 flares, 81.08 % of the X5.0–X9.9 flares, and 95.2 % of the ≥X10 flares were produced by SARs.     

Turbulent spectrum of Alfvén waves excited by a kinetic instability for explaining the modulations with multi-timescales in solar flares

A low-frequency wave is treated as a local oscillation to modulate the guiding center of electrons beam, which is considered as free energy to excite Alfvén waves by a kinetic plasma instability under low-frequency approximation. The nonlinearity of the model is shown by a critical value of the amplitude of the low-frequency wave, and Alfvén waves are growing in a broad turbulent spectrum with fractional harmonics, which strongly depend on the criterion. The instability is limited in the direction nearly perpendicular to the ambient magnetic field. The growth rates are very sensitive to the beam speed that perpendicular to the magnetic field, the propagational angle, and the magnetic field strength, but not sensitive to the beam speed parallel to the magnetic field. This model is used to explain the modulations with multiple timescales in the flare light curves at radio, hard X-ray and H-alpha bands.     

Binary star model of the γ-ray burst with special reference to that on 5 March, 1979 and subsequent bursts on 6 March, 4 April,and 24 April, 1979

The peculiar -ray burst phenomenon of 5 March, 1979, and the other subsequent bursts on 6 March, 4 April, and 24 April, 1979, are studied, using the physically more realistic exponentially increasing accretion rate on a highly magnetized neutron star from its companion, and the conclusions that pycnonuclear reaction flash for the first and thermonuclear flashes for the subsequent bursts as the most probable model for this series of bursts, are made.We further conclude that a huge -ray burst is a sequel to rapid X-ray transient or type-I X-ray bursts, i.e., an almost exactly similar burst as on 5 April, 1979 will never repeat from the same source, instead rapid X-ray transient burst, or type-I X-ray burst will be occured. A rough estimate gives that the next burst will occur within 0.5 yr since 24 April, 1979.     

On the association of predominant polarity of North-South component of IMF in the GSE system near 1 AU with solar polar magnetic fields during 1967–2020

The skewness of the monthly distribution of GSE latitudinal angles of Interplanetary Magnetic Field (IMF) observed near the Earth (Sk) is found to show anti-correlation with sunspot activity during the solar cycles 20–24. Sk can be considered as a measure of the predominant polarity of north-south component of IMF (B_z component) in the GSE system near 1 AU. Sk variations follow the magnitude of solar polar magnetic fields in general and polarity of south polar fields in particular during the years 1967–2020. Predominant polarity of Sk is found to be independent of the heliographic latitude of Earth. Sk basically reflects the variations of the solar dipolar magnetic field during a sunspot cycle. It is also found that IMF sector polarity variation is not a good indicator of the magnitude changes in solar polar magnetic fields during a sunspot cycle. This is possibly due to the influence of non-dipolar components of the solar magnetic field and the associated north-south asymmetries in the heliospheric current sheet.