Soft X-ray Bursts Research Articles

Observations of soft gamma-ray or hard X-ray bursts in the GRIF experiment on the Mir orbiting station

During the GRIF experiment onboard the Mir orbiting station, the sky was monitored with a PX-2 wide-field (∼1 sr) scintillation X-ray spectrometer to detect bursts in the photon energy range 10–300 keV. Because of the comprehensive instrumentation, which, apart from the X-ray and gamma-ray instruments, also included charged-particle detectors, the imitations of astrophysical bursts by magnetospheric electron precipitations and strongly ionizing nuclei were effectively filtered out. It was also possible to separate solar and atmospheric events. Several tens of bursts interpreted as being astrophysical were detected in the experiment at sensitivity levels S∼10−7 erg cm−2 (for bursts whose spectra were characterized by effective temperatures kT∼100 keV) and S∼3×10−8 erg cm−2 (for bursts with kT∼25 keV). Some of the soft gamma-ray or hard X-ray bursts with kT∼10–50 keV were identified with the bursting pulsar GRO J1744-28. Our estimate of the detection rate for cosmological soft gamma-ray or hard X-ray bursts from the entire sky suggests that the distributions of long-duration (>1 s) gamma-ray bursts (GRBs) in characteristic energy kT and duration are inconsistent with the steady-state cosmological model in which the evolution of burst sources is disregarded. Based on GRIF and BATSE/CGRO data, we conclude that most of the GRB sources originate at redshifts 1<z<5.

Studies on metric–decimetric Type II bursts in relation to soft X-ray flares

The relationship between metric type II radio bursts and soft X-ray (SXR) flares is studied. Type II bursts are highly associated with SXR flares. The duration and drift rate of type II bursts are found to depend on the duration, asymmetry in duration (ratio of rise time to duration), as well as on the peak flux of SXR bursts. Important results obtained are: (i) the durations of type II bursts are linearly correlated with the durations of associated SXR bursts in case of long-lived events (duration > 40 min), whereas in short-lived flares such a correlation is not found, (ii) the durations of type II bursts do not depend upon the SXR peak flux, (iii) more durable type II radio bursts are correlated with more symmetric SXR bursts, (iv) average drift rates of type II bursts are larger in the events associated with more powerful and more symmetric SXR bursts.

Solar Hard X-Ray Bursts and Electron Acceleration Down to 8 [CLC]ke[/CLC

In 2 days that were typical of solar maximum (1991 October 1-2), the CGRO/BATSE Spectroscopy Detectors observed more than five solar X-ray bursts per hour above 8 keV [8-13 keV flux greater than 0.4 (cm2 s keV)-1]. Only approximately one-third of these were detected above 25 keV [25-59 keV flux greater than 1.7 × 10-3 (cm2 s keV)-1] by the BATSE Large Area Detectors (LADs). Most of the >8 keV bursts are impulsive and similar to solar hard X-ray bursts observed above 20 keV—the bursts occurred on the most rapid rise of the GOES thermal soft X-ray burst, and the ratio of 8-13 keV to greater than 13 keV flux for the impulsive bursts fitted a power-law spectrum [dJ/d(hν) ~ (hν)-γ] with γ = 3-7. More than half of the bursts that should be detected by the LADs if the power law extends to high energies are not detected, indicating that the spectrum steepens or cuts off above ~25 keV. These findings suggest that the acceleration of electrons occurs for essentially all impulsive energy releases but may only be detectable at low energies in smaller releases. The burst occurrence frequency varies with energy W(>8 keV) in accelerated electrons as dn/dW ~ W-1.6 above W ~ 1029 ergs, with an average rate of energy deposition of ~1026 ergs s-1, about 100 times that above 20 keV. Comparison with the occurrence frequency of the energy content in active region transient brightenings (ARTBs) detected by the Yohkoh soft X-ray telescope suggests that accelerated electrons may provide a significant fraction of the total energy in ARTBs.

Comparative Analysis of Type ii Bursts and of Thermal and non-Thermal Flare Signatures

The relationship between metric type II radio bursts and solar flares is studied. Well-defined correlations between the properties of type II bursts and the characteristics of associated microwave and soft X-ray bursts are established in two entirely independent data sets. It is shown that the correlations are strongly affected by the wide range of coronal Alfven velocities involved, comprising values from only 150 up to 800 km s−1, with a typical value of 400 km s−1. After careful data analysis it was inferred that type II bursts are more closely related to the soft X-ray bursts than they are to microwave bursts. The correlations indicate that type II burst shocks are preferably generated by flares with a relatively strong thermal component, and that the shocks are probably ignited by the plasma expansion associated with the 'evaporation' process in the transition region. Although the results imply that the majority of metric type II bursts are caused by flares, a simple geometrical consideration shows that a fraction of non-flare type II bursts cannot be explained by behind-limb events and that roughly 10% of metric type II bursts should be attributed to non-flare coronal mass ejections.

Collaborative VLA and LASCO Observations of Nonthermal Energy Release in the Corona

We discuss recent collaborative observations of coronal mass ejections and related activity using the Very Large Array (VLA) and the Solar and Heliospheric Observatory (SOHO). VLA observations show the onset of intense 400 cm burst emission during an X1 GOES soft X-ray burst observed by the Yohkoh spacecraft and prior to a CME observed by the Large Angle Spectroscometric C2 Coronagraph (LASCO) on board SOHO. VLA snapshot maps show that the 400 cm burst site varied discontinuously throughout the course of the flare, possibly reflecting abrupt changes in the structure of the coronal magnetic fields along which the energetic particles traveled.

Characteristics of Flares with Hα Emission Protruding over Major Sunspot Umbrae

A sample of 47 importance ≥ 1 flares whose Hα emission occurred or protruded over umbrae of major sunspots (so called Z-flares) was studied to investigate characteristics of the associated dm – m radio, microwave and soft X-ray emission as the energy release site permeats into regions of strong magnetic fields. A close time association was found between the microwave burst peak and the `contact' of the Hα emission with the sunspot umbra. The Hα emission attained maximum close to or a few minutes after the contact. The soft X-ray bursts were delayed more, attaining maximum 0–10 min after the contact. The onset of bursts in the dm – m wavelength range was associated with the period of growth or the peak of the microwave burst. Two categories of type III and IV bursts could be recognized: the ones starting some ten minutes before the microwave peak, and those that begin close to the microwave burst peak. Type III bursts occur preferably when the microwave burst peaks simultaneously with or after the contact. The results are explained presuming that the contact reveals a permeation of the energy release process into a region of strong magnetic fields, where the process intensifies, and where the accelerated particles have access to magnetic field lines extending to large coronal heights. Different manifestations of the energy release process in various magnetic field topologies are considered to account for the various time sequences observed.

The Expulsion of Stellar Envelopes in Core‐Collapse Supernovae

We examine the relation between presupernova stellar structure and the distribution of ejecta in core-collapse supernovae of Types Ib, Ic, and II, under the approximations of adiabatic, spherically symmetric flow. We develop a simple yet accurate analytical formula for the velocity of the initial forward shock that traverses the stellar envelope. For material that does not later experience a strong reverse shock, the entropy deposited by this forward shock persists into the final, freely expanding state. We demonstrate that the final density distribution can be approximated with simple models for the final pressure distribution, in a way that matches the results of simulations. Our results indicate that the distribution of density and radiation pressure in a star's ejecta depends on whether the outer envelope is radiative or convective, and if convective, on the composition structure of the star. Our models are most accurate for the high-velocity ejecta cast away from the periphery of a star. For stellar structures that limit to a common form in this region, the resulting ejecta limit to a common distribution at high velocities because the blast wave forgets its history as it approaches the stellar surface. We present formulae for the final density distribution of this material as a function of mass, for both radiative and efficiently convective envelopes. These formulae limit to the well-known planar, self-similar solutions for mass shells approaching the stellar surface. However, the assumption of adiabatic flow breaks down for shells of low optical depth, so this planar limit need not be attained. The event of shock emergence, which limits adiabatic flow, also produces a soft X-ray burst of radiation. Formulae are given for the observable properties of this burst and their dependence on the parameters of the explosion. Motivated by the relativistic expansion recently inferred by Kulkarni et al. for the synchrotron shell around SN 1998bw, we estimate the criterion for relativistic mass ejection and the rest mass of relativistic ejecta. We base our models for the entire ejecta distribution on the high-velocity solution, on our shock-velocity formula, and on realistic radiation pressure distributions. We also present simpler, but less flexible, analytical approximations for ejecta distributions. We survey the ejecta of the polytropic hydrogen envelopes of red supergiants. Our models will be useful for studies of the light curves and circumstellar or interstellar interactions of core-collapse supernovae, and of the birth of pulsar nebulae in their ejecta.

The Hardness Distribution of Gamma‐Ray Bursts

It is often stated that gamma-ray bursts (GRBs) have typical energies of several hundred keV, where the typical energy may be characterized by the hardness H, the photon energy corresponding to the peak of νFν. Among the 54 BATSE bursts analyzed by Band et al. in 1993 and the 136 analyzed by us, more than 60% have 50 keV < H < 300 keV. Is the narrow range of H a real feature of GRBs, or is it due to an observational difficulty in detecting harder bursts? We consider a population of standard candle bursts with a hardness distribution ρ(H)d log H Hγ d log H and no luminosity-hardness correlation. We model the detection algorithm of BATSE as a function of H, including cosmological effects, detector characteristics, and triggering procedure, and we calculate the expected distribution of H in the observed sample for various values of γ. Both samples show a paucity of soft (X-ray) bursts, which may be real. However, we find that the observed samples are consistent with a distribution above H = 120 keV with γ ~ -0.5 (a slowly decreasing number of GRBs per decade of hardness). Thus, we suggest that a large population of unobserved hard gamma-ray bursts may exist.

Eruption of a Twisted Filament and Associated Phenomena

AbstractIn this paper we present CCD observations between February 14–20, 1994 and analysis of the giant twisted filament evolved in the active region NOAA 7671. The dynamic eruption of the filament was accompanied by a major flare (3B/M4), CME, long duration type II, IV radio bursts, great microwave bursts, a long duration soft X-ray burst, SIDs, strong geomagnetic storms and a very energetic proton flare. We analysed and estimated the twist, length, volume, mass and energy associated with filament system between February 14 and 20, 1994. The present study shows that the magnetic energy required for the solar flare came from the filament system associated with the solar flare and associated phenomena.

THE BRIGHT SURGE-LIKE SPIKE AT THE SOLAR LIMB SEPTEMBER 24, 1991

The 1N/M6.1 limb-flare-associated surge-spike phenomenon of 24 September, 1991 is investigated using Hα filtergrams taken at Kandilli Observatory. The kinematics and the morphological features of the moving plasma are discussed. Radio and soft X-ray bursts, ionospheric, and magnetic disturbances are detected in the oscillatory motions during the surge phenomenon.

SOLAR ENERGETIC ION AND ELECTRON LIMITS FROM HIREGS OBSERVATIONS

The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is ≲ 0.8 ph cm-2 in the flares, and the upper limit flux is 1.8 × 10-4 ph s-1 cm-2 in the absence of flares. These limits imply that ≲ 6 × 1030 (2σ) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons >30 MeV is ≲ 4 × 1026 ergs, but this limit can be more than 1030 ergs if the spectrum extends down to ∽1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 1030 ergs if the spectrum goes down to ∽ 7 keV. Quiet-Sun observations indicate that ≲ 1026erg s-1 are deposited by energetic protons >1 MeV, well below the1027 –1028 erg s-1 required for coronal heating, while <3 × 1027 erg s-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least ∽2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for ≳ 104 s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.

VERY LARGE ARRAY OBSERVATIONS OF EVOLVING NOISE STORM SOURCES ON THE SUN

The presence of coronal magnetic fields connecting active regions is inferred from decimetric observations of solar noise storms with the Very Large Array (VLA) and from soft X-ray images taken by Yohkoh. Temporal changes in the noise storms appear to be correlated with some soft X-ray bursts detected by both Yohkoh and the GOES satellite. Combined analysis of the radio and X-ray data suggests a re-arrangement of the coronal magnetic field during the onset of impulsive noise storm burst emission. On one day during the combined VLA–Yohkoh–GOES observations, two widely-separated active regions appear to be connected by a faint trans-equatorial 91 cm source as well as two distinct soft X-ray loops. The two active regions show anti-correlated fluctuations in decimetric radio emission. On another day of combined VLA–Yohkoh observations, a series of 91 cm noise storm bursts are observed along the major axis of the associated noise storm continuum. Time sequences of Yohkoh soft X-ray images show a contraction of coronal loops prior to the onset of this series of bursts and a corresponding increase in the X-ray flux in the apparent footpoint of the overarching loop containing the noise storm. These observations imply that energy from a realignment of the magnetic field is being transferred, possibly by accelerated particles, along loops connecting separated active regions on the Sun.

Damages to B4C/W multilayer mirrors by intense soft x-ray bursts

B4C/W multilayer mirrors designed for an x-ray beamline at the MEGAJOULE facility are susceptible to x-ray damage. The OCTAL laser was implemented to generate a gold plasma with a continuous x-ray spectrum (0.1 to 1.5 keV) used to irradiate the mirror. The fluence on the sample is 0.05 J/cm2 per shot. The pulse duration is about 1 ns. The temporally integrated spectrum reflected by the mirror is analyzed by a bent crystal spectrometer and recorded on photographic film. The spectral analysis suggests that an increase of the mean period of the multilayer happens during the second shot. This expansion is estimated to be 5% with respect to the unexposed mirror. After the second shot, destruction of the multilayer coating in the central part of the sample and irreversible damages in the other part characterized by a decrease of Bragg reflectivity have been observed.

Correlation between hard and soft X-ray bursts and small-scale acceleration processes during the rise phase of solar flares

Soft and hard X-ray flare bursts were compared during the rise phase of solar flares of moderate importance. The analysis shows that plasma heating in the flare region has two components: permanent heating (like Joule dissipation) and impulsive heating coinciding with relatively short hard X-ray bursts. It is shown that during the primary energy release phase the most probable mechanism of energy release is acceleration of charged particles in direct electric fields. Particles are accelerated within small regions where these fields are excited. Characteristics of such regions are obtained in the paper.

Large-scale active coronal phenomena in Yohkoh SXT images

We have found several occurrences of slowly rising giant arches inYohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3–5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches followeruptive flares with gradual soft X-ray bursts, and rise with speeds of 1.1–2.4 km s−1 which keep constant for >5 to 24 hours, reaching altitudes up to 250 000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnecting loop. In the event of 21–22 February 1992 one can see both the loop system, rising with a gradually decreasing speed to an altitude of 120 000 km, and the arch, emerging from behind the loops and continuing to rise with a constant speed for many more hours up to 240 000 km above the solar limb. In the event of 2–3 November 1991 three subsequent rising large-scale coronal systems can be recognized: first a fast one with speed increasing with altitude and ceasing to be visible at about 300 000 km. This most probably shows the X-ray signature of a coronal mass ejection (CME). A second one, with gradually decreasing speed, might represent very high rising flare loops. A third one continues to rise slowly with a constant speed up to 230 000 km (and up to 285 000 km after the speed begins to decay), and this is the giant arch. This event, including an arch revival on November 4–5, is very similar to rising giant arches observed by the SMM on 6–7 November 1980. Other events of this kind were observed on 27–28 April 1992, 15 March 1993, and 4–6 November 1993, all seen above the solar limb, where it is much easier to identify them. The temperature in the brightest part of the arch of 2–3 November 1991 was increasing with its altitude, from 2 to 4 × 106 K, which seems to be an effect of slower cooling at lower densities. Under an assumption of line-of-sight thickness of 50 000 km, the emission measure indicates densities from 1.1 × 1010 cm−3 at an altitude of 150 000 km to 1.0 × 109 cm−3 at 245 000 km 11.5 hours later. It appears that the arch is composed of plasma of widely different temperatures, and that hot plasma rises faster than the cool component. Thus the whole arch expands upward, and its density gradient increases with time, which explains whyYohkoh images show only the lowest and coolest parts of the expanding structure. The whole arch may represent an energy in excess of 1031 erg, and more if conduction contributes to the arch cooling. We suggest that the rise of the arch is initiated by a CME which removes the magnetic field and plasma in the upper corona, and the coronal structures remaining below this cavity begin to expand into the “vacuum” left behind the CME. However, we are unable to explain why the speed of rise stays constant for so many hours.

Large-amplitude oscillations of soft X-rays in a high current density plasma on TPE-1RM15 reversed field pinch

In the high current density plasma on TPE-1RM15 reversed field pinch, large-amplitude oscillations with m=0, n=0 are observed in soft X-ray (SXR) emission for a wide range of theta values, even at low- theta ( theta <1.55) discharges. Another small-amplitude oscillation with m=1 is observed quasicontinuously for almost all the discharge. The burst of high-energy soft X-rays is observed in the channels of the SBD array looking at the electron drift side of a metal limiter and seems to correlate with the crash of the large-amplitude oscillation. This SXR burst indicates the existence of energetic electrons flowing along the field lines near the plasma surface in the dynamo phase. These large-amplitude SXR oscillations are mainly due to the variations in the electron temperature and are roughly in antiphase with the oscillation of ion temperature.

Detection of X-rays from SN 1987A with ROSAT

Soft X-rays (0.5-2 keV) were detected with the ROSAT Position-Sensitive Proportional Counter (PSPC) from the direction of SN 1987A, which falls within an association in the Large Magellanic Cloud (LMC) that is rich in B stars. The emission is consistent with two point sources in the LMC with total luminosity of approximately 10(exp 34) ergs/s. The brighter source is identified with SN 1987A on the basis of its positional agreement. We interpret the emission as arising from the interaction of supernova ejecta with the pre-existing blue giant wind. The second source may be an unidentified Be/X-ray binary. Another possibility is that the second source is an X-ray echo from the supernova outburst. The X-ray echo interpretation reqires that the supernova emitted approximately 10(exp 47) ergs in a burst of soft X-rays.

Simultaneous Optical and ROSAT Soft X-Ray Observations of Impulsive Bursts on the Flare Star UV Ceti

Two small flares on the nearby flare star UV Ceti were simultaneously observed with ground-based, highspeed optical multicolor photometry and in soft X-rays using the ROSAT Observatory. Optically, each flare showed a brief (∼12 s duration) impulsive event. The second flare also showed a gradual phase lasting about 5 minutes. Upon closely examining the soft X-ray timing data, it was found that for both flares a soft X-ray burst followed the optical spike by about 30 s. The gradual phase of the second flare was accompanied by a rising and falling X-ray flux, as has been observed in numerous previous flares, but our timing data appear to show soft X-ray bursts continuing during this phase as well. These impulsive X-ray bursts may be the low energy tails of hard X-ray emission from particle beams

Radio and soft X-ray investigation of the solar flares of February 4, 1986

In this paper, the 3B flare of February 4, 1986 is studied comprehensively. The escape electrons accelerated to 10–100 keV at the top of coronal loop are confirmed by III type bursts. The energetic electron beams moved downward trigger the eruptions in the low layer of solar atmosphere. The radio and soft X-ray bursts are interpreted, respectively, by the maser mechanism and evaporation effect. Finally, the important role of energetic electron beams in solar flares is pointed out.

Stereoscopic observations of a solar flare hard X-ray source in the high corona

The vertical structure of the impulsive and gradual hard X-ray sources in high coronae and the characteristics of the impulsive soft X-ray emission are investigated on the basis of PVE, ICE, and GOES observations of the energetic flare on February 16, 1984. The average photon spectra observed by these instruments during the impulsive and gradual hard X-ray bursts are summarized. A comparison of these unocculted and partially occulted spectra shows that the sources of the impulsive hard X-ray (greater than about 25 keV) and impulsive soft X-ray (2-5 keV) emissions in this flare extended to coronal altitudes greater than about 200,000 km above the photosphere. At about 100 keV, the ratio of the coronal source brightness to the total source brightness was 0.001 during the impulsive phase and less than about 0.01 during the gradual hard X-ray burst. The sources of the gradual hard X-ray burst and gradual soft X-ray burst were almost completely occulted, indicating that these sources were located at heights less than 200,000 km above the photosphere.