Taro Sakao

Yohkoh observations of an impulsive solar flare which occurred on 16 December, 1991 are presented. This flare was a GOES M2.7 class event with a simple morphology indicative of a single flaring loop. X-ray images were taken with the Hard X-ray Telescope (HXT) and soft X-ray spectra were obtained with the Bragg Crystal Spectrometer (BCS) on board the satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the rise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Ca xix during the initial phase of the flare. The data are compared with observations from other space and ground-based instruments. A balance calculation is performed which indicates that the energy contained in non-thermal electrons is sufficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently verified in a manner which indicates that the loop filling factor is close to 100%. The production of 'superhot' plasma in impulsive events is shown to differ in detail from the morphology and mechanisms appropriate for more gradual events. © 1994 Kluwer Academic Publishers.

We analyze simultaneous Hα images and spectra (from Mees Solar Observatory), and soft and hard X-ray images and spectra (from YOHKOH) during the early phase of an X1.5/3B flare. We investigate the morphological relationship between chromospheric downflows, coronal upflows, and particle precipitation sites, and the energetic relationship between conductive heating, nonthermal particle heating, and the chromospheric response. We find that the observations consistently fit the chromospheric evaporation model. In particular, we demonstrate that the observed upflowing coronal and downflowing chromospheric plasma components originate in the same locations, and we show that our unique set of optical and X-ray observations can clearly distinguish between conductively driven and electron beam driven evaporation.

The Yohkoh soft X-ray telescope has observed impulsive, thermal, soft X-ray emission at the footpoints of magnetic loops during solar flares. The soft X-ray (thermal) time profiles at the footpoints closely match the hard X-ray (nonthermal) time profiles, directly demonstrating the heating of the lower solar atmosphere on short timescales during the interval of nonthermal energy release. This phenomenon is the rule, rather than the exception, occurring in the majority of flares that we have examined with the Yohkoh data. We illustrate the impulsive behavior with data from the major flare of 1992 January 26. For this flare, the soft X-ray peak times matched the hard X-ray peak times within the time resolution of the soft X-ray measurements (about 10 s), and the soft and hard X-ray locations match within the resolution of the hard X-ray imager. The impulsive soft X-ray emission clearly has a thermal spectral signature, but not at the high temperature of a "superhot" source. We conclude that the impulsive soft X-ray emission comes from material heated by precipitating electrons at loop footpoints and evaporating from the deeper atmosphere into the flaring flux tube.

This is a preliminary report concerning an impulsive flare, which occurred on 1992 August 17-18, and was observed with the Nobeyama Radioheliograph, Yohkoh, and ground-based instruments. Emphasis is put on the alignment of radio images as well as soft X-ray and hard X-ray maps, which axe compared in both impulsive and gradual phases. In the impulsive phase, which continued for about 30 s, nonthermal emission was most remarkable at 17 GHz, and was also seen in hard X-rays at the southern part of the SXT source, which was about 14'' width and 80'' long, extending in the north-east to south-west direction. Thermal emission was detected in soft X-rays and possibly in hard X-rays at the northern Part. During the gradual phase, about 80 s after the onset of the impulsive phase, thermal emission dominated and was located at the northern part of the source over the entire energy range. Difficulties are discussed concerning a possible simple topology of the.magnetic fields.

An impulsive burst which occurred on 1992 October 27 was observed simultaneously with the hard X-ray telescope on board the Yohkoh satellite and the Nobeyama Radioheliograph at 17 GHz. The hard X-ray images show a double-source structure during the main phase. One footpoint, A, shows a nonthermal spectrum, while another, B, shows a better fit to the X-rays from extremely hot thermal electrons with about 80 million degrees, flowing into the chromosphere. The loop top also shows a better fit to a quasi-thermal spectrum of 75 million degrees at the main peak. On the other hand, the radio images are a larger single source covering the X-ray source, and are highly polarized in the L-sense. The larger single image is mainly ascribed to the beamwidth, and the single polarization may be due to ''limiting polarization''. Consequently, the radio source at 17 GHz may be cospatial with the X-ray source around the loop top. The radio emission at 17 GHz and a part of the emission at 9.4 GHz are attributed to the thermal gyro emissions from the extremely hot thermal electrons emitting the X-rays. The rest of the radio emission at 9.4 GHz and the emission at 3.75 GHz are ascribed to thermal gyro emission at the outer layers with smaller magnetic fields and lower electron temperatures. The intense radio emission at 35 GHz at the main peak is ascribed to gyro-synchrotron emission from the nonthermal electrons in footpoint A.

The time variations of hard X-ray images of four impulsive bursts with simple source structures were investigated in a comparison with the magnetic structure. Two of them are limb bursts. Common variations during the early phase are as follows: i) The hard X-ray brightening seems to start at the top of a single coronal loop. ii) The X-ray source spreads during the increasing phase of the burst in both directions along the loop, and both ends become brighter, especially at higher energies with generally unequal brightness. The loop top is still bright, especially at lower energies, to show three peaks. The speed of the expansion of the X-ray source amounts to about 10(4) km s-1 in three cases. iii) At and after the peak of the X-ray flux, the source tends to be a single source at the loop top, especially at lower energies. iv) The effective temperature for quasi-thermal electrons and their number density during the early phase in the vicinity of the loop top are (4-6) x 10(7) K and (5-2) x 10(9) cm-3, respectively, so that the electron mean free path is greater than three-times the local temperature scale height. These observations axe consistent with the idea that anomalous resistivity, which triggers impulsive bursts, is caused by electron plasma waves generated in the process of heat conduction.

More than two hundred solar flares, including several GOES X-class events, were successfully observed with the Hard X-ray Telescope (HXT) on board Yohkoh during the initial six months of observations since 1991 October. Hard X-ray images taken simultaneously in four X-ray energy bands (14-23-33-53-93 keV), with angular and temporal resolutions of approximately 5" and 0.5 s, respectively, have been revealing how and where ha.rd X-rays are emitted in flaring magnetic loops, and further how and where electrons are accelerated and confined. These HXT observations are briefly reviewed from the viewpoint of the instrument capability and performance, with some new scientific results.

© Royal Astronomical Society. We report Ginga observations of the old nova GK Persei in quiescence, as well as a brief scanning observation during an outburst. The X-ray spectrum in quiescence is well fitted by thermal bremsstrahlung emission of very high temperature (kT~30 keV), plus an iron emission line. In contrast, the outburst spectrum is complex and comprises two continua with different column densities {Nu ~ 1023 and ~ 1024 cm-2). The 351-s spin modulation of GK Per was clearly detected in the quiescence observation, which confirms the results of previous EXOSAT observations. The folded light curve shows two peaks that are not separated by 180° in phase, which is indeed quite different from the EXOSAT outburst data. It is, however, similar to the EXOSAT observation at a similar flux level.

© 1992 Oxford University Press. All rights reserved. We present Ginga observations of the contact binary VW Cep. The observed X-ray luminosity is 1.1 x 1030 erg s-1 in the energy range of 2-10 keV, assuming a distance of 31 pc. No evidence for X-ray orbital modulation or for Hare events was seen. The observed X-ray spectrum is very hard, and can be represented well either by a thermal bremsstrahlung model with a temperature of 11.2+4.0-2.3keV or a power-law model with photon index of ? = 1.90+0.24-0.20. These observational results are interpreted in terms of thermal emission from hot coronal plasma extending beyond the stellar size. However the observed upper limit on the iron K-line intensity is considerably below the theoretical prediction.

The Hard X-ray Telescope (HXT) is a Fourier-synthesis imager; a set of spatially-modulated photon count data are taken from 64 independent subcollimators and are Fourier-transformed into an image by using procedures such as the maximum entropy method (MEM) or CLEAN. The HXT takes images of solar flares simultaneously in four energy bands, nominally 15 (or 19)-24, 24-35, 35-57, and 57-100 keV, with an ultimate angular resolution as fine as ∼ 5 arc sec and a time resolution 0.5 s. Each subcollimator has a field of view wider than the solar disk. The total effective area of the collimator/detector system reaches ∼ 70 cm2, about one order of magnitude larger than that of the HINOTORI hard X-ray imager. Thanks to these improvements, HXT will for the first time enable us to take images of flares at photon energies above ∼ 30 keV. These higher-energy images will be compared with lower-energy ones, giving clues to the understanding of nonthermal processes in solar flares, i.e., the acceleration and confinement of energetic electrons. It is of particular importance to specify the acceleration site with regard to the magnetic field figuration in a flaring region, which will be achieved by collaborative observations between HXT and the Soft X-ray Telescope on board the same mission. © 1991 Kluwer Academic Publishers.

An outburst of the transient X-ray pulsar X0115+634 was detected with the All Sky Monitor (ASM) on board Ginga on 1990 February 5. Follow-up observations with the large-area proportional counters (LACs) revealed complex changes in the energy spectrum which depend on the phase of the 3.6 s pulsation. We find that characteristic structures in the spectra above 10 keV can be best interpreted as two dips at ∼12 and ∼23 keV, although not at all phases. The center energies of the two dips are consistent with the harmonic relation of 1:2, showing phase-dependent ±10% variations with pulse phase. The results strongly suggest that the structures in the spectra are due to cyclotron resonant scattering and the two apparent absorption lines are ascribed to the fundamental and second harmonics. This indicates a magnetic field strength on the neutron star surface of ∼1 × 1012 G. Equivalent widths of the second harmonic line are about 2 times larger than those of the first harmonic line, depending on the pulse phase.

The Hard X-ray Telescope (HXT), now under fabrication for the SOLAR-A mission (scheduled for launch in August 1991), is an advanced Fourier-synthesis imager. An overview of the HXT instrument is given together with its scientific objectives, that is, the electron acceleration and confinement mechanisms in solar flares. Scientific return from HXT will be greatly increased if worldwide collaboration with other space and ground-based observations is well organized. © 1991.

© Royal Astronomical Society. Ginga observations of the X-ray binary pulsar GX301-2 are analysed for their temporal and spectral properties. We report the unambiguous detection of aperiodic intensity variations of 7-10 per cent rms relative amplitude over time-scales covering ∼16 to 0.1 s. These variations are self-similar, being well fitted by a power law in power spectral density versus frequency space. The pulse period measured is 689.80 s, the shortest yet measured for this source. The fraction of X-rays which is pulsed is energy dependent, being greater at higher energies. The fractional aperiodic variability shows no such energy dependence, nor do they appear to be significantly dependent on source intensity, pulse phase or absorbing column measured. These facts strongly suggest that the aperiodic variations are not caused by absorption variations, but are intrinsic to the source. The iron-line intensity shows little pulsations, but surprisingly shows significant aperiodic variability down to a time-scale as short as several seconds, thus giving a constraint to the size of the line emission region.

A remarkable absorption feature at 28.5 keV, attributable to electron cyclotron resonance, has been discovered in the 1.9-60 keV X-ray spectrum of the recurrent transient X-ray pulsar X0331+53 (V0332+53). The observed resonance energy implies a neutron star surface magnetic field of 2.5(1 + z) × 1012 G, where z is the gravitational redshift. The detection was made with the Ginga observatory in 1989 October, during an outburst of this transient with a flux level of ∼ 0.3 Crab. The feature is very deep and has been resolved with excellent statistics. This is the fourth unambiguous detection of cyclotron resonant scattering features from X-ray pulsars, suggesting that these features are a common phenomenon among these objects. An empirical relation found between the cyclotron resonance energy and the spectral cutoff energy suggests that the magnetic field strengths of the known X-ray pulsars are clustered in a range (1-4) × 1012 G.

A cyclotron absorption line near 20 keV has been found in the spectrum of the massive eclipsing binary X-ray pulsar 4U 1538 - 52 in observations with the Ginga observatory. The line is detected throughout the 529 s pulse cycle with a variable equivalent width that has its maximum value during the smaller peak of the two-peak pulse profile. We find that the profile of the pulse and the phase-dependence of the cyclotron line can be explained qualitatively by a pulsar model based on recent theoretical results on the properties of pencil beams emitted by accretion-heated slabs of magnetized plasma at the magnetic poles of a neutron star. The indicated field at the surface of the neutron star is 1.7(1 + z) × 1012 G, where z is the gravitational redshift.

ALTHOUGH neutron stars are generally believed to be born with intense (1011-1013 G) magnetic fields1,2, which then gradually decay3, measurements of their field strengths remain uncertain. In the special case of X-ray-emitting binary pulsars, a direct estimate of the field strength can be obtained by measuring the energy of spectral features that are due to electron cyclotron resonance4-13. With the Ginga satellite observatory14,15, we have measured a cyclotron feature in the hard X-ray spectrum of the 1.24-s binary pulsar Hercules X-1 with a much greater energy resolution than in previous observations4-9. The spectrum from 10-60 keV can be described with a simple analytical formula12,16,17, which indicates an absorption feature at ∼34 keV rather than an emission feature at ∼50keV. From this we estimate the surface magnetic field strength of Her X-1 to be (2.9±0.3) × 1012 G. © 1990 Nature Publishing Group.

Observations of a cyclotron absorption feature in the spectrum of X-ray pulsars are reported. The cyclotron absorption features have been discovered with Ginga in two X-ray pulsars, 4U 1538-52 (at 21keV) and X0331+53 (at 28keV). Ginga data indicate that the known cyclotron feature in the spectrum from Her X-1 should be regarded as an absorption feature at &acd;34keV rather than an emission feature at &acd;50keV. These results indicate that he cyclotron absorption feature should be common in binary X-ray pulsars. We infer that the high-energy spectral break, characteristic of X-ray pulsars, is a direct consequence of the cyclotron resonance, and closely related to the absorption feature. This implies that the spectral break, instead of the absorption feature, can be used as a convenient measure of the field strength for many X-ray pulsars. It is suggested that the X-ray pulsars exhibit a rather narrow scatter in the field strength, namely in the range of (1-4)×10^<12> G.

Observations of X-ray pulsars with the GINGA Large Area Counter (LAC) have been exploring a new frontier in the understanding of X-ray pulsar spectra. Amongst them, the GINGA observation of the binary X-ray pulsar 4U 1538-52 revealed a noticeable feature in its energy spectra near 20keV, which was strongly pulse-phase-dependent. We regard this structure as cyclotron absorption caused by electrons in an intense magnetic field of the pulsar. In this case, the magnetic field strength near the surface of the neutron star in 4U 1538-52 is estimated to be 2×10^<12> Gauss. In this report, we show the results of spectral analysis of 4U 1538-52,especially concentrating on the cyclotron structure.

X-ray pulsars1,2 are magnetized, spinning neutron stars accreting matter from their binary companions. Their pulse periods P, ranging over four orders of magnitude, increase and decrease in complex ways1,3,4. The more luminous ones tend to show faster spin-up1,5. A puzzle is that the spin-up timescales of many X-ray pulsars are much shorter than their binary-evolution timescales, thus apparently violating the steady-state condition. It has there-fore been suspected6 that there exist many 'turned-off X-ray pulsars currently spinning down undetected. An excellent test for this hypothesis became available using the X-ray pulsar GX1 +4, which used to show the fastest spin-up over a decade1,7-10and then faded away11. Using the X-ray satellite Ginga12, we detected GX1+4 at ∼1/40 the previous intensity, and found that it now has an average spin-down trend. This discovery apparently supports the above hypothesis. © 1988 Nature Publishing Group.