Y. Ogawara

Hot-Plasma Ejections Associated with Compact-Loop Solar Flares

Masuda et al. found a hard X-ray source well above a soft X-ray loop in impulsive compact-loop flares near the limb. This indicates that main energy release is going on above the soft X-ray loop, and suggests magnetic reconnection occurring above the loop, similar to the classical model for two ribbon flares. If the reconnection hypothesis is correct, a hot plasma (or plasmoid) ejection is expected to be associated with these flares. Using the images taken by the soft X-ray telescope aboard Yohkoh, we searched for such plasma ejections in eight impulsive compact-loop flares near the limb, which are selected in an unbiased manner and include also the Masuda flare, 1992 January 13 flare. We found that all these flares were associated with X-ray plasma ejections high above the soft X-ray loop and the velocity of ejections is within the range of 50-400 km s-1. This result gives further support for magnetic reconnection hypothesis of these impulsive compact-loop flares.

The Yohkoh Mission for High-Energy Solar Physics

The Japanese Yohkoh satellite is now in orbit observing the sun with a set of x-ray imagers and x-ray and gamma-ray spectrometers. The data from this successful mission provide new information on solar flares and the suns corona. This paper discusses the Yohkoh observations and presents a sample of the first scientific results from the mission.

The 1992 January 5 flare at 13.3 UT : observations from Yohkoh

We discuss X-ray spectra and soft X-ray images of an M1.9 flare that occurred on 1992 January 5 near 13.3 UT. These data were obtained with instrumentation on the Japanese Yohkoh spacecraft. They cover the entire rise phase of the flare. To supplement these data we have ground-based magnetograms and Halpha spectroheliograms. We calculate the electron temperature and emission measure of the flare as a function of time during the early rise phase using X-ray spectral line intensities and line ratios. Using spectral line widths, line profile asymmetries, and wavelength shifts due to the Doppler effect, we calculate the dynamical properties of the flare. The time development of the morphology of the flare, as revealed by the soft X-ray images and the Halpha spectroheliograms, and the physical quantities inferred from the X-ray spectra, are compared with chromospheric evaporation models. There is an enhancement of blueshifted emission that is closely correlated with the hard X-ray bursts. Heating of one loop in the flare is consistent with a conduction-evaporation model, but heating is found in several structures that do not appear to be physically associated with each other. No standard evaporation model can adequately explain all of the observations.

Observation of a New X-ray Source

THE purpose of this article is to present a preliminary report on a new cosmic X-ray source. The source was observed near Sco X-1 during recent rocket flights to observe Sco X-1 simultaneously with ground-based optical telescopes. It is very likely that we observed the source reported to have appeared between July 6 and July 9 by Conner et al. in an IAU telegram issued on July 30, 1969.

Shrinkage of Coronal X-Ray Loops

We present the first set of examples of the shrinkage of large-scale nonflare loops in the solar corona, observed by the Yohkoh Soft X-Ray Telescope in 1993 February. A large and isolated active region exhibited an unusual south-north asymmetry in coronal dynamics and heating. The northern part, referred to the main magnetic axis, showed episodic expansion and heating. In contrast, the southern part displayed obvious shrinking and cooling. This asymmetry was correlated with a severe asymmetry in the surface magnetic activity revealed by Huairou vector magnetograms. Observations suggest that this shrinkage is not an apparent motion, but a real contraction of coronal loops that brighten as a result of heating at footpoints followed by gradual cooling.

Delays of Optical Bursts in Simultaneous Optical and X-Ray Observations of MXB 1636-53

Observations of simultaneous optical and X-ray bursts from 4U/MXB 1636-53 were made using the Hakucho burst monitor system and optical telescopes at the European Southern Observatory during 1979 and 1980. The six best cases among the 10 coinciding observations are analyzed in terms of a model in which the optical emission is the result of reprocessing of X-rays (through blackbody heating). From this analysis, the temperature (spatially averaged) and size of a reprocessor, and the smearing and delay of the optical bursts are obtained. For the maximum temperatures of the optical reprocessor, the values differ from burst to burst, ranging from about 3 x 10 to the 4th to about 10 to the 5th K. The present analysis suggests that the size of the reprocessor varies by a factor of a few. For the smearing of the optical bursts an upper limit of a few seconds is derived. The most important result of this analysis is that the delay times are not the same for all bursts. The possible constraints which these results put on a low-mass binary model of this burst source are discussed.

Thirty-meter X-ray pencil beam line at the Institute of Space and Astronautical Science

A 30-m-long X-ray beam line has been built at the Institute of Space and Astronautical Science (ISAS) to evaluate the performance of X-ray optical instruments for space programs, in particular for the X-ray telescope onboard the Astro-D (Asca) satellite. This beam line consists of an X-ray generator, a 30-m-long vacuum duct, and measuring chambers. Strong and stable X-ray pencil beams from Al, Ti, Cu, Mo and W targets are available with the parallelism of several arcs [full width at half maximum (FWHM)]. Three kinds of detectors are prepared: a conventional gas proportional counter equipped with a thin plastic window, a one-dimensional position-sensitive proportional counter with a Be window, and a charge-coupled device (CCD) modified for X-ray measurements. At the present compact beam line, instead of giant systems of hundreds of meters, the combination of a strong X-ray (0.2-10 keV) pencil beam and translation stages enables us to examine the entire aperture of large X-ray optical instruments of up to 40 cm and 1 m in length.

γ-Ray burst observed at balloon altitude

SINCE the γ-ray burst was discovered in 1973, approximately 50 events have been observed using artificial satellites1,2. In addition, several bursts of smaller size have been found using balloon-borne detectors3–6 with large sensitive areas. No burst has yet been located on the celestial sphere, with an adequate precision to associate it with an astronomical object. To determine the precise position of a γ-ray burst which had not been predicted to occur, the detector must have a wide field of view and the capability of precise location of the source. A rotating cross-modulation-collimator (RCMC) proposed7 as a device to fulfill these apparently conflicting requirements was used in the series of balloon observations reported here. A small γ-ray burst was found during ∼ 150 h of observations and its celestial position was determined with a precision of ∼ 0.3°.

Calibrations of imaging gas scintillation proportional counters on ASTRO-D

The fourth Japanese X-ray astronomy satellite, ASTRO-D, was launched successfully by the Institute of Space and Astronautical Science on February 20, 1993 and was named ASCA. Two of the focal plane detectors are imaging gas scintillation proportional counters (Gas Imaging Spectrometer:GIS). The GIS sensors performed the energy resolution of 8% FWHM at 6 keV, and position resolution of 0.5 mm FWHM on-board, which confirmed their ultimate capability as gas counters. The non-Xray background counting rate was approximately 6 X 10-4 c/s/cm2/keV in the energy range of 2 - 10 keV, which was as low as that achieved by the Ginga instrument. The scientific results obtained by the GIS sensors are also presented.

Discovery of an X-ray burst source XB 1715-321

A new X-ray burst source, XB 1715-321, was discovered with the Hakucho satellite. Three type I X-ray bursts were observed from this source in 1979. The error region for XB 1715-321, about 0.3 square degrees in area, includes a persistent X-ray source MX/2S 1715-321 which has been suspected to be the source of several fast transient events. Those bursts detected by Hakucho are characterized by a slow rise time (5--10 s) and a relatively long burst interval.