Soft X-ray Telescope Observations Research Articles

Extensive Broadband X-Ray Monitoring During the Formation of a Giant Radio Jet Base in Cyg X-3 with AstroSat

Abstract We present X-ray spectral and timing behavior of Cyg X-3 as observed by AstroSat during the onset of a giant radio flare on 2017 April 1–2. Within a timescale of a few hours, the source shows a transition from the hypersoft state (HPS) to a more luminous state (we termed as the very high state), which coincides with the time of the steep rise in radio flux density by an order of magnitude. Modeling the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) spectra jointly in 0.5–70.0 keV, we found that the first few hours of the observation is dominated by the HPS with no significant counts above 17 keV. Later, an additional flat power-law component suddenly appeared in the spectra that extends to very high energies with the power-law photon index of . Such a flat power-law component has never been reported from Cyg X-3. Interestingly the fitted power-law model in 25–70 keV, when extrapolated to the radio frequency, predicts the radio flux density to be consistent with the trend measured from the RATAN-600 telescope at 11.2 GHz. This provides direct evidence of the synchrotron origin of flat X-ray power-law component and the most extensive monitoring of the broadband X-ray behavior at the moment of decoupling the giant radio jet base from the compact object in Cyg X-3. Using SXT and LAXPC observations, we determine the giant flare ejection time as MJD 57845.34 ± 0.08 when 11.2 GHz radio flux density increases from ∼100 to ∼478 mJy.

Temperature and Density Measurements in a Quiet Coronal Streamer

Many previous studies have used emission line or broadband filter ratios to infer the presence of temperature gradients in the quiet solar corona. Recently it has been suggested that these temperature gradients are not real, but result from the superposition of isothermal loops with different temperatures and density scale heights along the line of sight. A model describing this hydrostatic weighting bias has been developed by Aschwanden & Acton. In this paper we present the application of the Aschwanden & Acton differential emission measure model to Solar and Heliospheric Observatory Solar Ultraviolet Measurement of Emitted Radiation (SUMER) observations of a quiet coronal streamer. Simultaneous Yohkoh soft X-ray telescope (SXT) observations show increases in the filter ratios with height above the limb, indicating an increase in temperature. The application of the Aschwanden & Acton model to these SUMER data, however, show that the temperature is constant with height and that the distribution of temperatures in the corona is much too narrow for the hydrostatic weighting bias to have any effect on the SXT filter ratios. We consider the possibility that there is a tenuous hot component (~3 MK) that accounts for the SXT observations. We find that a hot plasma with an emission measure sufficient to reproduce the observed SXT fluxes would also produce significant count rates in the high-temperature emission lines in the SUMER wavelength range. These lines are not observed, and we conclude that the SUMER spectra are not consistent with the SXT filter ratio temperatures. Calculations from a hydrodynamic loop model suggest that nonuniform footpoint heating may be consistent with the temperatures and densities observed at most heights, consistent with the recent analysis of relatively cool (~1 MK) active region loops. We also find, however, that at the lowest heights the observed densities are smaller than those predicted by uniform or footpoint heating.

EIT and SXT Observations of a Quiet-Region Filament Ejection: First Eruption, Then Reconnection

We observe a slow-onset quiet-region filament eruption with the Extreme Ultraviolet Imaging Telescope (EIT) on the Solar Heliospheric Observatory (SOHO) and the Soft X-ray Telescope (SXT) on Yohkoh. This event occurred on 1999 April 18 and was likely the origin of a coronal mass ejection detected by SOHO at 08:30 UT on that day. In the EIT observation, one-half of the filament shows two stages of evolution: stage 1 is a slow, roughly constant upward movement at approximately 1 km/s lasting approximately 0.5 hr, and stage 2 is a rapid upward eruption at approximately 16 km/s occurring just before the filament disappears into interplanetary space. The other half of the filament shows little motion along the line of sight during the time of stage 1 but erupts along with the rest of the filament during stage 2. There is no obvious emission from the filament in the SXT observation until stage 2; at that time, an arcade of EUV and soft X-ray loops forms first at the central location of the filament and then expands outward along the length of the filament channel. A plot of EUV intensity versus time of the central portion of the filament (where the postflare loops initially form) shows a flat profile during stage 1 and a rapid upturn after the start of stage 2. This light curve is delayed from what would be expected if 'tether-cutting' reconnection in the core of the erupting region were responsible for the initiation of the eruption. Rather, these observations suggest that a loss of stability of the magnetic field holding the filament initiates the eruption, with reconnection in the core region occurring only as a by-product.

Relation between Flare-associated X-Ray Ejections and Coronal Mass Ejections.

In an attempt to identify the direct signatures of coronal mass ejections (CMEs) in soft X-ray wavelengths, we have searched for plasma ejections in Yohkoh soft X-ray telescope (SXT) images in a total of 17 limb flares, and compared the results with the Solar and Heliospheric Observatory LASCO data. A general correlation exists between the presence/absence of the X-ray ejection and the CME. Although the height versus time relation often indicates (under the assumption of constant speed) that the CME onset coincides with the X-ray ejection, the latter probably does not represent the CME front, because the CME speed must result from acceleration, which would put the estimated onset at an earlier time. In some cases, the estimated CME onset time comes well before the impulsive phase of the associated flare. Although the role of the flare-associated plasma ejection in a CME is still unclear, we propose that its occurrence depends on the presence of open field lines, which can be due to a preceding CME. Lastly, we present a rare example of SXT observations of what appeared to be the three-part structure of a CME, which was seen a few minutes before a major flare started.

Heliospheric tomography using interplanetary scintillation observations: 1. Combined Nagoya and Cambridge data

We have produced a computer assisted tomography program that optimizes a three‐dimensional model to fit observational data. We have used this program with interplanetary scintillation data from Nagoya, Japan, and Cambridge, England. The program iterates to a least squares solution fit of observed data using solar rotation and solar wind motion to provide perspective views of each point in space accessible to the observations. We plot the optimized model as Carrington maps in velocity V and density Ne for the two data sets with resolutions of 10° in heliographic longitude and latitude. We map the model to 1 AU and compare this to in situ observations from the IMP spacecraft. From this comparison we find ΔNe ∝ Ne0.3. We plot Carrington maps extrapolated to the solar surface to compare with Yohkoh Soft X ray Telescope (SXT), Sacramento Peak green line, and Mark III K‐coronameter observations. High velocities modeled at the solar surface for individual rotations trace coronal holes (including polar ones) observed in SXT data. Regions of high density modeled from the Cambridge scintillation level data generally show a high correlation with regions of high solar activity observed as bright in Yohkoh SXT and green line observations. There is also a general correspondence of the regions of high density and the areas which are bright in K‐coronameter observations.

YohkohSoft X‐Ray Determination of Plasma Parameters in a Polar Coronal Hole

The Yohkoh Soft X-Ray Telescope (SXT) has been used to study the emission from a coronal hole surrounding the north pole of the Sun. Stronger emission from closed coronal structures in the line of sight can interfere with attempts to measure properties of coronal hole plasma. SXT observations indicate that the north polar region was free of such contamination on 1992 October 3. Measured X-ray intensities, corrected for background and scattered X-rays, are compared with a theoretical coronal hole model. They are found to be broadly consistent with the model predictions for variation of intensity with height and for limb brightening, although the electron density is lower than would be appropriate for model predictions based on solar maximum densities. Electron temperatures estimated by the filter ratio method are also consistent with the model and with an in situ estimate of the maximum electron temperature in the solar wind by the Ulysses ionic charge composition experiment.

ITAL]Yohkoh[/ITAL] SXT Observations of X-Ray “Dimming” Associated with a Halo Coronal Mass Ejection

A sudden depletion or intensity of the X-ray corona sometimes accompanies a solar eruptive flare or coronal mass ejection (CME). We have identified a dimming that occurred just prior to a halo CME, observed on 1997 April 7 using the Soft X-ray Telescope (SXT) on Yohkoh. Halo CMEs are prime candidates for space weather effects. The dimming occurred in compact regions near a flare of 14 UT on April 7, over a projected area of about 1020 cm-2, and indicate that a mass of a few times 1014 g was ejected. This is a lower limit imposed by the obscuration of the dimming volume by the brightness of the accompanying flare and other factors. Most of the mass deficit comes from two regions close to the ends of a preflare S-shaped active-region structure, and the resulting dimmings in these regions persisted for more than three days following the flare. A cusp-shaped loop—not apparent prior to the flare—dominates the emission in the flare decay phase, and has a mass comparable to that lost in the dimming regions. Our findings are consistent with the source of the CME being a flux rope that erupted, leaving behind the dimming regions. The cusp-shaped loop probably represents magnetic fields reconfigured or reconnected by the eruption. We do not see an X-ray analog of the wavelike disturbance evident in SOHO EUV images.

Heliospheric tomography using interplanetary scintillation observations

We have produced a Computer Assisted Tomography (CAT) program that optimizes a three-dimensional model to fit observational data. We have used this program with interplanetary scintillation data from Nagoya, Japan and Cambridge, England. The program iterates to a least-squares solution fit of observed data using solar rotation and solar wind motion to provide perspective views of each point in space accessible to the observations. We plot the optimized model as Carrington maps in velocity, V, and density, N e , for the two data sets with resolutions of ten degrees in heliographic longitude and latitude. High velocities modeled at the solar surface for individual rotations trace the outlines of coronal holes (including polar ones) observed in Yohkoh Soft X-ray Telescope (SXT) observations. Regions of slow velocity generally map to bright regions in SXT data. Regions of high density modeled from the Cambridge scintillation level data generally show a high correlation with regions of high solar activity observed as bright in Yohkoh SXT observations.

Heliospheric tomography using interplanetary scintillation observations

We have produced a Computer Assisted Tomography (CAT) program that optimizes a three dimensional solar wind velocity, V, and density fluctuation, ΔN e , model to fit observed interplanetary scintillation (IPS) data from Nagoya, Japan and Cambridge, England. The multiple perspective views of the solar wind needed for the reconstruction solution are provided by solar rotation and outward solar wind motion. The CAT program iterates to a least squares fit solution to the observed IPS values. We map the model to one AU and compare this to in situ observations from the IMP spacecraft. From this comparison we find ΔN e ∝ N 0.3 e . We plot the optimized model as Carrington maps in velocity and ΔN e , and compare these with Yohkoh Carrington synoptic maps. We find that the model velocity projected to the solar surface for individual rotations shows regions of high velocity that map directly to coronal hole areas observed in Yohkoh Soft X-ray Telescope (SXT) observations. Regions of slow velocity generally map to bright regions in SXT data. Regions of high ΔN e show a high correlation with regions of high solar activity observed as bright in Yohkoh SXT observations.