Richard J. Edgar

RX J0852.0–4622: Another Nonthermal Shell-Type Supernova Remnant (G266.2–1.2)

The newly discovered supernova remnant G266.2-1.2 (RX J0852.0-4622), along the line of sight to the Vela supernova remnant (SNR), was observed with ASCA for 120 ks. We find that the X-ray spectrum is featureless and well described by a power law, extending to three of the class of shell-type SNRs dominated by nonthermal X-ray emission. Like G347.3-0.5, this low-latitude remnant displays discrete regions of enhanced emission along the rim as well as faint nonthermal emission from the interior. We derive limits on the thermal content of the remnant emission, although the presence of the Vela SNR compromises our ability to seriously constrain a low-temperature component. Limits placed on the amount of Sc-K emission are compared with the expected flux based on the reported 44Ti emission from G266.2-1.2. We also report on an unresolved X-ray source surrounded by diffuse emission near the center of the remnant. The properties of the source are not well determined but appear consistent with the interpretation that the source is a neutron star surrounded by a synchrotron nebula. Alternatively, the source may be associated with one of two stars located within the positional error circle, but this appears somewhat unlikely.

Plasma Properties in Coronal Holes Derived from Measurements of Minor Ion Spectral Lines and Polarized White Light Intensity

Recent observations of the Lyα λ1216, Mg X λ625, and O VI λ1038 spectral lines carried out with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO at distances in the range 1.35-2.1 RS in the northern coronal hole are used to place limits on the turbulent wave motions of the background plasma and the thermal motions of the protons and Mg+9 and O+5 ions. Limits on the turbulent wave motion are estimated from the measured line widths and electron densities derived from white light coronagraph observations, assuming WKB approximation at radial distances covered by the observations. It is shown that the contribution of the turbulent wave motion to the widths of the measured spectral lines is small compared to thermal broadening. The observations show that the proton temperature slowly increases between 1.35 and 2.7 RS and does not exceed 3×10 K in that region. The temperature of the minor ions exceeds the proton temperature at all distances, but the temperatures are neither mass proportional nor mass-to -charge proportional. It is shown, for the first time, that collision times between protons and minor ions are small compared to the solar wind expansion times in the inner corona. At 1.35 RS the expansion time exceeds the proton Mg+9 collision time by more than an order of magnitude. Nevertheless, the temperature of the Mg ions is significantly larger than the proton temperature, which indicates that the heating mechanism has to act on timescales faster than minutes. When the expansion time starts to exceed the collision times a rapid increase of the O+5 ion spectral line width is seen. This indicates that the heavier and hotter ions lose energy to the protons as long as collision frequencies are high, and that the ion spectral line width increases rapidly as soon as this energy loss stops.

Chandra Spectra of the Soft X-Ray Diffuse Background

We present an exploratory Chandra ACIS-S3 study of the diffuse component of the cosmic X-ray background (CXB) in the 0.3–7 keV band for four directions at high Galactic latitudes, with emphasis on details of the ACIS instrumental background modeling. Observations of the dark Moon are used to model the detector background. A comparison of the Moon data and the data obtained with ACIS stowed outside the focal area showed that the dark Moon does not emit significantly in our band. Point sources down to 3 � 10 � 16 ergs s � 1 cm � 2 in the 0.5–2 keV band are excluded in our two deepest observations. We estimate the contribution of fainter, undetected sources to be less than 20% of the remaining CXB flux in this band in all four pointings. In the 0.3–1 keV band, the diffuse signal varies strongly from field to field and contributes between 55% and 90% of the total CXB signal. It is dominated by emission lines that can be modeled by a kT ¼ 0:1 0:4 keV plasma. In particular, the two fields located away from bright Galactic features show a prominent line blend at E � 580 eV (O vii+O viii) and a possible line feature at E � 300 eV. The two pointings toward the North Polar Spur exhibit a brighter O blend and additional bright lines at 730–830 eV (Fe xvii). We measure the total 1–2 keV flux of 1:0 1:2 � 0:2 ðÞ �10 � 15 ergs s � 1 cm � 2 arcmin � 2 (mostly resolved) and the 2–7 keV flux of 4:0 4:5 � 1:5 ðÞ �10 � 15 ergs s � 1 cm � 2 arcmin � 2 .A tE > 2 keV, the diffuse emission is consistent with zero, to an accuracy limited by the short Moon exposure and systematic uncertainties of the S3 background. Assuming Galactic or local origin of the line emission, we put an upper limit of � 3 � 10 � 15 ergs s � 1 cm � 2 arcmin � 2 on the 0.3–1 keV extragalactic diffuse flux. Subject headings: intergalactic medium — ISM: general — methods: data analysis — X-rays: diffuse background — X-rays: ISM

CHANDRA OBSERVATIONS OF THE ''DARK'' MOON AND GEOCORONAL SOLAR WIND CHARGE TRANSFER

We have analyzed data from two sets of calibration observations of the Moon made by the Chandra X-Ray Observatory. In addition to obtaining a spectrum of the bright side that shows several distinct fluorescence lines, we also clearly detect time-variable soft X-ray emission, primarily O vii Kand O viii Ly� , when viewing the optically dark side. The apparent dark-side brightness varied in time by at least an order of magnitude, up to � 2 � 10 � 6 photons s � 1 arcmin � 2 cm � 2 between 500 and 900 eV, which is comparable to the typical 3 keV-band background emission measured in the ROSAT All-Sky Survey. The spectrum is also very similar to background spectra recorded by Chandra in low- or moderate-brightness regions of the sky. Over a decade ago, ROSAT also detected soft X-rays from the dark side of the Moon, which were tentatively ascribed to continuum emission from energetic solar wind electrons impacting the lunar surface. The Chandra observations, however, with their better spectral resolution, combined with contemporaneous measurements of solar wind parameters, strongly favor charge transfer between highly charged solar wind ions and neutral hydrogen in the Earths geocorona as the mechanism for this emission. We present a theoretical model of geocoronal emission and show that predicted spectra and intensities match the Chandra observations very well. We also model the closely related process of heliospheric charge transfer and estimate that the total charge transfer flux observed from Earth amounts to a significant fraction of the soft X-ray background, particularly in the ROSAT 3 keV band.

Highly ionized atoms in cooling gas

The ionization of low density gas cooling from a high temperature was calculated. The evolution during the cooling is assumed to be isochoric, isobaric, or a combination of these cases. The calculations are used to predict the column densities and ultraviolet line luminosities of highly ionized atoms in cooling gas. In a model for cooling of a hot galactic corona, it is shown that the observed value of N(N V) can be produced in the cooling gas, while the predicted value of N(Si IV) falls short of the observed value by a factor of about 5. The same model predicts fluxes of ultraviolet emission lines that are a factor of 10 lower than the claimed detections of Feldman, Brune, and Henry. Predictions are made for ultraviolet lines in cooling flows in early-type galaxies and clusters of galaxies. It is shown that the column densities of interest vary over a fairly narrow range, while the emission line luminosities are simply proportional to the mass inflow rate.

Coronal Structure and Abundances of Capella from Simultaneous EUVE and ASCA Spectroscopy

We report analysis of the simultaneous 1996 March EUVE and ASCA observations of the spectroscopic binary Capella. The EUVE spectrum is dominated by lines of highly ionized Fe, requiring a continuous emission-measure distribution over a wide range of temperatures. The ASCA spectrum shows He-like line emission features of S, Si, and Mg, as well as unresolved L-shell emission lines of Fe and Ni and H-like and He-like Ne lines. The flux in these line features cannot be determined independently from the continuum flux. The ASCA spectrum is relatively soft, with few counts above 4 keV. The emission-measure distribution determined by Line-Based Analysis of the EUV Fe line intensities is well constrained from Te ~ 6 × 105 to 2 × 107 K, but it is not constrained above this range since Fe XXIV is the highest temperature line observed with EUVE. Since repeated observations of Capella by EUVE have shown that emission-line intensities of the hottest EUV-emitting material (Fe XXI to XXIV) vary by factors up to 4, the ASCA spectrum is important for extending the temperature coverage. Thus, the high-energy cut-off of the ASCA spectrum provides a constraint on the highest temperature emission measures. In principle, elemental abundances are determined from global fits to the ASCA spectrum; however, no well-fitting model has been found for the high signal-to-noise ASCA performance verification spectrum of Capella (1993 September 2). The newer ASCA spectrum of Capella (1996 March 3-4) shows a similar pattern of fitting difficulties. Using the EUVE measurements (1996 March 3-7) to constrain models, we have conducted sensitivity studies of the atomic data, source physics, and instrument calibration. The plasma spectral emission models (Raymond-Smith, MEKAL, SPEX) around 1.2 keV appear to have flux deficits relative to the observed ASCA count spectrum. New atomic models by Liedahl and Brickhouse, calculated with the HULLAC code, provide a set of lines—missing from the existing plasma codes—to fill in this flux deficit. Incorporating these additional lines dramatically improves the spectral model fits to the data, allowing reliable determination of elemental abundances. The successful application of the new atomic models to the Capella problem can have widespread implications, affecting spectral models of galaxies, cluster cooling flows, and supernova remnants, as well as other stellar coronae. Analysis with the new atomic models of the simultaneous ASCA and EUVE data confirms the previous EUVE results that the continuous emission-measure distribution of Capella has a strong enhancement at Te ~ 6 × 106 K. While a two-temperature model actually provides a better fit to the ASCA spectrum than the EUVE-derived continuous model, the EUVE data are not well fitted with only two temperatures. We find that the abundances of Mg, Si, S, and Fe are consistent with solar photospheric values, while Ne appears to be underabundant by a factor of ~3 to 4.

Stability of radiative shocks with time-dependent cooling

A full evolutionary calculation of ion abundances and radiative cooling has been incorporated into an accurate one-dimensional gasdynamics calculation in order to investigate the dynamics of radiative shock instability and to determine the extent to which power-law cooling models describe the situation. Radiative shocks are shown to be unstable for velocities of greater than about 140 km/s. The temperature dependence of cooling functions behind steady shocks suggests that the stability limit may depend upon the amplitude of the perturbation. Oscillation is noted in the fundamental oscillation mode.

Suzaku Observations of the Local and Distant Hot ISM

Suzaku observed the molecular cloud MBM 12 and a blank field less than 3 ◦ away to separate the local and distant components of the diffuse soft X-ray background. Towards MBM 12, a local (D 275pc) O VII emission line was clearly detected with an intensity of 3.5 photons cm −2 s −1 sr −1 (or line units, LU), and the O VIII flux was < 0.34 LU. The origin of this O VII emission could be hot gas in the Local Hot Bubble (LHB), charge exchange within the heliosphere with oxygen ions from the solar wind (SWCX), or both. If entirely from the LHB, the emission could be explained by a region with emission measure of 0.0075cm −6 pc and a temperature of 1.2 ×10 6 K. However, this temperature and emission measure implies 1/4 keV emission in excess of observations. There is no evidence in the X-ray light curve or solar wind data for a significant contribution from geocoronal SWCX, although interplanetary SWCX is still possible. In any case, the observed O VII flux represents an upper limit to both the LHB emission and interplanetary SWCX in this direction. The blank field was observed immediately afterwards. The net off-cloud O VII and O VIII intensities were (respectively) 2.34 ±0.33 and 0.77 ±0.16 LU, after subtracting the on-cloud foreground emission. If this more distant O VII and O VIII emission is from a thermal plasma in collisional equilibrium beyond the Galactic disk, we infer it has a temperature of (2.1 ±0.1) ×10 6 K with an emission measure of (4 ±0.6) ×10 −3 cm −6 pc.

Performance expectation versus reality

The AXAF (Advanced X-ray Astrophysics Facility) high resolution mirror assembly (HRMA) now is complete and has been tested at the NASA Marshall Space Flight Center (MSFC) X-ray Calibration Facility (XRCF). The surface and alignment properties of the mirror were thoroughly measured before the x-ray test, which allowed accurate performance predictions to be performed. The preliminary analysis of the measured x-ray image distributions for all energies tested show excellent agreement with predictions made before the beginning of the test. There is a discrepancy between the measured and predicted effective areas; this typically is less than 5%, and is less than 13% for all energies measured. We present evidence that this discrepancy is due to uncertainties in the calibration of the test instrumentation, and therefore can be expected to be reduced when results from further instrument calibration tests now in progress are incorporated into the analysis. We predict that 65 - 80% (depending upon energy) of the flux from an imaged point source will be contained on a one arc second diameter aperture in flight. We expect the HRMA to more than fulfill the requirements necessary to achieve the AXAF scientific objectives.

Spectra of the ¼ keV X-Ray Diffuse Background from the Diffuse X-Ray Spectrometer Experiment

The Diffuse X-ray Spectrometer (DXS) flew as an attached payload on the STS-54 mission of the space shuttle Endeavour in 1993 January and obtained spectra of the soft X-ray diffuse background in the 148-284 eV (84-44 ?) band using a Bragg-crystal spectrometer. The spectra show strong emission lines, indicating that the emission is primarily thermal. Since the observations were made at low Galactic latitude, this thermal emission must arise from a nearby hot component of the interstellar medium, most likely the Local Hot Bubble, a region within ~100 pc of the Sun characterized by an absence of dense neutral gas. The DXS spectrum of the hot interstellar medium is not consistent with either collisional equilibrium models or with nonequilibrium ionization models of the X-ray emission from astrophysical plasmas. Models of X-ray emission processes appear not yet adequate for detailed interpretation of these data. The DXS data are most nearly consistent with models of thermal emission from a plasma with a temperature of 106.1 K and depletions of refractory elements magnesium, silicon, and iron to levels ~30% of solar.