Konrad Dennerl

Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2

During its close approach to Earth, comet C/Hyakutake 1996 B2 was observed at extreme ultraviolet and x-ray wavelengths with the Rœntgen X-ray Satellite and Rossi X-ray Timing Explorer. The emission morphology was symmetric with respect to a vector from the comets nucleus toward the sun, but not symmetric around the direction of motion of the comet with respect to interplanetary dust. A slowly varying emission and a large impulsive event that varied on time scales of 1 to 2 hours were observed. An interaction between the comet and the solar wind/solar magnetic field seems to be the most likely mechanism for the observed emission.

The ROSAT all - sky survey bright source catalogue

In order to ensure the quality of the source catalogue derived from the SASS processing an automatic as well as a visual screening procedure was applied to 1378 survey fields. Most (94%) of the 18,811 sources were confirmed by this screening process. The rest is flagged for various reasons. Broad band images are available for a subset of the flagged sources. Details of the screening process can be found at www.rosat.mpe-garching.mpg.de/survey/rass-bsc/doc.html.

Discovery of X-rays from Mars with Chandra

On 4 July 2001, X-rays from Mars were detected for the first time. The observation was performed with the ACIS-I detector onboard Chandra and yielded data of high spatial and temporal resolution, together with spectral information. Mars is clearly detected as an almost fully illuminated disk, with an indication of limb brightening at the sunward side, accompanied by some fading on the opposite side. The morphology and the X-ray luminosity of4 MW are fully consistent with fluorescent scattering of solar X-rays in the upper Mars atmosphere. The X-ray spectrum is dominated by a single narrow emission line, which is most likely caused by O-K fluorescence. No evidence for temporal variability is found. This is in agreement with the solar X-ray flux, which was almost constant during the observation. In addition to the X-ray fluorescence, there is evidence for an additional source of X-ray emission, indicated by a faint X-ray halo which can be traced to about three Mars radii, and by an additional component in the X-ray spectrum of Mars, which has a similar spectral shape as the halo. Within the available limited statistics, the spectrum of this component can be characterized by 0.2 keV thermal bremsstrahlung emission. This is indicative of charge exchange interactions between highly charged heavy ions in the solar wind and exospheric hydrogen and oxygen around Mars. Although the observation was performed at the onset of a global dust storm, no evidence for dust-related X-ray emission was found.

X-rays from comets generated by energetic solar wind particles

Abstract By means of axisymmetric hydrodynamic models with chemistry we study the X-ray emission of two processes in a comet: Bremsstrahlung of solar wind electrons interacting with the neutral gas coma, and charge exchange of heavy solar wind ions in high ionization states with the neutral cometary gas. Comparisons of morphology, intensity and spectrum with observations favour heavy ions as the emitting agent.

Discovery of X-rays from Venus with Chandra

On January 10 and 13, 2001, Venus was observed for the rst time with an X{ray astronomy satellite. The observation, performed with the ACIS{I and LETG/ACIS{S instruments on Chandra, yielded data of high spatial, spectral, and temporal resolution. Venus is clearly detected as a half{lit crescent, with considerable brightening on the sunward limb. The morphology agrees well with that expected from fluorescent scattering of solar X{rays in the planetary atmosphere. The radiation is observed at discrete energies, mainly at the O{K energy of 0.53 keV. Fluorescent radiation is also detected from C{K at 0.28 keV and, marginally, from N{K at 0.40 keV. An additional emission line is indicated at 0.29 keV, which might be the signature of the C 1s! ? transition in CO2 and CO. Evidence for temporal variability of the X{ray flux was found at the 2:6 level, with fluctuations by factors of a few times indicated on time scales of minutes. All these ndings are fully consistent with fluorescent scattering of solar X{rays. No other source of X{ray emission was detected, in particular none from charge exchange interactions between highly charged heavy solar wind ions and atmospheric neutrals, the dominant process for the X{ray emission of comets. This is in agreement with the sensitivity of the observation.

Cassiopeia A: On the origin of the hard X-ray continuum and the implication of the observed O vııı Ly-α/Ly-β distribution

We present the first results on the hard X-ray continuum image (up to 15 keV) of the supernova remnant Cas A measured with the EPIC cameras onboard XMM-Newton . The data indicate that the hard X-ray tail, observed previously, that extends to energies above 100 keV does not originate in localised regions, like the bright X-ray knots and filaments or the primary blast wave, but is spread over the whole remnant with a rather flat hardness ratio of the 8-10 and 10-15 keV energy bands. This result does not support an interpretation of the hard X-radiation as synchrotron emission produced in the primary shock, in which case a limb brightened shell of hard X-ray emission close to the primary shock front is expected. In fact a weak rim of emission near the primary shock front is discernable in the hardest X-ray image but it contains only a few percent of the hard X-ray emissivity. The equivalent width of the Fe-K line blend varies by more than an order of magnitude over the remnant, it is hard to explain this as Fe-emission from the reverse shock heated ejecta given the ejecta temperature and the age of the remnant. The uniquely high wavelength-dispersive RGS-spectrometer has allowed, for the first time, to extract monochromatic images in several highly ionised element species with high spectral resolution. We present here a preliminary result on the measurement of the Oviii Ly- α and Ly- β brightness distribution and brightness ratios. The large observed decrease of the Ly- α /Ly- β ratio going from the N to the SE can be explained by small-scale

Spectral analysis of the Chandra comet survey

(10\arcsec)

AN ACCRETION MODEL FOR THE ANOMALOUS X-RAY PULSAR 4U 0142+61

variations in the N H column over the remnant and the potential presence of resonance scattering of the Oviii Ly- α photons in the limb brightened shell.

XMM-Newton observations of the supernova remnant RX J0852.0-4622/GRO J0852-4642

Aims.We present results of the analysis of cometary X-ray spectra with an extended version of our charge exchange emission model (Bodewits et al. 2006). We have applied this model to the sample of 8 comets thus far observed with the Chandra X-ray observatory and ACIS spectrometer in the 300-1000 eV range. The surveyed comets are C/1999 S4 (LINEAR), C/1999 T1 (McNaught-Hartley), C/2000 WM1 (LINEAR), 153P/2002 (Ikeya-Zhang), 2P/2003 (Encke), C/2001 Q4 (NEAT), 9P/2005 (Tempel 1) and 73P/2006-B (Schwassmann-Wachmann 3) and the observations include a broad variety of comets, solar wind environments and observational conditions. Methods.The interaction model is based on state selective, velocity dependent charge exchange cross sections and is used to explore how cometary X-ray emission depend on cometary, observational and solar wind characteristics. It is further demonstrated that cometary X-ray spectra mainly reflect the state of the local solar wind. The current sample of Chandra observations was fit using the constrains of the charge exchange model, and relative solar wind abundances were derived from the X-ray spectra. Results.Our analysis showed that spectral differences can be ascribed to different solar wind states, as such identifying comets interacting with (I) fast, cold wind, (II), slow, warm wind and (III) disturbed, fast, hot winds associated with interplanetary coronal mass ejections. We furthermore predict the existence of a fourth spectral class, associated with the cool, fast high latitude wind.

High-resolution X-ray spectroscopy and imaging of supernova remnant N132D

We propose that the quiescent emission of anomalous X-ray pulsars/soft gamma-ray repeaters (AXPs/SGRs) is powered by accretion from a fallback disk, requiring magnetic dipole fields in the range 1012-1013 G, and that the luminous hard tails of their X-ray spectra are produced by bulk-motion Comptonization in the radiative shock near the bottom of the accretion column. This radiation escapes as a fan beam, which is partly absorbed by the polar cap photosphere, heating it up to relatively high temperatures. The scattered component and the thermal emission from the polar cap form a polar beam. We test our model on the well-studied AXP 4U 0142+61, whose energy-dependent pulse profiles show double peaks, which we ascribe to the fan and polar beams. The temperature of the photosphere (kT ~ 0.4 keV) is explained by the heating effect. The scattered part forms a hard component in the polar beam. We suggest that the observed high temperatures of the polar caps of AXPs/SGRs, compared with other young neutron stars, are due to the heating by the fan beam. Using beaming functions for the fan beam and the polar beam and taking gravitational bending into account, we fit the energy-dependent pulse profiles and obtain the inclination angle and the angle between the spin axis and the magnetic dipole axis, as well as the height of the radiative shock above the stellar surface. We do not explain the high-luminosity bursts, which may be produced by the classical magnetar mechanism operating in super-strong multipole fields.