Jakob Englhauser

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.

An improved limit on the axion–photon coupling from the CAST experiment

We have searched for solar axions or similar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of 8.8 x 10^{-11} GeV^{-1} at 95% CL for m_a<~ 0.02 eV. This result is the best experimental limit over a broad range of axion masses and for m_a<~ 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globular-cluster stars.

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.

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.

The x-ray telescope of CAST

The CERN Axion Solar Telescope (CAST) has been in operation and taking data since 2003. The main objective of the CAST experiment is to search for a hypothetical pseudoscalar boson, the axion, which might be produced in the core of the sun. The basic physics process CAST is based on is the time inverted Primakoff effect, by which an axion can be converted into a detectable photon in an external electromagnetic field. The resulting x-ray photons are expected to be thermally distributed between 1 and 7 keV. The most sensitive detector system of CAST is a pn-CCD detector combined with a Wolter I type x-ray mirror system. With the x-ray telescope of CAST a background reduction of more than 2 orders of magnitude is achieved, such that for the first time the axion photon coupling constant gaγγ can be probed beyond the best astrophysical constraints gaγγ < 1 × 10−10 GeV−1.

X-ray emission from comet Hale-Bopp

The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation. There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to an emission mechanism coupled to gas, and not dust, in the comet’s coma.

pnCCDs on XMM-Newton-42 month in orbit

Abstract XMM-Newton—a cornerstone mission of the European Space Agencys Horizon 2000 programme—was launched on December 10, 1999 into orbit. Since March 2000 more than 2000 scientific observations were made. An example will be shown. The X-ray pnCCD camera on EPIC is operating since then without severe problems. We will report about the system performance as a function of time, with special emphasis on the effect of radiation damage in orbit. Up to now the spectroscopic and imaging performance of the pnCCD camera system is stable with time, even at the lowest energies most sensitive to degradation. The deviations of the detector response compared to the prelaunch performance over the 3.5 years is below 0.5%. One year after launch, the pnCCD X-ray camera was hit by a micrometeoroid, damaging 35 pixels spread over the entire field of view. Details of the impact of the pnCCD with a cosmic dust particle will be given. X-ray fluorescence photons, stimulated by charged particles, constitute a significant part of the instrument background. Possible improvements for future missions will be discussed.

Search for 14.4 keV solar axions emitted in the M1-transition of 57Fe nuclei with CAST

We have searched for 14.4 keV solar axions or more general axion-like particles (ALPs), that may be emitted in the M1 nuclear transition of 57Fe, by using the axion-to-photon conversion in the CERN Axion Solar Telescope (CAST) with evacuated magnet bores (Phase I). From the absence of excess of the monoenergetic X-rays when the magnet was pointing to the Sun, we set model-independent constraints on the coupling constants of pseudoscalar particles that couple to two photons and to a nucleon g{sub ay}|-1.19g{sub aN}{sup 0}+g{sub aN}{sup 3}| < 1.36 x 10{sup -16} GeV{sup -1} for ma < 0.03 eV at the 95% confidence level.

A major step in understanding the X-ray generation in comets: recent progress obtained with XMM-Newton

The observation of comet C/2000 WM1 (LINEAR) with XMM-Newton is a highlight in the field of cometary research. During 17 hours of almost uninterrupted observations, more than one million photons were recorded with unprecedented spectral resolution. The results obtained so far clearly demonstrate that the X-ray emission is caused by charge exchange reactions between highly charged heavy ions in the solar wind - mainly oxygen and carbon - and cometary gas. Due to the high number of detected photons, an investigation of the X-ray morphology is also possible in considerable detail. The observation was also a highlight from the technological point of view, demonstrating that XMM-Newton has more capabilities than are currently utilized. In order to follow the comet, the orientation of the satellite had to be adjusted several times. This was done by just changing the guide star offsets, leaving the X-ray instruments switched on. Compared to the usual procedure with regular slews, where the X-ray instruments are reinitialized afterwards, this new technique increased the time available for scientific observations by 34% for MOS and by 83% for PN. The capability of performing quick attitude adjustments during an observation may be of interest also for other astrophysical applications.