Martin C. Weisskopf

IBIS: The Imager on-board INTEGRAL

The IBIS telescope is the high angular resolution gamma-ray imager on-board the INTEGRAL Observatory, suc- cessfully launched from Baikonur (Kazakhstan) the 17th of October 2002. This medium size ESA project, planned for a 2 year mission with possible extension to 5, is devoted to the observation of the gamma-ray sky in the energy range from 3 keV to 10 MeV (Winkler 2001). The IBIS imaging system is based on two independent solid state detector arrays optimised for low (15 1000 keV) and high (0:175 10:0 MeV) energies surrounded by an active VETO System. This high eciency shield is essential to minimise the background induced by high energy particles in the highly excentric out of van Allen belt orbit. A Tungsten Coded Aperture Mask, 16 mm thick and1 squared meter in dimension is the imaging device. The IBIS telescope will serve the scientific community at large providing a unique combination of unprecedented high energy wide field imaging capability coupled with broad band spectroscopy and high resolution timing over the energy range from X to gamma rays. To date the IBIS telescope is working nominally in orbit since more than 9 month.

An Overview of the Performance and Scientific Results From the Chandra X-Ray Observatory (CXO)

ABSTRACT The Chandra X‐Ray Observatory (CXO), the X‐ray component of NASA’s Great Observatories, was launched on 1999 July 23 by the space shuttle Columbia. After satellite systems activation, the first X‐rays focused by the telescope were observed on 1999 August 12. Beginning with the initial observation it was clear that the telescope had survived the launch environment and was operating as expected. Despite an initial surprise due to the discovery that the telescope was far more efficient for concentrating CCD‐damaging low‐energy protons than had been anticipated, the observatory is performing well and is returning superb scientific data. Together with other space observatories, most notably XMM‐Newton, it is clear that we have entered a new era of discovery in high‐energy astrophysics.

On searches for pulsed emission with application to four globular cluster X-ray sources - NGC 1851, 6441, 6624, and 6712

We present the results of searches for periodic pulsations in the X-ray emission from four globular cluster sources, NGC 1851, NGC 6441, NGC 6624, and NGC 6712. The data were obtained by the Monitor Proportional Counter aboard HEAO 2 (Einstein Observatory). The methods of analysis are presented in some detail because we have correctly accounted for several effects which have been routinely overlooked by others. The periods searched cover the range from approx.1 ms to approx.500 s. No pulsed emission was detected, and the (90% confidence) upper limits for the pulsed fraction are presented.

Discovery of Spatial and Spectral Structure in the X-Ray Emission from the Crab Nebula

The Chandra X-Ray Observatory observed the Crab Nebula and pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) readout by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray inner ring within the X-ray torus, the suggestion of a hollow-tube structure for the torus, and X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the nebula.

Discovery of powerful gamma-ray flares from the Crab Nebula.

Gamma-ray observations of the Crab Nebula by two different space telescopes challenge particle acceleration theory. The well-known Crab Nebula is at the center of the SN1054 supernova remnant. It consists of a rotationally powered pulsar interacting with a surrounding nebula through a relativistic particle wind. The emissions originating from the pulsar and nebula have been considered to be essentially stable. Here, we report the detection of strong gamma-ray (100 mega–electron volts to 10 giga–electron volts) flares observed by the AGILE satellite in September 2010 and October 2007. In both cases, the total gamma-ray flux increased by a factor of three compared with the non-flaring flux. The flare luminosity and short time scale favor an origin near the pulsar, and we discuss Chandra Observatory x-ray and Hubble Space Telescope optical follow-up observations of the nebula. Our observations challenge standard models of nebular emission and require power-law acceleration by shock-driven plasma wave turbulence within an approximately 1-day time scale.

A precision measurement of the X-ray polarization of the Crab Nebula without pulsar contamination

The linear X-ray polarization of the Crab Nebula has been precisely measured at 2.6 keV and 5.2 keV with the OSO 8 graphite crystal polarimeters. The 1.4 ms time reolution of these instruments permitted the removal of any contribution to the polarization from the pulsar. The nebular polarization is 19.2% +- 1.0% at a position angle of 156/sup 0/.4 +- 1/sup 0/.4 at 2.6 keV. At 5.2 keV the corresponding results are 19.5% +- 2.8% at 152/sup 0/.6 +- 4/sup 0/.0.

A pulsating auroral X-ray hot spot on Jupiter

Jupiters X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planets polar regions. Here we report high-spatial-resolution observations that demonstrate that most of Jupiters northern auroral X-rays come from a ‘hot spot’ located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared and ultraviolet emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft. These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.

Advanced X-Ray Astrophysics Facility (AXAF)

AXAF is an x-ray observatory designed to study x-ray emission from al categories of astronomical objects, from normal stars to quasars. AXAF has broad scientific objectives and outstanding capability to provide high resolution images, spectrometric imaging and high resolution dispersive spectroscopy over the energy bandwidth from 0.1 to 10-keV. This is a significant year in the development of AXAF, to be launched in late 1998. Major elements of the observatory, the optics and the scientific instruments, are now nearing completion in preparation for calibration later this year.

On searches for periodic pulsed emission - The Rayleigh test compared to epoch folding

The Rayleigh test has been recently invoked as a method of searching a time series for periodic pulsations. In this paper, this technique is discussed and compared to epoch folding, a technique which has had widespread use in X-ray astronomy. It is found that the Rayleigh test provides a more sensitive approach to the search for periodic pulsations when the pulses are sinusoidal or of broad duty cycle, such as those typical of the pulsing X-ray sources. Epoch folding, on the other hand, is more sensitive to the narrow pulses typical of radio pulsars.

Detection of X-ray polarization of the Crab Nebula.

Two different types of X-ray polarimeters were used in a sounding rocket to search for X-ray polarization of the Crab Nebula. Polarization was detected at a statistical confidence level of 99.7%. If the X-ray polarization is assumed to be independent of energy, the results of this and a previous experiment lead to a polarization of 15.4 (plus or minus 5.2) percent at a position angle of 156 (plus or minus 10) deg. This result confirms the synchrotron model for X-ray emission from the Crab Nebula.