Astrophysics
A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in the TeV energy regime. The observations were motivated by the positive results…
Cosmic microwave background anisotropies provide a vast amount of information on both structure formation in the universe and the background dynamics and geometry. The full physical content and detailed structure of anisotropies can be…
(Abridged) XTE J1810-197 and 1E 1547.0-5408 are two transient AXPs exhibiting radio emission with unusual properties. In addition, their spin down rates during outburst show opposite trends, which so far has no explanation. Here, we extend…
We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. The star has a dipole field misaligned relative to the rotation axis by a small…
Many proto-planetary nebulae (PPN) appear as narrow collimated structures sometimes showing multiple, roughly aligned lobes. In addition, many PPN flows have been shown to have short acceleration times. In this paper we explore whether jet…
It is suggested that dark energy in a brane world can help reconcile an infinitely cyclic cosmology with the second law of thermodynamics. A cyclic cosmology is described, in which dark energy with constant equation of state leads to a…
We measure variations in orientation of fourteen dynamic fibrils as a function of time in a small isolated plage and nearby network using a 10-min time sequence of H-alpha filtergrams obtained by the Dutch Open Telescope. We found motions…
HST and ground-based [OII} and [NII] images obtained from 1996 to 1999 reveal the existence of a ionised optical nebula around the symbiotic binary CH Cyg extending out to 5000 A.U. from the central stars. The observed velocity range of the…
The solar chromosphere and transition region are highly structured and complex regimes. A recent breakthrough has been the identification of dynamic fibrils observed in H alpha as caused by field-aligned magnetoacoustic shocks. We seek to…
We derive correlation coefficients between temperature and line-of-sight velocity as a function of optical depth throughout the solar photosphere for the non-magnetic photosphere and a small area of enhanced magnetic activity. The maximum…
Primordial stars are likely to be very massive >30 Msun, form in isolation, and will likely leave black holes as remnants in the centers of their host dark matter halos. We expect primordial stars to form in halos in the mass range…
[abridged] We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different…
The present common view about GRB origin is related to cosmology, what is based on statistical analysis, and on measurements of the redshifts in the GRB optical afterglows of long GRB. No correlation is found between redshifts, GRB…
Large-scale extragalactic jets, observed to extend from a few to a few hundred kiloparsecs from active galactic nuclei, are now studied over many decades in frequency of electromagnetic spectrum, from radio until (possibly) TeV gamma rays.…
I present here a short personal view of our understanding of the Chandra detected knots and hot spots of powerful Fanaroff Rilley (FR) II radio galaxies and quasars in the context of leptonic models. Observations of the knots and hot spots…
The NASA/ESA/CSA James Webb Space Telescope (JWST) will be the successor to the Hubble Space Telescope and may be launched as early as mid-2011. The key scientific goals for JWST are discovering and understanding the formation of the first…
We review several aspects of clusters of galaxies and their application to cosmology. We present first results of numerical simulations of the dynamics of the intra-cluster gas and of different interaction processes between cluster galaxies…
1RXS J232953.9+062814 was identified as a cataclysmic variable by Wei et al. (1999). Four low-resolution spectra of 1RXS J232953.9+062814 were obtained by using the 2.16-m telescope of the National Astronomical Observatories, in which two…
In paper 1 (Cheng et al. 2001) we have shown that the soft X-ray spectra of two types of Seyfert 1 galaxies statistically vary differently with increasing intensity. In order to understand how the spectrum of blazars changes, the spectral…
In order to understand how the soft X-ray spectra vary we present the Hardness Ratio 1 versus Count Rates (HR1-CTs) correlation of 8 Narrow-line Seyfert 1 Galaxies (NLS1s) and 14 Broad-line Seyfert1 Galaxies (BLS1s) obtained during the…