Danish Space Research Institute

About: Danish Space Research Institute is a based out in . It is known for research contribution in the topics: Telescope & Galaxy. The organization has 199 authors who have published 501 publications receiving 16067 citations.

Transient optical emission from the error box of the gamma-ray burst of 28 February 1997

Abstract: For almost a quarter of a century1, the origin of γ-ray bursts— brief, energetic bursts of high-energy photons—has remained unknown. The detection of a counterpart at another wavelength has long been thought to be a key to understanding the nature of these bursts (see, for example, ref. 2), but intensive searches have not revealed such a counterpart. The distribution and properties of the bursts3 are explained naturally if they lie at cosmological distances (a few Gpc)4, but there is a countervailing view that they are relatively local objects5, perhaps distributed in a very large halo around our Galaxy. Here we report the detection of a transient and fading optical source in the error box associated with the burst GRB970228, less than 21 hours after the burst6,7. The optical transient appears to be associated with a faint galaxy7,8, suggesting that the burst occurred in that galaxy and thus that γ-ray bursts in general lie at cosmological distance.

EGRET Observations of the Extragalactic Gamma-Ray Emission

Abstract: The all-sky survey in high-energy gamma rays (E > 30 MeV) carried out by EGRET aboard the Compton Gamma Ray Observatory provides a unique opportunity to examine in detail the diffuse gamma-ray emission. The observed diffuse emission has a Galactic component arising from cosmic-ray interactions with the local interstellar gas and radiation, as well as an almost uniformly distributed component that is generally believed to originate outside the Galaxy. Through a careful study and removal of the Galactic diffuse emission, the flux, spectrum, and uniformity of the extragalactic emission are deduced. The analysis indicates that the extragalactic emission is well described by a power-law photon spectrum with an index of -(2.10 ± 0.03) in the 30 MeV to 100 GeV energy range. No large-scale spatial anisotropy or changes in the energy spectrum are observed in the deduced extragalactic emission. The most likely explanation for the origin of this extragalactic high-energy gamma-ray emission is that it arises primarily from unresolved gamma-ray-emitting blazars.

JEM-X: The X-ray monitor aboard INTEGRAL ?

Abstract: The JEM-X monitor provides X-ray spectra and imaging with arcminute angular resolution in the 3 to 35 keV band. The good angular resolution and the low energy response of JEM-X plays an important role in the identification of gamma ray sources and in the analysis and scientific interpretation of the combined X-ray and gamma ray data. JEM-X is a coded aperture instrument consisting of two identical, coaligned telescopes. Each of the detectors has a sensitive area of 500 cm 2 , and views the sky through its own coded aperture mask. The two coded masks are inverted with respect to each other and provides an angular resolution of 3 0 across an eective field of view of about 10 diameter.

A catalog of 203 galaxy clusters serendipitously detected in the ROSAT PSPC pointed observations

Abstract: We present a catalog of 203 clusters of galaxies serendipitously detected in 647 ROSAT PSPC high Galactic latitude pointings covering 158 deg2. This is one of the largest X-ray-selected cluster samples, comparable in size only to the ROSAT All-Sky Survey sample of nearby clusters (Ebeling et al. 1997). We detect clusters in the inner 175 of the ROSAT PSPC field of view using the spatial extent of their X-ray emission. Fluxes of detected clusters range from 1.6 × 10-14 to 8 × 10-12 ergs s-1 cm-2 in the 0.5-2 keV energy band. X-ray luminosities range from 1042 ergs s-1, corresponding to very poor groups, to ~5 × 1044 ergs s-1, corresponding to rich clusters. The cluster redshifts range from z = 0.015 to z > 0.5. The catalog lists X-ray fluxes, core radii, and spectroscopic redshifts for 73 clusters and photometric redshifts for the remainder. Our detection method, optimized for finding extended sources in the presence of source confusion, is described in detail. Selection effects necessary for a statistical analysis of the cluster sample are comprehensively studied by Monte Carlo simulations. We have optically confirmed 203 of 223 X-ray sources as clusters of galaxies. Of the remaining 20 sources, 19 are likely false detections arising from blends of unresolved point X-ray sources. Optical identifications of the remaining object are hampered by a bright nearby star. Above a flux of 2 × 10-13 ergs s-1 cm-2, 98% of extended X-ray sources are optically confirmed clusters. The number of false detections and their flux distribution are in perfect agreement with simulations. The log N-log S relation for clusters derived from our catalog shows excellent agreement with counts of bright clusters derived from the Einstein Extended Medium Sensitivity Survey and ROSAT All-Sky Survey. At fainter fluxes, our log N-log S relation agrees with the smaller area WARPS survey. Our cluster counts appear to be systematically higher than those from a 50 deg2 survey by Rosati et al. In particular, at a flux of 2 × 10-13 ergs s-1 cm-2, we find a surface density of clusters of 0.57 ± 0.07 deg-2, which is a factor of 1.3 more than was found by Rosati et al. This difference is marginally significant at the ~2 σ level. The large area of our survey makes it possible to study the evolution of the X-ray luminosity function in the high luminosity range inaccessible with other, smaller area ROSAT surveys.

Low cloud properties influenced by cosmic rays

Abstract: The influence of solar variability on climate is currently uncertain. Recent observations have indicated a possible mechanism via the influence of solar modulated cosmic rays on global cloud cover. Surprisingly the influence of solar variability is strongest in low clouds $(\ensuremath{\le}3\mathrm{km})$, which points to a microphysical mechanism involving aerosol formation that is enhanced by ionization due to cosmic rays. If confirmed it suggests that the average state of the heliosphere is important for climate on Earth.