X-ray emission from stars in the cool half of the Hertzsprung-Russell diagram is generally attributed to the presence of a magnetic corona that contains plasma at temperatures exceeding 1 million K. Coronae are ubiquitous among these stars, yet many fundamental mechanisms operating in their magnetic fields still elude an interpretation through a detailed physical description. Stellar X-ray astr...

We use the previously-identified 15 infrared star-cluster counterparts to Xray point sources in the interacting galaxies NGC 4038/4039 (the Antennae) to study the relationship between total cluster mass and X-ray binary number. This significant population of X-Ray/IR associations allows us to perform, for the first time, a statistical study of X-ray point sources and their environments. We defi...

We present an R-band lightcurve of an optical transient which is likely to be associated with the gamma-ray burst event GRB971214. Our first measurement took place 17 hours after the gamma-ray event. The brightness decayed with a power-law exponent α = −1.380 ± 0.013, which is significantly faster than the other two optically identified bursts, GRB970228 and GRB970508 which had exponents of α =...

X-rays have tremendous potential for imaging at the highest angular resolution. The high surface brightness of many xray sources will reveal angular scales heretofore thought unreachable. The short wavelengths make instrumentation compact and baselines short. We discuss how practical x-ray interferometers can be built for astronomy using existing technology. We describe the Maxim Pathfinder and...

Recently, Dixon et al. (1998) have re-analyzed the EGRET data, finding a statistically significant diffuse γ-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which γ-rays are produced through the interaction of high-energy cosmic-ray protons with cold H2 clouds clumped into dark clusters th...

HESS J0632+057 is one of only two unidentified very-high-energy gamma-ray sources which appear to be point-like within experimental resolution. It is possibly associated with the massive Be star MWC 148 and has been suggested to resemble known TeV binary systems like LS I +61 303 or LS 5039. HESS J0632+057 was observed by VERITAS for 31 hours in 2006, 2008 and 2009. During these observations, n...

The ARGO-YBJ detector, located 4300 m a.s.l. on the Tibet plateau, is a ground-based, full-coverage array of Resistive Plate Chambers (RPCs) covering a surface of 78 × 74 m 2 , surrounded by a guard ring of RPCs enclosing a total surface of about 11000 m 2. ARGO-YBJ was designed to detect extensive air showers generated by cosmic rays and gamma rays with primary energy greater than few hundred ...

Phase Fresnel lenses have the same imaging properties as zone plates, but with the possibility of concentrating all of the incident power into the primary focus, increasing the maximum theoretical efficiency from 11% to close to 100%. For X-rays, and in particular for gamma-rays, large, diffraction-limited phase Fresnel lenses can be made relatively easily. The focal length is very long for exa...