Communications have come to rely heavily on ionospheric radio in spite of the sometimes unpredictable behavior of the ionosphere as a transmission medium. Far from being the static reflector of radio waves that the communicator would desire, the ionosphere continuously and sometimes suddenly, undergoes structural changes on almost all scales of size and time. Such changes upset the often delica...

Very recently, J 1128+5925 was found to show strong intraday variability at radio wavelengths and may be a new source with annual modulation of the timescale of its radio variability. Therefore, its radio variability can be best explained via interstellar scintillation. Here we present the properties of its optical variability for the first time after a monitoring program in 2007 May. Our obser...

We report on ROSAT HRI observations of the z = 0.61 radio galaxy 3C 220.1. The X-ray emission from this object consists of an extended component, which we attribute to luminous cluster emission, and a compact central source. The compact component is too bright to be modelled as a cooling flow under some plausible assumptions for the hot gas temperature and distribution; we suggest instead that ...

HST /WFPC2 images of the central part of the globular cluster M 3 (NGC 5272) in UV (F255W), U (F336W), V (F555W) and I (F814W) are used to search for faint blue stars down to anUVmagnitude slightly below the turnoff level. Three low-luminosity blue stars are found in the PC1 field of view; two are new detections. All are within ∼ 10′′ from the cluster centre. Two are likely horizontal branch st...

In order to further test our hypothesis that jets and disk around compact accreting objects are symbiotic features we investigate the newly discovered superluminal galactic radio jets GRS 1915+105 and GRO J1655-40 and the two famous galactic radio jets 1E1740-2942 and SS 433 within the framework of our couple jet/disk model developed initially for active galactic nuclei (AGN) and the galactic c...

Gamma ray bursts (GRB) occur at random points in the sky at cosmological distances. They emit intense γ rays for a brief period. The spectrum then evolves through X–ray, optical region to possibly radio frequency. Though there are some models, the origin and time evolution of GRB are not well understood. Extragalactic radio sources also exhibit a baffling array of features that are poorly under...

A candidate radio-loud broad absorption line quasar (RLBAL) has been reported by Becker et al. (1997). We present JHK observations of this object and three other radiodetected BALs taken with the new Michigan-Dartmouth-MIT/Ohio State/Aladdin IR Camera (MOSAIC) on the KPNO 4-meter. The candidate RLBAL 1556+3517 has B−K=6.63, redder than all but one or two known z>1 quasars. This strongly suggest...

Context. The multi-wavelength study of old (>100 Myr) radio pulsars holds the key to understanding the long-term evolution of neutron stars, including the advanced stages of neutron star cooling and the evolution of the magnetosphere. Optical/UV observations are particularly useful for such studies because they allow one to explore both thermal and non-thermal emission processes. In particular,...

Periodic radio bursts from very low mass stars and brown dwarfs simultaneously probe their magnetic and rotational properties. The brown dwarf 2MASSI J1047539+212423 (2M 1047+21) is currently the only T dwarf (T6.5) detected at radio wavelengths. Previous observations of this source with the Arecibo observatory revealed intermittent, 100%-polarized radio pulses similar to those detected from ot...

We describe the results of our multi-wavelength observing campaign on the young stellar objects in the LkHα101 cluster. Our simultaneous X-ray and multifrequency radio observations are unique in providing simultaneous constraints on short-timescale variability at both wavelengths as well as constraints on the thermal or nonthermal nature of radio emission from young stars. Focussing in on radio...