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The nature of the optical--radio correlations for powerful radio galaxies is investigated using spectroscopic observations of a complete sample of southern 2Jy radio sources. In line with previous work, we find that significant correlations…
We analyze HST spectra and Chandra observations of a sample of 21 LINERs, at least 18 of which genuine AGN. We find a correlation between the X-rays and emission lines luminosities, extending over three orders of magnitude and with a…
Hard X-ray emission in radio-quiet active galactic nuclei (AGNs) is believed to be produced via inverse Compton scattering by hot and compact coronae near the super massive black hole. However the origin and physical properties of the…
X-ray reflection from an accretion disc produces characteristic emission lines allowing us to probe the innermost regions in AGN. We investigate these emission lines under a framework of Riemannian geometrical optics where the corona has a…
(truncated version) The X-ray spectra of Galactic binary systems in the very high state show both strong disk emission and a strong, steep tail to high energies. We use simultaneous optical-ASCA-RXTE data from the black hole transient XTE…
We study the X-ray spectra of a sample of 19 obscured, optically-selected Seyfert galaxies (Sy 1.8, 1.9 and 2) in the local universe ($d \leq 175$~Mpc), drawn from the CfA Seyfert sample. Our analysis is driven by the high sensitivity of…
Resonant scattering (RS) of X-ray line emission in supernova remnants (SNRs) may modify the observed line profiles and fluxes and has potential impact on estimating the physical properties of the hot gas and hence on understanding the SNR…
The observations of the solar F-corona from space are reviewed emphasizing the 25 years of continuous monitoring achieved by the LASCO-C2 and C3 coronagraphs. Our work includes images obtained by the Clementine cameras, and the…
The observed relation between the soft X-ray and the optical-ultraviolet emission in active galactic nuclei (AGN) is non-linear and it is usually parametrized as a dependence between the logarithm of the monochromatic luminosity at 2500…
Magnetars are a subclass of neutron stars with ultra-strong surface magnetic fields. Some magnetars exhibit persistent hard X-ray emission, characterized by power-law tails with photon indices around 1--1.5, extending from ${\sim}$10 keV to…
Using self-consistent, physically motivated models, we investigate the X-ray obscuration in 19 Type 2 [OIII] 5007 \AA\ selected AGN, 9 of which are local Seyfert 2 galaxies and 10 of which are Type 2 quasar candidates. We derive reliable…
We study the formation of photospheric emission lines in O stars and show that the rectangular profiles, sometimes double peaked, that are observed for some stars are a direct consequence of rotation, and it is unnecessary to invoke an…
The hard X-ray flux ratio R of the footpoint sources to the loop-top source has been used to investigate non-thermal electron trapping and precipitation in solar flares. Considering the mission-long Yohkoh Hard X-ray Telescope database,…
Molecules and particles make up $\sim 40 - 70\%$ of carbon in the interstellar medium, yet the exact chemical structure of these constituents remains unknown. We present carbon K-shell absorption spectroscopy of the Galactic Interstellar…
Direct comparison of high-resolution X-ray spectra of the active dMe star AD Leo, observed three times with Chandra, shows variability of key density-sensitive lines, possibly due to flaring activity. In particular, a significant…
We present a reanalysis of the HeII Lyman alpha absorption towards the quasars HS1700+6416 and HE2347-4342 using new high S/N, optical observations. An alternative analysis method is applied, which fits the high quality, optical HI data…
We consider the Compton scattering in the optically thick uniform spherical corona around a neutron star in an X-ray binary. In the scattering, the low energy seed photons (0.1 - 2.5 keV) are scattered in low energy electrons (2.5 - 10 keV)…
Electron density diagnostics based on a line intensity ratio of Si X are applied to the X-ray spectra of Procyon, $\alpha$ Cen A$&$B, Capella and $\epsilon$ Eri measured with the Low Energy Transmission Grating Spectrometer (LETGS) combined…
We present He-like line ratios (resonance, intercombination and forbidden lines) for totally and partially photoionized media. For solar plasmas, these line ratios are already widely used for density and temperature diagnostics of coronal…
A cross-correlation of the SDSS DR7 with the FIRST radio survey makes it possible to conduct a joined multiwavelength statistical study of radio-optical galaxy properties on a very large number of sources. Our goal is to improve the study…