Does an orbiting star cause periodic modulation of X-rays from NGC6814?
USING data from the Ginga satellite, Done et al.1 have confirmed an Exosat observation2 of high-amplitude periodic modulation in the X-ray emission from the Seyfert galaxy NGC6814. To within observational errors of ∼1%, the period seems to have remained constant at ∼12,100 seconds between 1985 and 1989. An obvious candidate for the phenomenon underlying the periodicity is the orbital motion of a star or low-mass compact object around the central black hole3. As we show here, the presence of an orbiting star could be verified easily by looking for the effects of Lense–Thirring precession of the orbital plane, caused by the dragging of inertia! frames around a rotating black hole. Precession-induced variations in the waveform and in the phase of the observed periodicity should have a period of between a month and a year. Such variations could account for the different waveforms present in the Ginga and Exosat data sets4, and may be detectable in existing Ginga and future Rosat, OSSE/GRO, and Astro-D data.
Large third order optical nonlinearity and optical limiting properties of a 3,4-diaminopyridine
Abstract
Third-order nonlinear optical properties of aqueous solution of 3,4-diaminopyridine with different MgCl2 and NaCl concentrations are investigated by the use of the Z-scan technique at wavelength of 473 nm with a continues wave (CW) laser. The results showed that the samples exhibited self-defocusing optical nonlinearity. All the samples showed large nonlinear refractive indices and nonlinear absorption coefficients of the order of 10−8 cm2/W and 10−4 cm/W, respectively. Also the samples exhibits good optical power limiting properties under CW excitation. The nonlinear refractive index, nonlinear absorption coefficient and efficiency of limiting are found to increase with increases in the concentration MgCl2. Therefore, the organic compound investigated here are promising candidates for optical power limiting devices.
Consequences of relativistic proton injection in active galactic nuclei
Les processus non thermiques dans les noyaux galactiques actifs peuvent conduire a une production de protons ultra-relativistes avec des energies 10 7 -10 8 GeV. Ces protons peuvent interagir avec le champ de rayonnement intense ainsi qu'avec le plasma ambiant au travers d'une diversite de processus, incluant les collisions inelastiques proton-proton, la production de photomesons, et la creation de paires pγ. Les resultats de ces processus sont analyses afin de determiner comment ils affectent la distribution des energies protoniques et quels types de particules secondaires ils produisent.