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.
Diffuse fluxes of cosmic high-energy neutrinos
Production spectra of high-energy neutrinos from galactic cosmic-ray interactions with interstellar gas and extragalactic ultrahigh-energy cosmic-ray interactions with microwave blackbody photons are presented and discussed. These production processes involve the decay of charged pions and are thus related to the production of cosmic ..gamma..-rays from the decay of neutral pions. Estimates of the neutrino fluxes from various diffuse cosmic sources are then made, and the reasons for significant differences with previous estimates are discussed. Small predicted event rates for a DUMAND type detection system, combined with a possible significant flux of prompt neutrinos from the atmosphere above 50 TeV, may make the study of diffuse extraterrestrial neutrinos more difficult than previously thought.