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.
Reverse saturable absorption in Er +3 doped systems
Abstract This work reports on reverse saturable absorption (RSA) from Er 3+ doped systems. RSA is characterized by a decrease in the transmitted intensity as the incident intensity increases. This effect is interesting due to its potential application in optical devices such as optical inverters, optical limiters and switches. Particularly, Er doped systems pumped at wavelengths around 830 nm may exhibit RSA. In this case, the RSA process is achieved because the resonant excited-state ( 4 I 13/2 – 4 S 3/2 ) absorption cross section is larger than the quasiresonant ground state ( 4 I 15/2 – 4 I 9/2 ) absorption cross section. We have simulated the transmitted intensity of Er doped samples as a function of laser intensity, laser wavelength and Er concentration. Our results indicate that Er concentration plays an important role in the process. RSA is intensified as cross relaxation between Er ions increases. Moreover, the effect of optical limiting is observed and is dependent on the cross relaxation among Er ions.
5.63 and 5.80 MeV levels of Ne20; de-excitation branching
Abstract The ratio Г γ /Г for the 5.63 and 5.80 MeV levels of Ne 20 has been determined to be 0.077 (±0.008) and −4 , respectively, by using the C 12 (C 12 , α)Ne 20 reaction and determining the fraction of the events in which the alpha-particle and the Ne 20 recoil are present in coincidence at the kinematically correct angles. In both cases these results in themselves are shown to imply radiative E1 transition (5.63→1.63 and 5.80→0, respectively) probabilities ⪡5 × 10 −5 Weisskopf units. When taken together with the life-time for the 5.63 MeV level measured by Evans et al . values of Г( E1 ) ≲ 5 × 10 −6 and Г α ≈ 6% of the particle estimate are obtained for this state.