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.
Mo (W,V)Cu (Ag)S(Se) cluster compounds
Abstract This review summarizes the preparation, skeletal structure, reactions, spectroscopic properties and non-linear optical properties of Mo(W,V)Cu(Ag)S(Se) cluster compounds; 190 such compounds belong to 23 structural types; 25 reactions are classified into ligand substitution, addition and decomposition reactions. Recently, some Mo(W)Cu(Ag)S clusters were found to exhibit strong non-linear optical properties; the optical limiting threshold measured for [(PPh 3 ) 4 Ag 4 Mo 2 S 8 ] is one order of magnitude lower than that of C 60 .
Percussion marks on bone surfaces as a new diagnostic of hominid behaviour
Since stone tool cut marks and carnivore tooth marks were first described systematically on Plio-Pleistocene archaeological bone1,2, bone surface modification has played a prominent role in interpreting early archaeological site formation and hominid behaviour3–15. Here we introduce a new class of bone surface modification, which we call percussion marks. Percussion marks were produced during experimental breakage of marrow bones, and occur as pits or grooves impressed on a bone's surface by natural protrusions on the granitic hammerstone and anvil used. Although percussion marks can superficially mimic carnivore tooth marks, they nonetheless are closely associated with hammerstone impact notches and show consistent micromorphological features which distinguish them from tooth marks and other classes of bone surface modification. Given indications of prehistoric hammer-stone breakage of marrow bones1,7, an awareness of percussion marks is critical for accurately identifying the biological agents of bone modification at archaeological sites and provides a new diagnostic of carcass processing by hominids.