Periodic dips observed in ≈ 20% of low-mass X-ray binaries are thought to arise from obscuration of the neutron star by the outer edge of the accretion disk. We report the detection with the Rossi X-ray Timing Explorer of two dipping episodes in Aql X-1, not previously a known dipper. The X-ray spectrum during the dips exhibited an elevated neutral column density, by a factor between 1 and almost two orders of magnitude. Dips were not observed in every cycle of the 18.95-hr orbit, so that the estimated frequency for these events is 0.10 +0.07 −0.05 cycle −1 . This is the first confirmed example of intermittent dipping in such a system. Assuming that the dips in Aql X-1 occur because the system inclination is intermediate between the non-dipping and dipping sources, implies a range of 72-79 • for the source. This result lends support for the presence of a massive (> 2 M ⊙ ) neutron star in Aql X-1, and further implies that ≈ 30 additional LMXBs may have inclinations within this range, raising the possibility of intermittent dips in those systems also. Thus, we searched for dips from 24 other bursting systems, without success. For the system with the largest number of dip phases covered, 4U 1820−303, the nondetection implies a 95% upper limit to the dip frequency of 1.4 × 10 −3 cycle −1 .

This paper responds to the need for a theoretical model to describe the structural strength capacity of innovative hollow corrugated columns consisting of four cold-formed corrugated mild steel plates. Interest in these columns has been growing over a number of years, due to high ratio of axial load bearing capacity to weight. The paper utilises the Direct Strength Method (DSM) to calculate the axial strength of aforementioned columns while the results are compared with those obtained from experiments and finite element models. Existing formulations are extended to predict local failure of these columns over a vast range of slenderness with a good degree of accuracy. The results show the appropriateness of this method as an alternative for analytical capacity prediction of innovative corrugated columns. Figure 1: (a) Schematic view of an innovative column, and (b) plat's geometric layout (Nassirnia et al. 2015(Nassirnia et al. , 2017a