No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81

Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds are likely white dwarfs that steadily or cyclically burn accreted matter on their surface, which are promising type Ia supernova progenitors. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous, including some ultraluminous supersoft sources (ULSs) that are possibly intermediate mass black holes of a few thousand solar masses. In this paper we report our X-ray spectral and timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS observations spanning six years, and its spectrum can be described by either a $kT_{bb}\approx70$ eV blackbody for a $\sim1.2M_\odot$ white dwarf, or a $kT_{in} \approx 80$ eV multicolor accretion disk for a $\gtrsim10^3M_\odot$ intermediate mass black hole. In two observations, the light curves exhibited dramatic flux drop/rise on time scales of $10^3$ seconds, reminiscent of eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive search for periodicity in the reasonable range of 50 ksec to 50 days failed to reveal an orbital period. The failure to reveal any periodicity is consistent with the long period ($\ge30$ yrs) predicted for this system given the optical identification of the secondary with an asymptotic giant star. Also, the eclipse-like dramatic flux changes in hours are hard to explain under the white dwarf model, but can in principle be explained by disk temperature changes induced by accretion rate variations under the intermediate mass black hole model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ

Geometric constant defining shape transitions of carbon nanotubes under pressure

Journal ArticleWe demonstrate that when a single-walled carbon nanotube is under pressure it undergoes a series of shape transitions, first transforming from a circle to an oval and then from an oval to a peanut. Most remarkably, the ratio of the area of the tube cross sections at the second transition over that at the first transition appears as a constant, independent of the tube radius. Its accurate value is computed to be G = 0:819 469, by formulating a variational geometry problem to represent single-walled carbon nanotubes with a family of closed plane curves of fixed length and minimum bending energy. The implications of such a geometric constant in designing nanotube electromechanical pressure sensors are discussed

Mechanism for nanotube formation from self-bending nanofilms driven by atomic-scale surface-stress imbalance

Journal ArticleWe demonstrate, by theoretical analysis and molecular dynamics simulation, a mechanism for fabricating nanotubes by self-bending of nanofilms under intrinsic surface-stress imbalance due to surface reconstruction. A freestanding Si nanofilm may spontaneously bend itself into a nanotube without external stress load, and a bilayer SiGe nanofilm may bend into a nanotube with Ge as the inner layer, opposite of the normal bending configuration defined by misfit strain. Such rolled-up nanotubes can accommodate a high level of strain, even beyond the magnitude of lattice mismatch, greatly modifying the tube electronic and optoelectronic properties

Modified Timoshenko formula for bending of ultrathin strained bilayer films

Journal ArticleMechanical bending of nanoscale thin films can be quite different from that of macroscopic thick films. However, current understanding of mechanical bending of nanoscale thin strained bilayer films is often limited within the Timoshenko model [Timoshenko, J. Opt. Soc. Am. 11, 233 (1925)], which was originally derived for macroscopic thick films. Here, we derive a modified Timoshenko formula by including the prominent effect of surface stress played in the nanofilms, which gives a much better agreement with the experiments than the classical formula