DASCH 100-yr light curves of high-mass X-ray binaries

We analyzed the 100-yr light curves of Galactic high-mass X-ray binaries using the Harvard photographic plate collection, made accessible through the DASCH project (Digital Access to a Sky Century at Harvard). As scanning is still in progress, we focus on the four objects that are currently well covered: the supergiant X-ray binary Cyg X-1 (V1357 Cyg), and the Be X-ray binaries 1H 1936+541 (BD+53 2262), RX J1744.7-2713 (HD 161103), and RX J2030.5+4751 (SAO 49725), the latter two objects being similar to gamma Cas. The star associated with Cyg X-1 does not show evidence for variability with an amplitude higher than 0.3 magnitude over a hundred years. We found significant variability of one magnitude with timescales of more than 10 years for SAO 49725, as well as a possible period of 500-600 days and an amplitude of 0.05 magnitude that might be the orbital, or super-orbital period of the system. The data is insufficient to conclude for HD 161103 but suggests a similar long-term variability. We thus observe an additional characteristic of gamma Cas-like objects: their long-term variability. This variability seems to be due to the slow evolution of a decretion disk around the Be star, but may be triggered by the presence of a compact object in the system, possibly a white dwarf. This characteristic could be used to identify further similar objects otherwise difficult to detect.Comment: Accepted for publication in Proceedings of Science (INTEGRAL 2012), Eds. A. Goldwurm, F. Lebrun and C. Winkler, based on a presentation at the 9th INTEGRAL Workshop "An INTEGRAL view of the high-energy sky (the first 10 years)", October 15-19, 2012, Paris, Franc

Revivals, classical periodicity, and zitterbewegung of electron currents in monolayer graphene

Revivals of electric current in graphene in the presence of an external magnetic field are described. It is shown that when the electrons are prepared in the form of wave packets assuming a Gaussian population of only positive (or negative) energy Landau levels, the presence of the magnetic field induce revivals of the electron currents, besides the classical cyclotron motion. When the population comprises both positive and negative energy Landau levels, revivals of the electric current manifest simultaneously with zitterbewegung and the classical cyclotron motion. We relate the temporal scales of these three effects and discuss to what extent these results hold for real graphene samples

Harmful monitoring

Abstract We show that there may be circumstances in which a principal prefers not to observe the project choice of an agent that acts on her behalf. The ability of the agent is private information. Projects differ with respect to the amount of risk. If the principal can observe the project choice of the agent, the latter will use that choice as a signal. In the separating equilibrium, an agent with high ability then chooses a project that is too risky. If more difficult projects require more effort, there are two opposite effects. The shirking effect implies that the agent chooses a project that is too safe. The signaling effect implies that he chooses a project that is too risky. The net effect is ambiguous. We also discuss the implications of our model for promotion policies.