Periodic Modulations in an X-ray Flare from Sagittarius A*

We present the highly significant detection of a quasi-periodic flux modulation with a period of 22.2 min seen in the X-ray data of the Sgr A* flare of 2004 August 31. This flaring event, which lasted a total of about three hours, was detected simultaneously by EPIC on XMM-Newton and the NICMOS near-infrared camera on the HST. Given the inherent difficulty in, and the lack of readily available methods for quantifying the probability of a periodic signal detected over only several cycles in a data set where red noise can be important, we developed a general method for quantifying the likelihood that such a modulation is indeed intrinsic to the source and does not arise from background fluctuations. We here describe this Monte Carlo based method, and discuss the results obtained by its application to a other XMM-Newton data sets. Under the simplest hypothesis that we witnessed a transient event that evolved, peaked and decayed near the marginally stable orbit of the supermassive black hole, this result implies that for a mass of 3.5 x 10^{6} Msun, the central object must have an angular momentum corresponding to a spin parameter of a=0.22.Comment: 4 pages, 6 figures, submitted to ApJ

The yellow hypergiants HR 8752 and rho Cassiopeiae near the evolutionary border of instability

High-resolution near-ultraviolet spectra of the yellow hypergiants HR 8752 and rho Cassiopeiae indicate high effective temperatures placing both stars near the T_eff border of the ``yellow evolutionary void''. At present, the temperature of HR 8752 is higher than ever. For this star we found Teff=7900+-200 K, whereas rho Cassiopeiae has Teff=7300+-200 K. Both, HR 8752 and rho Cassiopeiae have developed strong stellar winds with Vinf ~ 120 km/s and Vinf ~ 100 km/s, respectively. For HR 8752 we estimate an upper limit for the spherically symmetric mass-loss of 6.7X10^{-6}M_solar/yr. Over the past decades two yellow hypergiants appear to have approached an evolutionary phase, which has never been observed before. We present the first spectroscopic evidence of the blueward motion of a cool super/hypergiant on the HR diagram.Comment: 13 pages including 3 figures. Accepted for publication in ApJ Letter

On the X-ray feature associated with the Guitar Nebula

Context: A mysterious X-ray nebula, showing a remarkably linear geometry, was recently discovered close to the Guitar Nebula, the bow-shock nebula associated with B2224+65, which is the fastest pulsar known. The nature of this X-ray feature is unknown, and even its association with pulsar B2224+65 is unclear. Aims: We attempt to develop a self-consistent scenario to explain the complex phenomenology of this object. Methods: We assume that the highest energy electrons accelerated at the termination shock escape from the bow shock and diffuse into the ambient medium, where they emit synchrotron X-rays. The linear geometry should reflect the plane-parallel geometry of its ambient field. Results: We estimate the Lorentz factor of the X-ray emitting electrons and the strength of the magnetic field. The former (~10^8) is close to its maximum possible value, while the latter, at ~45 uG, is higher than typical interstellar values and must have been amplified in some way. The magnetic field must also be turbulent to some degree to trap the electrons sufficiently for synchrotron X-ray emission to occur effectively. We propose a self-consistent scenario in which, by some streaming instability, the electrons themselves generate a turbulent field in which they then diffuse. Some numerical coincidences are explained, and tests are proposed to verify our scenario. Conclusions: Electron leaking may be common in the majority of pulsar bow-shock nebulae, even though the X-ray nebulosity in general is too diffuse to be detectable.Comment: 4 pages. Accepted for publication in A&A, Letters to the Edito

Does "free" higher education in South Africa make economic sense? Views of commerce students

AbstractStudent protests demanding ‘free’ higher education at South Africa universities in 2015 is a reminder that there are still a lot of unresolved issues regarding the inequalities due to the apartheid era in the higher education system. Some of these issues include insufficient state funding of higher education, the increase of tuition fee and unpaid student debt. Even though ‘free’ higher education sounds appealing, the question is whether implementing ‘free’ higher education would make economic sense in South Africa. The study aimed to investigate the perceptions of commerce students at one South African university regarding the economic feasibility of ‘free’ higher education and how it might affect the South African economy. The research aimed to give a descriptive analysis of the perceptions of commerce students and were empirically investigated by means of a questionnaire, containing both open and closed questions. Gaining understanding into student perceptions can be invaluable, as they are considered the livelihood of higher education institutions.The results show that the respondents do not think that ‘free’ higher education make economic sense in South Africa as the economy is too weak and there is too much corruption and wasteful spending in government. The respondents also indicated that the economy will mostly be negatively affected by ‘free’ higher education in that there will be an increase of the financial burden on the South African economy and tax payer. They also regard other government services to be of greater importance than implementing ‘free’ higher education.The results of this study are not necessarily generalisable beyond the scope of the particular institution, but the findings do create a platform for the continued debate on the importance, economic feasibility and implementation of ‘free’ higher education in South Africa.

Stiffness of planar tensegrity beam topologies

This paper demonstrates a symbolic procedure to compute the stiffness of structures. Geometrical design parameters enter in this computation. A set of equations linear in the degrees-of freedom, but nonlinear in the design parameters, is solved symbolically. The resulting expressions reveal the values of these parameters which yield desirable properties for the stiffness or stiffness-to-mass ratio. By enumerating a set of topologies, stiffness properties are optimized over this set of topologies. This procedure is applied to a planar tensegrity beam. The results make it possible to optimize the structure with respect to stiffness properties, not only by appropriately selecting (continuous) design parameters like dimensions, but also by selecting an appropriate topology for the structure (a discrete design decision)

Event Weighted Tests for Detecting Periodicity in Photon Arrival Times

This paper treats the problem of detecting periodicity in a sequence of photon arrival times, which occurs, for example, in attempting to detect gamma-ray pulsars. A particular focus is on how auxiliary information, typically source intensity, background intensity, and incidence angles and energies associated with each photon arrival should be used to maximize the detection power. We construct a class of likelihood-based tests, score tests, which give rise to event weighting in a principled and natural way, and derive expressions quantifying the power of the tests. These results can be used to compare the efficacies of different weight functions, including cuts in energy and incidence angle. The test is targeted toward a template for the periodic lightcurve, and we quantify how deviation from that template affects the power of detection

Detection of very-high-energy gamma-ray emission from the vicinity of PSR B1706-44 with H.E.S.S

The energetic pulsar PSR B1706-44 and the adjacent supernova remnant (SNR) candidate G 343.1-2.3 were observed by H.E.S.S. during a dedicated observational campaign in 2007. A new source of very-high-energy (VHE; E > 100 GeV) gamma-ray emission, HESS J1708-443, was discovered with its centroid at RA(J2000) = 17h08m10s and Dec(J2000) = -44d21', with a statistical error of 3 arcmin on each axis. The VHE gamma-ray source is significantly more extended than the H.E.S.S. point-spread function, with an intrinsic Gaussian width of 0.29 +/- 0.04 deg. Its energy spectrum can be described by a power law with a photon index Gamma = 2.0 +/- 0.1 (stat) +/- 0.2 (sys). The integral flux measured between 1-10 TeV is ~17% of the Crab Nebula flux in the same energy range. The possible associations with PSR B1706-44 and SNR G343.1-2.3 are discussed.Comment: 4+ pages, 2 figures; v1 submitted to ICRC Proceedings on 15 May 2009; v2 has additional references and minor change