Boundary layer scaling in Rayleigh-Benard convection

Accepted versio

Edge Elimination in TSP Instances

The Traveling Salesman Problem is one of the best studied NP-hard problems in combinatorial optimization. Powerful methods have been developed over the last 60 years to find optimum solutions to large TSP instances. The largest TSP instance so far that has been solved optimally has 85,900 vertices. Its solution required more than 136 years of total CPU time using the branch-and-cut based Concorde TSP code [1]. In this paper we present graph theoretic results that allow to prove that some edges of a TSP instance cannot occur in any optimum TSP tour. Based on these results we propose a combinatorial algorithm to identify such edges. The runtime of the main part of our algorithm is $O(n^2 \log n)$ for an n-vertex TSP instance. By combining our approach with the Concorde TSP solver we are able to solve a large TSPLIB instance more than 11 times faster than Concorde alone

Quiescent NIR and optical counterparts to candidate black hole X-ray binaries

We present near-infrared and optical imaging of fifteen candidate black hole X-ray binaries. In addition to quiescent observations for all sources, we also observed two of these sources (IGR J17451-3022 and XTE J1818-245) in outburst. We detect the quiescent counterpart for twelve out of fifteen sources, and for the remaining three we report limiting magnitudes. The magnitudes of the detected counterparts range between $K_s$ = 17.59 and $K_s$ = 22.29 mag. We provide (limits on) the absolute magnitudes and finding charts of all sources. Of these twelve detections in quiescence, seven represent the first quiescent reported values (for MAXI J1543-564, XTE J1726-476, IGR J17451-3022, XTE J1818-245, MAXI J1828-249, MAXI J1836-194, Swift J1910.2-0546) and two detections show fainter counterparts to XTE J1752-223 and XTE J2012+381 than previously reported. We used theoretical arguments and observed trends, for instance between the outburst and quiescent X-ray luminosity and orbital period $P_{orb}$ to derive an expected trend between $\Delta K_s$ and $P_{orb}$ of $\Delta K_s \propto \log P_{orb}^{0.565}$. Comparing this to observations we find a different behaviour. We discuss possible explanations for this result.Comment: 18 pages, 6 figures. Accepted for publication in MNRA

An investigation into factors impacting on exports from South Africa to the Southern African Development Community (SADC)

Manufacturers face vigorous competition in local and export markets and need to have a genuine competitive advantage in order to grow. The South African government has recognised the importance of developing national manufacturing capacity as a means of increasing employment and reducing poverty. To this end, the government provides substantial support to both the manufacturing and exporting sectors. The government also negotiated the Southern African Development Community (SADC) agreement which leverages competitive advantages for South African manufacturers exporting into the region. However, since the ratification of the SADC agreement in 2008, there has been no perceptible increase in export activity to the region when compared to other markets. This research study was conducted to determine why this is the case and what factors are influencing the process. A structured literature review was undertaken to encapsulate export barriers, the role of the South African government in the export process, and the SADC agreement. The findings of the reviewed literature form the basis of the survey that led to the compilation of the research primary data. The results indicate that export barriers do not pose a major obstacle to trade into the SADC region. The role that the South African government holds was less conclusive with some successes noted, but on the whole the impact is not meaningfully positive. The SADC agreement and the dynamics prevailing in the free trade area do have a positive impact on exports to the region. The level of awareness with regard to the government support initiatives appears to be low.RésuméLes fabricants font face à une vive concurrence sur les marchés locaux et d’exportation et ont besoin d’avoir un véritable avantage concurrentiel pour se développer. Le gouvernement sud-africain a reconnu l’importance de renforcer les capacités nationales de production comme un moyen d’accroître l’emploi et réduire la pauvreté . À cette fin, le gouvernement apporte un soutien important à la fois la fabrication et secteurs exportateurs. Le gouvernement a également négocié l’accord Southern African Development Community (SADC ), qui s’appuie sur des avantages concurrentiels pour les fabricants sud-africains exportateurs dans la région. Cependant , depuis la ratification de l’accord de la SADC en 2008 , il a eu aucune augmentation perceptible de l’activité d’exportation de la région par rapport à d’autres marchés. Cette étude a été menée afin de déterminer pourquoi c’est le cas et quels sont les facteurs qui influencent le processus . Une revue de la littérature structurée a été entreprise pour encapsuler les obstacles à l’exportation , le rôle du gouvernement sud-africain dans le processus d’exportation , et l’accord de la SADC . Les résultats de la littérature examinée forment la base de l’enquête qui a conduit à la compilation des données primaires de la recherche . Les résultats indiquent que les barrières à l’exportation ne constituent pas un obstacle majeur au commerce dans la région de la SADC . Le rôle que le gouvernement sud-africain détient était moins concluante avec quelques réussites constatées, mais dans l’ensemble l’impact n’est pas significative positive. L’accord de la SADC et la dynamique qui prévaut dans la zone de libre-échange ont un impact positif sur les exportations de la région . Le niveau de sensibilisation à l’égard des initiatives de soutien du gouvernement semble être faibl

On the mass of the neutron star in V395 Car/2S 0921-630

We report high-resolution optical spectroscopy of the low-mass X-ray binary V395 Car/2S 0921-630 obtained with the MIKE echelle spectrograph on the Magellan-Clay telescope. Our spectra are obtained near superior conjunction of the mass donor star and we exploit the absorption lines originating from the back-side of the K-type object to accurately derive its rotational velocity. Using K0-K1 III templates, we find vsini=32.9 +/- 0.8 km/s. We show that the choice of template star and the assumed limb darkening coefficient has little impact on the derived rotational velocity. This value is a significant revision downwards compared to previously published values. We derive new system parameter constraints in the light of our much lower rotational velocity. We find M_1=1.44 +/- 0.10 Msun, M_2=0.35 +/- 0.03 Msun, and q=0.24 +/- 0.02 where the errors have been estimated through a Monte-Carlo simulation. A possible remaining systematic effect is the fact that we may be over-estimating the orbital velocity of the mass donor due to irradiation effects. However, any correction for this effect will only reduce the compact object mass further, down to a minimum mass of M_1=1.05 +/- 0.08 Msun. There is thus strong evidence that the compact object in this binary is a neutron star of rather typical mass and that the previously reported mass values of 2-4Msun were too high due to an over-estimate of the rotational broadening.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

Low Temperature Magnetic Properties of the Double Exchange Model

We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener to explain ferromagnetism has unexpected properties when there is more than one itinerant electron. We find that, in general, the many-body ground state of the DE model is {\it not} globally FM ordered (except for special filled-shell cases). Also, the low energy excitations of this model are distinct from spin wave excitations in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.Comment: 5 pages, RevTeX, 5 Postscript figures include

Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091

In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The X-Shooter data are compared to the GMOS data analyzed previously by Nelemans et al. (2004). We confirm the presence of C III and O II emission features at ~ 4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler shifts that could be attributed to the motion of the donor star/hot spot around the center of mass of the binary. We note a weak periodic signal in the red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The signal occurs at P = 30.23 +/- 0.03 min in the X-Shooter and at P = 30.468 +/- 0.006 min in the GMOS spectra when the source was in the low/hard state. Due to aliasing effects the period in the GMOS and X-Shooter data could well be the same. We deem it likely that the orbital period is thus close to 30 min, however, as several photometric periods have been reported for this source in the literature already, further confirmation of the 30 min period is warranted. We compare the surface area of the donor star and the disc of 4U 0614+091 with the surface area of the donor star and the disc in typical hydrogen-rich low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue that the optical emission in 4U 0614+091 is likely dominated by the disc emission. Additionally, we search for periodic signals in all the publicly available RXTE PCA light curves of 4U 0614+091 which could be associated with the orbital period of this source. A modulation at the orbital period with an amplitude of ~ 10% such as those that have been found in other ultra-compact X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure