The upper kHz QPO: a gravitationally lensed vertical oscillation

We show that a luminous torus in the Schwarzschild metric oscillating along its own axis gives rise to a periodically varying flux of radiation, even though the source of radiation is steady and perfectly axisymmetric. This implies that the simplest oscillation mode in an accretion flow, axisymmetric up-and-down motion at the meridional epicyclic frequency, may be directly observable when it occurs in the inner parts of accretion flow around neutron stars and black holes. The high-frequency modulations of the X-ray flux observed in low-mass X-ray binaries at two frequencies (twin kHz QPOs) could then be a signature of strong gravity both because radial and meridional oscillations have different frequencies in non-Newtonian gravity, and because strong gravitational deflection of light rays causes the flux of radiation to be modulated at the higher frequency.Comment: 8 p., 4 fig

Analisis Rugi-Rugi Daya Akibat Ketidakseimbangan Beban pada Jaringan Distribusi Sekunder di PT. PLN (Persero) ULP Watang Sawitto

Pentingnya energi listrik bagi kehidupan di masyarakat dan industri menuntut PT. PLN (Persero) selaku penyedia energi listrik harus menyediakan energi listrik yang stabil. Penelitian ini dilakukan untuk menghitung rugi-rugi daya dan jatuh tegangan pada jaringan distribusi sekunder dalam keadaan beban tidak seimbang menggunakan perhitungan secara teori dan simulasi ETAP 12.6.0. Penelitian ini juga bertujuan untuk untuk menghitung rugi-rugi daya dan jatuh tegangan pada jaringan distribusi sekunder dalam keadaan beban seimbang menggunakan perhitungan secara teori dan simulasi ETAP 12.6.0. Sehubungan dengan itu, penelitian ini dilakukan dengan cara mengumpulkan data terkait data pembebanan transformator, data penghantar, dan data tegangan pangkal dan penerima, kemudian data dianalisis sehingga didapatkan perbandingan antara besar rugi-rugi daya dalam keadaan beban tidak seimbang dan keadaan beban seimbang. Berdasarkan hasil penelitian dan pembahasan, dapat disimpulkan bahwa besar rugi-rugi daya pada jaringan distribusi sekunder dalam keadaan beban tidak seimbang berdasarkan hasil perhitungan adalah 22,1 kW dengan jatuh tegangan sebesar 20,3 % dan rugi-rugi daya berdasarkan hasil simulasi ETAP 12.6.0 adalah 23,1 kW dengan jatuh tegangan sebesar 16,37%. Sedangkan rugi-rugi daya dalam keadaan beban seimbang berdasarkan hasil perhitungan 20,9 kW dan rugi-rugi daya berdasarkan hasil simulasi ETAP 12.6.0 adalah 22,3 kW dengan jatuh tegangan sebesar 13,69%.  Keywords: Ketidakseimbangan Beban, Rugi-Rugi Daya, Jatuh Tegangan

Vertical dissipation profiles and the photosphere location in thin and slim accretion disks

We calculate optically thick but geometrically thin (and slim) accretion disk models and perform a ray-tracing of photons (in the Kerr geometry) to calculate the observed disk spectra. Previously, it was a common practice to ray-trace photons assuming that they are emitted from the Kerr geometry equatorial plane, z = 0. We show that the spectra calculated with this assumption differ from these calculated under the assumption that photons are emitted from the actual surface of the disc, z = H(r). This implies that a knowledge of the location of the thin disks effective photosphere is relevant for calculating the spectra. In this paper we investigate, in terms of a simple toy model, a possible influence of the (unknown, and therefore ad hoc assumed) vertical dissipation profiles on the vertical structure of the disk and thus on the location of the effective photosphere, and on the observed spectra. For disks with moderate and high mass accretion rates (\dot m>0.01\dot m_C) we find that the photosphere location in the inner, radiation pressure dominated, disk region (where most of the radiation comes from) does not depend on the dissipation profile and therefore emerging disk spectra are insensitive to the choice of the dissipation function. For lower accretion rates the photosphere location depends on the assumed vertical dissipation profile down to the disk inner edge, but the dependence is very weak and thus of minor importance. We conclude that the spectra of optically thick accretion disks around black holes should be calculated with the ray-tracing from the effective photosphere and that, fortunately, the choice of a particular vertical dissipation profile does not substantially influence the calculated spectrum.Comment: 7 pages, 7 figure

SO(2N) and SU(N) gauge theories in 2+1 dimensions

We perform an exploratory investigation of how rapidly the physics of SO(2N) gauge theories approaches its N=oo limit. This question has recently become topical because SO(2N) gauge theories are orbifold equivalent to SU(N) gauge theories, but do not have a finite chemical potential sign problem. We consider only the pure gauge theory and, because of the inconvenient location of the lattice strong-to-weak coupling 'bulk' transition in 3+1 dimensions, we largely confine our numerical calculations to 2+1 dimensions. We discuss analytic expectations in both D=2+1 and D=3+1, show that the SO(6) and SU(4) spectra do indeed appear to be the same, and show that a number of mass ratios do indeed appear to agree in the large-N limit. In particular SO(6) and SU(3) gauge theories are quite similar except for the values of the string tension and coupling, both of which differences can be readily understood.Comment: 27 pages, 9 figure

The correlations and anticorrelations in QPO data

Double peak kHz QPO frequencies in neutron star sources varies in time by a factor of hundreds Hz while in microquasar sources the frequencies are fixed and located at the line \nu_2 = 1.5 \nu_1 in the frequency-frequency plot. The crucial question in the theory of twin HFQPOs is whether or not those observed in neutron-star systems are essentially different from those observed in black holes. In black hole systems the twin HFQPOs are known to be in a 3:2 ratio for each source. At first sight, this seems not to be the case for neutron stars. For each individual neutron star, the upper and lower kHz QPO frequencies, \nu_2 and \nu_1, are linearly correlated, \nu_2=A \nu_1 + B, with the slope A < 1.5, i.e., the frequencies definitely are not in a 1.5 ratio. In this contribution we show that when considered jointly on a frequency-frequency plot, the data for the twin kHz QPO frequencies in several (as opposed to one) neutron stars uniquely pick out a certain preferred frequency ratio that is equal to 1.5 for the six sources examined so far.Comment: 3 pages, 1 figure, Astronomische Nachrichten, in pres

Casimir scaling of domain wall tensions in the deconfined phase of D=3+1 SU(N) gauge theories

We perform lattice calculations of the spatial 't Hooft k-string tensions in the deconfined phase of SU(N) gauge theories for N=2,3,4,6. These equal (up to a factor of T) the surface tensions of the domain walls between the corresponding (Euclidean) deconfined phases. For T much larger than Tc our results match on to the known perturbative result, which exhibits Casimir Scaling, being proportional to k(N-k). At lower T the coupling becomes stronger and, not surprisingly, our calculations show large deviations from the perturbative T-dependence. Despite this we find that the behaviour proportional to k(N-k) persists very accurately down to temperatures very close to Tc. Thus the Casimir Scaling of the 't Hooft tension appears to be a `universal' feature that is more general than its appearance in the low order high-T perturbative calculation. We observe the `wetting' of these k-walls at T around Tc and the (almost inevitable) `perfect wetting' of the k=N/2 domain wall. Our calculations show that as T tends to Tc the magnitude of the spatial `t Hooft string tension decreases rapidly. This suggests the existence of a (would-be) 't Hooft string condensation transition at some temperature which is close to but below Tc. We speculate on the `dual' relationship between this and the (would-be) confining string condensation at the Hagedorn temperature that is close to but above Tc.Comment: 40 pages, 14 figure