222 research outputs found
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
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