72,497 research outputs found
Evolution of black-hole intermediate-mass X-ray binaries: the influence of a circumbinary disc
Justham, Rappaport & Podsiadlowski (2006) recently suggested that black-hole
low-mass X-ray binaries (BHLMXBs) with short orbital periods may have evolved
from black-hole intermediate-mass X-ray binaries (BHIMXBs). In their model the
secondaries in BHIMXBs are assumed to possess anomalously high magnetic fields,
so that magnetic braking can lead to substantial loss of angular momentum. In
this paper we propose an alternative mechanism for orbital angular momentum
loss in BHIMXBs. We assume that a small fraction of the transferred
mass from the donor star form a circumbinary disc surrounding the binary
system. The tidal torques exerted by the disc can effectively drain orbital
angular momentum from the binary. We have numerically calculated the
evolutionary sequences of BHIMXBs, to examine the influence of the circumbinary
disc on the binary evolution. Our results indicate when \delta\la 0.01-0.1
(depending on the initial orbital periods), the circumbinary disc can cause
secular orbital shrinking, leading to the formation of compact
BHLMXBs, otherwise the orbits always expand during the evolution. This
scenario also suggests the possible existence of luminous, persistent BHLMXBs,
but it suffers the same problem as in Justham, Rappaport & Podsiadlowski (2006)
that, the predicted effective temperatures of the donor stars are significantly
higher than those of the observed donor stars in BHLMXBs.Comment: 7 pages, 5 figures, accepted for publication in MNRA
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The demands of improving energy efficiency for high performance scientific applications arise crucially nowadays. Software-controlled hardware solutions directed by Dynamic Voltage and Frequency Scaling (DVFS) have shown their effectiveness extensively. Although DVFS is beneficial to green computing, introducing DVFS itself can incur non-negligible overhead, if there exist a large number of frequency switches issued by DVFS. In this paper, we propose a strategy to achieve the optimal energy savings for distributed matrix multiplication via algorithmically trading more computation and communication at a time adaptively with user-specified memory costs for less DVFS switches, which saves 7.5% more energy on average than a classic strategy. Moreover, we leverage a high performance communication scheme for fully exploiting network bandwidth via pipeline broadcast. Overall, the integrated approach achieves substantial energy savings (up to 51.4%) and performance gain (28.6% on average) compared to ScaLAPACK pdgemm() on a cluster with an Ethernet switch, and outperforms ScaLAPACK and DPLASMA pdgemm() respectively by 33.3% and 32.7% on average on a cluster with an Infiniband switch
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