19,201 research outputs found
Helicopter low-speed yaw control
A system for improving yaw control at low speeds consists of one strake placed on the upper portion of the fuselage facing the retreating rotor blade and another strake placed on the lower portion of the fuselage facing the advancing rotor blade. These strakes spoil the airflow on the helicopter tail boom during hover, low speed flight, and right or left sidewards flight so that less side thrust is required from the tail rotor
Conflict between background matching and social signalling in a colour-changing freshwater fish
The ability to change coloration allows animals to modify their patterning to suit a specific function. Many freshwater fishes, for example, can appear cryptic by altering the dispersion of melanin pigment in the skin to match the visual background. However, melanin-based pigments are also used to signal dominance among competing males; thus colour change for background matching may conflict with colour change for social status signalling. We used a colour-changing freshwater fish to investigate whether colour change for background matching influenced aggressive interactions between rival males. Subordinate males that had recently darkened their skin for background matching received heightened aggression from dominant males, relative to males whose coloration had not changed. We then determined whether the social status of a rival male, the focal male's previous social status, and his previous skin coloration, affected a male's ability to change colour for background matching. Social status influenced skin darkening in the first social encounter, with dominant males darkening more than subordinate males, but there was no effect of social status on colour change in the second social encounter. We also found that the extent of skin colour change (by both dominant and subordinate males) was dependent on previous skin coloration, with dark males displaying a smaller change in coloration than pale males. Our findings suggest that skin darkening for background matching imposes a significant social cost on subordinate males in terms of increased aggression. We also suggest that the use of melanin-based signals during social encounters can impede subsequent changes in skin coloration for other functions, such as skin darkening for background matching
Testing the binary hypothesis: pulsar timing constraints on supermassive black hole binary candidates
The advent of time domain astronomy is revolutionizing our understanding of
the Universe. Programs such as the Catalina Real-time Transient Survey (CRTS)
or the Palomar Transient Factory (PTF) surveyed millions of objects for several
years, allowing variability studies on large statistical samples. The
inspection of 250k quasars in CRTS resulted in a catalogue of 111
potentially periodic sources, put forward as supermassive black hole binary
(SMBHB) candidates. A similar investigation on PTF data yielded 33 candidates
from a sample of 35k quasars. Working under the SMBHB hypothesis, we
compute the implied SMBHB merger rate and we use it to construct the expected
gravitational wave background (GWB) at nano-Hz frequencies, probed by pulsar
timing arrays (PTAs). After correcting for incompleteness and assuming virial
mass estimates, we find that the GWB implied by the CRTS sample exceeds the
current most stringent PTA upper limits by almost an order of magnitude. After
further correcting for the implicit bias in virial mass measurements, the
implied GWB drops significantly but is still in tension with the most stringent
PTA upper limits. Similar results hold for the PTF sample. Bayesian model
selection shows that the null hypothesis (whereby the candidates are false
positives) is preferred over the binary hypothesis at about and
for the CRTS and PTF samples respectively. Although not decisive,
our analysis highlights the potential of PTAs as astrophysical probes of
individual SMBHB candidates and indicates that the CRTS and PTF samples are
likely contaminated by several false positives.Comment: 14 pages, 11 figures, 3 tables. Resubmitted to the Astrophysical
Journal after some major revision of the results including a proper estimate
of the intrinsic mass of the binary candidate
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