28,097 research outputs found
Neotropical Bird Migration During The Ice Ages: Orientation And Ecology
Reconstruction of breeding habitat of North American Neotropical migrants 18,000 years ago and 9,000 years ago indicated major shifts in both location and composition of plant communities relative to present conditions. Increased vegetation in xeric areas may have compensated, at least in part, for the reduction in breeding habitat due to glaciation. Autumnal flights of Neotropical passerine migrants flying on constant headings from North America to Central and South America were simulated under present wind conditions and for winds during periods of glaciation at 18,000 and 9,000 years ago. The 155 degrees average headings currently observed for Atlantic migrants were found to function well during periods of glaciation and may have been more generally useful during those times than at present
Comment on "Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars"
In their Letter [Phys. Rev. Lett. 92, 121302 (2004)] (also [Astron.
Astrophys. 417, 853 (2004)]), Srianand et al. analysed optical spectra of
heavy-element species in 23 absorption systems along background quasar
sight-lines, reporting limits on relative variations in the fine-structure
constant: da/a=(-0.06+/-0.06) x 10^{-5}. Here we demonstrate basic flaws in
their analysis, using the same data and absorption profile fits, which led to
spurious values of da/a and significantly underestimated uncertainties. We
conclude that these data and fits offer no stringent test of previous evidence
for a varying alpha.
In their Reply (arXiv:0711.1742) to this Comment, Srianand et al. state or
argue several points regarding their original analysis and our new analysis. We
discuss these points here, dismissing all of them because they are demonstrably
incorrect or because they rely on a flawed application of simple statistical
arguments.Comment: 1+2 pages, 1 EPS figure. Page 1 accepted as PRL Comment on
arXiv:astro-ph/0402177 . Further details available in arXiv:astro-ph/0612407
. v2: Added critical discussion of Reply from Srianand et al.
(arXiv:0711.1742
Keck constraints on a varying fine-structure constant: wavelength calibration errors
The Keck telescope's HIRES spectrograph has previously provided evidence for
a smaller fine-structure constant, alpha, compared to the current laboratory
value, in a sample of 143 quasar absorption systems:
da/a=(-0.57+/-0.11)x10^{-5}. This was based on a variety of metal-ion
transitions which, if alpha varies, experience different relative velocity
shifts. This result is yet to be robustly contradicted, or confirmed, by
measurements on other telescopes and spectrographs; it remains crucial to do
so. It is also important to consider new possible instrumental systematic
effects which may explain the Keck/HIRES results. Griest et al. (2009,
arXiv:0904.4725v1) recently identified distortions in the echelle order
wavelength scales of HIRES with typical amplitudes +/-250m/s. Here we
investigate the effect such distortions may have had on the Keck/HIRES varying
alpha results. We demonstrate that they cause a random effect on da/a from
absorber to absorber because the systems are at different redshifts, placing
the relevant absorption lines at different positions in different echelle
orders. The typical magnitude of the effect on da/a is ~0.4x10^{-5} per
absorber which, compared to the median error on da/a in the sample,
~1.9x10^{-5}, is relatively small. Consequently, the weighted mean value
changes by less than 0.05x10^{-5} if the corrections we calculate are applied.
Nevertheless, we urge caution, particularly for analyses aiming to achieve high
precision da/a measurements on individual systems or small samples, that a much
more detailed understanding of such intra-order distortions and their
dependence on observational parameters is important if they are to be avoided
or modelled reliably. [Abridged]Comment: 9 pages, 2 figures, 2 tables. Invited contribution to Proc. IAU
XXVIIth General Assembly, Joint Discussion 9, "Are the fundamental constants
varying with time?". To appear in P. Molaro, E. Vangioni-Flam, eds, Memorie
della Societa Astronomica Italiana (MmSAIt), Vol. 80. Complete version of
Table 1 available at http://astronomy.swin.edu.au/~mmurphy/pub.htm
A TiO study of the black-hole binary GRO J0422+32 in a very low state
We present 53 simultaneous photometric (I band) and spectroscopic (6900-9500
Angstroms) observations of J0422+32, taken during December 1997. From these we
determine that J0422+32 was in its lowest state yet observed, at
I=20.44+/-0.08. Using relative spectrophotometry, we show that it is possible
to correct very accurately for telluric absorption. Following this, we use the
TiO bands at 7055 Angstroms and 7589 Angstroms for a radial velocity study and
thereby obtain a semi-amplitude of 378+/-16kms-1, which yields
f(M)=1.191+/-0.021M_solar and q=9.0+2.2-2.7, consistent with previous
observations. We further demonstrate that this little explored method is very
powerful for such systems. We also determine a new orbital ephemeris of
HJD=2450274.4156+/-0.0009 + 0.2121600+/-0.0000002E.
We see some evidence for an ellipsoidal modulation, from which we determine
the orbital inclination of J0422+32 to be less than 45 degrees. We therefore
calculate a minimum mass for the primary of 2.22M_solar, consistent with a
black hole, but not necessarily the super-massive one proposed by Beekman et al
(1997). We obtain an M4-5 spectral type for the secondary star and determine
that the secondary contributes 38+/-2% of the flux that we observe from
J0422+32 over the range 6950-8400 Angstroms. From this we calculate the
distance to the system to be 1.39+/-0.15kpc.Comment: (1) Department of Physics, Keele University, Keele, Staffordshire,
ST5 5BG (2) Department of Astrophysics, Nuclear Physics Laboratory, Keble
Road, Oxfo rd, OX1 3RH Accepted, to appear in MNRAS 8 pages, 5 figure
Threshold detection in an on-off binary communications channel with atmospheric scintillation
The optimum detection threshold in an on-off binary optical communications system operating in the presence of atmospheric turbulence was investigated assuming a poisson detection process and log normal scintillation. The dependence of the probability of bit error on log amplitude variance and received signal strength was analyzed and semi-emperical relationships to predict the optimum detection threshold derived. On the basis of this analysis a piecewise linear model for an adaptive threshold detection system is presented. Bit error probabilities for non-optimum threshold detection system were also investigated
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