Small Angular Distances Research Articles

A Cosmic-Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?

Geminga and B0656+14 are the closest pulsars with characteristic ages in the range of 100 kyr to 1 Myr. They both have spin-down powers of the order 3 × 1034 ergs s−1 at present. The winds of these pulsars had most probably powered pulsar wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of the pulsars. Assuming that leptonic particles accelerated by the pulsars were confined in the PWNe and were released into the interstellar medium (ISM) on breakup of the PWNe, we show that, depending on the pulsar parameters, both pulsars make a nonnegligible contribution to the local cosmic ray (CR) positron spectrum, and they may be the main contributors above several GeV. The relatively small angular distance between Geminga and B0656+14 thus implies an anisotropy in the local CR positron flux at these energies. We calculate the contribution of these pulsars to the locally observed CR electron and positron spectra depending on the pulsar birth period and the magnitude of the local CR diffusion coefficient. We further give an estimate of the expected anisotropy in the local CR positron flux. Our calculations show that within the framework of our model, the local CR positron spectrum imposes constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar period at birth, and also the local interstellar diffusion coefficient for CR leptons.

The planet Mercury: Synthesis of resolved images of unknown part in the longitude range 250–290°W

Plans to send orbiter missions to Mercury (e.g., NASA's Messenger and ESA's BepiColombo) have prompted renewed efforts to investigate the surface of Mercury using ground-based remote sensing. While the highest resolution instrumentation optical telescopes (e.g. HST) cannot be used at small angular distances ( < 45 ° ) from the Sun (Mercury's elongation never exceeds 28° seen from Earth), advanced ground-based astronomical techniques and modern processing software can be used to construct resolved images of the poorly known part of Mercury. Our observations of the planet presented here were carried out mainly in April and May, 2002, at evening elongation of the planet, at the Skinakas astrophysical observatory of Heraklion University (Crete, Greece). A synthesis of the acquired images of the hemisphere of Mercury, which was not observed by the Mariner 10 mission (1974–1975), is presented. A double rim basin with an internal diameter of about 1000 km and an external rim about 2000 km is suggested by the data. We present the observational method, the data analysis approach, and the resulting images.

Observations of solar system objects with GAIA

This paper discusses the observability by gaia of solar system objects belonging to the group of near-earth objects, an important issue in the scientific program whose relevance for gaia has been questioned. A simulation based on the most up-to-date distribution of the orbital parameters has been conducted over five years to determine the probability that an object will be in the field of view of the instrument with an apparent brightness larger than the limiting magnitude. It is found (for an albedo of 0.1) that most objects with a diameter larger than 2 km will be observed at least five times by gaia and much more often for larger bodies. The detection survey will be 50% complete for objects of diameter 1 km, while smaller objects (D < 0:5 km) will usually escape detection. Very interestingly, it is shown that most observations will take place at small angular distance from the Sun, a zone where the ground-based surveys are the most biased, allowing to extend the existing surveys of atens and to probe into the virtually unknown population of objects orbiting inside the Earth's orbit. The distribution of the along-scan velocity averages near 40 mas/s (145 arcsec/h), permitting excellent detection during a transit over the focal plane of gaia. gaia scientists therefore will be in a good position to alert a ground-based follow-up program.

The incompatibilities between the standard theory of interstellar extinction and observations

In this paper I review a series of observations which do not agree with the standard interpretation of the extinction curve. The consequence is that light we receive from a reddened star must be contaminated by starlight scattered at very small angular distances from the star. The true extinction curve is a straight line from the near infrared to the far-UV. If so, all interstellar grains models must be questioned. Another conclusion concerns the average properties of interstellar grains which seem much more uniform than previously thought.

On the identification of absorption lines with small red shifts in the spectra of quasistellar objects

The reliability of identifications of narrow absorption lines with small red shifts is analyzed statistically on the basis of literature data on spectra with high spectral resolution for 38 quasi-stellar objects. The identifications were made using a reference spectrum including the most pronounced lines of the interstellar medium. Particular attention is devoted to the case of quasi-stellar objects in the region + or - 10 deg from the supergalactic equator or coincident in position on the celestial sphere with high-latitude high-velocity hydrogen clouds, and also situated at small angular distances from nearby galaxies. It is shown that in the majority of cases identifications with the line pairs of Ca II and Na I are random.

The ability of the Space Telescope to detect extra-solar planetary systems

The Space Telescope can play a key role in searching for and investigating the contents of extra-solar planetary systems. For about 90 nearby stars, positional variations due to major planets would be well within the astrometric capability of the Wide-Field/Planetary Camera system. Since the centroids of star images will be determined to within a milliarcsecond down to 22nd magnitude, there will be an abundance of reference stars at very small angular distances from each planetary system candidate, and they will have small enough motions of their own to provide a reference frame of the stability required.

The origins of the Greenwich observatory

Although the foundation of England's national observatory may be regarded as an inevitable consequence of progressive trends in seventeenth-century astronomy, cartography, and navigation, it was the chance conjunction of two related events which caused it to be established exactly 300 years ago. One was the attempt in 1674 of the Sieur de St. Pierre, a Frenchman at the Court of Charles II, to interest that king in a proposal for a lunar method of finding longitude at sea. The other was the condemnation of the practicability of his method by John Flamsteed on the same ground — the inadequacy of the necessary observational data — that had led a French scientific commission to reject methods of a similar nature put forward by Jean Morin some 40 years earlier. A reconstruction of the method ascribed by Flamsteed to St. Pierre serves to confirm St. Pierre's accusation that Flamsteed supplied him with calculations rather than observations by which to demonstrate his method, appears to be justified inasmuch as Flamsteed converted two of his own Derby observations of the Moon's altitude to corresponding values on arbitrary meridians close to those of Brunswick and Danzig respectively, before presenting these data to the Frenchman as if they had been directly observed at those places. Significantly, the method already being applied by Flamsteed 2 years previously when determining errors in the Moon's celestial position from micrometric measurements of small angular distances from certain bright zodiacal stars for the two dates in question, is the very same as that which he ascribed to St. Pierre.

Elliptic Polarization of Synchrotron Radiation

Although the synchrotron emission from a single electron moving with an ultrarelativistic velocity βc in a magnetic field B 0 is elliptically polarized, the characteristics of the polarization ellipse being determined by the position of the direction of observation n relative to the cone swept out by the direction u of the velocity vector about the direction of B 0, the resultant emission from a distribution of such gyrating electrons is, to a first approximation, linearly polarized in the direction perpendicular to the projection of B 0 on to the plane transverse to n. The reasons for this are that for a single electron the emission is effectively confined to within a small angular distance O(ξ) of u, where ξ = √ (1 −β 2) ≪ 1 and that (i) the fourth Stokes parameter is to this approximation an odd function of the angle ip between n and u at its closest approach, and (ii) the number of electrons passing within equal angular distances ψ = ±O(ξ) are equal. It follows that if ξ is large enough it is possible for the next approximation to the fourth Stokes parameter for a distribution of electrons to be significant, so that the resultant emission is elliptically polarized.

Elliptic Polarization of Synchrotron Radiation

Although the synchrotron emission from a single electron moving with an ultrarelativistic velocity βc in a magnetic field B0 is elliptically polarized, the characteristics of the polarization ellipse being determined by the position of the direction of observation n relative to the cone swept out by the direction u of the velocity vector about the direction of B0, the resultant emission from a distribution of such gyrating electrons is, to a first approximation, linearly polarized in the direction perpendicular to the projection of B0 on to the plane transverse to n. The reasons for this are that for a single electron the emission is effectively confined to within a small angular distance O(ξ) of u, where ξ = √ (1 −β2) ≪ 1 and that (i) the fourth Stokes parameter is to this approximation an odd function of the angle ip between n and u at its closest approach, and (ii) the number of electrons passing within equal angular distances ψ = ±O(ξ) are equal. It follows that if ξ is large enough it is possible for the next approximation to the fourth Stokes parameter for a distribution of electrons to be significant, so that the resultant emission is elliptically polarized.

Occultation of Taurus-A by the Solar Corona at 430 Mc/s in June 1964

DURING June of every year, the radio source Taurus-A passes within a small angular distance of the Sun. This particular situation, commonly referred to as the occultation of Taurus-A by the Sun, permits an investigation of the Sun's outer corona as the radio source is viewed through it. The measurements made over the past several years have demonstrated that the radiation from the source is scattered at the electron density irregularities in the corona; as a result one observes an apparent broadening of the source accompanied by a decrease of its intensity. The effect becomes larger with decreasing angular separation of the source from the Sun. This phenomenon has been observed in the range of frequencies 26–178 Mc/s1–5. At wave-lengths of 6 and 18 cm the effect has been reported to be negative6,7, except that a positive result has been reported on 10- and 25-cm wave-lengths8. The results on all metre and decametre wave-lengths are consistent and are in accordance with the theory of small-angle multiple scattering, in which the angular size of the scattered distribution is proportional to the square of the wave-length. However, the increase of intensity on 169 Mc/s at 7 R0 (R0 is the photospheric radius) from the centre of the Sun is not consistent with the theoretical prediction based on the lower frequency data5. In order to investigate if the coronal scattering is still effective at frequencies higher than 169 Mc/s, and in particular to see if the increase of intensity is also observed on 430 Mc/s, we undertook to observe at 430 Mc/s the occultation of Taurus-A by the solar corona in June 1964, using the 1,000-ft. dish of the Arecibo Ionospheric Observatory.

Quasi-free proton-proton scattering in Li 6 and Li 7

Angular correlation distributions for quasi-free proton-proton scattering in Li 6 and Li 7 have been calculated and compared with experiment. The single particle model of the nucleus was used and the wave functions were adjusted to give electron form factors in agreement with Stanford scattering results. It is found that the best agreement with measured (p, 2p) results is obtained when the p-state protons are moving in a potential of larger extension than that for the s-state protons. Configuration mixing or a different model such as the cluster model would have to be invoked in order to reproduce the small angular distance between the maxima of the Li 6 p-state distribution found experimentally.

A New Estimate of the Strength of the Magnetic Field of the Planet Venus

IN some earlier publications1, attention was directed to the behaviour of the geomagnetic activity around the dates of lower conjunction of Venus. It appeared that, especially at times of weak solar activity and with conjunctions, where Venus passed the Sun at a small angular distance, seen from the Earth, a screening effoct of Venus on the solar corpuscula was found from 6 days before up to 1 day after the date of conjunction. An estimate of the strength of the magnetic field of Venus met with difficulties.

The Scattering of Radio Waves in the Solar Corona

Observations of the radio emission from the Crab nebula during June, when it passes within a small angular distance of the Sun, have shown that the radiation suffers considerable scattering during its passage through the solar corona. In this paper an account is given of measurements carried out each year over the period 1952–1958. The following new results have been obtained: (i) measurements at a wave-length of 7.9 m have suggested the presence of refraction effects, in addition to scattering, which may provide information about the mean electron density in the corona; (ii) extension of the measurements to the shorter wave-length of 1.9 m has enabled the large scattering at the distance of closest approach to be determined more accurately; (iii) a pronounced sunspot-cycle variation has been observed in regions of the corona at a distance of |$8{R}_{\odot};$| and (iv) the scattering has been shown to exhibit anisotropy. This result strengthens the earlier conclusion that the scattering is caused by filamentary irregularities aligned in a magnetic field and suggests that the direction of the field is approximately radial at distances of |$15-20{R}_{\odot}.$|

On the interferometric measurement of small angular distances

The applicability of Young's fringes to the measurement of small angular distances is discussed, and an arrangement is described which gives much brighter images. An account is given of the use of a Mach interferometer for the study of scintillation.

IX. On the problem of the most efficient tests of statistical hypotheses

The problem of testing statistical hypotheses is an old one. Its origin is usually connected with the name of Thomas Bayes, who gave the well-known theorem on the probabilities a posteriori of the possible “causes" of a given event. Since then it has been discussed by many writers of whom we shall here mention two only, Bertrand and Borel, whose differing views serve well to illustrate the point from which we shall approach the subject. Bertrand put into statistical form a variety of hypotheses, as for example the hypothesis that a given group of stars with relatively small angular distances between them as seen from the earth, form a “system” or group in space. His method of attack, which is that in common use, consisted essentially in calculating the probability, P, that a certain character, x , of the observed facts would arise if the hypothesis tested were true. If P were very small, this would generally be considered as an indication that the hypothesis, H, was probably false, and vice versa . Bertrand expressed the pessimistic view that no test of this kind could give reliable results. Borel, however, in a later discussion, considered that the method described could be applied with success provided that the character, x , of the observed facts were properly chosen—were, in fact, a character which he terms “en quelque sorte remarquable.”

The Displacement of Light Rays Passing near the Sun

THE part of the earth's atmosphere within the conical shadow of the moon during a total solar eclipse may be regarded as approximately a right circular cylinder, the area of the base of which depends on the length of the shadow. Observations have shown that there are temperature and pressure gradients in this cylinder. The latter gradient at the surface of the earth is usually slight, but the temperature gradient may be considerable, so that, assuming that there is equilibrium, we have, roughly speaking, a cylinder of air the density of which decreases outwards in all directions perpendicular to its axis. When we remember that the light from stars at small angular distances from the sun's centre makes small angles with the axis of this cylinder, it is easy to see that a very small density gradient would be sufficient to account for the displacements that were observed in the total solar eclipse of the present year.