Kenneth Brecher

Straight lines, 'uncurved lines', and Helmholtz's 'great circles on the celestial sphere'.

Helmholtzs famous pincushioned chessboard figure has been used to make the point that straight lines in the world are not always perceived as straight and, conversely, that curved lines in the world can sometimes be seen as straight. However, there is little agreement as to the cause of these perceptual errors. Some authors have attributed the errors to the shape of the retina, or the amount of cortex devoted to the processing of images falling on different parts of the retina, while others have taken the effects to indicate that visual space itself is curved. Helmholtz himself claimed that the ‘uncurved lines on the visual globe’ corresponded to ‘direction circles’ defined as those arcs described by the line of fixation when the eye moves according to Listings law. Careful re-reading of Helmholtz together with some additional observations lead us to the conclusion that two other factors are also involved in the effect: (i) a lack of information about the distance of peripherally viewed objects and (ii) the preference of the visual system for seeing the pincushion squares as similar in size.

MicroObservatory Net: A Network of Automated Remote Telescopes Dedicated to Educational Use

Many students have a deep interest in astronomy, but a limited opportunity to use telescopes to explore the heavens. The MicroObservatory Network of automated telescopes is designed to provide access to classroom teachers who wish their students to conduct projects over the World Wide Web. The intuitive interface makes it easy for even 10-year-olds to take pictures. Telescopes can be remotely pointed and focused: filters, field of view, and exposure times can be changed easily. Images are archived at the website, along with sample challenges and a user bulletin board, all of which encourage collaboration among schools. Wide geographic separation of instruments provides access to distant night skies during local daytime. Since “first light” in 1995, we have learned much about remote troubleshooting, designing for unattended use, and for acquiring the kinds of images that students desire. This network can be scaled up from its present capability of 240,000 images each year to provide telescope access for all US students with an interest in astronomy. Our WWW address is http://mo-www.harvard.edu/MicroObservatory/

Electron-positron pair production by ultrarelativistic electrons in a soft photon field

The fully differential cross section for photon-electron pair production is integrated numerically over phase space. We obtain results for the astrophysically interesting case in which the interaction between an ultrarelativistic electron and a soft photon results in e/sup + -/ pair production. The positron spectrum is a function of the energies of both the photon and the electron, as well as the angle of interaction. We integrate over solid angle by assuming an isotropic distribution of photons. We find that the energy at which the positron distribution peaks is inversely proportional to the photon energy and independent of the electron energy. The positron spectrum is integrated once more over initial electron energies for a power-law energy distribution of primary electrons. The same procedure is repeated for the recoil particle; it is shown that the peak of the recoil energy distribution depends linearly on the energy of the primary electron. Finally, semianalytical expressions are obtained for the energy losses of the primary electrons.

LEMAITRE UNIVERSE, GALAXY FORMATION, AND OBSERVATIONS.

Lemaitre universe and galaxy formation effect on stability of quasi-static epoch, deriving limits for red shift, cosmic ray and electromagnetic fluxes

Project LITE: light inquiry through experiments

“Project LITE: Light Inquiry Through Experiments” is a science education project aimed at developing interactive hands-on and eyes-on curriculum, software and materials about light and optics. These are being developed for use in undergraduate astronomy courses, but they can also be used to advantage in physics, chemistry, Earth science and psychology courses throughout the K-12 and undergraduate curriculum.

On Compton models of the isotropic X-ray background.

Investigation of the compatibility of recent X-ray background energy flux observations with Compton models of the isotropic X-ray background. It is shown that the spectral shape of the X-ray flux arising from Compton scattering of fast electrons on the microwave background radiation, contrary to a recent assertion, is not inconsistent with recent observations of the isotropic X-ray background.

Search for 511 keV electron-positron annihilation radiation from mildly active galaxies using the HEAO 3 gamma-ray spectrometer

The HEAO 3 spacecrafts gamma-ray spectrometer has been used to obtain upper limits to the 511-keV electron-positron annihilation line flux from seven galaxies: M 81/M 82, NGC 4278, M 104, NGC 6500, NGC 2911, and NGC 262. These appear to contain scaled-up versions of the radio source in the galactic center, and their nonthermal activity appears to be confined to the central regions, implying that energetic particles produced in these nuclei do not escape into the low density regions of interstellar or intergalactic space. On the basis of the scaling law adopted, six of these seven galaxies should have been detected above the upper limits obtained for narrow 511 keV emission. Explanations are adduced which account for the scaling laws failure to predict the correct level of radiation annihilation.

Positron production by a hot, young pulsar

We consider a way of positron production in pulsars; pair production from electron‐photon collisions. We show that this process must be the dominant one for a young pulsar when its surface temperature is around 107 K. We apply this model to the observed positron flux coming from the Galactic Center and we show that/a pulsar with parameters close to those of the Crab at birth can explain, in principle, the flux.

Are mildly active galaxies sources of e± annihilation radiation?

The Galactic Center has been established as a source of 511 keV line radiation resulting from electron-position annihilation. The Galactic Center is also the site of a compact, nonthermal radio source similar to the scaled-down version of those found in the nuclei of active galaxies and quasars. This suggests that there is a mildly active central engine at the Galactic Center, which is capable of producing the high output of positrons required to explain the strength of the 511 keV line. If this is the case, then one would expect other active galaxies to also contain positron sources in their nuclei. Because of possible escape routes for positrons, it was decided that a promising candidate for 511 keV line detection would be any active galaxy whose nonthermal radio activity is contained within the galaxy. Seven galaxies were chosen using this criteria. The HEAO-3 ..gamma.. Ray Spectrometer was operable from 20 September 1979 until June 1980, and none of these galaxies were detected in the 511 keV line. 19 references, 1 table.

Electromagnetic cascades in the magnetosphere of a very young pulsar: a model for the positron production near the galactic center

A detailed model for positron production by a young pulsar is presented. It is shown that electromagnetic cascades can develop in a young pulsars magnetosphere, and the model results are applied to the pulsar which is hypothesized to lie near the Galactic center. It is found that such a pulsar would be expected to produce relatively low energy electron-positron pairs with an efficiency rating high enough to explain the observed luminosity of the Galactic center annihilation line. Virtually all of the gamma ray continuum radiation produced in the cascades would be beamed along the magnetic poles of the neutron star, and therefore probably would not be observed from earth. Some observational predictions generated by the proposed model for the Galactic center positron source are given. 47 references.