High-latitude Emission Research Articles

Afterglow Emission from Naked Gamma-Ray Bursts

We calculate the afterglow emission for gamma-ray bursts (GRBs) going off in an extremely low density medium, referred to as naked bursts. Our results also apply to the case where the external medium density falls off sharply at some distance from the burst. The observed afterglow flux in this case originates at high latitudes, i.e., where the angle between the fluid velocity and the observer line of sight is greater than Γ-1. The observed peak frequency of the spectrum for naked bursts decreases with observer time as t-1, and the flux at the peak of the spectrum falls off as t-2. The 2-10 keV X-ray flux from a naked burst of average fluence should be observable by the Swift satellite for time duration of about 103 longer than the burst variability timescale. The high-latitude emission contributes to the early X-ray afterglow flux for any GRB, not just naked bursts, and can be separated from the shocked interstellar medium emission by their different spectral and temporal properties. Measurements of the high-latitude emission could be used to map the angular structure of GRB-producing shells.

High-Latitude Radio Emission in a Sample of Edge-on Spiral Galaxies

We have mapped 16 edge-on galaxies at 20 cm using the Very Large Array in its C configuration, and a subset of these galaxies in the D configuration at 6 and/or 20 cm, in a search for extended (1 kpc) radio continuum emission above and below the plane. For five galaxies, we could form spectral index, energy, and magnetic field maps (assuming minimum energy). While the galaxies were partly chosen by radio flux density, they span a variety of star formation rates (SFRs), and only six might be considered starburst galaxies. A range of Hubble type and degree of isolation are also represented. The galaxies largely fall on the FIR–radio continuum correlation. They also display a correlation between IR surface brightness and warmth, extending the previously observed relation of Lehnert & Heckman to galaxies with lower star formation rates. We find that all but one galaxy show evidence for nonthermal high-latitude radio continuum emission, suggesting that cosmic-ray (CR) halos are common in star-forming galaxies. Of these, eight galaxies are new detections. The high-latitude emission is seen over a variety of spatial scales and in discrete and/or smooth features. In some cases, discrete features are seen on large scales, suggesting that smooth radio halos may consist, in part, of discrete features combined with low spatial resolution. In general, the discrete features emanate from the disk, but estimates of CR diffusion lengths suggest that diffusion alone is insufficient to transport the particles to the high latitudes seen (>15 kpc in one case). Thus CRs likely diffuse through low-density regions and/or are assisted by other mechanisms (e.g., winds). We searched for correlations between the prevalence of high-latitude radio emission and a number of other properties, including the global SFR, supernova input rate per unit star-forming area, A, and environment, and do not find clear correlations with any of these properties. A subset of the data allows, at best, for only a weak correlation with A. Our one nondetection (NGC 4517), however, occurs at a threshold level consistent with that found by Dahlem, Lisenfeld, & Golla. The lack of a good correlation with star formation indicators could be the result of the different timescales for star formation processes compared with the duration of the radio emission. Correlations with other properties, such as environment, are more difficult to assess. However, a few isolated galaxies display strong radio halos, indicating that an interaction is not necessary to produce the extraplanar emission.

AN ‘EXTENDED SOLAR CYCLE’ AS OBSERVED IN Fe XIV

Investigation of the behavior of coronal intensity above the limb in Fexiv emission (530.3 nm) obtained at the National Solar Observatory at Sacramento Peak over the last 23 years has resulted in the confirmation of a second set of zones of solar activity at high latitudes, separate from the Main Activity Zones (MAZ). Localized high-latitude intensity maxima, which I will call High-latitude Emission Features (HEF), are observed at 0.15 solar radii above the limb throughout the solar cycle. They persist long enough at a given latitude to be visible in long-term (e.g., annual) averages. I identify two types of HEF. Poleward-moving HEF, which may be identified with the ‘Rush to the Poles’ phenomenon seen in polar-crown prominences, were first seen to appear in this investigation near latitude 60° in 1978. In 1979 equatorward-moving HEF branched off from the poleward-moving HEF (which continued on to reach the pole in 1980) at a latitude of 70° to 80°. They evolved approximately parallel to the MAZ. Near solar minimum, these HEF evolved into the MAZ of cycle 22, and the emission continues its path towards the equator, where it should disappear soon. Currently, it is clear that the pattern seen earlier is repeating. The poleward-moving HEF became apparent near the beginning of 1988 near 50° to 60° latitude. The northern poleward-moving HEF reached the pole and disappeared in 1990. The southern poleward-moving HEF moved more slowly, reaching the pole and disappearing in 1991. The equatorward-moving HEF that are the precursors of cycle 23 appeared in 1989 to 1990 and began to move approximately parallel to the MAZ of cycle 22. Based on inferences from previous cycles, we can expect these HEF to continue to the equator, with emission ceasing there near 2009. These recent observations increase the evidence for an ‘extended’ solar cycle that begins every 11 years but lasts for approximately 19–20 years.

Diffuse Gamma Rays of Galactic and Extragalactic Origin: Preliminary Results from EGRET

The all-sky survey in high energy gamma rays (E>30 MeV) carried out by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory provides for the first time an opportunity to examine in detail diffuse gamma-ray emission of extra-galactic origin. The observed diffuse emission at high galactic latitudes is generally assumed to have a galactic component arising from cosmic-ray interactions with the local interstellar gas and radiation, in addition to an isotropic component presumably of extragalactic origin. The galactic component can be estimated from a model of the interstellar medium and cosmic-ray distribution. Since the derived extragalactic spectrum depends very much on the success of our galactic model, the consistency of the galactic diffuse emission model is examined both spectrally and spatially with existing EGRET observations. In conjunction with this model, EGRET observations of the high latitude emission are used to examine the flux and spectrum of the residual extragalactic emission. This residual emission could be either truly diffuse in origin or could arise from accumulated emission from unresolved sources particularly in the light of EGRET observations showing the presence of numerous gamma-ray bright active galactic nuclei.

Neptune radio emission in dipole and multipole magnetic fields

We study Neptune's smooth radio emission in two ways: we simulate the observations and we then consider the radio effects of Neptune's magnetic multipoles. A procedure to deduce the characteristics of radio sources observed by the Planetary Radio Astronomy experiment minimizes limiting assumptions and maximizes use of the data, including quantitative measurement of circular polarization. Study of specific sources simulates time variation of intensity and apparent polarization of their integrated emission over an extended time period. The method is applied to Neptune smooth recurrent emission (SRE). Time series are modeled with both broad and beamed emission patterns, and at two frequencies which exhibit different time variation of polarization. These dipole‐based results are overturned by consideration of more complex models of Neptune's magnetic field. Any smooth emission from the anticipated auroral radio source is weak and briefly observed. Dominant SRE originates in complex fields at midlatitude. Possible SRE source locations overlap that of “high‐latitude” emission (HLE) between +(out) and −(in) quadrupoles. This is the first identification of multipolar magnetic structure with a major source of planetary radio emission.

High-latitude molecular clouds

We report the detection of a large number of molecular clouds at high galactic latitude, a few of which were previously cataloged as dark nebulae. We have mapped a large fraction of the clouds detected at Vertical BarbVertical Bar>30/sup 0/ in order to determine the properties of this class of objects. The clouds are nearby (d-bar100 pc) with typical diameters and masses of about 2 pc and roughly 65 M. Allowing for incompleteness in sky coverage, there are approximately 64/epsilon such clouds withon 100 pc, where epsilon is the fractional completeness of our survey. A large fraction, perhaps a majority, of the clouds are not gravitationally bound. The internal velocity structure implies that many of the clouds are very young with ages < or =10/sup 6/ yr. The clouds are invariably associated with atomic hydrogen clouds, but the molecular column densities appear to dominate in many cases. The mean molecular density of the clouds is roughly 170 cm/sup -3/. The one-dimensional cloud-to-cloud velocity dispersion is 5.7 +- 1.2 km s/sup -1/. The surface density of these clouds projected on the galactic plane is approximately 0.1/epsilon M pc/sup -2/, a small fraction of the H I surface density, but an appreciablemore » fraction of the smoothed surface density of giant molecular clouds. Individual clouds subtend areas from a fraction of a square degree to several square degrees, implying that they may be responsible for at least some of the diffuse high-latitude emission detected by IRAS.« less

Green corona and solar sector structure

Analysis of the green line corona for the interval 1947–1970 suggests the existence of largescale organization of the emission. The green line emission at high northern latitudes (≈ 40°–60°) is correlated with the emission at high southern latitudes 6, 15 and 24 days later, while the low latitude green corona seems to be correlated on both sides of the equator with no time lag. These coronal features are recurrent with a 27-day period at all latitudes between ± 60 °, and we associate these large-scale structures with the solar magnetic sector structure. The high correlation between northern and southern high-latitude emission at 15 days time lag is explained as a signature of a two-sector structure, while four sectors are associated with the 6 and 24 day peaks.

Bright Nebulae Near Concentrations of High-Velocity Gas

Results of a systematic search for bright nebulae that are likely to be emission regions in the vicinity of high-velocity gas, drawing attention to some interesting coincidences that have been found. It is apparent that there is a tendency for many of the high-latitude nebulae to cluster in the vicinity of the high-velocity gas. Some of the most prominent cases of overlap are tabulated. It is emphasized that measurement of the radial velocity of the emission nebulae could confirm the existence of a physical association between high-latitude emission nebulae and high-velocity gas. The fact that Johnson (1972) had used narrow-band filters to find indications of H beta emission in similar high-latitude nebulae suggests that H alpha should readily be detectable with Fabry-Perot interferometry.

An Interpretation of the Distribution of Metre Wavelength Radio Emission

A recent 85 MHz survey of the southern sky had an absolute calibration accuracy and resolution comparable with a number of surveys made for the northern skies. By combining the results of these surveys in both hemispheres a complete sky map has been produced, and in this paper an analysis is made of the distribution of the medium and high latitude emission. A fundamental difficulty encountered is the identification and isolation of the spurs of emission projecting from the galactic plane. Two hypotheses are proposed. The first attributes the spurs to a large-scale feature associated with the galactic core and the remaining emission to a galactic halo. The second postulates the origin of the spurs within the local spiral arm, which is itself considered to contribute significantly to the high latitude background. An upper-limit estimate of the emissivity of the local arm is made from currently available independent data. Using this result a model local arm is proposed, which, together with an isotropic component from beyond the Galaxy and a small additional galactic component, explains the observed distribution.