Pulsar scintillation as a physical tool

1992 ◽  
Vol 341 (1660) ◽  
pp. 167-176 ◽  
Keyword(s):  
Spatial Coherence ◽  
Line Of Sight ◽  
Interstellar Gas ◽  
Pulsar Emission ◽  
Pulsar Radiation ◽  
Wide Range ◽  
Random Modulation ◽  
Radio Frequency Spectrum ◽  
The Galaxy ◽  
Diffraction Patterns

The interstellar gas contains irregularities of electron density having a wide range of physical scales. Pulsar radiation propagating through this inhomogeneous medium suffers a random modulation of phase which causes the received intensity to scintillate on a variety of timescales. Observations of the radio frequency spectrum and temporal variation of scintillation give information on the form of the irregularity spectrum and the distribution of density structure across the Galaxy. The high spatial coherence of pulsar radiation leads to the formation of extremely fine-scale diffraction patterns which also provide information on the motion of sources across the line of sight and the size of pulsar emission regions. Some uses of scintillation as a means of probing the interstellar gas and elucidating the physical properties of pulsars will be discussed.

scholarly journals Observations of interstellar Lyman-α absorption

1970 ◽  
Vol 36 ◽  
pp. 281-301 ◽  
Author(s):  
Edward B. Jenkins
Keyword(s):  
Neutral Hydrogen ◽  
Local Region ◽  
Line Of Sight ◽  
Small Scale ◽  
Line Broadening ◽  
Interstellar Gas ◽  
B Stars ◽  
Hydrogen Density ◽  
The Galaxy ◽  
To Come

Absorption at the Lyman-α transition from interstellar neutral hydrogen has been observed in the ultraviolet spectra of 18 nearby O and B stars. Radiation damping is the dominant cause of line broadening, which makes the derived line-of-sight column densities proportional to the square of the observed equivalent widths. An average hydrogen density on the order of 0.1 atom cm−3 has been found for most of the stars observed so far. This is in contrast to the findings from surveys of 21-cm radio emission, which suggest 0.7 atom cm−3 exists in the local region of the Galaxy. Several effects which might introduce uncertainties into the Lyman-α measurements are considered, but none seems to be able to produce enough error to explain the disagreement with the 21-cm data. The possibility that small-scale irregularities in the interstellar gas could give significantly lower values at Lyman-α is explored. However, a quantitative treatment of the factor of ten discrepancy in Orion indicates the only reasonable explanation requires the 21-cm flux to come primarily from small, dense, hot clouds which are well separated from each other. The existence of such clouds, however, poses serious theoretical difficulties.

scholarly journals Exploring the central molecular zone of the Galaxy using spectroscopy of H 3 + and CO

2012 ◽  
Vol 370 (1978) ◽  
pp. 5151-5161 ◽  
Author(s):  
T. R. Geballe
Keyword(s):  
Large Fraction ◽  
Level Structure ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Radiative Properties ◽  
Molecular Gas ◽  
Interstellar Gas ◽  
Energy Level Structure ◽  
Wide Range ◽  
The Galaxy

The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of and CO is a powerful way to determine the basic properties of molecular interstellar gas, because the abundance ratio of to CO in ‘dense’ clouds is quite different from that in ‘diffuse’ clouds. Moreover, the energy-level structure and the radiative properties of combined with the unusually warm temperatures of molecular gas in the CMZ make a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200 km s −1 and that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy.

scholarly journals Visibility of Pulsar Emission: Motion of the Visible Point

2014 ◽  
Vol 31 ◽  
Author(s):  
R. Yuen ◽  
D. B. Melrose
Keyword(s):  
Field Line ◽  
Vector Model ◽  
Line Of Sight ◽  
Dipolar Field ◽  
Pulsar Emission ◽  
Pulsar Radio ◽  
Wide Range ◽  
Recent Claim ◽  
Rotational Phase ◽  
Pulsar Radio Emission

AbstractA standard model for the visibility of pulsar radio emission is based on the assumption that the emission is confined to a narrow cone about the tangent to a dipolar field line. The widely accepted rotating vector model (RVM) is an approximation in which the line of sight is fixed and the field line is not strictly tangent to it. We refer to an exact treatment (Gangadhara, 2004) as the tangent model. In the tangent model (but not in the RVM) the visible point changes as a function of pulsar rotational phase, ψ, defining a trajectory on a sphere of radius r. We solve for the trajectory and for the angular velocity of the visible point around it. We note the recent claim that this motion is observable using interstellar holography (Pen et al., 2014). We estimate the error introduced by use of the RVM and find that it is significant for pulsars with emission over a wide range of ψ. The RVM tends to underestimate the range of ψ over which emission is visible. We suggest that the geometry alone strongly favors the visible pulsar radio being emitted at a heights more than ten percent of the light-cylinder distance, where our neglect of retardation effects becomes significant.

scholarly journals Interstellar sodium and Galactic structure. A high-resolution survey

1985 ◽  
Vol 106 ◽  
pp. 325-328
Author(s):  
E. Maurice ◽  
A. Ardeberg ◽  
H. Lindgren
Keyword(s):  
High Resolution ◽  
Spectral Resolution ◽  
Absorption Lines ◽  
High Spectral Resolution ◽  
Line Of Sight ◽  
Substantial Part ◽  
Galactic Rotation ◽  
Interstellar Gas ◽  
Interstellar Clouds ◽  
The Galaxy

Observation of absorption lines produced by interstellar gas is a straight-forward way to determine column densities and velocities along the line of sight of interstellar clouds. In practice, peculiar motions often mask galactic rotation and/or cause line blending. We have made a study of absorption lines of interstellar sodium covering a substantial part of the Galaxy at extremely high spectral resolution.

towards and within the Galactic centre

2006 ◽  
Vol 364 (1848) ◽  
pp. 3035-3042 ◽  
Author(s):  
T.R Geballe
Keyword(s):  
Rotational Level ◽  
Ionization Rate ◽  
High Resolution Spectroscopy ◽  
Line Of Sight ◽  
Galactic Centre ◽  
Long Line ◽  
Wide Range ◽  
The Galaxy ◽  
Infrared Sources ◽  
Diffuse Clouds

High-resolution spectroscopy of bright infrared sources in the centre of the Galaxy has resulted in the detection of in a remarkable array of dense and diffuse clouds along the 8000 parsec long line of sight, at a wide range of distances from the centre. Most prominent among these is a previously undetected, but very large amount of warm ( T ∼250 K) and diffuse ( n ∼100 cm 2 ) gas within a few hundred parsecs of the centre. The key to understanding the environment of the in this region is an absorption line at 3.53 μm from the metastable (3,3) rotational level, which has not been detected in dense or diffuse clouds outside of the Galactic centre (GC). We have used spectroscopy of this line along with other lines of and CO to characterize all of the clouds along the line of sight to the GC. The high abundance of in the central few hundred parsecs implies an ionization rate there that is several times larger than estimated for diffuse clouds outside the GC, and nearly two orders of magnitude greater than originally predicted for diffuse clouds.

Precise method for measuring spatial coherence in TEM beams using Airy diffraction patterns

Microscopy ◽  
2017 ◽  
Vol 67 (1) ◽  
pp. 1-10
Author(s):  
Jun Yamasaki ◽  
Yuki Shimaoka ◽  
Hirokazu Sasaki
Keyword(s):  
Coherence Length ◽  
Spatial Coherence ◽  
Beam Diameter ◽  
Remarkable Feature ◽  
Aperture Diameter ◽  
Point Spread ◽  
Transmission Electron ◽  
Wide Range ◽  
Electron Microscopes ◽  
Diffraction Patterns

Abstract We have developed a method to precisely measure spatial coherence in electron beams. The method does not require an electron biprism and can be implemented in existing analytical transmission electron microscopes equipped with a post-column energy filter. By fitting the Airy diffraction pattern of the selector aperture, various parameters such as geometric aberrations of the lens system and the point-spread function of the diffraction blurring are precisely determined. From the measurements of various beam diameters, components that are attributed to the partial spatial coherence are successfully separated from the point-spread functions. A linear relationship between the spatial coherence length and beam diameter is revealed, thus indicating that a wide range of coherence lengths can be determined by our proposed method as long as the coherence length remains >80% of the aperture diameter. A remarkable feature of this method is its ability to simultaneously determine diffraction blurring and lens aberrations. Possible applications of this method are also discussed.

Interstellar gas in the central region of the Galaxy

1967 ◽  
Vol 31 ◽  
pp. 239-251 ◽  
Author(s):  
F. J. Kerr
Keyword(s):  
Central Region ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Interstellar Gas ◽  
Hydrogen Recombination ◽  
The Galaxy ◽  
Centre Region ◽  
The Relationship

A review is given of information on the galactic-centre region obtained from recent observations of the 21-cm line from neutral hydrogen, the 18-cm group of OH lines, a hydrogen recombination line at 6 cm wavelength, and the continuum emission from ionized hydrogen.Both inward and outward motions are important in this region, in addition to rotation. Several types of observation indicate the presence of material in features inclined to the galactic plane. The relationship between the H and OH concentrations is not yet clear, but a rough picture of the central region can be proposed.

A New Detector System For The HB5 Stem Instrument

1985 ◽  
Vol 43 ◽  
pp. 134-135
Author(s):  
J.M. Cowley
Keyword(s):  
Dark Field ◽  
Detector System ◽  
Electron Holography ◽  
Small Specimen ◽  
Bright Field ◽  
Multiple Images ◽  
Practical Applications ◽  
Wide Range ◽  
Diffraction Patterns ◽  
Operational Modes

The HB5 STEM instrument at ASU has been modified previously to include an efficient two-dimensional detector incorporating an optical analyser device and also a digital system for the recording of multiple images. The detector system was built to explore a wide range of possibilities including in-line electron holography, the observation and recording of diffraction patterns from very small specimen regions (having diameters as small as 3Å) and the formation of both bright field and dark field images by detection of various portions of the diffraction pattern. Experience in the use of this system has shown that sane of its capabilities are unique and valuable. For other purposes it appears that, while the principles of the operational modes may be verified, the practical applications are limited by the details of the initial design.

scholarly journals Line-of-Sight Winds and Doppler Effect Smearing in ACE-FTS Solar Occultation Measurements

Atmosphere ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 680
Author(s):  
Chris D. Boone ◽  
Johnathan Steffen ◽  
Jeff Crouse ◽  
Peter F. Bernath
Keyword(s):  
Atmospheric Chemistry ◽  
Doppler Effect ◽  
Line Of Sight ◽  
Measured Signal ◽  
Mixing Ratio ◽  
Model Calculations ◽  
Doppler Shifts ◽  
Solar Occultation ◽  
Wide Range ◽  
Wind Profiles

Line-of-sight wind profiles are derived from Doppler shifts in infrared solar occultation measurements from the Atmospheric Chemistry Experiment Fourier transform spectrometers (ACE-FTS), the primary instrument on SCISAT, a satellite-based mission for monitoring the Earth’s atmosphere. Comparisons suggest a possible eastward bias from 20 m/s to 30 m/s in ACE-FTS results above 80 km relative to some datasets but no persistent bias relative to other datasets. For instruments operating in a limb geometry, looking through a wide range of altitudes, smearing of the Doppler effect along the line of sight can impact the measured signal, particularly for saturated absorption lines. Implications of Doppler effect smearing are investigated for forward model calculations and volume mixing ratio retrievals. Effects are generally small enough to be safely ignored, except for molecules having a large overhang in their volume mixing ratio profile, such as carbon monoxide.

scholarly journals Suppressing meta-holographic artifacts by laser coherence tuning

2021 ◽  
Vol 10 (1) ◽  
Author(s):  
Yaniv Eliezer ◽  
Geyang Qu ◽  
Wenhong Yang ◽  
Yujie Wang ◽  
Hasan Yılmaz ◽  
...  
Keyword(s):  
Spatial Coherence ◽  
Fine Tuning ◽  
Total Power ◽  
Image Sharpness ◽  
Continuous Tuning ◽  
Fine Spatial Resolution ◽  
Compact Size ◽  
Wide Range ◽  
Fabrication Defects ◽  
Holographic Displays

AbstractA metasurface hologram combines fine spatial resolution and large viewing angles with a planar form factor and compact size. However, it suffers coherent artifacts originating from electromagnetic cross-talk between closely packed meta-atoms and fabrication defects of nanoscale features. Here, we introduce an efficient method to suppress all artifacts by fine-tuning the spatial coherence of illumination. Our method is implemented with a degenerate cavity laser, which allows a precise and continuous tuning of the spatial coherence over a wide range, with little variation in the emission spectrum and total power. We find the optimal degree of spatial coherence to suppress the coherent artifacts of a meta-hologram while maintaining the image sharpness. This work paves the way to compact and dynamical holographic displays free of coherent defects.

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