scholarly journals Interstellar Broadening of Compact Low Galactic Latitude Radio Sources

1984 ◽  
Vol 110 ◽  
pp. 309-312
Author(s):  
Brian Dennison ◽  
M. Thomas ◽  
J. J. Broderick ◽  
R. S. Booth ◽  
Robert L. Brown ◽  
...  
Keyword(s):  
Interstellar Medium ◽  
Radio Source ◽  
Electron Density ◽  
Galactic Center ◽  
Galactic Disk ◽  
Angular Diameter ◽  
Radio Sources ◽  
Galactic Latitude ◽  
Radio Waves

Scattering of radio waves off inhomogeneities in electron density in the interstellar medium can produce an apparent broadening in the angular diameter of an intrinsically compact background radio source. The magnitude and distribution of this effect at low galactic latitudes (|b|<5°) is not well known, although several cases suggest substantial broadening in certain directions, such as the Cygnus X region (Anderson et al. 1972), and the galactic center (Davies, Walsh, and Booth 1976). Large scattering in the plane is consistent with the scintillation properties of pulsars seen through substantial thicknesses (≳ 1 kpc) of the galactic disk.

scholarly journals The magnetic structure of our Galaxy: a review of observations

2008 ◽  
Vol 4 (S259) ◽  
pp. 455-466 ◽  
Author(s):  
JinLin Han
Keyword(s):  
Magnetic Fields ◽  
Magnetic Structure ◽  
Large Scale ◽  
Galactic Halo ◽  
Galactic Center ◽  
Galactic Plane ◽  
Galactic Disk ◽  
Dust Emission ◽  
Measure Data ◽  
Radio Sources

AbstractThe magnetic structure in the Galactic disk, the Galactic center and the Galactic halo can be delineated more clearly than ever before. In the Galactic disk, the magnetic structure has been revealed by starlight polarization within 2 or 3 kpc of the Solar vicinity, by the distribution of the Zeeman splitting of OH masers in two or three nearby spiral arms, and by pulsar dispersion measures and rotation measures in nearly half of the disk. The polarized thermal dust emission of clouds at infrared, mm and submm wavelengths and the diffuse synchrotron emission are also related to the large-scale magnetic field in the disk. The rotation measures of extragalactic radio sources at low Galactic latitudes can be modeled by electron distributions and large-scale magnetic fields. The statistical properties of the magnetized interstellar medium at various scales have been studied using rotation measure data and polarization data. In the Galactic center, the non-thermal filaments indicate poloidal fields. There is no consensus on the field strength, maybe mG, maybe tens of μG. The polarized dust emission and much enhanced rotation measures of background radio sources are probably related to toroidal fields. In the Galactic halo, the antisymmetric RM sky reveals large-scale toroidal fields with reversed directions above and below the Galactic plane. Magnetic fields from all parts of our Galaxy are connected to form a global field structure. More observations are needed to explore the untouched regions and delineate how fields in different parts are connected.

scholarly journals The Compact Nonthermal Radio Source at the Galactic Center: an Update

1989 ◽  
Vol 136 ◽  
pp. 527-534
Author(s):  
K. Y. Lo
Keyword(s):  
Radio Source ◽  
Galactic Center ◽  
Central Star ◽  
Radio Sources ◽  
Stellar Objects ◽  
Nonthermal Radio ◽  
Compact Radio Source ◽  
The Galaxy ◽  
Observational Status ◽  
Sgr A

We review the current observational status of Sgr A∗, the compact nonthermal radio source at the galactic center. Sgr A∗ is a unique radio source at a unique location of the Galaxy. It is unlike any compact radio source associated with known stellar objects, but it is similar to extragalactic nuclear compact radio sources. The positional offset between Sgr A∗ and IRS16 places little constraint on the nature of the underlying energy source, since IRS16 need not be the core of the central star cluster. Sgr A∗ is still the best candidate for marking the location of a massive collapsed object.

scholarly journals Understanding our Galaxy: from the center to outskirts

2007 ◽  
Vol 3 (S248) ◽  
pp. 470-473
Author(s):  
Z. Q. Shen ◽  
Y. Xu ◽  
J. L. Han ◽  
X. W. Zheng
Keyword(s):  
Magnetic Fields ◽  
Radio Source ◽  
Large Scale ◽  
Milky Way ◽  
Galactic Center ◽  
Galactic Disk ◽  
Spiral Arms ◽  
Compact Radio Source ◽  
Sgr A ◽  
Spiral Arm

AbstractWe describe the efforts to understand our Milky Way Galaxy, from its center to outskirts, including (1) the measurements of the intrinsic size of the galactic center compact radio source Sgr A*; (2) the determination of the distance from the Sun to the Perseus spiral arm; and (3) the revealing of large scale global magnetic fields of the Galaxy.With high-resolution millimeter-VLBI observations, Shen et al. (2005) have measured the intrinsic size of the radio-emitting region of the galactic center compact radio source Sgr A* to be only 1 AU in diameter at 3.5 mm. When combined with the lower limit on the mass of Sgr A*, this provides strong evidence for Sgr A* being a super-massive black hole. Comparison with the intrinsic size detection at 7 mm indicates a frequency-dependent source size, posing a tight constraint on various theoretical models.With VLBI phase referencing observations, Xu et al. (2006) have measured the trigonometric parallax of W3OH in the Perseus spiral arm with an accuracy of 10 μas and also its absolute velocity with an accuracy of 1 km s−1. This demonstrates the capability of probing the structure and kinematics of the Milky Way by determining distances to 12 GHz methanol (CH3OH) masers in star forming regions of distant spiral arms and Milky Way's outskirts.With pulsar dispersion measures and rotation measures, Han et al. (2006) can directly measure the magnetic fields in a very large region of the Galactic disk. The results show that the large-scale magnetic fields are aligned with the spiral arms but reverse their directions many times from the most inner Norma arm to the outer Perseus arm.

scholarly journals Large scale magnetic fields of our Galaxy

2009 ◽  
Vol 5 (H15) ◽  
pp. 450-451
Author(s):  
JinLin Han
Keyword(s):  
Star Formation ◽  
Magnetic Fields ◽  
Large Scale ◽  
Galactic Halo ◽  
Galactic Center ◽  
Galactic Plane ◽  
Galactic Disk ◽  
Zeeman Splitting ◽  
Radio Sources ◽  
Star Formation Regions

AbstractLarge-scale magnetic fields in the Galactic disk have been revealed by distributions of pulsar rotation measures (RMs) and Zeeman splitting data of masers in star formation regions, which have several reversals in arm and interarm regions. Magnetic fields in the Galactic halo are reflected by the antisymmetric sky distribution of RMs of extragalactic radio sources, which have azimuthal structure with reversed directions below and above the Galactic plane. Large-scale magnetic fields in the Galactic center probably have a poloidal and toroidal structure.

scholarly journals Herschel/HIFI Discoveryof a Far-Infrared DIB Analog

2013 ◽  
Vol 9 (S297) ◽  
pp. 197-202
Author(s):  
H. S. P. Müller ◽  
P. Schilke ◽  
M. Gerin ◽  
D. C. Lis ◽  
E. A. Bergin ◽  
...  
Keyword(s):  
Interstellar Medium ◽  
Molecular Clouds ◽  
Galactic Center ◽  
Galactic Disk ◽  
Far Infrared ◽  
Line Of Sight ◽  
Absorption Feature ◽  
Substantial Part ◽  
Star Forming ◽  
Star Forming Regions

AbstractThe Herschel Space Observatory carried out observations at far-infrared wavelengths, which significantly increased our knowledge of the interstellar medium and the star-formation process in the Milky Way and external galaxies, as well as our understanding of astrochemistry.Absorption features, known, e.g., from observations at millimeter wavelengths, are more commonly observed in the far-infrared, in particular toward strong dust continuum sources. The lowest energy transitions are not only observed at LSR-velocities related to the source, but often also at velocities associated with diffuse molecular clouds along the line of sight toward the background source.Unbiased spectral line surveys of the massive and very luminous Galactic Center sources Sagittarius B2(M) and (N) were carried out across the entire frequency range of the high-resolution Heterodyne Instrument for Far-Infrared Astronomy (HIFI). An absorption feature was detected toward both sources at about 617.531 GHz, corresponding to 20.599 cm−1, 485.47 μm, or 2.5539 meV. This feature is unique in its appearance at all velocity components associated with diffuse foreground molecular clouds, together with its conspicuous absence at velocities related to the sources themselves. The carriers of at least a substantial part of the DIBs are thought to reside in the diffuse interstellar medium. Therefore, we consider this absorption feature to be a far-infrared DIB analog.Subsequent dedicated observations confirmed that the line is present only in the foreground clouds on the line of sight toward other massive star-forming regions in the Galactic disk. There is indication that the feature has substructure, possibly of fine or hyperfine nature. Attempts to assign the feature to atomic or molecular species have been unsuccessful so far.

scholarly journals High-Precision NBPP Timing Measurements at Nançay

2000 ◽  
Vol 177 ◽  
pp. 55-56
Author(s):  
I. Cognard ◽  
J.-F. Lestrade ◽  
D.C. Backer ◽  
P.S. Ray ◽  
R.S. Foster ◽  
...  
Keyword(s):  
Interstellar Medium ◽  
Real Time ◽  
Flux Density ◽  
Electron Density ◽  
San Francisco ◽  
High Precision ◽  
Line Of Sight ◽  
Radio Sources ◽  
Search Data ◽  
Scattering Events

The Nançay radiotelescope in France is a large collecting area (7000m2) with receivers around 1.4, 1.7 and 3.5GHz. At this observatory, we are conducting frequent high-precision timing observations of 5 millisecond pulsars (PSR B1937+21, B1821-24 since 1988; J1643-1224, J1713+0747 and B1620-26 since 1996) with a swept frequency oscilllator based on a DDS as a dedisperser.The most interesting result from these dense series of observations is the detection of several Extreme Scattering Events in direction of B1937+21 and possibly in direction of B1821-24 (Cognard, 1993,Nature,366, 320; Cognard &amp; Lestrade, 1996, in ASP Conf. Ser. Vol 105, Pulsars: Problems and Progress (San Francisco: ASP), 469; Lestrade, Rickett &amp; Cognard 1998, A&amp;A,334, 1068). The flux density variations and TOA fluctuations observed have been used to estimate the size (several AU) and electron density (a few hundreds electrons cm−3) of the discrete ionised clouds localized in the interstellar medium that are thought to be responsible for this phenomena. The number of events recorded at Nançay in direction of B1937+21 yields the space density 105– 106pc−3for these clouds. This density is very large and is about 100 times higher than the density estimated from Extreme Scattering Events observed in direction of extragalactic radio sources (Fiedler et al., 1994,ApJ,430, 581). This might mean that the line of sight to B1937+21 is peculiar. This is being investigated with additional observations of a larger array of pulsars at Nançay with the Navy-Berkeley-Pulsar Processor (Figure 1) This processor NBPP (Foster et al., in ASP Conf. Ser. Vol 105, Pulsars: Problems and Progress (San Francisco: ASP), 25) has been used to acquire pulsar search data for 2 years at Nancay and we are now using its real-time folding capability.

scholarly journals Interstellar Scattering

1984 ◽  
Vol 110 ◽  
pp. 303-307
Author(s):  
James M. Cordes
Keyword(s):  
Interstellar Medium ◽  
Electron Density ◽  
Angular Size ◽  
Density Fluctuations ◽  
Length Scale ◽  
Radio Sources ◽  
Fine Scale ◽  
Interstellar Scattering ◽  
Electron Density Fluctuations ◽  
Temporal Broadening

Fine scale electron density fluctuations in the interstellar medium (ISM) are manifest as scintillations and temporal broadening of pulsar signals and as angular broadening of galactic and extragalactic sources. Although scattering off the fluctuations is often a nuisance for conventional studies of radio sources, analysis or searches for interstellar scintillations (ISS) can lead to information about the ISM or radio sources that otherwise would not be obtainable. For example, the length scale probed by ISS in the ISM is typically 1011 cm and the characteristic angular size for quenching ISS is less than 1 μarc sec.

Multibaseline Observations of the Occultation of Crab Nebula by the Solar Corona at Decameter Wavelengths

2000 ◽  
Vol 179 ◽  
pp. 421-422
Author(s):  
K. R. Subramanian
Keyword(s):  
Refractive Index ◽  
Solar Corona ◽  
Radio Source ◽  
Crab Nebula ◽  
Angular Size ◽  
Radio Sources ◽  
Radio Waves ◽  
Apparent Increase ◽  
Pass Through ◽  
Point To Point

Extended abstractInformation about the outer solar corona can be obtained by observing the occultation of radio sources by the solar corona. As the radio waves pass through the corona they get scattered due to the fact that the electron density and consequently the refractive index varies from point to point. The effect of scattering is manifested by an apparent increase in the angular size of the radio source which can be measured by suitable interferometers. We present here multibaseline observations on the occultation of Crab Nebula at 34.5 MHz with baselines extending upto 4.9 km during June 1986 and 1987.

scholarly journals G359.3–0.82: An Unusual Radio Source

1989 ◽  
Vol 136 ◽  
pp. 197-204
Author(s):  
F. Yusef-Zadeh ◽  
F. Yusef-Zadeh
Keyword(s):  
Radio Emission ◽  
Interstellar Medium ◽  
Radio Source ◽  
Relative Velocity ◽  
Galactic Center ◽  
Extragalactic Radio Source ◽  
Compact Source ◽  
Nonthermal Radio ◽  
Galactic Object

An unusual radio source exhibiting an axisymmetric, cometary morphology was recently reported by Yusef-Zadeh and Bally (1987) near the Galactic center. This source, G359.3–0.82, consists of a bright head containing a compact source followed by a tail exhibiting sinuous structure. Radio emission is highly polarized and has a nearly flat spectrum between λ6cm and λ20cm. Its location in the sky, spectrum, and lack of resemblance to any other extragalactic radio source suggested to us that this radio source is a Galactic object possibly lying near the Galactic center. New high-reslolution radio images obtained using the VLA confirm the remarkable morphology and strengthen the distinction between G359.3–0.82 and any known extragalactic radio source. The characteristics of G359.3–0.82 suggest that it may be a nonthermal radio wake produced by an object moving through the interstellar medium at a high relative velocity.

scholarly journals The Vertical Structure of Warm Ionised Gas in the Milky Way

2008 ◽  
Vol 25 (4) ◽  
pp. 184-200 ◽  
Author(s):  
B. M. Gaensler ◽  
G. J. Madsen ◽  
S. Chatterjee ◽  
S. A. Mao
Keyword(s):  
Electron Density ◽  
Filling Factor ◽  
Milky Way ◽  
Galactic Disk ◽  
Emission Measure ◽  
Scale Height ◽  
Thick Disk ◽  
Joint Analysis ◽  
Dispersion Measure ◽  
Galactic Latitude

AbstractWe present a new joint analysis of pulsar dispersion measures and diffuse Hα emission in the Milky Way, which we use to derive the density, pressure and filling factor of the thick disk component of the warm ionised medium (WIM) as a function of height above the Galactic disk. By excluding sightlines at low Galactic latitude that are contaminated by Hii regions and spiral arms, we find that the exponential scale-height of free electrons in the diffuse WIM is 1830–250+120 pc, a factor of two larger than has been derived in previous studies. The corresponding inconsistent scale heights for dispersion measure and emission measure imply that the vertical profiles of mass and pressure in the WIM are decoupled, and that the filling factor of WIM clouds is a geometric response to the competing environmental influences of thermal and non-thermal processes. Extrapolating the properties of the thick-disk WIM to mid-plane, we infer a volume-averaged electron density 0.014 ± 0.001 cm−3, produced by clouds of typical electron density 0.34 ± 0.06 cm−3 with a volume filling factor 0.04 ± 0.01. As one moves off the plane, the filling factor increases to a maximum of ∼30% at a height of ≈1–1.5 kpc, before then declining to accommodate the increasing presence of hot, coronal gas. Since models for the WIM with a ≈1 kpc scale-height have been widely used to estimate distances to radio pulsars, our revised parameters suggest that the distances to many high-latitude pulsars have been substantially underestimated.

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