Pc Cm-3 Research Articles

Progress on Establishing the Spatial Distribution of Material Responsible for the \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\frac{1}{4}$ \end{document} keV Soft X‐Ray Diffuse Background Local and Halo Components

In this paper we present a new look at the distribution of emission responsible for the soft X-ray diffuse background (SXRB) using ROSAT all-sky survey data. This is prompted by the demonstration of the existence of extensive 1/4 keV emission in the Galactic halo that was not considered in the most successful previous model, in which the bulk of the observed 1/4 keV X-rays originated in a Local Hot Bubble (LHB) that surrounds the Sun. The basic distribution can be represented by two angularly varying Galactic components (the LHB and an X-ray halo) and an isotropic extragalactic component. In addition, there are the distinct enhancements of supernova remnants, superbubbles, and clusters of galaxies. Using the negative correlation between the SXRB and DIRBE-corrected IRAS 100 μm intensity, we find a LHB that is similar to previous models, although it is possibly more limited in extent. The emission of the LHB varies by a factor of ~3.3 with direction and produces ROSAT 1/4 keV intensities between ~250 and ~820 × 10-6 counts s-1 arcmin-2 (typically brighter at higher Galactic latitudes), with inferred emission measures of ~0.0018 to 0.0058 cm-6 pc (assuming thermal ionization equilibrium). The distribution of the emission temperature of the LHB is peaked at 106.07 K with a range of ±12% at FWHM. While this variation is small and is perhaps due to systematic uncertainties in the analysis, there is a suggestion of a cooler region in the anticenter direction consistent with previous studies. We derive halo plus extragalactic intensities that vary from ~400 to 3000 × 10-6 counts s-1 arcmin-2 outside the absorbing gas of the Galactic disk. As the low end of the range is comparable to what is expected for the extragalactic background, this implies that the halo emission varies considerably over the sky, with inferred emission measures ranging from near zero to >0.02 cm-6 pc. The distribution of emission temperatures in the halo, again derived from thermal equilibrium emission models, peaks near 106.02 K and varies over ±20% at FWHM.

Optical Spectroscopy of Diffuse Ionized Gas in M31

We have obtained sensitive long-slit spectra of diffuse ionized gas (DIG) in the Andromeda galaxy, M31, covering the wavelength range of 3550-6850 A. By co-adding extracted DIG spectra, we reached a 1 σ uncertainty of 9.3 × 10-19 ergs s-1 cm-2 arcsec-2 corresponding to 0.46 pc cm-6 in emission measure. We present average spectra of DIG at four brightness levels with emission measures ranging from 9 to 59 pc cm-6. We present the first measurements of [O II] λ3727 and [O III] λ5007 of the truly diffuse ionized medium in the disk of an external spiral galaxy. We find that I[O II]/IHα = 0.9-1.4. The [O III] line is weak (I[O III]/IHβ = 0.5), but it is stronger than found for the Galactic DIG. Measurements of [N II] λ6583 and [S II] (λ6717 + λ6731) are also presented. The [S II] lines are clearly stronger than typical H II regions (I[S II]/IHα = 0.5 compared to 0.2), confirming various imaging studies of spiral galaxies. Overall, the line ratios are in agreement with predictions of photoionization models for diffuse gas exposed to a dilute stellar radiation field, but the line ratios of the DIG in M31 are somewhat different than observed for Galactic DIG. The differences indicate a less diluted radiation field in the DIG of M31's spiral arms compared to DIG in the Solar Neighborhood of the Milky Way. Turbulent mixing layers can contribute at most 20% of the ionization budget of the DIG, with lower percentages producing better fits to the observed line ratios. We have also detected He I λ5876 emission from the brightest DIG in M31. The He I line appears to be stronger than in the Galactic DIG, possibly indicating that most of the Helium in the bright DIG in M31 is fully ionized. However, this result is somewhat tentative since bright night sky lines hamper an accurate measurement of the He I line strength.

A Search for Pulsar Companions to OB Runaway Stars

view Abstract Citations (38) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Search for Pulsar Companions to OB Runaway Stars Sayer, R. W. ; Nice, D. J. ; Kaspi, V. M. Abstract We have searched for radio pulsar companions to 40 nearby OB runaway stars. Observations were made at 575 and 770 MHz with the NRAO 140 foot (43 m) telescope. The survey was sensitive to long- period pulsars with flux densities of 1 mJy or more. No pulsar companions to OB runaways were discovered. One previously unknown pulsar was discovered, PSR J2044 +4614, while observing toward target 0 star BD +45°3260. Follow-up timing observations of the pulsar measured its position to high precision, revealing a 9' separation between the pulsar and the target star, unequivocally indicating they are not associated. The pulsar is ordinary, except that its dispersion measure, 315 pc cm-3, is unusually high given its Galactic longitude, l = 85°. The nondetection of pulsars bound to target stars places an upper limit of 8% (95% confidence) for the fraction of runaway OB stars with pulsar companions detectable via pulsed radio emission by surveys such as ours. Assuming standard models for the pulsar beaming fraction and luminosity function, we conclude most OB runaways do not have pulsar companions. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177063 Bibcode: 1996ApJ...461..357S Keywords: STARS: PULSARS: GENERAL; STARS: PULSARS: INDIVIDUAL ALPHANUMERIC: PSR J2044+4614; STARS: BINARIES: GENERAL; STARS: EARLY-TYPE full text sources ADS | data products SIMBAD (43)

A Very Luminous Binary Millisecond Pulsar

We report the discovery of a field binary millisecond pulsar, J0218+4232, with a period of 2.3 ms and in a 2.0 day binary orbit with a 0.16 M☉ companion. The new pulsar was serendipitously discovered as a steep-spectrum, highly polarized, compact radio source during imaging observations at Westerbork, and was later confirmed to be a pulsar with observations carried out with the 76 m antenna at Jodrell Bank. With a dispersion measure of 61 pc cm-3, it lies outside the electron layer in the direction l = 140°, b = -18°. At a distance of more than 5.7 kpc, it is the farthest known field millisecond pulsar and has a radio luminosity L400 comparable to that of PSR B1937+21. It appears that a significant fraction of the radio emission is not pulsed. This, together with the extremely broad pulse profile, suggests that we are looking at an aligned rotator.

The Multiphase Structure of the Galactic Halo: High-Velocity Clouds in a Hot Corona

view Abstract Citations (198) References (79) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Multiphase Structure of the Galactic Halo: High-Velocity Clouds in a Hot Corona Wolfire, Mark G. ; McKee, Christopher F. ; Hollenbach, David ; Tielens, A. G. G. M. Abstract Observations indicate that some high-velocity clouds (HVCs) have a two-phase structure consisting of cold cores and warm envelopes. We calculate the thermal equilibrium gas temperature and investigate the thermal stability of neutral gas in the Galactic halo. Phase diagrams (thermal pressure P vs. gas density n) are presented for gas at a range of heights z above the Galactic plane. Our method accounts for the photoelectric heating from small grains and PAHs and includes a detailed treatment of the ionization rates and heating due to the soft X-ray background and due to cosmic rays. We find that stable two-phase gas exists over a range of heights, but only within a narrow range of pressures at each height. Using a realistic Galactic gravitational potential and using halo parameters consistent with observed properties of the soft X-ray background, we show that a hot (T ∼ 1-2 × 106 K) Galactic corona can provide the necessary pressure for two-phase HVCs. We find that for an isothermal T = 106 K halo, the observed X-ray emission measure (EMh = 2.5 × 10-3 pc cm-6) yields a thermal pressure in the Galactic midplane of ≌2000-3000 K cm-3, similar to observed pressures. In addition, we find that the electrons in the hot halo make a nonnegligible contribution to the dispersion measure of pulsars far from the Galactic plane. A method is presented for estimating HVC distances from X-ray shadowing measurements. We demonstrate that the two-phase nature of HVCs can be used to constrain the distance to the clouds and their metallicity or origin. A primordial origin (extremely low metallicities) can be ruled out since two phases exist only for z <2 kpc, contrary to most distance lower limits to HVCs. An extragalactic origin for some clouds, a result of gas stripped from the LMC, is supported by our calculated range of distances over which two-phase clouds exist in a hot corona, the observed dust and metal content, and the high velocities observed for a few HVCs. We predict that no cold cores are expected to be found in a T = 106 K halo at distances greater than ∼20 kpc, which is consistent with the lack of cold cores seen in the Magellanic Stream. However, the mass infall rate to the Galactic plane, and the general velocity distribution of halo clouds is consistent with an origin from gas injected into the halo by a Galactic fountain. For this case, two-phase models require that the abundance of very small grains and PAHs in the fountain clouds must be similar to the standard abundances in the disk. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176429 Bibcode: 1995ApJ...453..673W Keywords: GALAXY: HALO; ISM: CLOUDS; ISM: STRUCTURE full text sources ADS | data products SIMBAD (8)

A Comparison of Diffuse Ionized and Neutral Hydrogen Away from the Galactic Plane: H alpha -emitting H i Clouds

view Abstract Citations (77) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of Diffuse Ionized and Neutral Hydrogen Away from the Galactic Plane: H alpha -emitting H i Clouds Reynolds, R. J. ; Tufte, S. L. ; Kung, D. T. ; McCullough, P. R. ; Heiles, C. Abstract We compare velocity-resolved maps of the interstellar Hα emission at 1° angular resolution within a 10° × 12° region of the sky, and 3' resolution within a 1° × 1° region, with corresponding maps of 21 cm emission. The results indicate that at least 30% of the Hα background and 10%-30% of the 21 cm emission are spatially and kinematically associated with "clouds" containing both neutral and ionized hydrogen. These Hα-emitting H I clouds, identified as prominent Hα and 21 cm emission enhancements of several degrees extent on narrow (12 km s-1) velocity interval maps, have H I column densities that range from about 2 × 1019 cm-2 to 2 × 1020 cm-2 and emission measures from 2 cm-6 pc to 10 cm-6 pc. Their radial velocities suggest distances |z| from the Galactic midplane that range from |z| ≲ 100 pc to |z| ≍ 1 kpc, with a |z|-distribution that is much thicker than that of the non-Hα-emitting H I. The high |z| Hα-emitting H I clouds identified on these maps are approximately 40% ionized, have densities n ≍ 0.2-0.3 cm-3, and are associated with large filament and loop structures. The relationship between the Ho and the H+ within these clouds is not clear; however, the available data suggest that the neutral and ionized components of each cloud are spatially separated. If the clouds are photoionized, the Hα surface brightnesses imply that at |z| ≍ 1 kpc the strength of the incident Lyman continuum flux 4πJ ≍ 2 × 106 photons cm-2 s-1. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/175999 Bibcode: 1995ApJ...448..715R Keywords: ISM: CLOUDS; ISM: GENERAL; ISM: H II REGIONS; RADIO LINES: ISM full text sources ADS | data products SIMBAD (1)

RADIO SPECTRA AND STAR FORMATION HISTORIES IN H II GALAXIES

ABSTRACT A sample of actively star forming H II galaxies has been observed in the radio continuum and several optical bands. These galaxies are currently undergoing very active star formation and have been selected on the basis of strong radio continuum emission. Most of the galaxies are irregular and have been associated with merger or interaction events, which may have triggered the star formation and the radio emission. Radio continuum observations at 0.32 GHz, 1.5 GHz, and in the range of 8-15 GHz were obtained at the NRAO-VLA, to determine their radio spectra. Several of the spectra were found to flatten towards lower frequencies, which is unusual. Surface and aperture photometry was obtained in the B,R, and I bands and in the H-alpha emission line. Radio emission, absorption, and relativistic electron loss mechanisms are reviewed and their suitability to account for the observed spectral shapes is discussed. Energy equipartition calculations led to galaxy magnetic fields of 10-30 mu-G; the radiation density inside the H II regions has been derived by a new method and was found to be in the range of 2 to 15 eVJcm-3. Mechanisms which may account for the observed radio spectra were fitted to the radio continuum data. The spectra resulting from a time variable relativistic electron injection ('synchrotron aging') have been calculated over a greater range of parameters than previously published. Fits of these spectra show variations of the injection rate with time scales of the order of a few Myrs. A fit based on free-free absorption uses the emission measure to balance both free-free absorption and thermal emission, thereby constraining the maximum size of the thermally emitting region. This permits a direct comparison with the optical H-alpha observations which typically show H II regions with sizes between 0.5 and 1 kpc. The fits allowed the derivation of a variety of physical parameters, such as the electron density, which is typically between 10 and 60 cm-3, and the emission measure, which is of the order of a few 105 pc cm-6. Fits of different mechanisms may describe the same spectrum well, thus, in some cases, a unique determination of a particular mechanism is not possible, although an estimate for its likelihood can be made. Parts of this work have been published by Deeg et al. (1993, ApJ 410, 626). The fraction of thermal emission in the galaxies has been determined with a higher reliability than usual by combining radio, H-alpha, and UV data. A correlation between the fraction of thermal emission and the size, as well as the total luminosity of the galaxies has been found. A simple model of relativistic electron diffusion losses--dependent on a galaxies' size--can reproduce the observed correlation well; its suitability and limits are discussed. This correlation may lead to a better understanding between the supernova rate in a galaxy and the expected non-thermal emission, a relation which is currently only very poorly known. Star formation rates based on thermal and nonthermal radio and FIR emission all indicate star formation which is enhanced during the last 106-7 years as compared to the long term (1 Gyr) rates based on B band photometry. 'Synchrotron aging', optical colors and the thermal to FIR ratio were used to derive typical ages of star formation. Based on the star formation rates and the age indicators, the galaxies were sorted into an sequence of their starbursts ages. The physical picture of a region in which star formation, subsequent SN explosions, and the resulting nonthermal radio emission takes place, can be accounted for well, by comparing the different star formation estimators which are based on a variety of radiative processes and across three regions of the electromagnetic spectrum.

Low density ionized gas in the inner galaxy — Interpretation of recombination line observations at 325 MHz

The recent survey of H 272α recombination line (324.99 MHz) in the direction of 34 Hn regions, 12 SNRs and 6 regions of continuum minimum (‘blank’ regions) in the galactic plane is used to derive the properties of diffuse ionized gas in the inner Galaxy. The intensity of radio recombination lines at high frequencies is dominated by spontaneous emission in high-density gas and that at low frequencies (325 MHz) by stimulated emission in low-density gas. We have used this property to obtain the electron density in the gas in the direction of blank regions and SNRs, by combining the H 272 α measurements (preceeding paper) with the published data at higher frequencies. Further, we have imposed constraints on the electron temperature and pathlength through this gas using the observed high-frequency continuum emission, average interstellar electron density and geometry of the line-emitting regions. The derived properties of the gas are (i) electron density 0.5–6 cm-3, (ii) electron temperature 3000–8000 K and (iii) emission measures 500–3000 pc cm-6 The corresponding pathlengths are 50–200 pc. As the derived sizes of the low-density regions are small compared to the pathlength through the Galaxy, the low-frequency recombination lines cannot be considered as coming from a widely distributed component of the interstellar medium. The Hn regions studied in the above survey cannot themselves produce the H 272α lines detected towards them because of pressure broadening, optical depth, and beam dilution. However, the agreement in velocity of these lines with those seen at higher frequencies suggests that the low-frequency recombination lines arise in low-density envelopes of the Hn regions. Assuming that the temperature of the envelopes are similar to those of the cores and invoking geometrical considerations we find that these envelopes should have electron densities in the range 1–10 cm-3 and linear sizes of 30–300 pc in order to produce the observed H 272α lines.

Optical Emission Line Studies and the Warm Ionized Component of the Local Interstellar Medium

AbstractObservations of diffuse, galactic Hα, [NII]λ6583, and [SII]λ6716 emission lines provide evidence for a warm (~ 104 K), primarily ionized component of the interstellar medium distributed throughout the galactic disk. This component of the interstellar gas has an electron density ≈ 0.1-0.2 cm-3 and occupies about 10-30% of the interstellar volume. Interstellar Hα emission near the galactic poles, the dispersion measure of a nearby pulsar, and observations of interstellar gas flowing into the solar system indicate that this ionized component is an important constituent of the interstellar medium in the solar neighborhood. The intensity of the Hα background at high galactic latitudes implies that this component is maintained by an average hydrogen ionization rate in the vicinity of the Sun of (2-4) x 106 s-1 per cm2 of galactic disk. The emission measure is 1.3-2.3 cm-6 pc toward the galactic poles. The sources of this ionization have not yet been identified but may include escaping Lyman continuum radiation from planetary nebulae, hot white dwarfs, and early type stars. Investigations of the regions surrounding ζ Oph (09V), the nearest (d ≃ 140 pc) 0 star, and a Vir (Bl IV), one of the nearest (d ≃ 87 pc) early B stars, have revealed areas of enhanced Hα emission extending 6°-12° from each star. However, it appears that these stars do not contribute significantly to the more diffuse ionization within the local interstellar medium.

The Soft X-Ray Diffuse Background: Implications for the Nature of the Local Interstellar Medium

AbstractObservations of the diffuse X-ray background in the B and C bands (130-188 eV and 160-284 eV, respectively) provide convincing evidence for the existence of high-temperature interstellar gas. Since the opacity of normal interstellar material is very high, we must assume that the soft X-ray flux observed in the galactic plane originates within a few hundred parsecs of the Sun. The intensity and B/C ratio of this low-latitude flux can be provided by emission from an equilibrium plasma with normal abundances, T = 106.0 K, and 0.0019 cm-6 pc emission measure. More sophisticated nonequilibrium models of material heated by a supernova blast wave would reduce the required emission measure somewhat, but not by so much as a factor of two. Arbitrarily limiting the pressure to 104 cm-3 K gives a maximum density of 0.005 cm-3 and a minimum radius for the emitting region of 75 pc. This fits in well with UV interstellar absorption measurements which indicate that the ISM is very deficient in neutral hydrogen out to ~100 pc from the Sun.The constancy of the observed B/C ratio implies that there are less than 5x1019 cm-2 variations in any cooler material lying between us and the bulk of the emission. This is consistent with the much smaller column densities of neutral or partially ionized material detected in the solar neighborhood through UV absorption measurements, but the organization of the cooler gas and its interaction with the coronal material require further investigation.X-ray intensities at high latitudes are larger than those in the plane by as much as a factor of three. The fluctuations show a global anti-correlation with H I column density which suggests that they might be caused by variations in the transmission of X-rays from an extragalactic source, such as a galactic halo or corona. Such models are very difficult to reconcile quantitatively with existing H I measurements, however, and it seems more likely that the bulk of the high-latitude excess is produced by an extension of the unabsorbed interstellar emission in those directions with the remaining fluctuations produced by a combination of absorption and displacement by embedded cooler material, or possibly by transmission of flux from a hot halo.While it would be most interesting to resolve this point because of its impact on the nature of the galactic halo and the high-latitude H I distribution, it does not greatly affect the amount of hot gas which apparently exists in the solar neighborhood: an isotropic emitting region with only the emission measure required by the average intensity near the galactic plane would account for about two-thirds of the integrated B and C band flux observed at the Earth. The total energy flux is ~1x10-6 ergs cm-2 s-1 if we assume a thermal equilibrium spectrum.

Compact H II Regions and OB Star Formation

In 1967 radio observations led to the discovery of very small (d 104 cm -3) H II regions. Ryle & Downes (1967) found that the obscured object DR 2 1 has a very high emission measure E.M. ( = n/d) = 5 x 107 pc cm-6, while Mezger, Schraml & Terzian (1967a) deduced from the measured radio spectrum that the similarly obscured galactic source W49A contains a high density component with E.M. = 108 pc cm 6. Subsequently, Mezger et al. (1967b) coined the term com­ pact H II regions, showed the close relationship to OH main-line masers, and proposed that compact H II regions/OH masers should be recognized as a new class of galactic sources. The recombination rate of the compact H II regions requires ionization by one or more early 0 type star; from the observed high turbulent velocities (V;:;:: 25 km s -1 ) Mezger et al. (1967b) concluded that the objects must be in a state of rapid expansion, and must represent a very early stage in the life of an OB star. In fact, Vandervoort (1963) and W. G. Mathews (1965) had already pointed out that H II regions with such densities must of necessity undergo rapid expansion and therefore have short lifetimes. The discovery of compact H II regions coincided with their theoretical prediction by Davidson & Harwit (1967), who also drew attention to the strong infnired emission expected from the (predicted) dust cocoon (see also Davidson 1970). The first compact H II regions to be recognized as strong sources of near-infrared emission were W 51-IRS2 and K3-50

Radio Recombination Lines and Dark Nebulae

view Abstract Citations (14) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Recombination Lines and Dark Nebulae Gordon, M. A. Abstract An unsuccessful search for the 1 65a and 1 66a radio recombination lines provides upper limits of 6 x 10- cm6 pc for the emission measures of H+, He+, C+, H2+, and H3+ in four dark nebulae having photographic absorptions from 1 to 5 mag. These results limit the net ionization rate of hydrogen to be less than 0.7 x 10-15 in these dark clouds. Subject headings: excitation and ionization - interstellar matter - nebulae - radio lines Publication: The Astrophysical Journal Pub Date: August 1973 DOI: 10.1086/152306 Bibcode: 1973ApJ...184...77G full text sources ADS |

Radio Observations of the Gum Nebula below 20 MHz

Low frequency radio waves propagating in the interstellar medium are attenuated through absorption by thermal electrons, the optical depth being given bywhere f is the radio frequency in MHz, the emission measure in cm-6 pc, and Te the electron temperature.

The Galactic Radio Source DR 21 AT 3.3 MM

view Abstract Citations (13) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Galactic Radio Source DR 21 AT 3.3 MM Riegel, Kurt W. ; Epstein, Eugene E. Abstract The flux of DR 21 at 3.3 mm is (21 ± 4) X 1O~ W m~ Hz'; this value is consistent with a model of DR 21 consisting of a single high-density thermal component. Ryle and Downes (1967) pointed out the high-intensity, small-diameter nature of the radio source DR 21 in the Cygnus X complex of thermal radio sources. This source and DR 23 form W75. There exist contour maps of the continuum distribution of Cygnus X at 21 cm (Pike and Drake 1964) and at 6 cm (Downes and Rinehart 1966). The very high observed emission measure of DR 21, about 1O~ cm6 pc, places in it the class of very dense H ii regions discussed by Mezger, Schraml, and Terzian (1967). They found that the 2-cm continuum flux of DR 21 agrees well with the continuum spectrum derived by Downes and Rinehart. This spectrum, together with our point at 3.3 mm (91 GHz), appears in Figure 1. Mezger et al. (1967) have also found 158a hydrogen recombination line emission at a velocity of +7.8 km/sec with respect to the local standard of rest. Weaver, Dieter, and Williams (1968) report 1665-MHz OH line emission at a position 4~ 7 north of DR 21 at +1, +6, and +12 km/sec with respect to the local standard of rest. The +1 km/sec component is 42 per cent linearly polarized. To continue our study of very dense H ii regions, which began with a study of W49 (Riegel and Epstein 1967), we have observed DR 21 to extend its spectrum to higher frequencies and to test for the existence of very high-density structure. The instrument and observing and reduction procedures were identical to those used for W49. We ob- served the source position R.A. = 20h37m14 ~2, deci. = +42°09'07", for 30 hours with the 15-foot antenna of the Space Radio Systems Facility of the Aerospace Corporation. The antenna temperature of the source, corrected for atmospheric attenuation by the use of radiosonde measurements of precipitable water vapor (Epstein 1968), was (71 ± 6) X 10~ ° K. The main spatial component of the source has a half-power width of 20" at 21 cm, an amount small compared with our antenna half-power beam width of 2 8. Therefore, if we assume that the 3-mm and the 21-cm brightness distributions are similar, the effective antenna pattern f' defined in equation (2) of Riegel and Epstein (1967), under the assumption that the source was at the beam center, is 0.986, nearly unity. Using equation (1) of the same paper, we find that the total flux is S = (21 ± 4) X 1O_26 W m2 Hz-'. (1) The uncertainty includes an estimate of the systematic errors, plus the rms noise of 2 flux units. We have also assumed that there is no difference between the shapes of the background brightness distributions at 6 cm and at 3.3 mm. From the contour maps of the continuum brightness distribution produced by Mezger et al. (1967) at 6 cm, we conclude that no significant error is introduced because of our switched-beam mode of * Supported by the United States Air Force under contract F04695-67-C-0158. K. W. R. acknowledges the support of NSF and NASA Publication: The Astrophysical Journal Pub Date: January 1968 DOI: 10.1086/180134 Bibcode: 1968ApJ...151L..33R full text sources ADS |

Observations of the Cosmic Noise Spectrum between 2 and 5 Mc/sec.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the Cosmic Noise Spectrum between 2 and 5 Mc/sec. Stone, R. G. ; Alexander, J. K. Abstract -The average cosmic noise intensity at four frequencies below the ionospheric cutoff was measured from a rocket probe at altitudes near 1100 km on 23 October 1964. The observed intensities were (2.2+00.69) X 10-20 W rn-2 cps-2 sr-2 at 1.91 Mc/sec, (2.7+0.5) X 10-20 W m-2 cps-1 sr-2 at 2.85 Mc/sec, (~.~+0171)x~o-20 W rn-2 cps-1 sr-' at 3.60 Mc/sec, (2.2+ ()149) X 10-20 W rn-2 cps-2 sr-1 at 4.70 Mc/sec. At the time of the flight the zenith was centered within 10~ of the North Galactic Pole, and the results are compared with other similar observations made from the ground and from space vehicles. Although these results are consistent with theoretical interpretations giving an emission measure of S cm-6 pc, it is suggested that the uncertainties in most measurements do not permit precise interpretation of the spectrum below S Mc/sec. The most important need in the future is for measurements at lower frequencies with improved accuracy and with better angular resolution than that of a dipole antenna. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109543 Bibcode: 1965AJ.....70Q.331S full text sources ADS |