449 research outputs found
Evolution of the luminosity function of extragalactic objects
A nonparametric procedure for determination of the evolution of the luminosity function of extragalactic objects and use of this for prediction of expected redshift and luminosity distribution of objects is described. The relation between this statistical evolution of the population and their physical evolution, such as the variation with cosmological epoch of their luminosity and formation rate is presented. This procedure when applied to a sample of optically selected quasars with redshifts less than two shows that the luminosity function evolves more strongly for higher luminosities, indicating a larger quasar activity at earlier epochs and a more rapid evolution of the objects during their higher luminosity phases. It is also shown that absence of many quasars at redshifts greater than three implies slowing down of this evolution in the conventional cosmological models, perhaps indicating that this is near the epoch of the birth of the quasar (and galaxies)
Structure of impulsive phase of solar flares from microwave observations
Variation of the microwave intensity and spectrum due to gyro-synchrotron radiation from semi-relativistic particles injected at the top of a closed magnetic loop is described. Using the recent high spatial resolution X-ray observations from the HXIS experiment of Solar Maximum Mission and from observations by the Very Large Array (VLA), it is shown that the high microwave brightness observed at the top of the flare loop can come about if (1) the magnetic field from top to footpoints of the loop does not increase very rapidly, and (2) the accelerated particles injected in the loop have a nearly isotropic pitch angle distribution. The limits on the rate of increase of the magnetic field and/or the average pitch angle depend on the geometry and location of the loop on the solar disk
The luminosity function of quasars and its evolution: A comparison of optically selected quasars and quasars found in radio catalogs
The luminosity function of quasars and its evolution are discussed, based on comparison of available data on optically selected quasars and quasars found in radio catalogs. It is assumed that the red shift of quasars is cosmological and the results are expressed in the framework of the Lambda = 0, Q sub Q = 1 cosmological model. The predictions of various density evolution laws are compared with observations of an optically selected sample of quasars and quasar samples from radio catalogs. The differences between the optical luminosity functions, the red shift distributions and the radio to optical luminosity ratios of optically selected quasars and radio quasars rule out luminosity functions where there is complete absence of correlation between radio and optical luminosities. These differences also imply that Schmidt's (1970) luminosity function, where there exists a statistical correlation between radio and optical luminosities, although may be correct for high red shift objects, disagrees with observation at low red shifts. These differences can be accounted for by postulating existence of two classes (1 and 2) of objects
Directivity of Bremsstrahlung radiation from relativistic beams and the Gamma-Rays from solar flares
It has been observed that flares with greater than 10 Mev gamma-ray emission are concentrated around the solar limb with a dispersion of 10 to 20 degrees. It is shown that the bremsstrahlung by relativistic electrons is responsible for such gamma-rays and that the expected relativistic beaming cannot explain this dispersion. It is argued that this dispersion is predominately a reflection of the pitch angle distribution of the electrons. Then it is shown that this requires a small variation of the magnetic field from the point where the electrons are injected to the photosphere and a nearly isotropic (in the downward direction) pitch angle distribution at the injection. The influence of other effects on the observed distribution is also discussed
Correlation of hard X-ray and type 3 bursts in solar flares
Correlations between X-ray and type 3 radio emission of solar bursts are described through a bivariate distribution function. Procedures for determining the form of this distribution are described. A model is constructed to explain the correlation between the X-ray spectral index and the ratio of X-ray to radio intensities. Implications of the model are discussed
The impulsive phase of solar flares. 2: Characteristics of the hard X-rays
The results of an investigation into the electron distribution in the general non-thermal models of solar flares were used to calculate the characteristics of the impulsive hard X-rays. The height distribution, the spectrum, the polarization, and directivity of the X-rays were observed and how these X-ray characteristics are affected by the parameters defining the model was investigated. An expression is obtained for the X-ray intensity as a function of source height which is an excellent fit under certain constraints. Some available data with spatial resolution are examined and it is shown that it is possible to reproduce these data adequately with the non-thermal model and to determine the values of the parameters describing the flares
Emission, absorption and polarization of gyrosynchrotron radiation of mildly relativistic paricles
Approximate analytic expressions for the emissivity and absorption coefficient of synchrotron radiation of mildly relativistic particles with an arbitrary energy spectrum and pitch angle distribution are given. From these, an expression for the degree of polarization is derived. To accomplish this, previously developed methods of integration are used. The analytic results are compared with numerical results for both thermal and non-thermal (power law) distributions of particles
Flux and Photon Spectral Index Distributions of Fermi-LAT Blazars And Contribution To The Extragalactic Gamma-ray Background
We present a determination of the distributions of photon spectral index and
gamma-ray flux - the so called LogN-LogS relation - for the 352 blazars
detected with a greater than approximately seven sigma detection threshold and
located above +/- 20 degrees Galactic latitude by the Large Area Telescope of
the Fermi Gamma-ray Space Telescope in its first year catalog. Because the flux
detection threshold depends on the photon index, the observed raw distributions
do not provide the true LogN-LogS counts or the true distribution of the photon
index. We use the non-parametric methods developed by Efron and Petrosian to
reconstruct the intrinsic distributions from the observed ones which account
for the data truncations introduced by observational bias and includes the
effects of the possible correlation between the two variables. We demonstrate
the robustness of our procedures using a simulated data set of blazars and then
apply these to the real data and find that for the population as a whole the
intrinsic flux distribution can be represented by a broken power law with high
and low indexes of -2.37 +/- 0.13 and -1.70 +/- 0.26, respectively, and the
intrinsic photon index distribution can be represented by a Gaussian with mean
of 2.41 +/- 0.13 and width of 0.25 +/- 0.03. We also find the intrinsic
distributions for the sub-populations of BL Lac and FSRQs type blazars
separately. We then calculate the contribution of Fermi blazars to the diffuse
extragalactic gamma-ray background radiation. Under the assumption that the
flux distribution of blazars continues to arbitrarily low fluxes, we calculate
the best fit contribution of all blazars to the total extragalactic gamma-ray
output to be 60%, with a large uncertainty.Comment: 13 pages, 13 figures, 2 tables, updated to published version with
additional figure
Gamma-ray Luminosity and Photon Index Evolution of FSRQ Blazars and Contribution to the Gamma-ray Background
We present the redshift evolutions and distributions of the gamma-ray
luminosity and photon spectral index of flat spectrum radio quasar (FSRQ) type
blazars, using non-parametric methods to obtain the evolutions and
distributions directly from the data. The sample we use for analysis consists
of almost all FSRQs observed with a greater than approximately 7 sigma
detection threshold in the first year catalog of the Fermi Gamma-ray Space
Telescope's Large Area Telescope, with redshfits as determined from optical
spectroscopy by Shaw et al. We find that FSQRs undergo rapid gamma-ray
luminosity evolution, but negligible photon index evolution, with redshift.
With these evolutions accounted for we determine the density evolution and
luminosity function of FSRQs, and calculate their total contribution to the
extragalactic gamma-ray background radiation, resolved and unresolved, which is
found to be 16(+10/-4)%, in agreement with previous studies.Comment: 9 pages, 10 figures, Accepted to Ap
Impulsive solar X-ray bursts. 3: Polarization and directivity of bremsstrahlung radiation from a beam of electrons directed toward the photosphere
The spectrum, directivity and state of polarization is presented of the bremsstrahlung radiation expected from a beam of high energy electrons spiraling along radial magnetic field lines toward the photosphere. The results are used for calculation of the characteristics of the reflected plus direct flux
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