Average Spectral Index Research Articles

Radio observations of peculiar emission-line Algol binary stars

Twelve peculiar emission-line Algol (PELA) binaries were observed at cm wavelengths. Six of these objects, WW And (tentative), KX And, AM Aur, V644 Mon, DN Ori, and AY Per were detected at the 100 to 400 micro-Jy level, corresponding to luminosity densities of order 10 exp 17 erg/s/Hz. These luminosity densities, combined with nearly flat cm spectra and high brightness temperatures, are consistent with the nonthermal gyrosynchrotron radiation model proposed as the result of observations from the previous PELA survey. Also, the star V367 Cyg was reobserved at thirteen different orbital phases; no variability was found within the errors. V367 Cyg's lack of variability and well-determined average spectral index, however, imply the possibility of a different emission mechanism, thermal bremsstrahlung, although the nonthermal model cannot be entirely ruled out. The characteristic brightness temperature of thermal bremsstrahlung is only about 10,000 K, which means that the radius of the emission region would have to be much larger than previously assumed, about 10 AU. Suggestions for further observations of all previously observed PELAS are presented in order to determine the correct emission mechanism.

The optical to near-infrared emission of BL Lac objects - Simultaneous observations

We present simultaneous optical to near-IR observations of 34 blazars, most of which classical BL Lacertae objects, with repeated observations for 21 of them. The observations cover a period of 3 years with a typical time interval of 6 months. With the sole exception of PKS 2208-137, all observed optical to near-IR flux distributions can be described either by a single power law (f ν ∞ν −α ) or by a power law plus the contribution of a standard elliptical galaxy. The optical and near-IR emission are strongly correlated and usually maintain the spectral index even under substantial flux variations. The average spectral index of the nonthermal emission, in the range 8×10 13 to 7×10 14 Hz, is 1.08±0.06 with significantly flatter spectra for the x-ray selected subsample

High energy gamma rays from the sun as seen by the gamma-1 telescope

The gamma-ray telescope GAMMA-1 has been operated from July 11, 1990, to February 28, 1992. During this period, the Sun was observed as a primary target several times, when its activity was on an increasing phase. Four outstanding flares were recorded, with energy in the range 30 – 200 MeV. Spectral analysis with the use of maximum likelihood and maximum entropy methods has revealed differences in gamma-ray production mechanisms. In the impulsive March 26, 1991 event high energy gamma-rays originate exclusively as a bremstrahlung of primary accelerated electrons. In contrast, the gamma ray emission of the extended phase of June 15, 1991 flare is mainly due to the decay of neutral pions. The average spectral index for primary nucleons was - 3.6. Evolution of the spectra for both flares shows tendency to a decrease of the primary particles mean energies with time. The third and fourth flares were recorded on October 15 and 16, 1991, but the poor quality of the telemetry has delayed the data analysis.

High‐latitude distributions of plasma waves and spatial irregularities from DE 2 alternating current electric field observations

An 18‐month data base from the Dynamics Explorer 2 AC electric field spectrometers is used to obtain average high‐latitude magnetic local time (MLT) versus invariant latitude (INL) distributions of signal intensities in 12 frequency bands between 4 Hz and 512 kHz. Three distinctly different distributions are obtained, corresponding to (1) Doppler‐shifted signals from spatial structures in the electric field (i.e., irregularities) and Alfven waves between 4 and 512 Hz, (2) ELF waves between 256 Hz and 4.1 kHz, and (3) VLF waves between 4.1 and 64 kHz with extensions into the 128–512 kHz band. The ELF and VLF distributions closely resemble previously published results based on more limited sampling. Comparable distributions for the seven channels between 4 and 512 Hz, showing a prominent zone of maximum intensities at 72.5°–80° INL between 0500 and 1300 MLT, have not previously been reported. The power law frequency dependence of average power spectral densities (PSDs) between 4 and 512 Hz is also mapped in MLT‐INL coordinates. At all locations, two power law indices (slopes) are required to closely fit the PSDs with an inverted knee joining the two slopes in the 32–64 Hz band. This knee band corresponds to the range of O+ cyclotron frequencies encountered, and it lends credence to Gurnett et al.'s (1984) contention that Alfven waves are an essential ingredient in explaining the low‐frequency in situ satellite signals which were previously attributed to polarization fields accompanying spatial irregularities in plasma densities. However, other aspects of the 4–512 Hz observations, including seasonal variations, favor the earlier spatial irregularity interpretation. As discussed, the difficulties encountered in seeking interpretations exclusively in terms of either spatial irregularities or Alfven waves can be resolved with a synthesis approach requiring both types of signals. It is proposed that the averaged intensities and corresponding spectral characteristics in the 4–512 Hz band represent the consequence of intermittently superimposing shear Alfven waves on a spatially irregular medium. There are then three principal contributions: (1) an omnipresent 4–512 Hz signal from Doppler‐shifted responses to 2000–15 m spatial irregularities having an average power law spectral index near −1.9, (2) intermittent signals from locally generated shear Alfven waves having maximum power at frequencies of <4 Hz and average power law spectral indices of ≤(−2.8) extending only to fc(O+), and (3) spatial irregularity modulations of shear Alfven waves originating both locally and in the distant magnetosphere.

EXOSAT X-ray spectra of quasars

view Abstract Citations (93) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS EXOSAT X-Ray Spectra of Quasars Comastri, Andrea ; Setti, Giancarlo ; Zamorani, Giovanni ; Elvis, Martin ; Giommi, Paolo ; Wilkes, Belinda J. ; McDowell, Jonathan C. Abstract We have measured the X-ray spectra in the 0.1-10 keV energy range for 17 AGNs (mostly PG quasars) using the EXOSAT Low and Medium Energy arrays. The sample has been selected on the basis of soft X-ray flux and includes all the AGNs in the 1981 Zamorani et al. and 1986 Tananbaum et al. publications with an Einstein flux greater than 6x10^-12^ ergs cm^-2^ s^-1^. For each object we have determined the best-fit spectral parameters in the medium-energy range (2-10 keV), i.e., the power-law slope, the normalization, and the absorbing column density. In this energy range, all the spectra are well described by a single power law model, with a wide distribution of energy spectral indices in the range 0.4 < α < 1.3. The average spectral index is <α> = 0.89 +/- 0.06, but most of the objects are distributed around α ~ 1.0. The overall distribution of spectral indices is significantly displaced toward steeper spectra, when compared to that found in the same energy band by Turner & Pounds, for a large sample of hard X-ray-selected Seyfert galaxies. This difference can be understood in terms of the different selection criteria of the two samples (soft versus hard X-ray selection), and it is concluded that the true distribution of hard X-ray spectral indices in AGNs is probably wider than previously suggested. Inclusion of lower energy data (0.1-2.0 keV) shows the presence of significant soft excess in six out of 17 cases. This soft excess emission can be modeled by either a steep power law with α ranging from 2.7 to 4.2 or the high-energy tail of a blackbody with temperatures in the range 40-80 eV. The intersection of this soft excess emission with the hard X-ray power law is in the energy interval 0.3-0.75 keV, in the source rest frame. A correlation analysis performed on our data set does not reveal any correlation between the medium-energy spectral properties and other physical parameters of the sources. However, fits performed in the energy range 0.1-1.0 keV (comparable with the Einstein IPC energy range) show a significant correlation between radio-loudness and X-ray spectral properties, with the radio-loud objects having on average, flatter slopes than the radio-quiet ones, in agreement with the findings of Wilkes & Elvis. The fact that the same correlation is not seen in our data at higher energy seems to be explained by a correlation between presence and strength of soft excess and "radio-quietness": the objects with significant soft excess emission tend to be the most radio-quiet quasars in our sample. Publication: The Astrophysical Journal Pub Date: January 1992 DOI: 10.1086/170851 Bibcode: 1992ApJ...384...62C Keywords: Exosat Satellite; Quasars; X Ray Spectra; Galactic Nuclei; Seyfert Galaxies; Astrophysics; GALAXIES: NUCLEI; GALAXIES: QUASARS: GENERAL; GALAXIES: SEYFERT; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (17) NED (17)

The EXOSAT high Galactic latitude survey

We have performed a survey of ~570 square degrees of high galactic latitude sky using a large number of EXOSAT X-ray images. 130 serendipitous sources were detected and ~ 60% of them have been identified with catalogued objects. The large majority of the remaining sources have faint optical counterparts on POSS or ESO plates and are expected to be extragalactic. The comparison of our results with those of the Einstein Medium Sensitivity Survey and an analysis of the correlation between the spatial distribution of EXOSAT serendipitous sources and the amount of galactic Hydrogen column density indicate that the average spectral index of extragalactic X-ray sources is steeper than the canonical AGN slope.

Electron-positron pairs, Compton reflection, and the X-ray spectra of active galactic nuclei

Recent X-ray observations by the Japanese satellite Ginga have given new insight into the X-ray emission from active galactic nuclei (AGN). In particular, they have shown the importance of a spectral component reflected from cold, optically thick material (T &lt; 10^6^ K) very close to the central engine in these objects. Here we show that this result revives models in which much of the luminosity generated in X- and γ-ray energy bands in AGN is reprocessed via electron-positron pairs. In those models, nonthermal electron-positron pair injection in a compact region results in X-ray spectra with an energy spectral index of α_x_ ~ 1. We show that if a substantial fraction of this radiation is incident upon cold material, the reflected component results in a composite spectrum with an average spectral index of α_x_ ~ 0.7 in the 1-30 keV energy range for a wide range of initial assumptions. Furthermore, our predicted spectra steepen above ~ 1 MeV and exhibit soft excesses below ~ 0.5 keV. Such spectra agree well with the X-ray and γ-ray observations of AGN and the constraints imposed by the cosmic γ-ray background.

Eight years of ultraviolet spectra of the variable BL Lacertae object PKS 2155-304

IUE data obtained on the variable BL Lacertae object PKS 2155-304 over an eight-year period are presented and analyzed in a fashion that minimizes the SNR of individual spectra. In general, the ultraviolet continuum can be fitted by a power law model with an average spectral index of roughly 0.89 + or - 0.12(sigma). The observations show that PKS 2155-304 has been variable on a time scale as short as 10 days and by an overall factor of two. The spectral index has changed very little and is uncorrelated with intensity in an absolute sense.

Radio-continuum survey of the Coma/A1367 supercluster. IV - 1.4 GHz observations of CGCG galaxies

1.4 GHz radio-continuum observations of 148 CGCG galaxies in the Coma supercluster region were obtained with the VLA in C array configuration. Comparison with previous measurements at 0.6 GHz leads to an average spectral index of 0.8. The structures of 29 galaxies in this region determined with high-resolution VLA (A array) observations are presented. 43 references.

The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

The X-ray energy distribution of a large sample of faint extragalactic X-ray sources is studied. Two independent methods are used to determine that the average spectral index for the sources is about 0.95. The intrinsic dispersion of the spectral index distribution is about 0.36. The sources are characterized by a range of power law slopes in the soft X-ray band with a mean of 1.03 + 0.05 or - 0.06 and an intrinsic dispersion of about 0.36. The present analysis extends knowledge of the AGN spectral properties to sources which are about three times more distant and more than 10 times fainter than those previously studied. There is no evidence in the data for any change in the mean spectral index of either extragalactic sources as a whole or AGN in particular, as a function of the X-ray flux in the flux range 10 to the -11th to 10 to the -13th ergs/sq cm/s.

VLA Observations of NGC 5793

AbstractThe edge-on spiral galaxy NGC 5793 has been observed at frequencies of 4860 and 14940 MHz with the Very Large Array. The corresponding half-intensity beamwidths in the final maps were 0.65 × 0.40 and 0.19 × 0.11 arcsec2. The maps show a continuum source close to the optical nucleus with dimensions of ≤0.01 and 0.04 arcsec in right ascension and declination. The average spectral index is –0.6.

The EXOSAT High Galactic Latitude Survey

We have performed a survey of ∼570 square degrees of high galactic latitude sky using a large number of EXOSAT X-ray images. 130 serendipitous sources were detected and ∼60% of them have been identified with catalogued objects. The large majority of the remaining sources have faint optical counterparts on POSS or ESO plates and are expected to be extragalactic. The comparison of our results with those of the Einstein Medium Sensitivity Survey and an analysis of the correlation between the spatial distribution of EXOSAT serendipitous sources and the amount of galactic Hydrogen column density indicate that the average spectral index of extragalactic X-ray sources is steeper than the canonical AGN slope.

Inferences from a study of Fe II emission in radio-loud and radio-quiet quasars⋆

From intermediate-resolution spectrophotometry of 33 quasars with redshift z < 0.7, significant statistical differences in the line properties emerge between the radio and radio-quiet objects. Fe II optical emission is rarer in radio quasars and less intense on average when present, whereas Fe II UV emission is always detected in both classes. Moreover, for the radio sample, Fe II optical emission is not correlated with the radio morphology, contrary to previous suggestions. Finally, in radio quasars the full-width at half maximum of Hβ and/or Mg II is statistically larger by nearly a factor of 2 than in radio-quiet ones. The continua are well approximated by a power law. The average spectral index for the whole sample is 0.16 ± 0.48, significantly smaller than the value found in the sample of higher redshift quasars studied by Richstone & Schmidt. Seven absorption-line systems are present in our sample and, for the first time, Ca II H and K absorptions are identified in two Mg II absorption systems. We suggest a model to account for the differences found between radio and radio-quiet quasars; it involves a hot diffuse plasma which constrains the surface density of the emissive clumps.

The temporal structure of intensity scintillations near the magnetic equator

The temporal structure of weak and strong intensity scintillations observed with a geostationary satellite at 249 MHz near the magnetic equator is studied. It is shown that for weak intensity scintillations, the power spectra exhibit an asymptote immediately beyond the Fresnel frequency, which signifies a power law variation of power spectral density with frequency yielding spectral indices ranging between −3 and −3.5. By the use of simultaneous irregularity drift measurements, it is found that the power law portion of the spectrum for weak scintillations is caused by F region irregularities in the scale length range of about 700 m to 100 m. On the other hand, for strong intensity scintillations, the asymptote signifies a somewhat higher spectral index ranging between −3 and −4.5. Considering both weak and strong scintillations, an average spectral index of −3.5 is obtained which is found to be consistent with the average one‐dimensional in situ spectral index for irregularities with scale lengths less than 1 km. In addition, it is shown that for strong intensity scintillations, a substantial decrease of the correlation time occurs which is dictated not only by the increased strength of scattering but by the increased irregularity drift as well. At 249 MHz the shortest correlation length transverse to the propagation path is found to be about 70 m near the magnetic equator. This is a factor of 5 larger than the corresponding value obtained away from the equator near the crest of the equatorial anomaly at Ascension Island.

A Possible Evolutionary Feature in the Spectra of Radio Galaxies

A recent analysis (Slee, 1981) of the spectra of extragalactic sources derived by Slee et al. (1981) has revealed a relationship between spectral index and redshift in the sense that the spectra of identified radio galaxies steepen with increasing redshift; this behaviour is consistent with an effect first reported by Kellermann et al. (1969) that the average spectral index of identified radio galaxies is lower than that of sources in empty optical fields, which are now generally regarded as distant radio galaxies (rather than QSOs). However, because of the selection effects present in the sample, Slee was not able to decide whether the basic correlation is between spectral index and redshift (implying an evolutionary origin), between spectral index and radio power, or between spectral index and linear dimension.

Measurement of the Neutron Spectrum from a Fast Pulse Reactor in the Presence of the Experimenter's Table

The measurement of the neutron spectrum from the White Sands Missile Range Fast Burst Reactor (FBR) in the presence of the experimenter's table is reported herein. A reinvestigation of this neutron spectrum was performed as a result of a recently published work, which indicates that the experimenter's table has a considerable softening effect on the neutron spectrum from the FBR. Based on such spectral parameters as average energy, integral fluence, and spectral index, we find the neutron spectrum associated with the experimenter's table to be essentially identical to the free-field leakage spectrum.

Energetic and relativistic electrons near the polar magnetopause

More than 2 years of Geos 2 observations in the high-latitude outer magnetosphere and magnetosheath have confirmed the presence of a layer of energetic and relativistic electrons near the polar tail magnetopause. This layer, appearing as 'electron spike' on almost every magnetopause crossing, contains electrons over a wide range of energies up to relativistic energies of >2MeV. Their differential energy spectrum, if fitted by a power law, an average spectral index between 3 and 4.5. The spikes are typically 1--2R/sub E/ wide extending more into the mangetosheath than into magnetosphere. The electron intensity in the magnetopause layer decreases with distance along the tail, away from the polar cusp, and increase with K/sub P/ values. It tends to be higher when the magnetosheath field is parallel or antiparallel to the tail magnetic field. In many cases the magnetopause spikes in the magnetosheath which often follow closely the fluctuations of the magnetosheath field direction and/or intensity. A map of the average electron distribution in the polar magnetosphere and magnetosheath is presented.

Further joint X-ray, infrared, and radio observations of Cygnus X-3

Observations of Cygnus X-3 were carried out at 2.5 - 7.5 keV, 2.2 micron, 8.1 GHz and 2.7 GHz over a two week period. The X-ray data show the periodic structure which is typical of Cyg X-3. At times the X-ray and infrared measurements show very similar periodic structure, both in phase and shape, while at other times the infrared data show no periodic variability. The radio fluxes were usually low during the period of observation; both the daily average radio flux levels and spectral index remained nearly constant.

Radio spectra of some markarian galaxies

AbstractRadio spectra of 16 Markarian galaxies are studied between 2.7 and 15.5 GHz. The Seyfert‐type galaxies have power‐law spectra with an average spectral index of ‐0.82, except for MRK 348, which is an active galaxy with a complex spectrum. The galaxies with featureless optical spectra have rather flat radio spectra, similar to BL Lac‐type objects.

High-energy electron spikes at high latitudes

Over 750 spikes of precipitating electrons with E ≥ 425 kev were observed aboard the low-altitude polar orbiter Ogo 4 between July 30, 1967, and December 31, 1967. The spikes may be divided into three distinct populations depending on whether they occur at latitudes below, at, or above the local limit of trapping. These spikes are designated type 1, 2, and 3, respectively. Type 3 spikes occur in a narrow latitude band about 3° wide, centered at invariant latitude Λ ≈ 78° at 1000 MLT (magnetic local time) and 68° at 2000 MLT. Their relative frequency of occurrence, intensity, and hardness do not depend significantly on magnetic local time. Type 3 spikes appear to be associated with spikes observed near the magnetopause and the neutral sheet. Type 2 spikes also occur in a latitude band about 3° wide, centered at about 71° at 1000 MLT and 67° at 2200 MLT. Their frequency of occurrence is highly dependent on magnetic local time, a large maximum occurring near 2300 and very few events occurring between 0600 and 1200 MLT. Type 2 spikes appear to be related to island fluxes in the neutral sheet, although they occur on closed field lines and may persist for many hours. Type 1 spikes occur in a wider band of latitudes, from about 62° to 68° near midnight and 66° to 68° near noon. The local-time dependence of their frequency of occurrence is similar to that of type 2 spikes but less pronounced. Although they are observed on closed field lines, type 1 spikes do not persist for periods longer than about 1 hour, and we conclude that they are produced by strong pitch-angle scattering from the stably trapped population. The average spectral index (assuming a power-law spectrum) is 3 to 5, and the median flux (>425 kev) is about 150 el cm^(-2) sec^(-1) ster^(-1), although type 2 events near midnight tend to be larger and harder. All types tend to be more intense and to occur at lower latitudes when Kp is large.