On the formation of energy spectra in synchrotron sources

The observed energy spectra in synchrotron sources are power laws,N (E)=KE ^–, with the distribution in peaked around 2.5. These are consistent with initially injected spectra with between 1 and 2, subsequently steepened by synchrotron losses. Contrary to the results of Kardashev (1962), it is shown that statistical acceleration when coupled with synchrotron losses lead naturally to the formation of flat stationary spectra with 1. These stationary spectra have bends near the energy at which synchrotron losses balance the energy gains by acceleration. Above this bend the spectrum tends to =2. The time evolution of an initial spectrum towards the stationary spectrum is investigated. It is suggested that the initially flat spectra with 1 to 1.5 observed in some variable sources result from an incomplete approach to the stationary spectrum, and that in sources with constant acceleration spectra with 2 are to be expected.     

Spectrophotometric study of P/Halley during post perihelion passage

Spectrophotometric observations of P/Halley are presented for four nights in the optical region (3200–7000 ) during post-perihelion period. Emission features due to CN, CH, C₃ and C₂ molecules are identified and the total apparent fluxes in each emission band are measured. It is found that the comet display large variations in the emission bands as well as continuum.     

The EM Algorithm for Imaging with Gaps in the Detector Plane

-ray telescopes based on coded-mask techniques does not give adirect image of the sky sources but its correlation with the mask responsefunction. To obtain an image of the sky sources one has to treatmathematically the detected image, usually by correlating it with areconstruction array (correlation methods). Unfortunately this kindof methods, although quite fast, have some limitations in the reconstructedimage and in handling the real telescope response.To improve these results, several reconstruction methods have been developedand used. In the case of LEGRI (a -ray coded-mask telescopedeveloped for the University of Valencia) we have used the EM algorithm,an iterative maximum likelihood algorithm with very good response andwith good handling of the telescope response.     

TACTIC Imaging γ-Ray Telescope: A Simulation Study of the Effect of Switched-off Pixels on Its Quality Factor

The 349-pixel Cerenkov Light Imaging Camera of the -raytelescope array, TACTIC, has been recently commissioned at Mt.Abu, India. This telescope is being used for gamma-ray sourceobservations at photon energies above 0.5 TeV. Comprehensivesimulation studies, using the CORSIKA air-shower code, are inprogress to predict and optimize the performance of the TACTIC.In this short note, we study the expected performance of theImaging Element of the TACTIC under conditions of a pixelswitch-off, forced either by the well-known star-rotationphenomenon or by a defective instrumentation channel. Thechanges in the image parameters, and the resulting telescopequality factor, are studied after simulating two representativecases of pixel shut-down.     

Interferometric study of the Milky Way between Carina and Aquila

Large field H observations of the Milky Way between Carina and Aquila were made through a narrow interference filter 15 wide. Characteristic large-scale features of the observed region are extended emission areas in Carina, Norma-Scorpius and Scutum-Sagittarius and some weak isolated nebulosities near the Coal Sac, Centauri and Normae. H photographs, a chart mapping the emission, and a list of identified emission regions are given.     

High-energy proton,helium, and gamma-ray observations on OSO-III

A report on preliminary results obtained from the analysis of the first 700 orbits of data obtained in the University of Rochester particle telescope, carried in the wheel section of OSO-III, is presented. The telescope is sensitive to high-energy -rays (threshold 50 MeV) and the nuclear component of the cosmic radiation. An upper limit of 3.2 × 10^–4 /cm²secster. is set on the intensity of the diffuse primary -radiation, on the assumption it arises from the decay of ° mesons produced in nuclear interactions. An upper limit to the flux from the sun, on the same assumptions, is set at 5.5 × 10^–5 /cm² sec. The analysis of the charged particle data yields the integral rigidity spectra of proton and helium nuclei from 3 to 15 GV; the results indicate that the He spectrum is slightly steeper than the proton spectrum and that the ratio P/He increases slowly from a value of approximately 6 at 3 GV to 8 at 15 GV.NASA Predoctoral Trainee.     

Morphological study of the time histories of gamma-ray bursts

Time histories of -ray bursts have revealed a great diversity, both in total duration and in the details of fine time structure. Results of the -ray experiment on the Helios-2 spacecraft have suggested some similarities in time profiles of events. The possibility of some characteristic separation times in events may also be indicated.Paper presented at the Symposium on Cosmic gamma-Ray Bursts, held at Toulouse, France, 26–29 November, 1979.     

Observation of MKN 501 at gamma-ray energies

During a balloon flight in September 1979 of the MISO low-energy -ray telescope, the BL Lac-object MkN 501 was studied in the hard X-ray range above 30 keV and in the low energy -ray range up to 19 MeV. No statistically significant X- and -ray fluxes were detected. The implications of the upper limits obtained are discussed in the light of the relativistic jet theories recently proposed.     

A momentum distribution of high energy particles around Crab pulsar

We determine the momentum distribution of the relativistic particles near the Crab pulsar from the observed X- and -ray spectra (10³10⁹ eV), provided that the curvature radiation is responsible for it. The power law spectrum for the relativistic electrons,f() ^–5, reproduces a close fit to the observed high-energy photon spectrum. The theoretically determined upper limit to the momentum (due to radiation damping), _M 8×10⁶, corresponds to the upper cut-off energy of the -ray spectrum, 10⁹ eV. The lower limit to the momentum, _m 1.8×10⁵, is chosen such that flattening of the X-ray spectrum below 10 keV is simulated. The number density of these electrons is found to be much higher than the Goldreich-Julian density. We also discuss pulse shape and polarization of high-energy photons. The extremely high density of particles and the steep momentum spectrum are difficult to understand. This may imply that another, more efficient, mechanism is in operation.     

Constraints on the high-energy properties of active galactic nuclei

Number-flux relations at -ray energies for active galactic nuclei have been computed, starting from their X-ray luminosity function and making different assumptions on the spectral shape and cosmological evolution of these sources.The radiation resulting from their integrated emission has been computed and compared to the observed intensity and spectrum of the extragalactic -ray diffuse background (0.3–20 MeV), in order to derive quantitative constraints on the evolution and spectral properties of active galactic nuclei at -ray energies.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Immediate reports of GRBs in progress from HETE

The High Energy Transient Experiment (HETE) will be able to perform multiwavelength observations of-ray and X-ray bursts. HETE will potentially be able to localize-ray bursts to a precision of 20 arc-minutes if significant X-ray flux is detected from the burst; a precision of 20 arc-seconds is possible if there is also significant UV radiation from the burst. HETE will broadcast information about bursts detected within seconds of their detection. This VHF-band broadcast will be received at a number of secondary ground stations (SGS) dedicated to HETE, and forwarded to a central distribution site at MIT, from which it is sent to interested observers via Internet.     

Two visual observations of relevance to the search for optical counterparts of gamma-ray sources

We draw attention to a visual observation of a brief flash from Lyrae, observed by Heis in 1850, which resembles the optical burst detected electronically by Wdowiak and Clifton (1985) from Cam in 1969.Visual observation by the author of a second magnitude flash of very short duration is shown to originate from planar reflection from a very distant satellite. Such flashes will contribute to the noise in all-sky searches for optical counterparts of -ray bursters.     

Observations of helium and hydrogen emission in quiescent prominences

Observations of a number of helium triplet ( 10830, 4713, 4471, 3889, 4026) and hydrogen (H, , , ) emission line intensities in six quiescent prominences are presented. The regions of prominence and neighboring corona were raster-scanned by the telescope, and all lines were measured concurrently at each point. The instrumental field of view was 5 × 20. The results are compared with previous observations and theory. In particular, the intensity of the 10830 emission relative to the other triplets is found to differ strongly from the predictions of the recent detailed calculations of Heasley et al. (1974) for model quiescent prominences.     

Spectrophotometry of some Be stars

The continuum energy distribution of six Be stars, namely 25 Cyg, 31 Peg, HR 8758, 14 Lac, 12 Vul, and Psc, in the wavelength region 3200–7800 Å, are presented. Comparing the observed energy distributions with those of theoretical models given by Kurucz (1979), their effective temperatures are determined.A near-infrared excess emission at wavelengths above 6000 Å is seen in most of the stars.     

The empirical determination of damping constants in the solar photosphere

We determine empirical damping constants for 73 selected Fe i lines following the method of Gurtovenko and Kondrashova (1980), employing high-quality observations and the accurate list of Fe i oscillator strengths by Gurtovenko and Kostik (1980).The results show: (i) No increase of the enhancement factor to van der Waals broadening with excitation potential, as predicted by Edmunds (1975), and with the frequency of the transition (Figure 1); (ii) a substantial part of the commonly-used enhancement factor for weaker lines is not due to collisional damping (Figure 2), but to a misrepresentation of the inhomogeneous structure of the deep photosphere. This false damping effect is not seen in the stronger lines which yield an average damping constant : 1.3₆ 1.5 ₆.     

Magnetically distorted polytropes

The fundamental frequencies of the non-radial mode of oscillation belonging to the second harmonic (l=2) of magnetically distorted polytropic gas spheres are evaluated in the second approximation by a variational method. The magnetic field is assumed to have both the toroidal and the poloidal components. We find that the frequencies of oscillation are increased due to the presence of the magnetic field and that these depend only slightly on the value of , the ratio of the specific heats. We have also determined the value of <1+1/n for the mode of oscillation which exhibits convective instability. This value is lower than the one which is obtained in the absence of a magnetic field.     

The contribution of normal galaxies to the low energy gamma-ray cosmic diffuse background

Theories to explain the origin of the cosmic diffuse -ray background generally fall into one of two broad categories: those which attribute the emission to particle interctions in intergalactic space and those which attribute it to the summation of numerous, unresolved discrete sources, including normal field galaxies, active galactic nuclei and clusters of galaxies. Strong support for the latter interpretation is given by recent measurements of -ray emission from external galaxies, mainly Seyfert galaxies. Their summed contribution has been evaluated elsewhere; here instead, we use recent observational data on the -ray emission from our own galaxy to estimate the contribution of normal galaxies to the cosmic diffuse -radiation. Our result indicates that this contribution is limited to less than 0.1% and can therefore be neglected.     

Scanned superresolution in theory and practice

We discuss some theoretical and practical considerations relating to achieving spatial resolution higher than /D, the nominal diffraction-limited resolution of a telescope of diameter D. Although the Nyquist criterion is often invoked as a justification for choosing pixel scales in array cameras, higher sampling densities are favored for imaging at and somewhat beyond the diffraction limit.     

A reexamination of the Gum nebula

A detailed analysis of interstellar measurements in the direction of the Gum nebula is carried out. The ionized region is shown to have an angular radius of 18° and appears to be bounded by a shell of neutral gas. The mean electron temperature deduced from radio-frequency absorption measurements is found to be 8500K. These parameters suggest that the nebula is the normal and possibly evolved Hii region of Pup and ² Vel, rather than the fossil Strömgren sphere of the Vela supernova as suggested by Brandtet al. (1971).     

AGN Detection Ranges of MACE and TACTIC Imaging Gamma-Ray Telescopes

Two high-sensitivity imaging gamma-ray telescopes, MACE and TACTIC, are being set up at Mt. Abu, India, for making detailed spectral and temporal investigations of galactic and extragalactic gamma-ray sources in the photon energy bands. E 20–200 GeV and 0.5–5TeV respectively. Here, we estimate the effective red-shift ranges of these telescopes for detecting gamma-ray signals from the EGRET-detected AGN's and identify the most likely candidate-sources for detection by the MACE and the TACTIC, under the assumption that the EGRET-inferred power-law spectra for these AGN's extend into the GeV-TeV range without any change in shape. Extremum bounds of the relevant intergalactic background radiation fields, suggested by various observational and theoretical considerations, have been used to estimate the attenuation of VHE gamma-ray fluxes due to photon-photon pair-production process, leading to the desired z-ranges for these instruments.