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NGC 6992



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New λ6 cm observations of the Cygnus Loop
Radio continuum and polarization observations of the entire Cygnus Loopat λ6 cm wavelength were made with the Urumqi 25 m telescope. Theλ6 cm map is analysed together with recently published maps fromthe Effelsberg 100 m telescope at λ21 cm and λ11 cm. Theintegrated flux density of the Cygnus Loop at λ6 cm is 90± 9 Jy, which implies a spectral index of α=-0.40 ±0.06. This rules out any global spectral steepening up to λ6 cm.However, small spectral index variations in some regions of the sourceare possible, but there are no indications for any spectral curvature.The linear polarization data at λ6 cm show percentagepolarizations up to 35% in some areas of the Cygnus Loop, exceedingthose observed at λ11 cm. The Rotation Measure is around -21 radm-2 in the southern area, which agrees with previousobservations. However, the distribution of Rotation Measures is rathercomplex in the northern part of the Cygnus Loop, where the λ21 cmemission is totally depolarized. Rotation Measures based on λ11cm and λ6 cm data are significantly larger than in the southernpart. The difference in the polarization characteristics between thenorthern and southern part supports previous ideas that the Cygnus Loopconsists of two supernova remnants.

Hubble Space Telescope Imaging of the Primary Shock Front in the Cygnus Loop Supernova Remnant
We present Hubble Space Telescope WFPC2 narrowband Hα (F656N) and[O III] λ5007 (F502N) imaging of two fields on the northeasternlimb of the Cygnus Loop supernova remnant. This region provides anoutstanding example of the initial encounter between the primary blastwave and the surrounding interstellar medium. The Hα images showthe primary nonradiative shock front, which, when viewed edge-on, isunresolved at WFPC2 resolution. The [O III] images show portions offilaments that are beginning to become radiative, and in these imagesthe filaments are resolved, appearing fuzzy at WFPC2 resolution. The [OIII] filament regions are not bounded directly by Hα filaments,indicating that the shock emission from the nascent radiative region issufficient to fully ionize the local preshock gas. One field, imaged 4yr earlier in Hα with WFPC2, is used to study the proper motion ofthe filament and constrain any brightness variations over this timeperiod. In conjunction with improved models of nonradiative shocks,these data are used to place limits on the possible deceleration of thefilament and refine the distance to the Cygnus Loop, arriving at arevised value of d=540+100-80 pc (assuming nodeceleration). The second field imaged contains examples of coherentHα filaments with much more dramatic curvatures than identifiedpreviously. We discuss the possible reasons for the different morphologyof these filaments and conclude that they can be accommodated withrelatively modest variations in local density and shock velocity.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute (STScI), which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.

Radio emission from the Cygnus Loop and its spectral characteristics
We present a new sensitive 2675 MHz radio continuum map of the CygnusLoop, which is used in conjunction with 408 MHz, 863 MHz and 1420 MHzmaps from both the Effelsberg 100-m telescope and the DRAO SynthesisTelescope for a spectral analysis. Between 408 MHz and 2675 MHz we findan overall integrated spectral index of α = -0.42 ± 0.06 (S˜ να), close to previous results. There is noindication of a spectral break in the integrated spectrum. Spatiallyhighly varying and rather strong spectral curvature was previouslyreported, but is not confirmed on the basis of new, higher sensitivityobservations. We found spectral variations across the Cygnus Loopreaching up to Δα = 0.2 from a TT-plot analysis. Theflattest spectra are seen towards enhanced emission areas. Spectralindex maps produced between different frequency pairs, as well as allfour maps, revealed that there are at least three flat spectrum regions.In regions interior to the high emission filaments, we have detected atleast two spectral components across the whole object with α=-0.4and α=-0.6 towards northern and southern parts of the object,respectively.Based on observations with the Effelsberg 100-m radio telescope operatedby the Max-Planck-Institut für Radioastronomie (MPIfR), near Bonn,Germany.

Is the Cygnus Loop two supernova remnants?
The Cygnus Loop is classified as a middle-aged supernova remnant (SNR)located below the Galactic equator (l=74o,b=-8.6o) and 770 pc away from us. Its large size and littleconfusion with Galactic emission makes it an ideal test ground forevolutionary and structural theories of SNRs. New radio continuummapping of the Cygnus Loop at 2695 MHz with the Effelsberg 100-mtelescope provides indications that the Cygnus Loop consists of twoseparate SNRs. Combining this result with data from the literature weargue that a secondary SNR exists in the south with a recently detectedneutron star close to its center. Two interacting SNRs seem to be thebest explanation to account for the Cygnus Loop observations at allwavelengths.

A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction
The XA region of the Cygnus Loop is a complex zone of radiative andnonradiatve shocks interacting with interstellar clouds. We combine fivefar ultraviolet spectral observations from the Hopkins UltravioletTelescope (HUT), a grid of 24 IUE spectra, and a high-resolutionlong-slit Hα spectrum to study the spatial emission linevariations across the region. These spectral data are placed in contextusing ground-based optical emission-line images of the region and afar-UV image obtained by the Ultraviolet Imaging Telescope (UIT). Thepresence of high-ionization species (O VI, N V, and C IV) indicates ashock velocity near 170 km s-1 while other diagnosticsindicate vsh~140 km s-1. It is likely that a largerange of shock velocities may exist at a spatial scale smaller than weare able to resolve. By comparing C IV λ1550, C III λ977,and C III] λ1909, we explore resonance scattering across theregion. We find that a significant column depth is present at allpositions, including those not near bright optical or UV filaments.Analysis of the O VI doublet ratio suggests an average optical depth ofabout unity in that ion, while flux measurements of [Si VIII]λ1443 suggest a hot component in the region at just below106 K. Given the brightness of the O VI emission and the ageof the interaction, we rule out the mixing-layer interpretation of theUV emission. Furthermore, we formulate a picture of the XA region asthat of an encounter of the blast wave with a finger of dense gasprotruding inward from the pre-supernova cavity.

The rise and rise of the deep sky image
Presidential Address to the British Astronomical Association, 2000October 25

Small-Scale Anisotropy of Cosmic Rays above 10^19 eV Observed with the Akeno Giant Air Shower Array
With the Akeno Giant Air Shower Array, 581 cosmic rays above 10^19 eV,47 above 4x10^19 eV, and seven above 10^20 eV were observed until 1998August. The arrival direction distribution of these extremely highenergy cosmic rays has been studied. While no significant large-scaleanisotropy is found on the celestial sphere, some interesting clustersof cosmic rays are observed. Above 4x10^19 eV, there are one triplet andthree doublets within a separation angle of 2.5d, and the probability ofobserving these clusters by a chance coincidence under an isotropicdistribution is smaller than 1%. The triplet is especially observedagainst expected 0.05 events. The cos(theta_GC) distribution expectedfrom the dark matter halo model fits the data as well as an isotropicdistribution above 2x10^19 and 4x10^19 eV, but the fit with the darkmatter halo model is poorer than the isotropic distribution above 10^19eV. The arrival direction distribution of seven 10^20 eV cosmic rays isconsistent with that of lower energy cosmic rays and is uniform. Threeof the seven are members of doublets above about 4x10^19 eV.

Panoramic Views of the Cygnus Loop
We present a complete atlas of the Cygnus Loop supernova remnant in thelight of [O III] (lambda5007), Hα, and [S II] (lambdalambda6717,6731). We include low-resolution (25") global maps and smaller fields at6" resolution from observations using the Prime Focus Corrector on the0.8 m telescope at McDonald Observatory. Despite its shell-likeappearance, the Cygnus Loop is not a current example of a Sedov-Taylorblast wave. Rather, the optical emission traces interactions of thesupernova blast wave with clumps of gas. The surrounding interstellarmedium forms the walls of a cavity through which the blast wave nowpropagates, including a nearly complete shell in which nonradiativefilaments are detected. We identify nonradiative shocks around half theperimeter of the Cygnus Loop, and they trace a circle of radius R = 1.4d(19 pc) in the spherical cavity walls. The Cygnus Loop blast wave is notbreaking out of a dense cloud but is instead running into confiningwalls. Modification of the shock velocity and gas temperature due tointeraction of the blast wave with the surrounding medium introduceserrors in estimates of the age of this supernova remnant. The opticalemission of radiative shocks arises only where the blast wave encountersinhomogeneities in the ambient medium; it is not a consequence ofgradual evolution to a global radiative phase. Distance measurementsthat rely on this uniform blast wave evolution are uncertain, but theradiative shocks can be used as distance indicators because of thespherical symmetry of the surrounding medium. The interstellar mediumdominates not only the appearance of the Cygnus Loop but also thecontinued evolution of the blast wave. If this is a typical example of asupernova remnant, then global models of the interstellar medium mustaccount for such significant blast wave deceleration.

The ROSAT HRI X-Ray Survey of the Cygnus Loop
We describe and report progress on the joint U.S. and German campaign tomap the X-ray emission from the entire Cygnus Loop with the ROSAT HighResolution Imager. The Cygnus Loop is the prototype for a supernovaremnant that is dominated by interactions with the interstellar mediumand supplies fundamental physical information on this basic mechanismfor shaping the interstellar medium. The global view that thesehigh-resolution (FWHM ~ 10") observations provide emphasizes theinhomogeneity of the interstellar medium and the pivotal nature ofcloud--blast-wave interactions in determining the X-ray morphology ofthe supernova remnant. While investigating the details of the evolutionof the blast wave, we also describe the interstellar medium in thevicinity of the Cygnus Loop, which the progenitor star has processed.Although we do not expect the X-ray observations to be complete until1997 September, the incomplete data combined with deep H alpha imagesprovide definitive evidence that the Cygnus Loop was formed by anexplosion within a preexisting cavity.

Comet Hale-Bopp: Your Viewing Guide
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What lies at the Milky Way's Center ?
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Celestron Schmidt cameras: Theory and practice
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Glittering realms of the summer Milky Way.
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H-alpha images of the Cygnus Loop - A new look at shock-wave dynamics in an old supernova remnant
Attention is given to deep H-alpha images of portions of the east, west,and southwest limbs of the Cygnus Loop which illustrate several aspectsof shock dynamics in a multiphase interstellar medium. An H-alpha imageof the isolated eastern shocked cloud reveals cloud deformation and gasstripping along the cloud's edges, shock front diffraction andreflection around the rear of the cloud, and interior remnant emissiondue to upstream shock reflection. A faint Balmer-dominated filament isidentified 30 arcmin further west of the remnant's bright line ofwestern radiative filaments. This detection indicates a far morewesterly intercloud shock front position than previously realized, andresolves the nature of the weak X-ray, optical, and nonthermal radioemission observed west of NGC 6960. Strongly curved Balmer-dominatedfilaments along the remnant's west and southwest edge may indicate shockdiffraction caused by shock wave passage in between clouds.

Der Blick in die Milchstrasse.
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Forbidden coronal iron emission in the Cygnus Loop
Forbidden iron line images of parts of the Cygnus Loop supernova remnantare reported and discussed. Images in both the red and green lines onthe rim of NGC 6995 cannot be well interpreted in terms of cloudevaporation, and instead support the reflected shock model of Hester andCox (1986). On the northeast rim both lines are brightest at theradiative filaments of NGC 6992 and fade to invisibility in theremnant's interior, in agreement with the 'sheet model' for the CygnusLoop. Forbidden Fe X emission is also found just behind some of thenonradiative filaments lying northeast of the main optical nebulosity,at a location quantitatively consistent with the cosmic-ray shock modelof Boulares and Cox (1988). However, the forbidden Fe X and forbidden FeIV data taken together also qualitatively agree with a hydrodynamicshock and cavity explosion model for the event which created the CygnusLoop.

The Cygnus Loop at 408 MHz - Spectral variations, and a better overall view
Observations of the Cygnus Loop made at 408 MHz are presented. Radiospectral variations within the remnant are revealed by comparing the newobservations with existing data taken at 2.695 GHz. The brightnortheastern arc of the Cygnus Loop has a radio spectral index alpha ofroughly 0.35, whereas the southern portion of the remnant generallyshows steeper spectra, with alpha up to roughly 0.55. There is probablya portion of the faint radio emission from the northeastern arc that hasa flatter spectrum than the brighter radio emission nearby. Thesedifferences are discussed in relation to mechanism for producing radioemission in SNRs and to the overall structure of the remnant. Theobservation confirm that there is faint radio emission outside thebright limb-brightened 'shell' of the remnant, but this is limited tothe west only. The faint halo of X-rays around the remnant is discussedand interpreted as originating from dust scattering.

Find a supernova remnant.
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The Cygnus Loop - an Older Supernova Remnant
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A two-dimensional spectrum of a nonradiative shock filament in the Cygnus Loop
A two-dimensional spectrum of a nonradiative filament in the Cygnus Loopcovering the spectral region 4600-7500 A is presented and discussed.Emission lines of H, He II, and forbidden O III, as well as extremelyfaint N II and S II forbidden lines, were detected. Small spatialdifferences for some of the filament's emission lines and a possiblevelocity gradient across the filament's western edge were also observed.Measured relative line intensities are combined with previousnonradiative data and are compared to new nonradiative shock modelswhich treat one, two, and three postshock temperature distributions. Itis found that two- and three-temperature models provide a good fit tothe observed relative line intensities. The observed weak N II and S IIforbidden line emissions are interpreted as due to either radiativeshock emissions produced by small density fluctuations in the shockedcloud (e.g., cloud cores), or faint dust grain reflection of theremnant's bright radiative filament emission.

La constel lacio del cigne.
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Coronal (Fe) lines in supernova remnants - Nonequilibrium ionization models
Intensity distributions of forbidden coronal Fe lines have been computedfor Sedov blast-wave models of Cygnus Loop, Puppis A, and IC 443. Themodels assumed both equilibrium and nonequilibrium collisionalionization. The results show that, at shock speeds over 350 km/s,departures from equilibrium ionization are important in the forbidden Feintensities, while at slower shock speeds equilibrium calculations areadequate to represent the line intensities. The calculated intensitiesfail to match observed published observations in spatial distributionand relative intensities. This disagreement supports the view that localconditions at the site of forbidden Fe observations depart from thosepredicted by blast-wave models. It is concluded that publishedobservations are more closely related to evaporative shock/cloudletinteractions than to direct emission from collisionally ionized Fe inthe shock fronts.

When Explosions Collide
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On the Distance of the Supernova Remnant Cygnus Loop
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The Cygnus Loop at 1420 MHz
Abstract image available at:http://adsabs.harvard.edu/abs/1971MNRAS.153..401M

Radio emission from the Cygnus Loop
Abstract image available at:http://adsabs.harvard.edu/abs/1960MNRAS.121..543M

Optical observations of nonthermal galactic radio sources
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Photographic Studies of Nebulae
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1937ApJ....86..496D&db_key=AST

A general study of diffuse galactic nebulae.
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Stars having peculiar spectra. Thirteen new variable stars.
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Observation and Astrometry data

Right ascension:20h56m20.00s
Apparent magnitude:99.9

Catalogs and designations:
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NGC 2000.0NGC 6992

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