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NGC 6995



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Far Ultraviolet Spectroscopic Explorer Spectroscopy of the XA Region in the Cygnus Loop Supernova Remnant
Spectra of the XA shock-cloud interaction region in the Cygnus Loopobtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) arepresented and analyzed. Several weak emission lines never beforedetected in the spectra of remnants are identified in a combinedspectrum from four bright regions. The intensities of the silicon linesmeasured in this spectrum show that the 120-200 km s-1 shocksare present in the XA region and are effective at liberating siliconfrom grains. Differences among the spectra imply that the shocked gashas a complex ionization structure. The strongest lines in the spectra,and also the most uniformly distributed, are O VI λλ1032,1038. Approximately 10% of the O VI emission from regions interior tothe main shock interaction arises from coronal (million-degree) gas.Model calculations show that the shock at the boundary between the cloudand remnant has a velocity of about 180 km s-1 and has sweptup a hydrogen column NH=1.66×1018cm-2, indicating that the blast wave encountered the cloudabout 5000 years ago. The O VI line profiles imply that the boundaryshock is dominated by a single broad component with an intrinsicvelocity width parameter, b~48 km s-1, which is probably theresult of bulk motions due to curvature of the shock front. Differencesbetween the O VI velocity profiles of two regions separated by 20" onthe sky suggest the presence of a concentrated region of dust, whichabsorbs emission from the shock on the far side of the cloud.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.

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.

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

[NE V] Imaging of the Cygnus Loop
We present continuum-subtracted images of a20'x20' region of the eastern Cygnus Loop (NGC6995) in the 3425 Å forbidden line of Ne V. The images revealbright linear filaments which are associated with, but not positionallycoincident with, bright features seen in Hα, [O III], and othernarrowband images. In some areas, the [Ne V] filaments define the edgeof X-ray-emitting regions. The filaments exhibit a peak surfacebrightness of 1.2x10-4 photons cm-2 s-1arcsec-2 at the top of the atmosphere in images with typicaldetection limits of ~10-5 photons cm-2s-1 arcsec-2. We present arguments that thesestructures are produced by radiative shock waves and discussimplications for the shock velocities and the three-dimensionalstructure of this section of the Cygnus Loop. We place limits on theimportance of thermal conduction-driven evaporation as a contribution tothe mass of X-ray-emitting gas. Lack of evidence of [Ne V] emissionresulting from thermal evaporation may have significance for supernovaremnants and interstellar medium models that rely heavily on theimportance of thermal evaporation.

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.

What lies at the Milky Way's Center ?
Not Available

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.

UIT Observations of the Cygnus Loop
Not Available

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.

Fabry-Perot observations of forbidden Fe X and Fe XIV in the Cygnus loop
Abstract image available at:http://adsabs.harvard.edu/abs/1989A&A...211..217B

An Fe X forbidden line 6374 A image of part of the Cygnus Loop
An image of part of the Cygnus Loop in the forbidden Fe X 6374 A line ata resolution of 2.5 arcsec is obtained, using a CCD preceded by aninterference filter with a 9.5 A bandpass. The data show a smoothintensity distribution associated with the supernova blast wave;superposed on this is a cloudlike surface-brightness pattern looselyassociated with the optical nebulosity. The structures have propertiesconsistent with evaporation of cloudlets into the blast wave interior.

Spectrophotometric Investigations of Filamentary Nebulae.
Not Available

The Cygnus Loop at 1420 MHz
Abstract image available at:http://adsabs.harvard.edu/abs/1971MNRAS.153..401M

Photographische Beobachtungen von Nebeln am Spiegelteleskop der Hamburger Sternwarte in Bergedorf
Not Available

Some of the dark markings on the sky and they suggest.
Not Available

Photographs of the Milky Way near 15 Monoceros[sic] and near epsilon Cygni.
Not Available

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Observation and Astrometry data

Right ascension:20h57m00.00s
Apparent magnitude:99.9

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 6995

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