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At least 181 records · Page 10
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NASA NTRS1982

Spectrophotometry of the Cygnus Loop

New spectrophotometry for 17 filaments in the Cygnus Loop are presented. A wide variety of spectral properties is observed within the remnant. Comparisons with published shock models indicate significant differences between the models and observations with the range of observed line intensities not well described by models with varying shock velocity. Interference-filter photographs of the remnant's northeastern, northwestern, and western nebulae in H-alpha + N II, S II, O III, and O I show radial variations of line emission intensity with a scale much larger than that expected from a shock moving through a uniform medium. It is suggested that the emission variations and apparent ionization structure are the result of the shock front encountering an inhomogeneous interstellar medium.

Fesen, R. A.↗
NASA NTRS1982

Optical spectrophotometry of the Cygnus Loop

Cygnus Loop filament optical spectral properties are discussed, with emphasis on comparisons between observational and model data. Some line intensities are correlated among filaments in such a way as to suggest that a well resolved ionization structure behind the shock front is being observed. Departures from the steady flow condition assumed in the present model are found to be necessary in order to improve agreement with observations.

Fesen, R. A.↗
NASA NTRS1983

The nature of NML Cygnus

Since the discovery of NML Cyg by Neugebauer et al. (1965), its nature has been uncertain because its distance and luminosity are not known. NML Cyg is partially surrounded by an H II region which is heavily obscured by intervening interstellar matter. This H II region has been mapped in the continuum at 21 cm, and it seems clearly associated with the star. In the present investigation it is proposed that the H II region has been created by ionizing radiation from the Cyg OB2 association. The close agreement between the observed structure of the H II region and that predicted by the considered model provides strong evidence that NML Cyg is indeed at the distance of the association, 2 kpc. At this distance it is among the most luminous red stars shown. Because of its high luminosity, NML Cygnus is probably a massive star of perhaps 50 solar masses.

Morris, M.↗
NASA NTRS1983

The broad-band X-ray spectrum of Cygnus X-2

Cygnus X-2 was observed with the broad-band X-ray spectroscopy experiment, HEAO 1 A-2, in the energy range 0.4-18 keV for four intervals of approximately 31 s over the course of 5 days in 1977. The spectra can be adequately represented by single-temperature thermal bremmstrahlung continua with temperatures ranging from 3.7 x 10 to the 7th K to 6.4 x 10 to the 7th K. An examination of the spectra and the spectra-luminosity relationship effectively rules out one degenerate dwarf model for the X-ray emission. The far-UV continuum emission could be dominated by this continuum component during X-ray high states, an effect which would be detected in optical UV line observations. A Comptonized X-ray cloud around a neutron star remains a viable model for the observed X-ray spectra.

Pravdo, S. H.↗
NASA NTRS1983

Digital analysis of narrow-band imagery of the Cygnus Loop

Calibrated surface brightness and spectral line ratio maps which involve emission from six different ionic species are derived from the digital analysis of narrow passband direct imagery of a field in the southeast part of the Cygnus Loop SNR, and the spatial structure is categorized on the basis of the presence or absence of features in images isolating different emission lines. While features present in all emission lines are found to have spectral behavior consistent with steady flow shock model calculations for a range of v(s), the features present only in such 'hot' lines as the O III forbidden line 5007 A are found to account for relative intensities of that line greater than can be matched by steady flow model calculations.

Parker, R. A. R.↗
NASA NTRS1983

A thermal interpretation of the X-ray spectra of quasars, active galactic nuclei, and Cygnus X-1

It is shown that spherically accreting black hole models emitting bremsstrahlung radiation provide an excellent fit to the observed X-ray spectrum of typical active galactic nuclei, as well as of 3C 273 and Cygnus X-1. The spectral slope remains approximately constant under changes of luminosity, as required by observations. Only one parameter, T(max), is needed for the fit.

Meszaros, P.↗
NASA NTRS1983

The states of Cygnus X-1

HEAO 3 High Resolution Gamma-Ray Spectrometer observations of Cygnus X-1 in the 48-300 keV range, along with correlated 3-6 keV Ariel 5 data, have led to the identification of a new state for this object with low flux in both the soft and hard X-ray regions. This is followed by a slow increase in the hard X-ray flux to the normal low state of the source, characterized by a low intensity soft X-ray emission and a high intensity hard X-ray emission. The new 'superlow' state is interpreted in terms of the Compton model, where the X-ray emission arises from the scattering of low energy photons and one-billion K hot electrons. The superlow state may be seen as one extreme of a continuum of low state behavior, given that Compton cooling should begin to quench the hot electrons at higher soft X-ray fluxes, giving rise to the softer X-ray spectrum and to the anticorrelation in changes in the soft and hard X-ray fluxes.

Ling, J. C.↗
NASA NTRS1983

The structure and emission spectrum of a nonradiative shock wave in the Cygnus Loop

The ultraviolet spectrum of a Balmer-line filament located just outside the main body of optical filaments in the Cygnus Loop confirms the nonradiative shock wave theory for its origin. This theory is extended to include the hydrogen two-photon continuum, and the possibility of slow electron-ion equilibration in the postshock gas is considered. The shock velocity is inferred from the H-alpha profile. Comparison of model calculations with optical and ultraviolet spectra favors Coulomb equilibration behind a 170 km/s shock over models with rapid equilibration due to plasma turbulence. Elemental abundances in the preshock gas are found to be typical of diffuse interstellar clouds. The ram pressure behind the shock is higher than the pressures inferred for bright optical filaments. It is suggested that the bright optical filaments are regions of thermally unstable cooling behind shocks faster than the shock velocities inferred from their optical and ultraviolet spectra.

Raymond, J. C.↗
NASA NTRS1984

Millisecond variability of Cygnus X-1

The May 7, 1978 HEAO 1 spacecraft observations of Cygnus X-1 with a 10 microsec time resolution have revealed 3 ms variability, as well as a 300 ms component and features which extend to 6 s. Emission on time scales shorter than 1 s can be approximately modeled as a superposition of uncorrelated 3 ms and 300 ms shot contributions, both with peak luminosities near 10 to the 37th ergs/s, and with shot rates of about 10/s and 0.2/s, respectively. The 3 ms variability is consistent with the time scales expected for variability near the inner edge of a viscous accretion disk that surrounds a 10-solar mass black hole.

Meekins, J. F.↗
NASA NTRS1984

Einstein Observations of the Cygnus Loop

The imaging instruments on the Einstein Observatory were used to study the detailed X-ray structure of the Cygnus Loop. An arc minute resolution map of the 0.1-4.0 keV emission from the entire remnant was obtained with the imaging proportional counter (IPC). The bright northeastern and western limbs were mapped with the high resolution imager (HRI). The large-scale structure of the Loop is well modeled as a limb-brightened shell of hot gas at 2 x 10 to the 6th K created by the expansion of a blast wave into an inhomogeneous interstellar medium. The pulse-height data from the IPC indicate that the center of the remnant is generally hotter than the limb with temperatures as high as 4 x 10 to the 6th K. The X-ray maps show good general correlation with large-scale radio and optical structure, and in some cases good detailed correlation with faint optical filaments dominated by hydrogen line emission.

Ku, W. H.-M.↗
NASA NTRS1984

The S201 far-ultraviolet imaging survey. II - A field in Cygnus

Far-ultraviolet imagery of a region in Cygnus, a 20 deg diameter field centered near (1950) R. A. 21 h 31.2 m decl. +37 deg 25 arcmin, was obtained by the S201 far-ultraviolet camera during the Apollo 16 mission. In a 10 minute exposure covering the 1250-1600 A wavelength range (effective wavelength 1400 A), 730 star images can be detected, corresponding to a limiting ultraviolet magnitude of about m (1400) = 10. Assuming nominal interstellar extinction values in this region near the galactic plane, this result corresponds to the detection of A0 V stars to a distance of 300 pc and of B0 V stars to 1500 pc. Uncertainties in spectral classification and interstellar extinction for individual objects are probably more significant than calibration or measurement errors. Most of the objects detected are identified with stars in the Smithsonian Astrophysical Observatory Star Catalog (1966), or the Catalog of Stellar Identifications (1979) or both, but 87 objects remain unidentified (or are identified with late-type stars).

Carruthers, G. R.↗
NASA NTRS1984

Evidence for weak X-ray burst emission from Cygnus X-2 and GX 17 + 2

Events resembling weak X-ray bursts have been detected from the two strong persistent sources, Cygnus X-2 and GX 17 + 2. Both events exhibit rise times less than 2.56 s and 1.2-14.4 keV decay times of approximately 5-10 s. The event from Cyg X-2 is similar to a type-I burst in terms of blackbody temperature and peak luminosity, and it can be plausibly interpreted as a thermonuclear flash from an accreting neutron star. The GX 17 + 2 event exhibits a considerably harder X-ray spectrum which makes it unlikely to be a thermonuclear flash. It is suggested that this event arises from an accretion instability as in type-II bursts.

Kahn, S. M.↗
NASA NTRS1984

Cygnus X-1 revisited

The X- and gamma-ray observations of Cygnus X-1 and their theoretical interpretations are reviewed, with emphasis on new developments since the mid-1970s. The overall data base at present is most consistent with the inverse Compton model by hot thermal electrons of T(e) of about 10 billion K, for the hard X-ray luminosity (10-200 keV). However, the origin of the soft X-rays (not greater than 10 keV) in high states and gamma rays (greater than 200 keV) remain unsettled.

Liang, E. P.↗
NASA NTRS1985

Observations of Cygnus X-3 above 10(15) eV from 1979 - 1984

The ultra high energy gamma-ray source, cygnus X-3, has been observed more or less continuously with an array sensitive to 10 to the 15th power ev primaries between 1 Jan. 1979 and 31 Dec. 1984. There is evidence for time variability in the phase of gamma-ray emission over this period.

Lambert, A.↗
NASA NTRS1985

1000 GeV gamma rays from Cygnus X-3: An update

Measurements of 1000 GeV gamma-rays from Cygnus X-3 made with the University of Durham facility at Dugway, Utah in 1981/82 are reviewed. The light curve of the 4.8 hour modulated emission is updated and shows evidence significant at the 4.4 sigma level for strong emission (9% of the cosmic ray rate) at phase 0.625 and less significant (1.4 sigma level) indications of weaker emission (3% of the cosmic ray rate) at phase 0.125. The effect constituting the excess on the few nights showing the strongest emission appears to arise from the smallest Cerenkov light signals suggesting a steep gamma-ray spectrum. The 1982 data have been searched unsuccessfully for evidence of emission at phase 0.2, in coincidence with the results from the ultra-high energy (extensive Air Showers (EAS) measurements in 1979-1982. A systematic investigation of a long term variation in the strength of the peak of the 4.8 hr modulated 1000 GeV gamma-ray emission has been made. We find that in addition to the approximately 34 d variation reported by us previously, a stronger effect exists at around 19d.

Chadwick, P. M.↗
NASA NTRS1985

High-resolution spectrum of Cygnus X-1

A high resolution spectrum of Cygnus X-1 in the 45 to 600 keV range is presented. The measurement was made by the HEAO 3 gamma ray spectrometer during 82 days in the fall of 1979 and spring of 1980, when the source was in its normal low state. Results of a search for narrow emission lines from the source are reported. The spectrum shows no significant narrow features. The 3 delta upper limit to a narrow 511 keV annihilation line is 3 x 0.0001 photons/sq cm/s. There is also no evidence in HEAO 3 broadband data above 500 keV for the broad annihilation feature observed by HEAO 1.

Ling, J. C.↗
NASA NTRS1985

A two-dimensional spectrum of a nonradiative shock filament in the Cygnus Loop

A two-dimensional spectrum of a nonradiative filament in the Cygnus Loop covering the spectral region 4600-7500 A is presented and discussed. Emission lines of H, He II, and forbidden O III, as well as extremely faint N II and S II forbidden lines, were detected. Small spatial differences for some of the filament's emission lines and a possible velocity gradient across the filament's western edge were also observed. Measured relative line intensities are combined with previous nonradiative data and are compared to new nonradiative shock models which treat one, two, and three postshock temperature distributions. It is found that two- and three-temperature models provide a good fit to the observed relative line intensities. The observed weak N II and S II forbidden line emissions are interpreted as due to either radiative shock emissions produced by small density fluctuations in the shocked cloud (e.g., cloud cores), or faint dust grain reflection of the remnant's bright radiative filament emission.

Fesen, R. A.↗
NASA NTRS1985

Anatomy of a cosmic-ray neutrino source and the Cygnus X-3 system

The effects of an intense beam of ultra-high energy cosmic rays from a compact object in the Cygnus X-3 binary system hitting the companion star, and of the subsequent production of secondary neutrinos, are examined. A maximum allowable beam luminosity of about 10 to the 42nd erg/s is found for a system containing a 1-10 solar mass main sequence target star. The proton beam must heat a relatively small area of the target star to satisfy observational constraints on the resulting stellar wind. With such a model, the neutrino to gamma-ray flux ratio of about 1000 can result from a combination of gamma-ray absorption and a large neutrino to gamma-ray duty cycle ratio. It is found that the high density of the atmosphere resulting from compression by the beam leads to pion cascading and a neutrino spectrum peaking at 1-10 GeV energies.

Stecker, F. W.↗