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Charles, P. A.

Publications and source records attributed to Charles, P. A..

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NASA NTRS1980

The 1978 X-ray and optical outburst of Aquila X-1 /4U 1908+00/

During the summer of 1978 the recurrent transient X-ray source, Aquila X-1, underwent its first major outburst in two years. This paper presents the results of extensive X-ray and optical observations of this event, which lasted for about two months. The peak X-ray luminosity was about 1.3 times that of the Crab and exhibited spectrum-dependent flickering on time scales of about 5 minutes. In addition, one very large flare was observed about one month after maximum that was also correlated with spectral changes. During this flare the previously identified optical counterpart brightened from V = 19 to a peak of V = 14.8, where it was distinctly blue (U - B = 0.4), and then reddened during the decay. These observations are interpreted in terms of a standard accretion disk model with particular emphasis on the similarities to Sco -1 and other dwarf X-ray systems.

Charles, P. A.↗
NASA NTRS1980

X-rays from RS Canum Venaticorum systems - A HEAO 1 survey and the development of a coronal model

The paper presents HEAO 1 low-energy X-ray observations of 59 known or suspected RS CVn systems cited in the lists of Hall (1976), Eggen (1978), and the circulars of the Working Group on RS CVn systems of IAU Commission 42. The data are used to argue against the validity of the minimum flux coronal models. A coronal loop model is used to derive expressions for the loop parameters in terms of observable quantities, and find acceptable solutions for RS CVn systems. It is concluded that the difference between solar activity and that observed in RS CVn systems may be merely a matter of scale.

Walter, F. M.↗
NASA NTRS1980

X-ray line emission from the Tycho supernova remnant

The A-2 experiment onboard HEAO 1 has observed the X-ray spectrum of the Tycho supernova remnant in the energy range 0.5-20 keV. The spectrum reveals four significant line features: the K(alpha) lines of silicon, sulfur, and iron; and the L lines of iron. Comparisons between the silicon and sulfur equivalent widths and K(alpha) iron line energies of Tycho and Cas A suggest that the X-ray emitting plasma in Tycho is further from collisional ionization equilibrium than that in Cas A.

Pravdo, S. H.↗
NASA NTRS1979

X-ray line emission from the Tycho supernova remnant

The observation of the X-ray spectrum of the Tycho supernova remnant in the energy range 0.5 to 20 keV is discussed. Four significant line features in the spectrum: The K alpha lines of silicon, sulphur, and iron; and the L lines of iron are examined. Comparisons between the silicon and sulphur equivalent widths and K alpha iron line energies of Tycho and Cas A are discussed. Suggest that the X-ray emitting plasma in Tycho is further from collisional ionization equilibrium than that of Cas A.

Pravdo, S. H.↗
NASA NTRS1979

Soft X-ray emission from 2A 0311-227, an AM Herculis-type system

The low-energy detectors on board the HEAO 1 satellite have observed a variable soft X-ray source consistent with the location of 2A 0311-227. The maximum observed intensity is about 2 x 10 to the -11th erg/sq cm-sec. (0.18-0.43 keV) and the temperature must be less than 10 million K. The source is interpreted as a member of the AM Herculis class on the basis of the optical identification with a magnetic binary system.

Charles, P. A.↗
NASA NTRS1979

Simultaneous X-ray and optical observations of the flaring X-ray source, Aquila A-1

During the summer of 1978 the recurrent transient X-ray source, Aquila X-1, underwent its first major outburst in two years. The results of extensive observations at X-ray and optical wavelengths throughout this event, which lasted for approximately two months are presented. The peak X-ray luminosity was approximately 1.3 times that of the Crab and exhibited spectral dependent flickering on timescales approximately 5 minutes. The observations are interpreted in terms of a standard accretion disk model withparticular emphasis on the similarities to Sco X-1 and other dward X-ray systems, although the transient nature of the system remains unexplained. It was found that Aquila X-1 can be described adequately by the semi-detached Roche lobe model and yields a mass ratio of less than or approximate to 3.5.

Bowyer, C. S.↗
NASA NTRS1979

Soft X-ray spectrum and structure of the supernova remnant G65.2+5.7

The HEAO 1 low energy detectors have observed the supernova remnant G65.2+5.7 in the energy range 0.2-2.5 keV. A strong, extended source of X-ray emission is observed coincident with the eastern side of the optical filamentary structure. The characteristic temperature of the emission is in the range 2-3.8 x 10 to the 6th K, and evidence is found for line emission at about 0.8 keV in the X-ray spectrum. These observations imply a shock velocity (425 km/s) and age (about 20,000 yr) that markedly disagree with the estimates from optical and radio observations.

Mason, K. O.↗
NASA NTRS1979

The interstellar oxygen edge in the X-ray spectrum of the Crab Nebula

Measurement of the X-ray spectrum of the Crab Nebula by the HEAO 1 A-2 low-energy (0.15-3 keV) detectors is reported. The spectral restoration techniques described by Blissett and Cruise (1979) and Kahn and Blissett (1979) are used to demonstrate explicitly the existence of a 'negative' feature near 0.5 keV that is consistent with the edge expected due to absorption by interstellar oxygen. For a simple power-law model with photon index -2.08, the data require a column density of 3.0 + or - 0.1 x 10 to the 21st per sq cm and an oxygen abundance of 1.0 + or - 0.1 times the Brown and Gould (1970) value. Satisfactory fits can also be achieved with the more sophisticated Fireman (1974) models, but no model can be adjusted to fit the data without the inclusion of interstellar oxygen.

Charles, P. A.↗
NASA NTRS1979

Discovery of X-ray emission from two southern supernova remnants

Two soft X-ray sources positionally coincident with the supernova remnants PKS 1209-52 and RCW 103 have been discovered by using the A-2 experiment on HEAO 1. Their measured fluxes are, respectively, about 1.4 x 10 to the -10th erg/cm-sec (0.2-1.0 keV) and about 1.8 x 10 to the -10th erg/cm-sec (0.6-2.0 keV). Spectral data are used to derive physical parameters for each remnant. For PKS 1209-52 the parameters are suggestive of the remnant's being in an advanced evolutionary phase, with shock-heated interstellar material producing the soft X-ray emission. RCW 103, in contrast, is known from radio and optical data to be in an earlier evolutionary phase, and the soft X-ray flux is most likely due to emission originating in a reflected shock wave or in plasma evaporated from shock-heated interstellar clouds.

Tuohy, I. R.↗
NASA NTRS1978

Positions and identifications for galactic X-ray sources 2A1822-371 and 2S1254-690

Precise positions and optical identifications for the galactic X-ray sources 2A1822-371 (4U 1822-37) and 2S 1254-690 (4U 1254-69) were obtained by used of a described scanning modulation collimator on HEAO 1 and are reported. Neither is thought to be substantially reddened since both show strong ultraviolet excesses and are located at a relatively high galactic latitude. The distance of 4U 1254-69 is estimated to be approximately 10 kpc.

Griffiths, R. E.↗
NASA NTRS1978

Spectroscopy of the extreme-ultraviolet source Feige 24 - The binary orbit and the mass of the white dwarf

Results are reported for coude spectroscopy of the extreme-ultraviolet white dwarf Feige 24. Radial velocities of the H-alpha, He I 5876-A, and He I 6678-A emission lines, and the underlying M-dwarf absorption features, were determined from spectrograms obtained with the Lick 3-m telescope. The velocities show a binary period of 4.239(+ or - 0.0015) days. The emission-line and absorption-line velocities agree in phase, which indicates that the emission lines originate in the atmosphere of the M-dwarf secondary as a result of reprocessing of the EUV radiation. This effect is modeled, and the observed amplitude of the emission-line variability is used to place a lower limit on the orbital inclination. From these and other data it is shown that the mass of the white dwarf lies between 0.46 and 1.24 solar masses. Some possible implications for the evolution of binary stars are briefly discussed.

Thorstensen, J. R.↗
NASA NTRS1978

The soft X-ray spectrum of Capella - Discovery of intense line emission

A soft X-ray spectrum of Capella has been obtained with the HEAO 1 satellite. Only models which include intense line emission at 0.85 keV can adequately explain the data. The spectrum is consistent with emission from an isothermal solar-abundance plasma with a temperature of 10.5 million (+ or - 2.8 million) K. It is also consistent with a blend of temperatures between 4 and 20 million K. The intensity of the X-ray emission is constant over 2 days and similar to the intensity observed by ANS.

Cash, W.↗
NASA NTRS1977

Radio and X-ray observations of NGC 1851 and NGC 1904

The globular clusters NGC 1851 and NGC 1904, have been observed simultaneously at radio and X-ray wavelengths with the aid of the Green Bank interferometer and the proportional counter on OAO Copernicus. The combined radio and X-ray observations provided simultaneous coverage of each subject for a total of about one hour. Data concerning the radio frequency observations of the two globular clusters are presented in a table. The X-ray counts per frame are shown in a graph as a function of time, taking into account a five day interval in December 1975.

Johnson, H. M.↗
NASA NTRS1976

The X-ray structure of the Perseus cluster of galaxies

The X-ray source in the Perseus cluster has been studied both by the Copernicus satellite and by sounding-rocket instruments flown by Columbia University. The spatial and spectral data from these observations are examined. A surface brightness distribution is obtained which shows that the source consists of a compact core associated with NGC 1275 and a more extended emission volume. The structure of the central core region has been examined by means of an image reconstruction technique, revealing a steeply declining luminosity function and north-south elongation. The spectrum of the emission of the core region is distinguished from that of the surrounding area and found to be consistent with a hydrogen column density of 2.5 x 10 to the 21th/sq cm. A comparison between the isothermal and adiabatic gas sphere models is presented which shows the need for at least two components to provide the X-ray emission.

Wolff, R. S.↗
NASA NTRS1976

The X-ray variability of Vela X-1 (3U 0900-40)

From observations of Vela X-1 with the MSSL 2.5-7.5 keV detector onboard Copernicus, the behavior of the source can be characterized by three phases: (1) high intensity, (2) low intensity, and (3) eclipse. Combining data from the 1972 Uhuru observations with eclipse observation yields a binary period of 8.963 + or - 0.001 days with zero phase on 1975 Feb. 6.97 + or - 0.04 UT. The low intensity phase is interpreted as being due to increased absorption in an accretion wake traveling across the line of sight (the spectral slope remains relatively constant throughout the cycle). Another period of enhanced absorption immediately after exit from eclipse may be due to a bow shock. Comparison of the two observations suggests that these structures vary from cycle to cycle and, since the orbital period is long, probably during each cycle.

Charles, P. A.↗
NASA NTRS1975

Soft X-ray search of centre of Cygnus Loop

Equipment on the Copernicus satellite has been used to search for evidence of a compact object in the center of the Cygnus Loop supernova remnant. Rocket measurements reported by Rappaport et al. (1973) indicate that a central object exists. However, the study conducted with the aid of the satellite was negative. This negative result could indicate that the X-ray source was simply not in its high-intensity mode at the time of observation, or could arise because the source is at some other location in the Loop.

Snyder, W. A.↗