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  • 1
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    Discovery of low-redshift X-ray selected quasars - New clues to the QSO phenomenon (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The identification of six X-ray sources discovered by the Einstein Observatory with X-ray quasars is reported, and the properties of these X-ray selected quasars are discussed. The four high-latitude fields of 1 sq deg each in which the Einstein imaging proportional counter detected serendipitous X-ray sources at intermediate exposures of 10,000 sec were observed by 4-m and 1.5-m telescopes, and optical sources with uv excesses and emission line spectra typical of many low-redshift quasars and Seyfert 1 galaxies were found within the 1-arcsec error boxes of the X-ray sources. All six quasars identified were found to be radio quiet, with low redshift and relatively faint optical magnitudes, and to be similar in space density, colors and magnitude versus redshift relation to an optically selected sample at the same mean magnitude. X-ray luminosity was found to be well correlated with both continuum and broad-line emission luminosities for the known radio-quiet quasars and Seyfert 1 galaxies, and it was observed that the five objects with the lowest redshifts have very similar X-ray/optical luminosity ratios despite tenfold variations in X-ray luminosity. It is concluded that photoionization by a continuum extending to X-ray energies is the dominant excitation mechanism in radio-quiet quasars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 239
    Format: text
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  • 2
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    Detection of large-scale alignment of Lick counts around Abell clusters (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The possible tendency of galaxies to be distributed in an aligned fashion, on sheets or filaments, on scales greater than the Abell radius, has been tested by a variety of statistics. The subject remains controversial because the statistical tests have either not been sensitive to the alignment or capable of unambiguously signalling alignment in a general, clumpy distribution of galaxies. The present approach combines ideas of Bingelli (1982) and Fry and Peebles (1980) in an examination of the cross correlation of Lick counts relative to the preferred direction defined by the cluster. This can substantially reduce the noise, and it is suggested on this basis that alignment has been found on large scales for Lick galaxy counts and Abell cluster positions and their angles.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 91; 471-477
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  • 3
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    The location and intensity of the X-ray source Centaurus A observed by the Astronomical Netherlands Satellite (1975)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 201; Nov. 1
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  • 4
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    A spectroscopic study of the optical counterparts of three X-ray bursters (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: One of the major puzzles in galactic X-ray astronomy is the nature of the galactic bulge sources in general and the X-ray burst sources in particular. The answer to this puzzle is likely to come from studies of the faint blue optical counterparts which have been identified with several of these sources. As part of such a study, some results of a detailed investigation of the spectroscopic properties of the optical counterparts to three X-ray bursters - 4U 1735-44, 4U 1636-53, and MXB 1659-29.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 234
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  • 5
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    Discovery of the optical counterpart of the transient X-ray burster Centaurus X-4 (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The paper deals with the discovery and subsequent study of the optical counterpart to an X-ray nova which is almost certainly the historical transient Centaurus X-4, first discovered in 1969 and then dormant for the past decade. It is shown that Cen X-4 is a clear example of a soft, transient X-ray burster. The most important consequence of the connection between bursters and soft transients is the support it gives to the hypothesis that bursters are accreting neutron stars in binary systems. The observations support the hypothesis that at least some of the light comes from an accretion disk, and that X-ray heating plays an important role in the optical emission.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 236
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  • 6
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    Identification of two hard X-ray emitting Be stars using the HEAO 1 scanning modulation collimator (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Using precise positions from the HEAO 1 Scanning Modulation Collimator experiment, two hard X-ray sources, 4U 0728 - 25 = 3A 0726 - 260 and 4U 2206 + 54 = 3A 2206 + 543, are identified with early-type stars. In both cases broad (10 A FWHM) H-alpha emission is detected. The UBV colors suggest that the optical counterparts are main-sequence B0-B2 stars at 2-6 kpc, implying a mean X-ray luminosity of order 10 to the 35th ergs/sq cm s (2-10 keV). The X-ray emission in both cases is highly variable, and it is suggested that they belong to the class of X-ray emitting Be stars, containing a neutron star in a widely separated binary system.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 280; 688-694
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  • 7
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    A 3-s delay in an optical burst from X-ray burst source MXB1735-44 (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Analysis of the arrival times of X-ray and optical bursts from the X-ray burst source MXB1735-44 shows that the onset of one optical burst was delayed 2.8 sec with respect to the X-ray burst. Observations of the pulses are accurate to less than or equal to five ms. The delay in the optical pulse may be attributed to light travel time in the system and/or the time needed to reprocess X-rays into light.
    Keywords: ASTROPHYSICS
    Type: Nature; 279; May 3
    Format: text
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  • 8
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    Determination of the mass of globular cluster X-ray sources (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The precise positions of the luminous X-ray sources in eight globular clusters have been measured with the Einstein X-Ray Observatory. When combined with similarly precise measurements of the dynamical centers and core radii of the globular clusters, the distribution of the X-ray source mass is determined to be in the range 0.9-1.9 solar mass. The X-ray source positions and the detailed optical studies indicate that (1) the sources are probably all of similar mass, (2) the gravitational potentials in these high-central density clusters are relatively smooth and isothermal, and (3) the X-ray sources are compact binaries and are probably formed by tidal capture.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 282; L13-L16
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  • 9
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    Optical studies of the X-ray globular cluster NGC 6624 (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Photographic, photometric, and spectroscopic studies of the core of the globular cluster NGC 6624 have been undertaken with the aim of obtaining some evidence regarding the location and nature of the associated X-ray source 3U 1820-30. The studies include an extended simultaneous observation with the SAS 3 satellite, which was carried out to search (unsuccessfully) for optical emission during X-ray bursts. All the results reported are shown to be negative, but serve to set some constraints on the source properties. The photometric results are used to derive a core radius of 5.0 + or - 0.5 arcsec (0.19 + or 0.02 pc at 8 kpc) and a central density of 110,000 solar masses per cu pc for the cluster. It is found that NGC 6624 is one of the most centrally dense globular clusters but otherwise normal and that the colors and spectrum of the nucleus are the same as those of the cluster as a whole. An X-ray source similar to HZ Her at maximum light is ruled out.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 224
    Format: text
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  • 10
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    Discovery of optical bursts from an X-ray burst source, MXB1735-44 (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The optical counterpart of 4U 1735-44, X-ray burst source MXB 1735-44, was studied with the 1.5-m telescope at Cerro Tololo, and by SAS 3, during the period between June 1 and 3, 1978. Explanations for the optical activity are discussed in terms of accretion instabilities onto a neutron star, or thermonuclear flashes on the neutron star itself. Attention is given to the timing of the bursts, which indicate that the burst source probably is located in a region within 1-2 light seconds from the X-ray source. In the binary case, it is noted that the optical emission is probably removed from the X-ray heated stellar atmosphere of the possible companion.
    Keywords: ASTROPHYSICS
    Type: Nature; 274; Aug. 10
    Format: text
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