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  • 1
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    Observations of the X-ray Nova GRO J0422+32. 1: Outburst and the decay to quiescence (1995)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We present optical photometry and spectroscopy and Burst and Transient Source Experiment (BATSE) observations of the X-ray nova GRO J0422+32, obtained during outburst and its subsequent decay to quiescence. Although the X-ray and optical properties of GRO J0422+32 are broadly similar to those of other X-ray novae, it is unique in several respects. The unusually protracted decay to quiescence of the optical light curve has been punctuated by at least two minioutbursts of approximately 4 mag. The BATSE and optical outbursts are each separated by approximately 120 days. We find that the optical luminosity of GRO J0422+32 during the primary outburst is dominated by reprocessing of E greater than 10 keV X-rays. In contrast, the optical minioutbursts are most likely generated by an intrinsically bright disk rather than X-ray reprocessing: they do not appear to have any X-ray counterparts. Extremely broad (up to 6000 km/s FWZI) absorption lines have also been observed during both primary outbursts and minioutbursts. During the second minioutburst, H-alpha and H-beta emission was observed superposed on redshifted absorption features. We find that the interoutburst light curve of GRO J0422+32 may be inconsistent with an accretion disk instability as the origin of the minioutbursts. Finally, a transient 5.1/10.2 hr modulation, which may be related to the orbital period, has been observed during roughly half of our observations. However, confirmation of the orbital period must await observations in quiescence.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 441; 2; p. 786-799
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  • 2
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    The Extreme Spin of the Black Hole Cygnus X-1 (2011)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: Remarkably, an astronomical black hole is completely described by the two numbers that specify its mass and its spin. Knowledge of spin is crucial for understanding how, for example, black holes produce relativistic jets. Recently, it has become possible to measure the spins of black holes by focusing on the very inner region of an accreting disk of hot gas orbiting the black hole. According to General Relativity (GR), this disk is truncated at an inner radius 1 that depends only on the mass and spin of the black hole. We measure the radius of the inner edge of this disk by fitting its continuum X-ray spectrum to a fully relativistic model. Using our measurement of this radius, we deduce that the spin of Cygnus X-1 exceeds 97% of the maximum value allowed by GR.
    Keywords: Astronomy
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  • 3
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    The Extreme Spin of the Black Hole in Cygnus X-1 (2011)
    In:  Other Sources
    Details
    Publication Date: 2019-08-26
    Description: The compact primary in the X-ray binary Cygnus X-1 was the first black hole to be established via dynamical observations. We have recently determined accurate values for its mass and distance, and for the orbital inclination angle of the binary. Building on these results, which are based on our favored (asynchronous) dynamical model, we have measured the radius of the inner edge of the black hole s accretion disk by fitting its thermal continuum spectrum to a fully relativistic model of a thin accretion disk. Assuming that the spin axis of the black hole is aligned with the orbital angular momentum vector, we have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a spin parameter a* 〉 0.95 (3(sigma)). For a less probable (synchronous) dynamical model, we find a. 〉 0.92 (3 ). In our analysis, we include the uncertainties in black hole mass, orbital inclination angle, and distance, and we also include the uncertainty in the calibration of the absolute flux via the Crab. These four sources of uncertainty totally dominate the error budget. The uncertainties introduced by the thin-disk model we employ are particularly small in this case given the extreme spin of the black hole and the disk s low luminosity.
    Keywords: Astronomy
    Type: GSFC.JA.5657.2011 , The Astrophysical Journal; 742; 2
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  • 4
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    Evidence for a black hole in the X-ray binary Nova Muscae 1991 (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Optical photometry and spectroscopy of the X-ray Nova Muscae 1991 in quiescence reveal an orbital period of 10.398 +/- 0.014 hr and an absorption-line velocity curve consistent with a sinusoidal modulation at a half-amplitude of 409 +/- 18 km/s. The spectral type of the secondary star is in the range K0 V to K4 V. The value of the mass function, 3.1 +/- 0.4 solar mass, is a conservative lower limit on the mass of the compact primary and suggests that the primary is a black hole. Further considerations of the binary inclination angle and the mass of the secondary strengthen the black hole model. The folded light curves in the I band and the B + V band resemble ellipsoidal variations, with an additional brightening near one of the maxima in the B + V band. The orbital period is 1.4 percent shorter than the photometric period observed during outburst, as expected if the outburst modulations are analogs of 'superhumps' in dwarf novae. In quiescence, the optical properties of the X-ray binary Nova Muscae 1991 bear a striking resemblance to the black hole binary A0620-00, which extends the basis of similarity that was demonstrated during outburst at X-ray and optical wavelengths.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 399; 2; p. L145-L149.
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  • 5
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    Quiescent accretion disks in black hole X-ray novae (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We present detailed time-resolved spectroscopy of the Balmer emission lines from two black hole binary systems in quiescence, A0620-00 and Nova Muscae 1991. We find extraordinary similarities between the two systems. There are 30-40 km/s velocity variations of the emission lines over the orbital period, the phases of which are not aligned with the expected phase of the motion of the compact primary. Detailed modeling of both systems is complicated by variable hot spot components, regions of optical thickness, and intermittent excess emission in the blue line wings of the H-alpha lines. Both sources also display low velocities at the outer edge of the accretion disk, implying a large primary Roche lobe and extreme mass ratios. These complications suggest that although simple optically thin, Keplerian alpha-disk models provide a useful parameterization of emission lines from these systems, the straightforward physical models they imply should be treated with great caution.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 436; 2; p. 848-858
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  • 6
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    The dim inner accretion disk of the quiescent black hole A0620-00 (1995)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: We observed the X-ray nova A0620-00 with the Hubble Space Telescope (HST) Faint object Spectrograph 16 yr after its 1975 outburst. We present a single spectrum (1250-4750 A), which is approximately an average over a full 7.8 hr orbital cycle of the source. The continuum can be fitted approximately by a blackbody model with T = 9000 K and a small projected source area, which is approximately 1 % of the expected area of the accretion disk. AS0620-00 is faint in the far-UV band; its luminosity is comparable to the luminosity of the quiescent dwarf-nova accretion disk (i.e., excluding the white dwarf). By analogy with dwarf novae, the optical luminosity of the disk (M(sub nu) approximately = 7) and the orbital period of A0620-00 imply that the rate of mass transfer onto the outer disk in M(sub d) approximately 10(exp -10) solar mass/yr. We also observed A0620-00 with the ROSAT PSPC X-ray detector for 3 x 10(exp 4) s and detected a faint source (5 sigma) at the location of the X-ray nova. For an assumed blackbody spectrum the source temperature and luminosity are approximately 0.16 keV and 6 x 10(exp 30) ergs/s, respectively (d = 1 kpc). This luminosity implies that the rate of mass transfer into the black hole is extraordinarily small: M(sub BH) less than 5 x 10(exp -15) solar mass/yr. The much larger mass transfer rate onto the outer disk, and the UV/X-ray faintness of the inner disk confirm key predictions of the disk instability model for the nova outburst of A0620-00 published by Huang and Wheeler and by Mineshige and Wheeler.
    Keywords: ASTRONOMY
    Type: The Astrophysical Journal, Part 1 (ISSN 0004-637X); 442; 1; p. 358-365
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  • 7
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    A Synoptic Study Of An X-Ray Nova In Outburst (ADP 2000) (2003)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This grant is in support of an RXTE target-of-opportunity program (PI: J. McClintock) to study bright, black-hole X-ray novae. We request a second, one-year no-cost extension in order to (1) complete the study of X-ray nova 4U1543-47, which went into outburst on 2002 June 17, and (2) provide support during this work for Harvard graduate student Shinae Park who joined our team on 2003 February 10.
    Keywords: Astronomy
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  • 8
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    EXOSAT guest observer program. Binary parameters of the X-ray Pulsar 4U1626-67 (1987)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The pulsing X-ray source 4U1626-67 is an accreting neutron star in a binary system with a very low mass companion. The source was observed with EXOSAT continuously for 23 hr on 30 to 31 March 1986 UT. These observations allowed the setting of a stringent upper limit on the projected semimajor axis of the orbit of the neutron star of approx. 10 light msec for the 2485-s orbital period found by Middleditch et al., and a limit of approx. 13 light msec for any other plausible orbital period. The corresponding upper limit on the mass function for the 2485-s orbital period is 1.3 x 0.000001 solar mass. It was concluded that if the orbital inclination angle, i, equals 90 deg, then the optical companion star has a mass greater than 0.02 solar mass. However, it was found that a companion star mass greater than 0.06 solar mass is required if gravitational radiation is responsible for driving the mass transfer in this system. Only for i less than 16 deg can a companion star mass this large be accommodated by the limits set on the orbital amplitude. Also presented are results on the flaring activity in 4U1626-67 on time scales of approx. 1000 s, the energy dependent pulse profiles, and the pulse period history over the past decade.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-182374 , NAS 1.26:182374
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  • 9
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    The Effects of High Density on the X-ray Spectrum Reflected from Accretion Discs Around Black Holes (2016)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Current models of the spectrum of X-rays reflected from accretion discs around black holes and other compact objects are commonly calculated assuming that the density of the disc atmosphere is constant within several Thomson depths from the irradiated surface. An important simplifying assumption of these models is that the ionization structure of the gas is completely specified by a single, fixed value of the ionization parameter (xi), which is the ratio of the incident flux to the gas density. The density is typically fixed at n(sub e) = 10(exp 15) per cu cm. Motivated by observations, we consider higher densities in the calculation of the reflected spectrum. We show by computing model spectra for n(sub e) approximately greater than 10(exp 17) per cu cm that high-density effects significantly modify reflection spectra. The main effect is to boost the thermal continuum at energies 2 approximately less than keV. We discuss the implications of these results for interpreting observations of both active galactic nuclei and black hole binaries. We also discuss the limitations of our models imposed by the quality of the atomic data currently available.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN41170 , Monthly Notices of the Royal Astromical Society (ISSN 0035-8711) (e-ISSN 1365-2966); 462; 1; 751-760
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