Robert W. Nelson

California Institute of Technology, Pasadena, California, United States

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Publications (12)81.75 Total impact

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    ABSTRACT: Dramatic torque reversals between spin-up and spin-down have been observed in half of the persistent X-ray pulsars monitored by the BATSE all-sky monitor on the Compton Gamma Ray Observatory. Theoretical models developed to explain early pulsar timing data can explain spin-down torques via a disk-magnetosphere interaction if the star nearly corotates with the inner accretion disk. To produce the observed BATSE torque reversals, however, these equilibrium models require the disk to alternate between two mass accretion rates, with ˙M5 producing accretion torques of similar magnitude but always of opposite sign. Moreover, in at least one pulsar (GX 114) undergoing secular spin-down, the neutron star spins down faster during brief (~20 day) hard X-ray flares—this is opposite the correlation expected from standard theory, assuming that BATSE pulsed flux increases with mass accretion rate. The 10 day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or viscous timescale near the inner disk boundary and are more suggestive of a global disk phenomenon. We discuss possible explanations of the observed torque behavior. Despite the preferred sense of rotation defined by the binary orbit, the BATSE observations are urprisingly consistent with an earlier suggestion for GX 1+4: the disks in these systems somehow alternate between episodes of prograde and retrograde rotation. We are unaware of any mechanism that could produce a stable retrograde disk in a binary undergoing Roche lobe overflow, but such flip-flop behavior does occur in numerical simulations of wind-fed systems. One possibility is that the disks in some of these binaries are fed by an X-ray–excited wind.
    Preview · Article · Jan 2009 · The Astrophysical Journal
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    ABSTRACT: Over 5 years of daily hard X-ray (.20 keV) monitoring of the 2 minute accretion-powered pulsar GX 114 with the Compton Gamma Ray ObservatoryyBATSE large-area detectors has found nearly continuous rapid spin-down, interrupted by a bright 200 day spin-up episode. During spin-down, the torque becomes more negative as the luminosity increases (assuming that the 20 -- 60 keV pulsed flux traces bolometric luminosity), the opposite of what is predicted by standard accretion torque theory. No changes in the shape of the 20--100 keV pulsed energy spectrum were detected, so that a very drastic change in the spectrum below 20 keV or the pulsed fraction would be required to make the 20 -- 60 keV pulsed flux a poor luminosity tracer. These are the first observations that flatly contradict standard magnetic disk accretion theory, and they may have important implications for understanding the spin evolution of X-ray binaries, cataclysmic variables, and protostars. We briefly discuss the possibility that GX 114 may be accreting from a retrograde disk during spin-down, as previously suggested.
    No preview · Article · Jun 2002
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    ABSTRACT: Dramatic torque reversals between spin-up and spin-down have been observed in half of the persistent X-ray pulsars monitored by the BATSE all-sky monitor on the Compton Gamma Ray Observatory. Theoretical models developed to explain early pulsar timing data can explain spin-down torques via a disk-magnetosphere interaction if the star nearly corotates with the inner accretion disk. To produce the observed BATSE torque reversals, however, these equilibrium models require the disk to alternate between two mass accretion rates, with producing accretion torques of similar magnitude but always of opposite sign. Moreover, in at least one pulsar (GX 114) undergoing secular spin-down, the neutron star spins down faster during brief (#20 day) hard X-ray flares---this is opposite the correlation expected from standard theory, assuming that BATSE pulsed flux increases with mass accretion rate. The 10 day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or viscous timescale near the inner disk boundary and are more suggestive of a global disk phenomenon.
    No preview · Article · Jun 2002
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    ABSTRACT: We present near-infrared medium-resolution ($R \sim 875$) spectra of the microquasar GRS 1915+105 on 1997 August 13-15 UTC from the Hale 200-inch telescope. The spectra showed broad emission lines of He I (2.058 $\mu$m) and H I (2.166 $\mu$m - Br$\gamma$), consistent with previous work. On August 14 UTC, we took spectra with $\sim 6$-minute time resolution during infrared flaring events similar to those reported in Eikenberry et al. (1998a), which appear to reveal plasma ejection from the system. During the flares, the emission line fluxes varied in approximately linear proportionality to the IR continuum flux, implying that the lines are radiatively pumped by the flares. We also detected a weak He II (2.189 $\mu$m) emission line on August 14 UTC. The nature of the line variability and the presence of the He II feature indicate that the emission lines in GRS 1915+105 arise in an accretion disk around the compact object, rather than in the circumstellar disk of a proposed Oe/Be companion. The radiative line pumping also implies that the flare emission originates from ejecta which have moved out of the accretion disk plane. Comment: 13 pages plus 4 figures, to appear in ApJ Letters
    Preview · Article · Jul 1998 · The Astrophysical Journal
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    JA Sellwood · Robert W. Nelson · Scott Tremaine
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    ABSTRACT: We study the vertical heating and thickening of galaxy disks due to accretion of small satellites. Our simulations are restricted to axial symmetry, which largely eliminates numerical evolution of the target galaxy but requires the trajectory of the satellite to be along the symmetry axis of the target. We find that direct heating of disk stars by the satellite is not important because the satellite's gravitational perturbation has little power at frequencies resonant with the vertical stellar orbits. The satellite does little damage to the disk until its decaying orbit resonantly excites large-scale disk bending waves. Bending waves can damp through dynamical friction from the halo or internal wave-particle resonances; we find that wave-particle resonances dominate the damping. The principal vertical heating mechanism is therefore dissipation of bending waves at resonances with stellar orbits in the disk. Energy can thus be deposited some distance from the point of impact of the satellite. The net heating from a tightly bound satellite can be substantial, but satellites that are tidally disrupted before they are able to excite bending waves do not thicken the disk. Comment: 13 pages, 8 figures, to appear in ApJ, latex (aaspp4.sty)
    Preview · Article · May 1998 · The Astrophysical Journal
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    ABSTRACT: We summarize 5 years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered five new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115 + 634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948 + 32, EXO 2030 + 375, GRO J1008-57, A0535 + 26, GRO J2058 + 42, 4U 1145-619, and A1118-616), and also measured the accretion torque history during outbursts of six of those transients whose orbital parameters were also known. We have also continuously measured the pulsed flux and spin frequency for eight persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1 + 4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic neutron stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long timescales, which blurs the conventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars but are uncorrelated, or even anti-correlated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA/Goddard Space Flight Center.
    Preview · Article · Sep 1997 · The Astrophysical Journal Supplement Series
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    Robert W. Nelson · Scott Tremaine
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    ABSTRACT: We apply linear response theory to a general, inhomogeneous, stationary stellar system, with particular emphasis on dissipative processes analogous to Landau damping. Assuming only that the response is causal, we show that the irreversible work done by an external perturber is described by the anti-Hermitian part of a linear response operator, and damping of collective modes is described by the anti-Hermitian part of a related polarization operator. We derive an exact formal expression for the response operator, which is the classical analog of a well-known result in quantum statistical physics. When the self-gravity of the response can be ignored, and the ensemble-averaged gravitational potential is integrable, the expressions for the mode energy, damping rate, and polarization operator reduce to well-known formulae derived from perturbation theory in action-angle variables. In this approximation, dissipation occurs only via resonant interaction with stellar orbits or collective modes. For stellar systems in thermal equilibrium, the anti-Hermitian part of the response operator is directly related to the correlation function of the fluctuations. Thus dissipative properties of the system are completely determined by the spectrum of density fluctuations---the fluctuation-dissipation theorem. In particular, we express the coefficient of dynamical friction for an orbiting test particle in terms of the fluctuation spectrum; this reduces to the known Chandrasekhar formula in the restrictive case of an infinite homogeneous system with a Maxwellian velocity distribution.
    Preview · Article · Aug 1997 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The accreting pulsar GX 301-2 (P = 680 s) has been observed continuously by the large-area detectors of the Burst and Transient Source Experiment (BATSE) instrument on the Compton Gamma Ray Observatory since 1991 April 5. Orbital parameters determined from these data are consistent with previous measurements, with improved accuracy in the current orbital epoch. The most striking features in the pulsar frequency history are two steady and rapid spin-up episodes, with ν˙~(3-5)×10^(-12) Hz s^(-1), each lasting for about 30 days. They probably represent the formation of transient accretion disks in this wind-fed pulsar. Except for these spin-up episodes, there are virtually no net changes in the neutron star spin frequency on long timescales. We suggest that the long-term spin-up trend observed since 1984 (ν˙~2×10^(-13) Hz s^(-1)) may be due entirely to brief (~20 days) spin-up episodes similar to those we have discovered. We assess different accretion models and their ability to explain the orbital phase dependence of the observed flux. In addition to the previously observed preperiastron peak at orbital phase 0.956 +/- 0.022, we also find a smaller peak close to apastron at orbital phase 0.498 +/- 0.057. We show that if the companion star's effective temperature is less than 22,000 K, then it must have a mass M_c < 70 M_⊙ and a radius R_c < 85 R_⊙ so as not to overfill the tidal lobe at periastron. In order not to overflow the Roche lobe at periastron, the corresponding values are M_c < 55 M_⊙ and R_c < 68 R_⊙. These constraints are nearly at odds with the reclassification by Kaper et al. of the companion as a B1 Ia + hypergiant.
    Preview · Article · May 1997
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    ABSTRACT: An unusual astronomical source of hard X-ray bursts, GRO J1744 - 28, was discovered recently. The properties of this source differ markedly from those of other known high-energy burst sources-X-ray bursters, soft gamma-ray repeaters and gamma-ray bursters- suggesting that it may represent a new type of source. The bursts probably arise from unstable accretion of matter onto a compact object2, such as a neutron star, but the nature of the object and the origin of the burst instability have not been revealed by observations of the bursts themselves. Here we report the detection of coherent X-ray pulsations, with a period of 467 milliseconds, from GRO J1744 - 28; these are the first persistent pulsations seen in a bursting X-ray source. These pulses and their timing indicate that the object is a magnetized neutron star, accreting gas from a low-mass companion star. The pulsation rate has been increasing during the period of our observations, indicating that an accretion disk has been formed and that the transfer of matter from the disk is spinning up the neutron star. The source of the instability that leads to the bursts remains unknown.
    Preview · Article · Mar 1997 · Nature
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    ABSTRACT: The accreting pulsar GX 301-2 (P = 680 s) has been observed continuously by the large-area detectors of the Burst and Transient Source Experiment (BATSE) instrument on the Compton Gamma Ray Observatory since 1991 April 5. Orbital parameters determined from these data are consistent with previous measurements, with improved accuracy in the current orbital epoch. The most striking features in the pulsar frequency history are two steady and rapid spin-up episodes, with ≈ (3-5)×10−12 Hz s-1, each lasting for about 30 days. They probably represent the formation of transient accretion disks in this wind-fed pulsar. Except for these spin-up episodes, there are virtually no net changes in the neutron star spin frequency on long timescales. We suggest that the long-term spin-up trend observed since 1984 ( ≈ 2×10−13 Hz s-1) may be due entirely to brief (≈ 20 days) spin-up episodes similar to those we have discovered. We assess different accretion models and their ability to explain the orbital phase dependence of the observed flux. In addition to the previously observed preperiastron peak at orbital phase 0.956 ± 0.022, we also find a smaller peak close to apastron at orbital phase 0.498 ± 0.057. We show that if the companion star's effective temperature is less than 22,000 K, then it must have a mass Mc < 70 M☉ and a radius Rc < 85 R☉ so as not to overfill the tidal lobe at periastron. In order not to overflow the Roche lobe at periastron, the corresponding values are Mc < 55 M☉ and Rc < 68 R☉. These constraints are nearly at odds with the reclassification by Kaper et al. of the companion as a B1 Ia + hypergiant.
    Preview · Article · Mar 1997 · The Astrophysical Journal
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    Robert W. Nelson · Scott Tremaine
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    ABSTRACT: Large-scale warps in the outer parts of spiral galaxy discs have been observed for almost forty years, but their origin remains obscure. We review the dynamics of warped galaxy discs. We identify several mechanisms that could excite warps, all involving the gravitational interaction between the disc and the dark-matter halo.
    Preview · Article · Nov 1995
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    Robert W. Nelson · Scott Tremaine
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    ABSTRACT: We investigate the dynamical interaction of galactic warps with the surrounding dark matter halo, using analytic perturbation theory. A precessing warp induces a density wake in the collisionless dark matter, which acts back on the original warp, transferring energy and angular momentum between the warp and halo (dynamical friction). In most cases dynamical friction damps the warp, but in unusual circumstances (such as a halo that rotates in the same direction as the precession of the warp, or a warp in the equatorial plane of an axisymmetric prolate halo) friction can excite a warp. The damping/excitation time is usually short compared to the Hubble time for realistic systems. Thus most warps cannot be primordial; they must be maintained by some ongoing excitation mechanism.
    Preview · Article · Sep 1994 · The Journal of the Royal Astronomical Society of Canada. Royal Astronomical Society of Canada

Publication Stats

764 Citations
81.75 Total Impact Points

Institutions

  • 1997-2009
    • California Institute of Technology
      • Department of Physics
      Pasadena, California, United States
  • 1997-1998
    • University of Toronto
      • Canadian Institute for Theoretical Astrophysics
      Toronto, Ontario, Canada