Robert W. Nelson

California Institute of Technology, Pasadena, California, United States

Are you Robert W. Nelson?

Claim your profile

Publications (12)87.29 Total impact

  • Source
    [Show abstract] [Hide abstract]
    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 1+4) 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 surprisingly 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.
    The Astrophysical Journal 01/2009; 488(2):L117. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    06/2002;
  • Article: Unknown
    [Show abstract] [Hide abstract]
    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.
    06/2002;
  • Source
    [Show abstract] [Hide abstract]
    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
    The Astrophysical Journal 07/1998; · 6.73 Impact Factor
  • Source
    J. A. Sellwood, Robert W. Nelson, Scott Tremaine
    [Show abstract] [Hide abstract]
    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)
    The Astrophysical Journal 05/1998; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: this paper we summarize over #ve years of observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE's principal advantage over previous instruments for studying accreting pulsars is its continuous monitoring capability. The timing data we present here represent a
    The Astrophysical Journal Supplement Series 09/1997; · 16.24 Impact Factor
  • Source
    Robert W. Nelson, Scott Tremaine
    [Show abstract] [Hide abstract]
    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.
    Monthly Notices of the Royal Astronomical Society 08/1997; · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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 a dot-nu approximately equal to (3_5) x 10(exp -12) Hz/s, 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 (dot-nu is approximately equals 2x10(exp -13) Hz/s) may be due entirely to brief (approximately 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 - 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(sub c) < 70 solar mass and a radius R(sub c) < 85 solar radius 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(sub c) < 55 solar mass and R(sub c) < 68 solar radius. These constraints are nearly at odds with the reclassification of the companion as a B1 Ia + hypergiant.
    05/1997;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The source of the instability that leads to the bursts remains unknown. We discovered pulsations from GRO J1744-28 as part of our X-ray pulsar monitoring and detection program, which uses 20-50 keV data from the Burst and Transient Source Experiment (BATSE) [3] on the Compton Gamma-Ray Observatory (CGRO). After discovery of the pulsations we initiated on-board epoch folding. The earliest detection of pulsations from the source occurred on 15 December 1995, 13 days after the first detected burst. Daily pulse frequencies were obtained by epochfolding each day's data at a range of frequencies and determining the frequency, , that maximized the pulse amplitude. Using a fit to these frequencies the data were then epoch-folded over 8 hour intervals and a pulse phase obtained for each interval by correlating it with a profile model. Local fits to the pulse phases were used as the basis for multi-day epochfolding in order to study the pulse profile, wh
    Nature 03/1997; · 38.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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) Theta10 Gamma12 Hz s Gamma1 , 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 time scales. We suggest that the long-term spin-up trend observed since 1984 ( 2 Theta 10 Gamma13 Hz s Gamma1 ) may be due entirely to brief ( 20 d) spin-up episodes similar to those we have discovered. 1 Current address: Department of Physics and Department of Astronomy, University of California, Berkeley, CA 94720 2 Current address: Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 3 Universities Space Research Association 4 Current address: RIKEN Institute, Wako-shi, Saitama 351-01, Japan 2 We assess different accretion models and their ability to explain the orbital phase dependence of the observed flux. In addition to the previously observed pre-periastron peak at orbital phase 0:956 Sigma 0:022, we also find a smaller peak close to apastron at orbital phase 0:498 Sigma 0:057. We show that if the companion star's effective temperature is ! 22; 000 K, then it must have a mass M c ! 70M fi and radius R c ! 85R fi so as to not overfill the tidal lobe at pe...
    The Astrophysical Journal 03/1997; · 6.73 Impact Factor
  • Source
    Robert W. Nelson, Scott Tremaine
    [Show abstract] [Hide abstract]
    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.
    11/1995;
  • Source
    Robert W. Nelson, Scott Tremaine
    [Show abstract] [Hide abstract]
    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.
    The Journal of the Royal Astronomical Society of Canada. Royal Astronomical Society of Canada 09/1994;

Publication Stats

579 Citations
87.29 Total Impact Points

Institutions

  • 1997–2009
    • California Institute of Technology
      • Department of Physics
      Pasadena, California, United States
  • 1998
    • University of Cambridge
      Cambridge, England, United Kingdom