Robert W. Nelson

Observations of accreting pulsars

We discuss recent observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE has detected and studied nearly half of the known accreting pulsar systems. Continuous timing studies over a two-year period have yielded accurate orbital parameters for 9 of these systems, as well as new insights into long-term accretion torque histories.

Rapid Spin-Up Episodes in the Wind-Fed Accreting Pulsar GX 301-2

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 stars 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.

On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars

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.

Resonant thickening of disks by small satellite galaxies

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 which 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 satellites 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.

On the Correlation of Torque and Luminosity in GX 1+4

Over 5 years of daily hard X-ray (>20 keV) monitoring of the 2 minute accretion-powered pulsar GX 1+4 with the Compton Gamma Ray Observatory/BATSE 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 1+4 may be accreting from a retrograde disk during spin-down, as previously suggested.

Nonthermal cyclotron emission from low-luminosity accretion onto magnetic neutron stars

The ROSAT, Astro D, and AXAF imaging surveys could detect large numbers of low-luminosity X-ray pulsars (L≤10 -4 ergs s -1 ), undergoing low-state wind accretion in Be/X-ray transient systems, or possibly isolated neutron stars accreting directly from the interstellar medium. If these pulsars were purely thermal emitters with polar cap temperatures T e ∼100(L/10 32 ergs s -1 ) 1/4 (A cap /10 12 cm 2 ) -1/4 eV, only nearby sources could be detected because of strong UV absorption by the intervening H I gas

Spectroscopy of Infrared Flares from the Microquasar GRS 1915+105

We present near-infrared medium-resolution (R~875) spectra of the microquasar GRS 1915+105 on 1997 August 13-15 UTC from the Hale 5 m telescope. The spectra show broad emission lines of He I (2.058 μm) and H I (2.166 μm; Brγ), which are consistent with previous work. On August 14 UTC, we took spectra with an ~6 minute time resolution during infrared flaring events similar to those reported in Eikenberry et al., 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 μm) 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 that have moved out of the accretion disk plane.

Trompe L'Oeil binary pulsars

A freely precessing pulsar produces pulse phase residuals which can mimic those of a pulsar in a binary orbit. In particular, discrete sets of phase residuals due to precessional motion of an isolated pulsar are sampled; it is shown that this data is well fit by residuals from a binary pulsar in a sufficiently tight orbit. Analytic and numerical relationships between the projected orbital size, a(p) sin i, and the orbital eccentricity, e, of a misidentified binary pulsar; are found the observations that would distinguish between these models are discussed. Regardless of the mechanism that causes the precession, the maximum amplitude of the phase residual is pi/2: consequently, a(p)sin i is (approximately) bounded by cP(puls)/4. The newly discovered binary millisecond pulsars in the globular cluster 47 Tuc is discussed, and it is shown that the periodic frequency modulation reported cannot be explained by free precession. 14 refs.

Synchrotron radiation with radiation reaction. [relativistic electron motion in strong astrophysical magnetic fields]

A rigorous discussion is presented of the classical motion of a relativistic electron in a magnetic field and the resulting electromagnetic radiation when radiation reaction is important. In particular, for an electron injected with initial energy gamma(0), a systematic perturbative solution to the Lorentz-Dirac equation of motion is developed for field strengths satisfying gamma(0) B much less than 6 x 10 to the 15th G. A particularly accurate solution to the electron orbital motion in this regime is found and it is demonstrated how lowest-order corrections can be calculated. It is shown that the total energy-loss rate corresponds to what would be found using the exact Larmor power formula without including radiation reaction. Provided that the particle energy and field strength satisfy the same contraint, it is explicitly demonstrated that the intuitive prescription for calculating the time-integrated radiation spectrum described above is correct. 40 refs.

Low-cost telepresence at technical conferences

Physical attendance at technical conferences maximizes both the professional and personal benefits of participation, and is often required as a condition of publishing. However, circumstances such as injury, literally being at the South Pole, and/or lack of funding may “get in the way”, to say the least. This paper describes how an inexpensive, remote-site controllable camera was used with the existing laptop-and-projector configuration at IEEE Aeroconference 2011 to establish a workable point-to-point telepresence for presenting papers, attending presentations, participating in panel discussions, and co-chairing a technical session. The discussion includes suggestions for camera, audio, and screen/chart-sharing configurations for these purposes in both point-to-point and multi-point situations, use of telepresence as part of traditional conference room sessions, and possibilities for and challenges of having a “telepresence hall” for presentations and/or professional networking.