R. P. Breton

Evidence for a Massive Neutron Star from a Radial-velocity Study of the Companion to the Black-widow Pulsar PSR B1957+20

The most massive neutron stars constrain the behavior of ultra-dense matter, with larger masses possible only for increasingly stiff equations of state. Here, we present evidence that the black-widow pulsar, PSR B1957+20, has a high mass. We took spectra of its strongly irradiated companion and found an observed radial-velocity amplitude of K_(obs) = 324 ± 3 km s^(-1). Correcting this for the fact that, due to the irradiation, the center of light lies inward relative to the center of mass, we infer a true radial-velocity amplitude of K_2 = 353 ± 4 km s^(-1) and a mass ratio q = M_(PSR)/M_2 = 69.2 ± 0.8. Combined with the inclination i = 65° ± 2° inferred from models of the light curve, our best-fit pulsar mass is M_(PSR) = 2.40 ± 0.12 M_⊙. We discuss possible systematic uncertainties, in particular, in the light curve modeling. Taking an upper limit of i 343 km s^(-1) (q > 67.3), we infer a lower limit to the pulsar mass of M_(PSR) > 1.66 M_⊙.

Observations of Doppler Boosting in Kepler Light Curves

Among the initial results from Kepler were two striking light curves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km s(-1). As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22 +/- 0.03 M(circle dot). We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems.

DISCOVERY OF THE OPTICAL COUNTERPARTS TO FOUR ENERGETIC FERMI MILLISECOND PULSARS

In the last few years, over 43 millisecond radio pulsars have been discovered by targeted searches of unidentified γ-ray sources found by the Fermi Gamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in compact binaries with low-mass companions. These systems often show eclipses of the pulsar signal and are commonly known as black widows and redbacks because the pulsar is gradually destroying its companion. In this paper, we report on the optical discovery of four strongly irradiated millisecond pulsar companions. All four sources show modulations of their color and luminosity at the known orbital periods from radio timing. Light curve modeling of our exploratory data shows that the equilibrium temperature reached on the companions dayside with respect to their nightside is consistent with about 10%-30% of the available spin-down energy from the pulsar being reprocessed to increase the companions dayside temperature. This value compares well with the range observed in other irradiated pulsar binaries and offers insights about the energetics of the pulsar wind and the production of γ-ray emission. In addition, this provides a simple way of estimating the brightness of irradiated pulsar companions given the pulsar spin-down luminosity. Our analysis also suggests that two of the four new irradiated pulsar companions are only partially filling their Roche lobe. Some of these sources are relatively bright and represent good targets for spectroscopic follow-up. These measurements could enable, among other things, mass determination of the neutron stars in these systems.

THE EVOLUTION OF PSR J0737–3039B AND A MODEL FOR RELATIVISTIC SPIN PRECESSION

We present the evolution of the radio emission from the 2.8 s pulsar of the double pulsar system PSR J0737- 3039A/B. We provide an update on the Burgay et al. analysis by describing the changes in the pulse profile and flux density over five years of observations, culminating in the B pulsars radio disappearance in 2008 March. Over this time, the flux density decreases by 0.177 mJy yr{sup -1} at the brightest orbital phases and the pulse profile evolves from a single to a double peak, with a separation rate of 2.{sup 0}6 yr{sup -1}. The pulse profile changes are most likely caused by relativistic spin precession but cannot be easily explained with a circular hollow-cone beam as in the model of Clifton and Weisberg. Relativistic spin precession, coupled with an elliptical beam, can model the pulse profile evolution well and the reappearance is expected to happen in {approx}2035 with the same part of the beam or in {approx}2014 if we assume a symmetric beam shape. This particular beam shape predicts geometrical parameters for the two bright orbital phases which are consistent with and similar to those derived by Breton et al. However, the observed decrease in flux over timemorexa0» and Bs eventual disappearance cannot be easily explained by the model and may be due to the changing influence of A on B.«xa0less

KOI 1224: A Fourth Bloated Hot White Dwarf Companion Found with Kepler

We present an analysis and interpretation of the Kepler binary system KOI?1224. This is the fourth binary found with Kepler that consists of a thermally bloated, hot white dwarf in a close orbit with a more or less normal star of spectral class A or F. As we show, KOI?1224 contains a white dwarf with T eff = 14, 700 ? 1000?K, mass = 0.22 ? 0.02 M ?, and radius = 0.103 ? 0.002 R ?, and an F-star companion of mass 1.59 ? 0.06 M ? that is somewhat beyond its terminal-age main sequence. The orbital period is quite short at 2.69802?days. The ingredients that are used in the analysis are the Kepler binary light curve, including the detection of the Doppler boosting effect; the NUV and FUV fluxes from the GALEX images of this object; an estimate of the spectral type of the F-star companion; and evolutionary models of the companion designed to match its effective temperature and mean density. The light curve is modeled with a new code named Icarus which we describe in detail. Its features include the full treatment of orbital phase-resolved spectroscopy, Doppler boosting, irradiation effects, and transits/eclipses, which are particularly suited to irradiated eclipsing binaries. We interpret the KOI?1224 system in terms of its likely evolutionary history. We infer that this type of system, containing a bloated hot white dwarf, is the direct descendant of an Algol-type binary. In spite of this basic understanding of the origin of KOI?1224, we discuss a number of problems associated with producing a system with an orbital period this short.

MULTIBAND STUDIES OF THE OPTICAL PERIODIC MODULATION IN THE X-RAY BINARY SAX J1808.4–3658 DURING ITS QUIESCENCE AND 2008 OUTBURST

We report on time-resolved optical imaging of the X-ray binary SAX J1808.4-3658 during its quiescent state and 2008 outburst. The binary, containing an accretion-powered millisecond pulsar, has a large sinusoidal-like modulation in its quiescent optical emission. We employ a Markov chain Monte Carlo technique to fit our multi-band light curve data in quiescence with an irradiated star model, and derive a tight constraint of 50(-5)(+6) deg on the inclination angle i of the binary system. The pulsar and its companion are constrained to have masses of 0.97(-0.22)(+0.31) M-circle dot and 0.04(-0.01)(+0.02) M-circle dot (both 1 sigma ranges), respectively. The dependence of these results on the measurements of the companions projected radial velocity is discussed. We also find that the accretion disk had nearly constant optical fluxes over a similar to 500 day period in the quiescent state our data covered, but started brightening 1.5 months before the 2008 outburst. Variations in modulation during the outburst were detected in our four observations made 7-12 days after the start of the outburst, and a sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller amplitude of 0.1 mag. The modulation variations are discussed. We estimate the albedo of the companion during its quiescence and the outburst, which was approximately 0 and 0.8 (for isotropic emission), respectively. This large difference probably provides additional evidence that the neutron star in the binary turns on as a radio pulsar in quiescence.

The double pulsar: evolutionary constraints from the system geometry

The double pulsar system PSR J0737–3039A/B is a highly relativistic double neutron star (DNS) binary, with a 2.4‐hour orbital period. The low mass of the second‐formed NS, as well the low system eccentricity and proper motion, point to a different evolutionary scenario compared to other known DNS systems. We describe analysis of the pulse profile shape over 6 years of observations, and present the resulting constraints on the system geometry. We find the recycled pulsar in this system, PSR 0737–3039A, to have a low misalignment between its spin and orbital angular momentum axes, with a 68.3% upper limit of 6.1°, assuming emission from both magnetic poles. This tight constraint lends credence to the idea that the supernova that formed the second pulsar was relatively symmetric, possibly involving electron‐capture onto an O‐Ne‐Mg core.

THE DOUBLE PULSAR ECLIPSES. I. PHENOMENOLOGY AND MULTI-FREQUENCY ANALYSIS

The double pulsar PSR J0737–3039A/B displays short, 30xa0s eclipses that arise around conjunction when the radio waves emitted by pulsar A are absorbed as they propagate through the magnetosphere of its companion pulsar B. These eclipses offer a unique opportunity to directly probe the magnetospheric structure and the plasma properties of pulsar B. We have performed a comprehensive analysis of the eclipse phenomenology using multi-frequency radio observations obtained with the Green Bank Telescope. We have characterized the periodic flux modulations previously discovered at 820xa0MHz by McLaughlin et al. and investigated the radio frequency dependence of the duration and depth of the eclipses. Based on their weak radio frequency evolution, we conclude that the plasma in pulsar Bs magnetosphere requires a large multiplicity factor (~105). We also found that, as expected, flux modulations are present at all radio frequencies in which eclipses can be detected. Their complex behavior is consistent with the confinement of the absorbing plasma in the dipolar magnetic field of pulsar B as suggested by Lyutikov & Thompson and such a geometric connection explains that the observed periodicity is harmonically related to pulsar Bs spin frequency. We observe that the eclipses require a sharp transition region beyond which the plasma density drops off abruptly. Such a region defines a plasmasphere that would be well inside the magnetospheric boundary of an undisturbed pulsar. It is also two times smaller than the expected standoff radius calculated using the balance of the wind pressure from pulsar A and the nominally estimated magnetic pressure of pulsar B.

The Unusual Binary Pulsar PSR J1744–3922: Radio Flux Variability, Near-Infrared Observation, and Evolution

PSR J1744-3922 is a binary pulsar exhibiting highly variable pulsed radio emission. We report on a statistical multifrequency study of the pulsed radio flux variability which suggests that this phenomenon is extrinsic to the pulsar and possibly tied to the companion, although not strongly correlated with orbital phase. The pulsar has an unusual combination of characteristics compared to typical recycled pulsars: a long spin period (172 ms); a relatively high magnetic field strength (1.7 × 1010 G); a very circular, compact orbit of 4.6 hr; and a low-mass companion (0.08 M☉). These spin and orbital properties are likely inconsistent with standard evolutionary models. We find similarities between the properties of the PSR J1744-3922 system and those of several other known binary pulsar systems, motivating the identification of a new class of binary pulsars. We suggest that this new class could result from: a standard accretion scenario of a magnetar or a high magnetic field pulsar; common envelope evolution with a low-mass star and a neutron star, similar to what is expected for ultracompact X-ray binaries; or accretion induced collapse of a white dwarf. We also report the detection of a possible K = 19.30(15) infrared counterpart at the position of the pulsar, which is relatively bright if the companion is a helium white dwarf at the nominal distance, and discuss its implications for the pulsars companion and evolutionary history.

Using the Double Pulsar Eclipses to Probe Fundamental Physics

The double pulsar system exhibits unique and spectacular eclipses when pulsar A passes behind pulsar B. We present 3.5 years of eclipse monitoring which provides an unprecedented way of probing pulsar magnetospheres, geometric orientation as well as testing general relativity The eclipse light curve of pulsar A has a rich phenomenology. This includes little observed radio frequency dependence and periodic flux modulations in phase with the rotation of pulsar B, which allow the flux to reach uneclipsed levels in narrow windows. Over the monitoring period, the eclipse profile noticeably changed and we observe a modification in the modulation behavior. We quantitatively analyzed the data using the magnetospheric synchrotron absorption eclipse model proposed by Lyutikov and Thompson under the assumption of a simple dipolar magnetic field geometry The success of the model at reproducing the eclipse profile indicates that the magnetic field configuration is mainly dipolar at a distance of about 0.1 light‐cylind...