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Register a new userMicroarcsecond VLBI pulsar astrometry with PSRPI I. Two binary millisecond pulsars with white dwarf companions
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Model-independent distance constraints to binary millisecond pulsars (MSPs) are of great value to both the timing observations of the radio pulsars, and multiwavelength observations of their companion stars. Very Long Baseline Interferometry (VLBI) astrometry can be employed to provide these model-independent distances with very high precision via the detection of annual geometric parallax. Using the Very Long Baseline Array, we have observed two binary millisecond pulsars, PSR J1022+1001 and J2145-0750, over a two-year period and measured their distances to be 700 +14 -10 pc and 613 +16 -14 pc respectively. We use the well-calibrated distance in conjunction with revised analysis of optical photometry to tightly constrain the nature of their massive (M ~ 0.85 Msun) white dwarf companions. Finally, we show that several measurements of their parallax and proper motion of PSR J1022+1001 and PSR J2145-0750 obtained by pulsar timing array projects are incorrect, differing from the more precise VLBI values by up to 5 sigma. We investigate possible causes for the discrepancy, and find that imperfect modeling of the solar wind is a likely candidate for the timing model errors given the low ecliptic latitude of these two pulsars.
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Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the Universe is not yet old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant host systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of a LMWD in an eccentric (e=0.13) orbit with a millisecond pulsar, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature T_eff = 8600 +/- 190 K) and 3D systemic velocity (179.5 kms). We also place model-independent constraints on the WD radius (R_WD = 0.024+/- 0.004/0.002 R_sun) and surface gravity (log g = 7.11 +/- 0.08/0.16 dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavour a three-body formation channel. In the case of the high eccentricity being the result of a spontaneous phase transition, we infer a mass of 1.6 M_sun for the progenitor of the pulsar, which is too low for the quark-nova mechanism proposed by Jiang et al. (2015). Similarly, the scenario of Freire & Tauris (2014), in which a WD collapses onto a neutron star via an rotationally-delayed accretion-induced collapse, requires both a high-mass differentially rotating progenitor and a significant momentum kick at birth under our constraints. Contrarily, we find that eccentricity pumping via interaction with a transient circumbinary disk is consistent with all inferred properties. Finally, we report tentative evidence for pulsations which, if confirmed, would transform the star into an unprecedented laboratory for WD physics and stellar convection.
The number of spatially unresolved white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ~30 in 2003 to over 3000. However, in the majority of known systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive FGK type companions have remained elusive due to the large difference in optical brightness between the two stars. In this paper we identify 934 main-sequence FGK stars from the Radial Velocity Experiment (RAVE) survey in the southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey in the northern hemisphere, that show excess flux at ultraviolet wavelengths which we interpret as the likely presence of a white dwarf companion. We obtained Hubble Space Telescope ultraviolet spectra for nine systems which confirmed that the excess is indeed caused, in all cases, by a hot compact companion, eight being white dwarfs and one a hot subdwarf or pre-helium white dwarf, demonstrating that this sample is very clean. We also address the potential of this sample to test binary evolution models and type Ia supernovae formation channels.
We present optical high-speed photometry of three millisecond pulsars with low-mass ($< 0.3 M_{odot}$) white dwarf companions, bringing the total number of such systems with follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is under-constrained, but the mode periods we observe are consistent with expectations for a $M_{star} = 0.16 - 0.19 M_{odot}$ white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.
Millisecond Pulsars (MSPs) are fast rotating, highly magnetized neutron stars. According to the canonical recycling scenario, MSPs form in binary systems containing a neutron star which is spun up through mass accretion from the evolving companion. Therefore, the final stage consists of a binary made of a MSP and the core of the deeply peeled companion. In the last years, however an increasing number of systems deviating from these expectations has been discovered, thus strongly indicating that our understanding of MSPs is far to be complete. The identification of the optical companions to binary MSPs is crucial to constrain the formation and evolution of these objects. In dense environments such as Globular Clusters (GCs), it also allows us to get insights on the cluster internal dynamics. By using deep photometric data, acquired both from space and ground-based telescopes, we identified 5 new companions to MSPs. Three of them being located in GCs and two in the Galactic Field. The three new identifications in GCs increased by 50% the number of such objects known before this Thesis. They all are non-degenerate stars, at odds with the expectations of the canonical recycling scenario. These results therefore suggest either that transitory phases should also be taken into account, or that dynamical processes, as exchange interactions, play a crucial role in the evolution of MSPs. We also performed a spectroscopic follow-up of the companion to PSR J1740-5340A in the GC NGC 6397, confirming that it is a deeply peeled star descending from a ~0.8$M_{odot}$ progenitor. This nicely confirms the theoretical expectations about the formation and evolution of MSPs.
We conducted multi-epoch VLBA phase reference observations of LS I +61 303 in order to study its precessing radio jet. Compared to similar observations in 2006, we find that the observed elliptical trajectory of emission at 8.4 GHz repeats after the 9-year gap. The accurate alignment of the emission patterns yields a precession period of 26.926 +- 0.005 d, which is consistent with that determined by Lomb-Scargle analysis of the radio light curve. We analytically model the projection on the sky plane of the peak position of a precessing, synchrotron-emitting jet, which traces an elliptical trajectory on the sky. Comparing the simulation with the VLBA astrometry we improve our knowledge of the geometry of the system.We measure the LS I +61 303 absolute proper motion to be -0.150 +- 0.006 mas/yr eastward and -0.264 +- 0.006 mas/yr northward. Removing Galactic rotation, this reveals a small, < 20 km/s, non-circular motion, which indicates a very low kick velocity when the black hole was formed.
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