[en] We report the optical discovery of the companion to the 2 M_sun millisecond pulsar PSR J1614-2230. The optical colors show that the 0.5 M_sun companion is a 2.2 Gyr old He-CO white dwarf. We infer that M-dot during the accretion phase is <10^-2 M-dot_edd. We show that the pulsar was born with a spin close to its current value, well below the rebirth line. The spin-down parameters, the mass of the pulsar, and the age of the system challenge the simple recycling model for the formation of millisecond pulsars.

[en] We present optical spectroscopic measurements of the eclipsing high-mass X-ray binary (HMXB) XMMU J013236.7+303228 in M 33. Based on spectra taken at multiple epochs of the 1.73 day binary orbital period we determine physical as well as orbital parameters for the donor star. We find the donor to be a B1.5IV subgiant with effective temperature T = 22, 000-23, 000 K. From the luminosity, temperature, and known distance to M 33 we derive a radius of R 8.9 ± 0.5 R_☉. From the radial-velocity measurements, we determine a velocity semi-amplitude of K_opt = 63 ± 12 km s^–1. Using the physical properties of the B star determined from the optical spectrum, we estimate the star's mass to be M_opt = 11 ± 1 M_☉. Based on the X-ray spectrum, the compact companion is likely a neutron star, although no pulsations have yet been detected. Using the spectroscopically derived B star mass we find the neutron star companion mass to be M_X = 2.0 ± 0.4 M_☉, consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_☉ found for many millisecond pulsars. We attempt to use as an additional constraint that the B star radius inferred from temperature, flux, and distance should equate to the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but by trying to apply the technique to known systems, we find that the masses are consistently overestimated. Attempting to account for that in our uncertainties, we derive M_X = 2.2^+0.8_–0.6 M_☉ and M_opt = 13 ± 4 M_☉. We conclude that precise constraints require detailed modeling of the shape of the Roche surface.

[en] We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic variable with a period of 120.21 minutes. The white dwarf primary has a magnetic field of B = 42⁺⁶_-5 MG and the secondary is an M3 dwarf. The system shows highly symmetric double-peaked photometric modulation in the active state as well as in quiescence. These arise from a combination of cyclotron beaming and ellipsoidal modulation. The projected orbital velocity of the secondary is K₂ = 390 ± 4 km s^-1. We place an upper limit of 830 ± 65 pc on the distance.

[en] We report positions, velocities, and metallicities of 50 ab-type RR Lyrae (RRab) stars observed in the vicinity of the Orphan stellar stream. Using about 30 RRab stars classified as being likely members of the Orphan stream, we study the metallicity and the spatial extent of the stream. We find that RRab stars in the Orphan stream have a wide range of metallicities, from –1.5 dex to –2.7 dex. The average metallicity of the stream is –2.1 dex, identical to the value obtained by Newberg et al. using blue horizontal branch stars. We find that the most distant parts of the stream (40-50 kpc from the Sun) are about 0.3 dex more metal-poor than the closer parts (within ∼30 kpc), suggesting a possible metallicity gradient along the stream's length. We have extended the previous studies and have mapped the stream up to 55 kpc from the Sun. Even after a careful search, we did not identify any more distant RRab stars that could plausibly be members of the Orphan stream. If confirmed with other tracers, this result would indicate a detection of the end of the leading arm of the stream. We have compared the distances of Orphan stream RRab stars with the best-fit orbits obtained by Newberg et al. We find that model 6 of Newberg et al. cannot explain the distances of the most remote Orphan stream RRab stars, and conclude that the best fit to distances of Orphan stream RRab stars and to the local circular velocity is provided by potentials where the total mass of the Galaxy within 60 kpc is M₆₀ ∼ 2.7 × 10¹¹ M_☉, or about 60% of the mass found by previous studies. More extensive modeling that would consider non-spherical potentials and the possibility of misalignment between the stream and the orbit is highly encouraged

[en] We present spectral and timing analysis of NuSTAR observations of RX J0520.5–6932 in the 3-79 keV band collected during its outburst in 2014 January. The target was observed on two epochs and we report the detection of a cyclotron resonant scattering feature with central energies of E_CRSF=31.3_−0.7^+0.8 keV and 31.5_−0.6^+0.7 keV during the two observations, respectively, corresponding to a magnetic field of B ≈ 2 × 10¹² G. The 3-79 keV luminosity of the system during the two epochs, assuming a nominal distance of 50 kpc, was 3.667 ± 0.007 × 10³⁸ erg s^–1 and 3.983 ± 0.007 × 10³⁸ erg s^–1. Both values are much higher than the critical luminosity of ≈1.5 × 10³⁷ erg s^–1, above which a radiation-dominated shock front may be expected. This adds a new object to the sparse set of three systems that have a cyclotron line observed at luminosities in excess of 10³⁸ erg s^–1. A broad (σ ≈ 0.45 keV) Fe emission line is observed in the spectrum at a central energy of 6.58_−0.05^+0.05 keV in both epochs. The pulse profile of the pulsar was observed to be highly asymmetric with a sharply rising and slowly falling profile of the primary peak. We also observed minor variations in the cyclotron line energy and width as a function of the rotation phase.

[en] PSR J2129−0429 is a “redback” eclipsing millisecond pulsar binary with an unusually long 15.2 hr orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified Fermi gamma-ray sources. The pulsar companion is optically bright (mean m_R = 16.6 mag), allowing us to construct the longest baseline photometric data set available for such a system. We present 10 years of archival and new photometry of the companion from the Lincoln Near-Earth Asteroid Research Survey, the Catalina Real-time Transient Survey, the Palomar Transient Factory, the Palomar 60 inch, and the Las Cumbres Observatory Global Telescope. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200 inch indicates that the pulsar is massive: 1.74 ± 0.18 $M_{\odot <!--\; ???\; -->}$. The G-type pulsar companion has mass 0.44 ± 0.04 $M_{\odot <!--\; ???\; -->}$, one of the heaviest known redback companions. It is currently 95 ± 1% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr⁻¹ secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical light curve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129−0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar–white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.