PSR J1753−2240: a mildly recycled pulsar in an eccentric binary system

We report the discovery of PSR J1753−2240 in the Parkes Multibeam Pulsar Survey data base. This 95-ms pulsar is in an eccentric binary system with a 13.6-d orbital period. Period derivative measurements imply a characteristic age in excess of 1 Gyr, suggesting that the pulsar has undergone an episode of accretion-induced spin-up. The eccentricity and spin period are indicative of the companion being a second neutron star, so that the system is similar to that of PSR J1811−1736, although other companion types cannot be ruled out at this time. The companion mass is constrained by geometry to lie above 0.48 solar masses, although long-term timing observations will give additional constraints. If the companion is a white dwarf or a main-sequence star, optical observations may yield a direct detection of the companion. If the system is indeed one of the few known double neutron star systems, it would lie significantly far from the recently proposed spin-period/eccentricity relationship.     

An interference removal technique for radio pulsar searches

Searches for radio pulsars are becoming increasingly difficult because of a rise in impulsive man-made terrestrial radio-frequency interference. Here, we present a new technique, zero dispersion measure filtering, which can significantly reduce the effects of such signals in pulsar search data. The technique has already been applied to a small portion of the data from the Parkes multi-beam pulsar survey, resulting in the discovery of four new pulsars, so illustrating its efficacy.     

Do superfluid instabilities prevent neutron star precession?

We discuss short wavelength (inertial wave) instabilities present in the standard two-fluid neutron star model when there is sufficient relative flow along the superfluid neutron vortex array. We demonstrate that these instabilities may be triggered in precessing neutron stars, since the angular velocity vectors of the neutron and proton fluids are misaligned during precession. Our results suggest that the standard (Eulerian) slow precession that results for weak drag between the vortices and the charged fluid (protons and electrons) is not seriously affected by the instability. In contrast, the fast precession, which results when vortices are strongly coupled to the charged component, is generally unstable. The presence of this instability renders the standard (solid body) rotation model for free precession inconsistent and makes unsafe conclusions that have recently been drawn regarding neutron star interiors based on observations of precession in radio pulsars.     

Oscillations of rapidly rotating superfluid stars

Using time evolutions of the relevant linearized equations, we study non-axisymmetric oscillations of rapidly rotating and superfluid neutron stars. We consider perturbations of Newtonian axisymmetric background configurations and account for the presence of superfluid components via the standard two-fluid model. Within the Cowling approximation, we are able to carry out evolutions for uniformly rotating stars up to the mass-shedding limit. This leads to the first detailed analysis of superfluid neutron star oscillations in the fast rotation regime, where the star is significantly deformed by the centrifugal force. For simplicity, we focus on background models where the two fluids (superfluid neutrons and protons) corotate, are in β-equilibrium and co-exist throughout the volume of the star. We construct sequences of rotating stars for two analytical model equations of state. These models represent relatively simple generalizations of single fluid, polytropic stars. We study the effects of entrainment, rotation and symmetry energy on non-radial oscillations of these models. Our results show that entrainment and symmetry energy can have a significant effect on the rotational splitting of non-axisymmetric modes. In particular, the symmetry energy modifies the inertial mode frequencies considerably in the regime of fast rotation.     

Quark-nova remnants: application to RRATs

This is the third paper of a series of papers where we explore the evolution of iron-rich ejecta from quark-novae. In the first paper, we explored the case where a quark-nova ejecta forms a degenerate shell, supported by the star's magnetic field, with applications to SGRs. In the second paper, we considered the case where the ejecta would have sufficient angular momentum to form a degenerate Keplerian torus and applied such a system to two AXPs, namely 1E2259+586 and 4U0142+615. Here, we explore the late evolution of the degenerate torus and find that it can remain unchanged for  ∼10⁶ yr  before it becomes non-degenerate. This transition from a degenerate torus (accretion-dominated) to a non-degenerate disc (no accretion) occurs about 10⁶ yr following the quark-nova, and exhibits features that are reminiscent of observed properties of Rotation RAdio Transients (RRATs). Using this model, we can account for the duration of both the radio bursts and the quiet phase, as well as the observed radio flux from RRATs. We discuss a connection between XDINs and RRATs and argue that some XDINs may be 'dead RRATs' that have already consumed their non-degenerate disc.     

A very rare triple-peaked type-I X-ray burst in the low-mass X-ray binary 4U 1636−53

We have discovered a triple-peaked X-ray burst from the low-mass X-ray binary (LMXB) 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ). This is the first triple-peaked burst reported from any LMXB using RXTE , and it is only the second burst of this kind observed from any source. (The previous one was also from 4U 1636−53, and was observed with EXOSAT .) From fits to time-resolved spectra, we find that this is not a radius-expansion burst, and the same triple-peaked pattern seen in the X-ray light curve is also present in the bolometric light curve of the burst. Similar to what was previously observed in double-peaked bursts from this source, the radius of the emitting area increases steadily during the burst, with short periods in between during which the radius remains more or less constant. The temperature first increases steeply, and then decreases across the burst also showing three peaks. The first and last peak in the temperature profile occur, respectively, significantly before and after the first and last peaks in the X-ray and bolometric light curves. We found no significant oscillations during this burst. This triple-peaked burst, as well as the one observed with EXOSAT and the double-peak bursts in this source, all took place when 4U 1636−53 occupied a relatively narrow region in the colour–colour diagram, corresponding to a relatively high (inferred) mass-accretion rate. No model presently available is able to explain the multiple-peaked bursts.     

Discovery of three new pulsars in a 610-MHz pulsar survey with the GMRT

We report on the discovery of three new pulsars in the first blind survey of the north Galactic plane  (45° < l < 135°; | b | < 1°)  with the Giant Meterwave Radio telescope (GMRT) at an intermediate frequency of 610 MHz. The survey covered 106 deg² with a sensitivity of roughly 1 mJy to long-period pulsars (pulsars with period longer than 1 s). The three new pulsars have periods of 318, 933 and 1056 ms. Their timing parameters and flux densities, obtained in follow-up observations with the Lovell Telescope at Jodrell Bank and the GMRT, are presented. We also report on pulse nulling behaviour in one of the newly discovered pulsars, PSR J2208+5500.     

Early evolution of newly born magnetars with a strong toroidal field

We present a state-of-the-art scenario for newly born magnetars as strong sources of gravitational waves (GWs) in the early days after formation. We address several aspects of the astrophysics of rapidly rotating, ultra-magnetized neutron stars (NSs), including early cooling before transition to superfluidity, the effects of the magnetic field on the equilibrium shape of NSs, the internal dynamical state of a fully degenerate, oblique rotator and the strength of the electromagnetic torque on the newly born NS. We show that our scenario is consistent with recent studies of supernova remnant surrounding Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs) in the Galaxy that constrains the electromagnetic energy input from the central NS to be  ≤ 10⁵¹  erg. We further show that if this condition is met, then the GW signal from such sources is potentially detectable with the forthcoming generation of GW detectors up to Virgo cluster distances where an event rate ∼1 yr⁻¹ can be estimated. Finally, we point out that the decay of an internal magnetic field in the 10¹⁶ G range couples strongly with the NS cooling at very early stages, thus significantly slowing down both processes: the field can remain this strong for at least 10³ yr, during which the core temperature stays higher than several times 10⁸ K.     

Optical spectroscopy of the quiescent counterpart to EXO 0748−676

We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748−676 after the source returned to quiescence in the autumn of 2008. The X-ray light curve displays eclipses consistent in orbital period, orbital phase and duration with the predictions and measurements before the return to quiescence. Hα and He  i emission lines are present in the optical spectra and show the signature of the orbit of the binary companion, placing a lower limit on the radial velocity semi-amplitude of   K ₂ > 405 km s⁻¹  . Both the flux in the continuum and the emission lines show orbital modulations, indicating that we observe the hemisphere of the binary companion that is being irradiated by the neutron star. Effects due to this irradiation preclude a direct measurement of the radial velocity semi-amplitude of the binary companion; in fact, no stellar absorption lines are seen in the spectrum. Nevertheless, our observations place a stringent lower limit on the neutron star mass of   M ₁ > 1.27 M_⊙  . For the canonical neutron star mass of   M ₁= 1.4 M_⊙  , the mass ratio is constrained to  0.075 < q < 0.105  .     

Warping modes in discs around accreting neutron stars

The origin and stability of a thin sheet of plasma in the magnetosphere of an accreting neutron star are investigated. First, the radial extension of such a magnetospheric disc is explored. Then a mechanism for magnetospheric accretion is proposed, reconsidering the bending wave explored by Agapitou, Papaloizou & Terquem, that was found to be stable in ideal magnetohydrodynamics. We show that this warping becomes unstable and can reach high amplitudes, in a variant of Pringle's radiation-driven model for the warping of active galactic nucleus accretion discs. Finally, we discuss how this mechanism might give a clue to explain the observed X-ray kilohertz quasi-periodic oscillation of neutron star binaries.