ALBERT

Description: In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on time-scales 〈10 4 yr, while continuing accretion may produce variations on a range of time-scales. While some quiescent neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal components on a range of time-scales, several others, particularly those in globular clusters with no detectable non-thermal hard X-rays (fit with a power law), have shown no measurable variations. Here, we constrain the spectral variations of 12 low-mass X-ray binaries in three globular clusters over ~10 years. We find no evidence of variations in 10 cases, with limits on temperature variations below 11 per cent for the seven qLMXBs without power-law components, and limits on variations below 20 per cent for three other qLMXBs that do show non-thermal emission. However, in two qLMXBs showing power-law components in their spectra (NGC 6440 CX 1 and Terzan 5 CX 12) we find marginal evidence for a 10 per cent decline in temperature, suggesting the presence of continuing low-level accretion. This work adds to the evidence that the thermal X-ray component in quiescent neutron stars without power-law components can be explained by heat deposited in the core during outbursts. Finally, we also investigate the correlation between hydrogen column density ( N H ) and optical extinction ( A V ) using our sample and current models of interstellar X-ray absorption, finding N H (cm –2 ) = (2.81 ± 0.13)  x  10 21 A V .

Description: We consider the local dynamics of a realistic neutron-star core, including composition gradients, superfluidity and thermal effects. The main focus is on the gravity g-modes, which are supported by composition stratification and thermal gradients. We derive the equations that govern this problem in full detail, paying particular attention to the input that needs to be provided through the equation of state and distinguishing between normal and superfluid regions. The analysis highlights a number of key issues that should be kept in mind whenever equation of state data is compiled from nuclear physics for use in neutron-star calculations. We provide explicit results for a particular stellar model and a specific nucleonic equation of state, making use of cooling simulations to show how the local wave spectrum evolves as the star ages. Our results show that the composition gradient is effectively dominated by the muons whenever they are present. When the star cools below the superfluid transition, the support for g-modes at lower densities (where there are no muons) is entirely thermal. We confirm the recent suggestion that the g-modes in this region may be unstable, but our results indicate that this instability will be weak and would only be present for a brief period of the star's life. Our analysis accounts for the presence of thermal excitations encoded in entrainment between the entropy and the superfluid component. Finally, we discuss the complete spectrum, including the normal sound waves and, in superfluid regions, the second sound.

Description: In the standard scenario for spin evolution of isolated neutron stars, a young pulsar slows down with a surface magnetic field B that does not change. Thus the pulsar follows a constant B trajectory in the phase space of spin period and spin period time derivative. Such an evolution predicts a braking index n  = 3 while the field is constant and n  〉 3 when the field decays. This contrasts with all nine observed values being n  〈 3. Here we consider a magnetic field that is buried soon after birth and diffuses to the surface. We use a model of a growing surface magnetic field to fit observations of the three pulsars with lowest n : PSR J0537–6910 with n  = –1.5, PSR B0833–45 (Vela) with n  = 1.4, and PSR J1734–3333 with n  = 0.9. By matching the age of each pulsar, we determine their magnetic field and spin period at birth and confirm the magnetar-strength field of PSR J1734–3333. Our results indicate that all three pulsars formed in a similar way to central compact objects (CCOs), with differences due to the amount of accreted mass. We suggest that magnetic field emergence may play a role in the distinctive glitch behaviour of low braking index pulsars, and we propose glitch behaviour and characteristic age as possible criteria in searches for CCO descendants.

Description: Pulsars are known for their superb timing precision, although glitches can interrupt the regular timing behavior when the stars are young. These glitches are thought to be caused by interactions between normal and superfluid matter in the crust of the star. However, glitching pulsars such as Vela have been shown to require a superfluid reservoir that greatly exceeds that available in the crust. We examine a model in which glitches tap the superfluid in the core. We test a variety of theoretical superfluid models against the most recent glitch data and find that only one model can successfully explain up to 45 years of observational data. We develop a new technique for combining radio and x-ray data to measure pulsar masses, thereby demonstrating how current and future telescopes can probe fundamental physics such as superfluidity near nuclear saturation.

Description: Recently Strohmayer and Mahmoodifar presented evidence for a coherent oscillation in the X-ray light curve of the accreting millisecond pulsar XTE J1751–305, using data taken by RXTE during the 2002 outburst of this source. They noted that a possible explanation includes the excitation of a non-radial oscillation mode of the neutron star, either in the form of a g mode or an r mode. The r-mode interpretation has connections with proposed spin-evolution scenarios for systems such as XTE J1751–305. Here, we examine in detail this interesting possible interpretation. Using the ratio of the observed oscillation frequency to the star's spin frequency, we derive an approximate neutron star mass–radius relation which yields reasonable values for the mass over the range of expected stellar radius (as constrained by observations of radius-expansion burst sources). However, we argue that the large mode amplitude suggested by the Strohmayer and Mahmoodifar analysis would inevitably lead to a large spin-down of the star, inconsistent with its observed spin evolution, regardless of whether the r mode itself is in a stable or unstable regime. We therefore conclude that the r-mode interpretation of the observed oscillation is not consistent with our current understanding of neutron star dynamics and must be considered unlikely. Finally we note that, subject to the availability of a sufficiently accurate timing model, a direct gravitational-wave search may be able to confirm or reject an r-mode interpretation unambiguously, should such an event, with a similar inferred mode amplitude, recur during the advanced detector era.

Description: We use Chandra and XMM–Newton observations of the globular clusters Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favoured by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.

Description: Several persistent, low-luminosity ( L X ~ 10 34 erg s –1 ), long-spin-period ( P  〉 100 s) high-mass X-ray binaries have been reported with blackbody components with temperatures 〉1 keV. These hot thermal excesses have correspondingly small emitting regions (〈1 km 2 ) and are attributed to the neutron star polar caps. We present a recent XMM–Newton target of opportunity observation of the newest member of this class, Swift J045106.8–694803. The period was determined to be 168.5 ± 0.2 s as of 2012 July 17 (MJD = 561 25.0). At L X ~ 10 36 erg s –1 , Swift J045106.8–694803 is the brightest member of this new class, as well as the one with the shortest period. The spectral analysis reveals for the first time the presence of a blackbody with temperature $kT_{\rm BB}=1.8^{+0.2}_{-0.3}$ keV and radius R BB = 0.5 ± 0.2 km. The pulsed fraction decreases with increasing energy and the ratio between the hard (〉2 keV) and soft (〈2 keV) light curves is anticorrelated with the pulse profile. Simulations of the spectrum suggest that this is caused by the pulsations of the blackbody being ~ out of phase with those of the power-law component. Using a simple model for emission from hot spots on the neutron star surface, we fit the pulse profile of the blackbody component to obtain an indication of the geometry of the system.

Description: The radio and gamma-ray pulsar PSR J2032+4127 was recently found to be in a decades-long orbit with the Be star MT91 213, with the pulsar moving rapidly towards periastron. This binary shares many similar characteristics with the previously unique binary system PSR B1259–63/LS 2883. Here, we describe radio, X-ray, and optical monitoring of PSR J2032+4127/MT91 213. Our extended orbital phase coverage in radio, supplemented with Fermi LAT gamma-ray data, allows us to update and refine the orbital period to 45–50 yr and time of periastron passage to 2017 November. We analyse archival and recent Chandra and Swift observations and show that PSR J2032+4127/MT91 213 is now brighter in X-rays by a factor of ~70 since 2002 and ~20 since 2010. While the pulsar is still far from periastron, this increase in X-rays is possibly due to collisions between pulsar and Be star winds. Optical observations of the Hα emission line of the Be star suggest that the size of its circumstellar disc may be varying by ~2 over time-scales as short as 1–2 months. Multiwavelength monitoring of PSR J2032+4127/MT91 213 will continue through periastron passage, and the system should present an interesting test case and comparison to PSR B1259–63/LS 2883.