Bulent Kiziltan

THE NEUTRON STAR MASS DISTRIBUTION

In recent years, the number of pulsars with secure mass measurements has increased to a level that allows us to probe the underlying neutron star (NS) mass distribution in detail. We critically review the radio pulsar mass measurements. For the first time, we are able to analyze a sizable population of NSs with a flexible modeling approach that can effectively accommodate a skewed underlying distribution and asymmetric measurement errors. We find that NSs that have evolved through different evolutionary paths reflect distinctive signatures through dissimilar distribution peak and mass cutoff values. NSs in double NS and NS-white dwarf (WD) systems show consistent respective peaks at 1.33 M and 1.55 M, suggesting significant mass accretion (Δm 0.22 M) has occurred during the spin-up phase. The width of the mass distribution implied by double NS systems is indicative of a tight initial mass function while the inferred mass range is significantly wider for NSs that have gone through recycling. We find a mass cutoff at 2.1 M for NSs with WD companions, which establishes a firm lower bound for the maximum NS mass. This rules out the majority of strange quark and soft equation of state models as viable configurations for NS matter. The lack of truncation close to the maximum mass cutoff along with the skewed nature of the inferred mass distribution both enforce the suggestion that the 2.1 M limit is set by evolutionary constraints rather than nuclear physics or general relativity, and the existence of rare supermassive NSs is possible.

Millisecond Pulsar Ages: Implications of Binary Evolution and a Maximum Spin Limit

In the absence of constraints from the binary companion or supernova remnant, the standard method for estimating pulsar ages is to infer an age from the rate of spin-down. While the generic spin-down age may give realistic estimates for normal pulsars, it can fail for pulsars with very short periods. Details of the spin-up process during the low-mass X-ray binary (LMXB) phase pose additional constraints on the period (P) and spin-down rates that may consequently affect the age estimate. Here, we propose a new recipe to estimate millisecond pulsar (MSP) ages that parametrically incorporates constraints arising from binary evolution and limiting physics. We show that the standard method can be improved by this approach to achieve age estimates closer to the true age while the standard spin-down age may overestimate or underestimate the age of the pulsar by more than a factor of ~10 in the millisecond regime. We use this approach to analyze the population on a broader scale. For instance, in order to understand the dominant energy loss mechanism after the onset of radio emission, we test for a range of plausible braking indices. We find that a braking index of n = 3 is consistent with the observed MSP population. We demonstrate the existence and quantify the potential contributions of two main sources of age corruption: the previously known age bias due to secular acceleration and age contamination driven by sub-Eddington progenitor accretion rates. We explicitly show that descendants of LMXBs that have accreted at very low rates will exhibit ages that appear older than the age of the Galaxy. We further elaborate on this technique, the implications and potential solutions it offers regarding MSP evolution, the underlying age distribution, and the post-accretion energy loss mechanism.

THE INCLINATION ANGLE AND EVOLUTION OF THE BRAKING INDEX OF PULSARS WITH PLASMA-FILLED MAGNETOSPHERE: APPLICATION TO THE HIGH BRAKING INDEX OF PSR J1640–4631

The recently discovered rotationally powered pulsar PSR J1640–4631 is the first to have a braking index measured, with high enough precision, that is greater than 3. An inclined magnetic rotator in vacuum or plasma would be subject not only to spin-down but also to an alignment torque. The vacuum model can address the braking index only for an almost orthogonal rotator, which is incompatible with the single-peaked pulse profile. The magnetic dipole model with the corotating plasma predicts braking indices between 3 and 3.25. We find that the braking index of 3.15 is consistent with two different inclination angles, 185 ± 3° and 56° ± 4°. The smaller angle is preferred given that the pulse profile has a single peak and the radio output of the source is weak. We infer the change in the inclination angle to be at the rate −023 per century, three times smaller in absolute value than the rate recently observed for the Crab pulsar.

Constraints on Pulsar Evolution: The Joint Period-Spin-Down Distribution of Millisecond Pulsars

We calculate the joint period-spin-down (P-) distributions of millisecond radio pulsars (MSRPs) for the standard evolutionary model in order to test whether the observed MSRPs are the unequivocal descendants of millisecond X-ray pulsars (MSXPs). The P- densities implied by the standard evolutionary model compared with observations suggest that there is a statistically significant overabundance of young/high magnetic field MSRPs. Taking biases due to observational selection effects into account, it is unlikely that MSRPs have evolved from a single coherent progenitor population that loses energy via magnetic dipole radiation after the onset of radio emission. By producing the P- probability map, we show with more than 95% confidence that the fastest spinning millisecond pulsars with high magnetic fields, e.g., PSR B1937+21, cannot be produced by the observed MSXPs within the framework of the standard model.

Reassessing The Fundamentals: New Constraints on the Evolution, Ages and Masses of Neutron Stars

The ages and masses of neutron stars (NSs) are two fundamental threads that make pulsars accessible to other sub‐disciplines of astronomy and physics. A realistic and accurate determination of these two derived parameters play an important role in understanding of advanced stages of stellar evolution and the physics that govern relevant processes. Here I summarize new constraints on the ages and masses of NSs with an evolutionary perspective. I show that the observed P‐Ṗ demographics is more diverse than what is theoretically predicted for the standard evolutionary channel. In particular, standard recycling followed by dipole spin‐down fails to reproduce the population of millisecond pulsars with higher magnetic fields (B > 4 × 108 G) at rates deduced from observations. A proper inclusion of constraints arising from binary evolution and mass accretion offers a more realistic insight into the age distribution. By analytically implementing these constraints, I propose a “modified” spin‐down age (τ) for mil...

Do all millisecond pulsars share a common heritage

The discovery of millisecond pulsations from neutron stars in low mass X‐ray binary (LMXB) systems has substantiated the theoretical prediction that links millisecond radio pulsars (MSRPs) and LMXBs. Since then, the process that produces millisecond radio pulsars from LMXBs, followed by spin‐down due to dipole radiation has been conceived as the ‘standard evolution’ of millisecond pulsars. However, the question whether all the observed millisecond radio pulsars could be produced by LMXBs has not been quantitatively addressed until now.The standard evolutionary process produces millisecond pulsars with periods (P) and spin‐down rates (Ṗ) that are not entirely independent. The possible P‐Ṗ values that millisecond radio pulsars can attain are jointly constrained. In order to test whether the observed millisecond radio pulsars are the unequivocal descendants of millisecond X‐ray pulsars (MSXP), we have produced a probability map that represents the expected distribution of millisecond radio pulsars for the st...