From Statistics to In-Depth Analysis: X-ray Insights into Neutron Stars

Abstract

Neutron stars, the remnants of massive stellar explosions, hold a unique place in astrophysics. Their extreme densities and magnetic fields, coupled with their diversity, make them captivating objects for study.

Despite being discovered almost 60 years ago, numerous questions about the nature of neutron stars remain unanswered. These include their inner composition, population density across the galaxy, evolutionary paths between different types of neutron stars, as well as the physics of their thermal and non-thermal emission processes.

This Ph.D. thesis aims to address these gaps in knowledge through the presentation of two distinct yet complementary projects. The first project focuses on X-ray properties, delving into population statistics, while the second project offers a detailed exploration of the distribution of different emission components over pulse phase and their origin from a specific neutron star.\\

The correlation between the spin down-energy loss and X-ray luminosity in pulsars has been a focal point in numerous studies. This so-called X-ray efficiency appears to converge within the range of 10$^{-4}$ to 10$^{-5}$ at elevated spin down-energy loss values. Due to the (few) detections of older pulsars, there are indications that the X-ray efficiency increases with lower rotational energy loss. The intriguing question persists regarding whether this clustering is attributed to the existence of higher multipole magnetic field components, or if it stems from an escalation in electron-positron pair heating luminosity.

One challenge is that we often focus on the brightest pulsars, which might give us a biased view. Looking specifically at ``interesting'' pulsars found through other methods might lead to a biased understanding of their X-ray emissions.

The first project, ``Towards an X-ray inventory of nearby neutron stars, \citep{Vahdat2022}'', aimed to enhance our understanding of the population of nearby neutron stars by conducting a survey using the \emph{XMM-Newton} telescope. Although many neutron stars had been observed in X-rays, some remained elusive due to challenges related to uncertain distances, ages, or locations. To address this, we took a systematic approach, analyzing X-ray data from various sources, including \emph{XMM-Newton}, Chandra, and other catalogs covering multiple wavelengths. Our efforts resulted in the discovery of X-ray sources that were previously unidentified, offering a more comprehensive overview of these celestial objects within a distance of about 2 kiloparsecs. This project expanded the sample of nearby X-ray-emitting neutron stars, providing essential input for future investigations that require an unbiased representation of these sources.\\

In our second project, ``Multiwavelength Pulsations and Surface Temperature Distribution in the Middle-Aged Pulsar B1055–52, \citep{Vahdat2024}'', we conducted a detailed analysis of an isolated, middle-aged pulsar. Observations from the \emph{XMM-Newton} telescope, we carefully examined the X-ray emission of PSR B1055–52. Our investigations found that the phase-averaged spectra is best fitted with two blackbody components and a power law.

Furthermore, the phase-resolved spectral analysis of B1055 revealed variations in the thermal emission parameters throughout the pulsar's rotational cycle. This discovery suggests the non-uniformity of the blackbody components, especially the hotter one. Applying different statistical analysis techniques and multiwavelength pulse profile analysis allowed for comparisons with other isolated pulsars and facilitated discussions on temperature and magnetic field distributions, as well as the mechanisms driving the underlying heating processes. Additionally, the observation of a second hot spot confirmed the orthogonal geometry indicated by the radio observation.