Transient Low-Mass X-Ray Binary Populations in Elliptical Galaxies NGC 3379 and NGC 4278

TL;DR

This paper develops a self-consistent model for transient neutron star low-mass X-ray binaries and compares it with observations in elliptical galaxies, revealing the importance of variable duty cycles and the role of primordial binaries.

Contribution

Introduces a physically motivated prescription for transient LMXB properties and demonstrates its consistency with observed populations in elliptical galaxies.

Findings

Models with variable duty cycles match observed transient populations.

Most observed transients likely have red giant donors.

Primordial binary evolution dominates in GC-poor ellipticals.

Abstract

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star (NS) low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration and recurrence time. We apply this prescription to the population synthesis (PS) models of field LMXBs presented by Fragos et al. (2008), and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources (Brassington et al., 2008, 2009). We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Our comparison suggests that LMXBs formed through evolution of primordial field binaries are dominant in globular cluster (GC) poor elliptical galaxies, while they still have a significant contribution in GC rich ones.