The 17 min orbital period in the Ultra Compact X-ray Binary 4U 0513-40

TL;DR

This paper reports the discovery of a 17-minute orbital period in the ultracompact X-ray binary 4U 0513-40, observed through spectral state changes and flux variations, indicating a high-inclination binary system.

Contribution

The study provides the first detection of a precise orbital period in 4U 0513-40 using multi-mission X-ray data, linking spectral states to orbital modulation.

Findings

Detected a 17-minute sinusoidal orbital period in high/soft state

Period disappears in low/hard state, indicating state-dependent visibility

Suggests a high inclination angle (>80°) for the binary system

Abstract

The ultracompact low-mass X-ray binary 4U 0513-40 in the globular cluster NGC1851 exhibits large amplitude X-ray flux variations with spectral changes from low/hard to high/soft states which have not been reported previously in other ultracompact X-ray binaries. Using BeppoSAX, CHANDRA and XMM Newton archival data together with recent INTEGRAL observations, we reveal a clear sinusoidal periodic signal with a period of ~17 minutes when the source is in a typical high/soft state with a dominant soft thermal component. The periodicity disappears when the source is in a low/hard state and the thermal soft component is not required any more to model the data. These properties indicate the orbital nature of the detected signal and imply an high inclination angle of the binary system (>80{\deg}).