Staring at 4U 1909+07 with Suzaku

TL;DR

This study analyzes Suzaku data of the neutron star HMXB 4U 1909+07, revealing its spectral and pulse profile characteristics, and suggests different physical conditions at the magnetic poles without detecting a cyclotron feature.

Contribution

It provides the first detailed spectral and pulse phase analysis of 4U 1909+07, highlighting the absence of a cyclotron line and insights into the accretion geometry.

Findings

Pulse period shows random-walk behavior, indicating wind accretion.

Spectral data fit well with a power-law and exponential cutoff model.

No cyclotron resonant scattering feature detected.

Abstract

We present an analysis of the neutron star High Mass X-ray Binary (HMXB) 4U 1909+07 mainly based on Suzaku data. We extend the pulse period evolution, which behaves in a random-walk like manner, indicative of direct wind accretion. Studying the spectral properties of 4U 1909+07 between 0.5 to 90 keV we find that a power-law with an exponential cutoff can describe the data well, when additionally allowing for a blackbody or a partially covering absorber at low energies. We find no evidence for a cyclotron resonant scattering feature (CRSF), a feature seen in many other neutron star HMXBs sources. By performing pulse phase resolved spectroscopy we investigate the origin of the strong energy dependence of the pulse profile, which evolves from a broad two-peak profile at low energies to a profile with a single, narrow peak at energies above 20 keV. Our data show that it is very likely that a higher folding energy in the high energy peak is responsible for this behavior. This in turn leads to the assumption that we observe the two magnetic poles and their respective accretion columns at different phases, and that these accretions column have slightly different physical conditions.