Timing and spectral properties of the accreting millisecond pulsar SWIFT J1756.9-2508

TL;DR

This study analyzes the timing and spectral properties of the accreting millisecond pulsar SWIFT J1756.9-2508 during its 2007 outburst, revealing characteristics typical of low-luminosity neutron star systems in a hard state.

Contribution

First detailed timing and spectral analysis of SWIFT J1756.9-2508, classifying it as an atoll source in the extreme island state based on X-ray variability and spectral features.

Findings

Detected flat-topped broadband noise (~35%) with low characteristic frequencies (~0.1 Hz)

Identified a hard X-ray tail extending up to 100 keV

Classified the source as an atoll in the extreme island state

Abstract

SWIFT J1756.9-2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi X-ray Timing Explorer during the 2007 outburst of the source. We detect strong (~35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (~0.1 Hz). This makes SWIFT J1756.9-2508 similar to the rest of AMPs and to other low luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.