Spectral and timing properties of IGR J00291+5934 during its 2015 outburst

TL;DR

This paper analyzes the spectral and timing behavior of the accreting millisecond X-ray pulsar IGR J00291+5934 during its 2015 outburst, revealing details about its emission components, orbital evolution, and pulse profile characteristics.

Contribution

It provides the first detailed spectral-timing analysis of IGR J00291+5934 during its 2015 outburst, including orbital parameters and pulse profile energy dependence.

Findings

Spectral state dominated by Comptonization and blackbody components.

Orbital period derivative constrained within a 3-sigma confidence interval.

Pulse fractional amplitude and lags vary with energy, with blackbody component linked to the neutron star surface.

Abstract

We report on the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015 outburst. The source is in a hard state dominated at high energies by a comptonization of soft photons ($\sim0.9$ keV) by an electron population with kT$_e\sim30$ keV, and at lower energies by a blackbody component with kT$\sim0.5$ keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a $3\sigma$ confidence level interval $-6.6\times 10^{-13}$ s/s $< \dot{P}_{orb} < 6.5 \times 10^{-13}$ s/s on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse-profile, indicating that this component resides at the neutron star surface (hot-spot).