Updated Spin and Orbital Parameters and Energy Dependent Pulse Behaviors of the Accreting Millisecond X-ray Pulsar IGR J17591$-$2342

TL;DR

This study refines the spin and orbital parameters of the accreting millisecond X-ray pulsar IGR J17591-2342 using NICER data, revealing energy-dependent pulse behaviors and phenomena explained by a two-component spectral model.

Contribution

The paper provides the first precise measurements of the pulsar's spin and orbital parameters and analyzes energy-dependent pulse behaviors with updated timing techniques.

Findings

Estimated magnetic field strength of the neutron star is approximately 4 x 10^8 G.

Observed soft phase lag from ~4 keV to 12 keV with a value of 0.06 cycles.

Pulsed fractional amplitude varies with energy, increasing up to ~5 keV then decreasing.

Abstract

We present our works in updating the spin and orbital parameters for the newly discovered accreting millisecond X-ray pulsar (AMXP) IGR J17591$-$2342 through pulsar timing and analyzing its energy dependent pulse behaviors. The data being analyzed were collected by Neutron Star Interior Composition ExploreR (NICER) that observed this AMXP from August to October 2018. Using the pulse arrival time delay technique, more accurate spin and orbital parameters were evaluated. From the measured spin frequency derivative, it is estimated that the magnetic field of the neutron star in IGR J17591$-$2342 is approximately $4 \times 10^{8}$ G. Precise pulse profiles can be made using the updated spin and orbital parameters. The soft phase lag phenomenon that is usually seen in other AMXPs is also observed from $\sim$4 keV to 12 keV with a value of 0.06 cycles (1.14 $\mu$s). Additionally, the pulsed fractional amplitude increases from 1 to $\sim$5 keV and then decreases for higher energy bands. We found that these phenomena, as well as the energy spectrum, can be explained by the two-component model with a relatively strong blackbody component and an additional unpulsed disk blackbody component.