Probing the emission geometry of the X-ray pulsar 2S 1417$-$624 during a weak outburst with NICER, IXPE, and NuSTAR

TL;DR

This study combines spectral and polarimetric observations from NICER, IXPE, and NuSTAR to analyze the emission geometry and physical conditions of the X-ray pulsar 2S 1417-624 during its 2025 outburst, revealing phase-dependent variability.

Contribution

It presents the first polarimetric measurements of 2S 1417-624 and constrains its magnetic obliquity, advancing understanding of emission geometry in transient X-ray pulsars.

Findings

Polarization degree of 3.3% with a polarization angle of 18°

Magnetic obliquity estimated at 64°

Pronounced phase-dependent spectral and polarization variability

Abstract

We report results from a multi-mission observational campaign of the transient X-ray pulsar 2S~1417$-$624 during its 2025 outburst, using data from NICER, IXPE, and NuSTAR. Phase-averaged and phase-resolved spectroscopy with NICER and NuSTAR reveal that a typical power-law model with a high-energy cut-off well describes the broadband spectra. Several spectral parameters, however, show clear and systematic modulations with pulse phase, indicating variations in the physical conditions of the emitting plasma over the neutron star's rotation. In particular, IXPE provides the first polarimetric measurements of this source, yielding a phase-averaged polarization degree of $3.3 \pm 1.7\%$ and a polarization angle of ${18}^{\circ} \pm {15}^{\circ}$, both at the $1\sigma$ confidence level. Fitting with the rotating vector model yields a magnetic obliquity of $\theta = 64_{-26}^{+17}$ deg, indicating a significantly inclined magnetic geometry that may approach a quasi-orthogonal configuration. Together, these findings demonstrate pronounced phase-dependent spectral and polarization variability, offering valuable constraints on the geometry and emission processes within the accretion region of this transient X-ray pulsar.