Probing the emission geometry of the X-ray pulsar 2S 1417$-$624 during a weak outburst with NICER, IXPE, and NuSTAR
Year: 2025
Type: preprint
Abstract: We report results from a multi-mission observational campaign of the transient X-ray pulsar 2S~1417$-$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 broken 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. IXPE provides the first polarimetric measurements of this source, yielding a phase-averaged polarization degree (PD) of $4.8 \pm 1.2$% and a polarization angle (PA) of ${17}^{\circ} \pm {7}^{\circ}$, both quoted at the $1σ$ confidence level. Fitting the phase-resolved PA with the rotating vector model (RVM) gives a magnetic obliquity of $θ= 69_{-29}^{+13}$ deg, indicating a significantly inclined magnetic geometry that may approach a quasi-orthogonal configuration. In addition, using the unbinned photon-by-photon method, we obtain a PD of $5.9 \pm 1.2$% across the pulse phase, together with a pulsar geometry consistent with that inferred from the binned analysis, assuming the variable PA predicted by the RVM. A simultaneous RVM fit across the three energy bands, 2--5 keV, 5--6 keV, and 6--8 keV, provides the strongest constraints on the geometrical parameters, yielding $θ= {84}_{-6}^{+4}$ deg. 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.
Source: arXiv (Cornell University)
Authors: Zhou, Menglei, Wang, Pengju, Liu, Honghui, Ducci, Lorenzo, Tsygankov, Sergey S., Zhao, Qingchang, Poutanen, Juri, Ji, Long, Suleimanov, Valery F., Mushtukov, Alexander A., Liu, Qi, Diez, Camille M., Abalo, Luis, Grinberg, Victoria, Santangelo, Andrea
Institutions: University of Tübingen, Qinghai University, Chinese Academy of Sciences, Institute of High Energy Physics, University of Chinese Academy of Sciences, University of Turku, Sun Yat-sen University, University of Oxford, Hebei Normal University, European Space Agency, European Space Astronomy Centre, Leiden University, European Space Research and Technology Centre
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Aeronautics and Space Administration, California Institute of Technology, UK Research and Innovation, Deutscher Akademischer Austauschdienst, Deutsche Forschungsgemeinschaft, Hebei Normal University, China Scholarship Council, Jet Propulsion Laboratory, Marshall Space Flight Center
OpenAlex ID: W7077674379
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