Dynamical constraints on the companion mass from pulsar timing and cyclotron line confirmation in the SFXT IGR J16418-4532

@misc{devaraj2026dynamical,
  title = {{Dynamical constraints on the companion mass from pulsar timing and cyclotron line confirmation in the SFXT IGR J16418-4532}},
  author = {Ashwin Devaraj and Rahul Sharma and Ketan Rikame and Biswajit Paul},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.36103},
  eprint = {2610.36103},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.36103},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Dynamical constraints on the companion mass from pulsar timing and cyclotron line confirmation in the SFXT IGR J16418-4532
AU  - Ashwin Devaraj
AU  - Rahul Sharma
AU  - Ketan Rikame
AU  - Biswajit Paul
PY  - 2026
DA  - 2026/10/26
PB  - arXiv
DO  - 10.48550/arxiv.2610.36103
UR  - https://arxiv.org/abs/2610.36103
AB  - Supergiant Fast X-ray Transients (SFXTs) exhibit extreme X-ray variability on short timescales of minutes to hours, driven by accretion processes that are still poorly understood. IGR J16418–4532 is an intermediate SFXT in which X-ray pulsations have been detected and which has been proposed as a candidate cyclotron line source, making it a key system for probing neutron-star accretion physics and magnetic field properties in this class of sources. We aim to constrain the projected semi-major axis of the SFXT IGR J16418--4532, through timing analysis, and confirm the presence of a Cyclotron Resonant Scattering Feature (CRSF), thereby characterising the nature of the compact object in this system. We analysed a 148 ks long NuSTAR observation and several archival RXTE observations using epoch-folding techniques to search for pulsations while correcting the photon arrival times for a range of a_x sin(i) values. We performed phase-averaged spectroscopy using several different continuum models and assessed the significance of the cyclotron line feature. We detect coherent pulsations with P_spin = 1208.67 ± 0.04 s and P_spin = 1209.4178 ± 0.0084 s in NuSTAR and RXTE/PCA light curves, respectively. The pulsations exhibit clear dependence on the assumed projected semi-major axis, and are detected with the highest significance for a_xsin i = 55.1 ± 3.5 lt-s (NuSTAR) and 51.3 ± 2.6 lt-s (RXTE/PCA). The light curves show several flares, and the pulse profiles are found to evolve over the course of the observation. Sliding window pulse profiles reveal the pronounced effect of the orbital delay on the pulse arrival times in uncorrected data, which is fully removed post-orbital correction. Using the inferred orbital parameters, we constrain the mass and radius of the companion star to be in the range of ∼12--18 M_⊙ and ∼13--16 R_⊙, respectively. We report the first successful constraint on the binary mass function of an SFXT obtained by considering the orbital motion of the neutron star. Spectral analysis reveals a robust cyclotron absorption line at ∼50 keV, independent of the choice of the continuum, suggesting a magnetic field strength of ∼5 × 10^12~G. The cyclotron line confirmation and dynamical constraints on the companion's mass indicate that IGR J16418--4532 hosts a neutron star with a magnetic field of B≈ 5 × 10^12 G accreting from a ∼12--18 M_⊙ companion. We demonstrate that optimising the pulse detection by correcting the photon arrival times with trial a_x sin(i) values rather than modelling the pulse arrival time delays offers a route to obtain dynamical mass measurements in other SFXTs whose variable pulse profiles limit the scope of conventional timing techniques.
ER  -