Drop in the hard pulsed fraction and a candidate cyclotron line in IGR J16320-4751 seen by NuSTAR

Arash Bodaghee; Alan J. -L. Chiu; John A. Tomsick; Varun Bhalerao; Eugenio Bottacini; Maica Clavel; Cody Cox; Felix Fürst; Matthew J. Middleton; Farid Rahoui; Jerome Rodriguez; Pat Romano; Joern Wilms

doi:10.3847/1538-4357/acd541

arXiv · 2305.07068

Drop in the hard pulsed fraction and a candidate cyclotron line in IGR J16320-4751 seen by NuSTAR

Arash Bodaghee·Alan J. -L. Chiu·John A. Tomsick·Varun Bhalerao·Eugenio Bottacini·Maica Clavel·Cody Cox·Felix Fürst·Matthew J. Middleton·Farid Rahoui·Jerome Rodriguez·Pat Romano·Joern Wilms

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Abstract

We report on a timing and spectral analysis of a 50-ks NuSTAR observation of IGR J16320-4751 (= AX J1631.9-4752); a high-mass X-ray binary hosting a slowly-rotating neutron star. In this observation from 2015, the spin period was 1,308.8+/-0.4 s giving a period derivative dP/dt ~ 2E-8 s s-1 when compared with the period measured in 2004. In addition, the pulsed fraction decreased as a function of energy, as opposed to the constant trend that was seen previously. This suggests a change in the accretion geometry of the system during the intervening 11 years. The phase-averaged spectra were fit with the typical model for accreting pulsars: a power law with an exponential cutoff. This left positive residuals at 6.4 keV attributable to the known iron K-alpha line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5-sigma. We found no significant differences in the spectral parameters across the spin period, other than the expected changes in flux and component normalizations. A flare lasting around 5 ks was captured during the first half of the observation where the X-ray emission hardened and the local column density decreased. Finally, the binary orbital period was refined to 8.9912+/-0.0078 d thanks to Swift/BAT monitoring data from 2005-2022.

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Arash Bodaghee, Alan J. -L. Chiu, John A. Tomsick, Varun Bhalerao, Eugenio Bottacini, Maica Clavel, Cody Cox, Felix Fürst, Matthew J. Middleton, Farid Rahoui, Jerome Rodriguez, Pat Romano, Joern Wilms. 2023-05-11. Drop in the hard pulsed fraction and a candidate cyclotron line in IGR J16320-4751 seen by NuSTAR. https://doi.org/10.3847/1538-4357%2Facd541

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