NuSTAR rules out a cyclotron line in the accreting magnetar candidate 4U2206+54

TL;DR

Using NuSTAR data, the study rules out the existence of a cyclotron line in 4U2206+54's spectrum, indicating a strongly magnetized neutron star with a significant spin-down rate.

Contribution

This work provides the first definitive exclusion of a cyclotron line in 4U2206+54 up to 60 keV, refining the understanding of its magnetic properties.

Findings

No cyclotron line detected up to 60 keV.

Significant increase in spin period to 5750 s.

Confirmation of rapid spin-down rate.

Abstract

Based on our new NuSTAR X-ray telescope data, we rule out any cyclotron line up to 60 keV in the spectra of the high mass X-ray binary 4U2206+54. In particular, we do not find any evidence of the previously claimed line around 30 keV, independently of the source flux, along the spin pulse. The spin period has increased significantly, since the last observation, up to $5750\pm10$ s, confirming the rapid spin down rate $\dot \nu=-1.8\times 10^{-14}$~Hz s$^{-1}$. This behaviour might be explained by the presence of a strongly magnetized neutron star ($B_{\rm s}>$ several times $10^{13}$ G) accreting from the slow wind of its main sequence O9.5 companion.