Discovery of the 5 keV cyclotron line followed by three harmonics in SwiftJ1626.6-5156

TL;DR

This paper reports the discovery of four cyclotron resonance scattering features in SwiftJ1626.6-5156, revealing a complex magnetic field structure and making it a key target for future X-ray polarimetry studies.

Contribution

It is the first to identify more than three cyclotron lines in SwiftJ1626.6-5156, indicating a weaker magnetic field than previously known in X-ray pulsars.

Findings

Four cyclotron lines detected at ~4.9, 9, 13, and 17 keV.

SwiftJ1626.6-5156 has the weakest confirmed magnetic field among X-ray pulsars.

The spectrum's complexity suggests multiple harmonics of the cyclotron resonance.

Abstract

We report on observations of the Be/X-ray binary system SwiftJ1626.6-5156 performed with NuSTAR during a short outburst in March 2021, following its detection of by the MAXI monitor and Spektrum-Roentgen-Gamma (SRG) observatory. Our analysis of the broadband X-ray spectrum of the source confirms the presence of two absorption-like features at energies ~9 and ~17 keV previously reported in literature and interpreted as the fundamental cyclotron resonance scattering feature (CRSF) and its first harmonic (based on RXTE data). The better sensitivity and energy resolution of NuSTAR, combined with the low energy coverage of NICER, allowed us to detect two additional absorption-like features at ~4.9 keV and ~13 keV. We conclude, therefore, that in total four cyclotron lines are observed in the spectrum of SwiftJ1626.6-5156: the fundamental CRSF at ~4.9 keV and three higher spaced harmonics. This discovery makes SwiftJ1626.6-5156 the second accreting pulsar, after 4U0115+63, whose spectrum is characterized by more than three lines of a cyclotronic origin, and implies the source has the weakest confirmed magnetic field among all X-ray pulsars B~4E11 G. This discovery makes SwiftJ1626.6-5156 one of prime targets for the upcoming X-ray polarimetry missions covering soft X-ray band such as IXPE and eXTP.