  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.
