Detection of a cyclotron line in SXP 15.3 during its 2017 outburst

We report the results of AstroSat and NuSTAR observations of the Be/X-ray binary pulsar SXP 15.3 in the Small Magellanic Cloud during its outburst in late 2017, when the source reached a luminosity level of ~ 10^{38} erg s^{-1}, close to the Eddington limit. The unprecedented broadband coverage of the source allowed us to perform timing and spectral analysis between 3 and 80 keV. The pulse profile exhibits a significant energy dependence, and morphs from a double peaked profile to a single broad pulse at energies >15 keV. This can be explained by a spectral hardening during an intensity dip seen between the two peaks of the pulse profile. We detect a Cyclotron Resonance Scattering Feature (CRSF) at ~5 keV in the X-ray spectrum, independent of the choice of the continuum model. This indicates a magnetic field strength of 6x10^{11} G for the neutron star.