X-ray and radio view of the Be/X-ray binary pulsar 1A 1118−61 during the 2026 giant outburst

Abstract We present a detailed X-ray and radio study of the Be/X-ray binary pulsar 1A 1118−61 during its 2026 giant outburst using NuSTAR, Swift/XRT, and MeerKAT observations. Timing analysis reveals coherent pulsations with spin periods of ~408.77 s and ~408.30 s at luminosities of ~2.44 × 1037 erg s−1 and ~9.7 × 1035 erg s−1, respectively. The pulse profiles show a clear dependence on both energy and luminosity. At higher luminosity, the profile is smooth and single-peaked, whereas at lower luminosity, an absorption-like feature appears, indicating changes in the accretion geometry. A quasi-periodic oscillation (QPO) at 0.106 Hz is detected during the NuSTAR observation at higher luminosity. Broadband spectral analysis using NuSTAR data reveals a cyclotron resonance scattering feature (CRSF) at ~53.9 keV during the high-luminosity observation, corresponding to a neutron star magnetic field strength of ~6.0 × 1012 G. However, no CRSF is detected in the spectrum at the lower luminosity. An iron Kα line at 6.4 keV is evident in both NuSTAR observations, while an additional weak higher-energy iron emission feature at ~6.9 keV is detected in the high-luminosity NuSTAR observation. The Swift/XRT spectra in the 1-10 keV energy range are well described using an absorbed power law and a blackbody component. Radio observation with MeerKAT during the decay phase of the X-ray outburst results in a non-detection, providing a stringent upper limit on the radio luminosity. This indicates weak or absent radio emission from 1A 1118-61 at the time of observation.