Firm detection of a cyclotron resonance feature with Suzaku in the X-ray spectrum of GRO J1008-57 during a giant outburst in 2012

We report on the firm detection of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of the Be X-ray binary pulsar, GRO J1008-57, achieved by the Suzaku Hard X-ray Detector during a giant outburst which was detected by the MAXI Gas Slit Camera in 2012 November. The Suzaku observation was carried out on 2012 November 20, outburst maximum when the X-ray flux reached similar to 0.45 Crab in 4-10 keV, which corresponds to a luminosity of 1.1 x 10(38) erg s(-1) in 0.5-100 keV at 5.8 kpc. The obtained broadband X-ray spectrum from 0.5 keV to 118 keV revealed a significant absorption feature, considered as the fundamental CRSF, at similar to 76 keV. This unambiguously reconfirms the previously suggested similar to 80 keV spectral feature in GRO J1008-57. The implied surface magnetic field, 6.6 x 10(12) G, is the highest among binary X-ray pulsars from which CRSFs have ever been detected.