Temporal variations and spectral properties of the Be/X-ray pulsar GRO J1008—57 studied by INTEGRAL

Abstract

The spin period variations and hard X-ray spectral properties of the Be/X-ray pulsar GRO J1008—57 are studied with INTEGRAL observations during two outbursts in 2004 June and 2009 March. The pulsation periods of ∼ 93.66 s in 2004 and ∼ 93.73 s in 2009 are determined. Pulse profiles of GRO J1008—57 during outbursts are strongly energy dependent with a double-peaked profile from 3–7 keV and a single-peaked profile in hard X-rays above 7 keV. Combined with previous measurements, we find that GRO J1008—57 has undergone a spin-down trend from 1993–2009 with a rate of ∼ 4.1 × 10−5 s d−1, and could have changed into a spin-up trend after 2009. We find a relatively soft spectrum in the early phase of the 2009 outburst with cutoff energy ∼ 13 keV. Above a hard X-ray flux of ∼ 10−9 erg cm−2 s−1, the spectra of GRO J1008—57 during outbursts need an enhanced hydrogen absorption with column density ∼ 6 × 1022 cm−2. The observed dip-like pulse profile of GRO J1008—57 in soft X-ray bands could be caused by this intrinsic absorption. Around the outburst peaks, a possible cyclotron resonance scattering feature at ∼ 74 keV is detected in the spectra of GRO J1008—57 which is consistent with the feature that was reported in MAXI/GSC observations, making the source a neutron star with the highest known magnetic field (∼ 6.6 × 1012 G) among accreting X-ray pulsars. This marginal feature is supported by the present detections in GRO J1008—57 following the correlation between the fundamental line energies and cutoff energies in accreting X-ray pulsars. Finally we discovered two modulation periods at ∼ 124.38 d and ∼ 248.78 d using RXTE/ASM light curves of GRO J1008—57. Two flare peaks appearing in the folded light curve had different spectral properties. The normal outburst lasting 0.1 of an orbital phase had a hard spectrum and could not be significantly detected below 3 keV. The second flare lasting ten days showed a very soft spectrum without significant detections above 5 keV. GRO J1008—57 is a good candidate of an accreting system with an equatorial circumstellar disk around the companion star. The neutron star passing the disk of the Be star near periastron and apastron produces two X-ray flares. The soft spectral properties in the secondary flares still need further detailed studies with soft X-ray spectroscopy.