SRG/ART-XC and NuSTAR Observations of the X-Ray pulsar GRO J1008–57 in the Lowest Luminosity State
A. Lutovinov, S. Tsygankov, S. Molkov, V. Doroshenko, A. Mushtukov, V. Arefiev, I. Lapshov, A. Tkachenko, M. Pavlinsky
Abstract
Abstract We report results of the first broadband observation of the transient X-ray pulsar GRO J1008−57 performed in the quiescent state. Observations were conducted quasi-simultaneously with NuSTAR and the Mikhail Pavlinsky ART-XC telescope on board SRG right before the beginning of a Type I outburst. GRO J1008−57 was detected in the state with the lowest observed luminosity around several ×10 34 erg s −1 and consequently accreting from the cold disk. Timing analysis allowed us to detect pulsations during this state for the first time. The observed pulsed fraction of about 20% is, however, almost three times lower than in brighter states when the accretion proceeds through the standard disk. We traced the evolution of the broadband spectrum of the source on a scale of three orders of magnitude in luminosity and found that at the lowest luminosities the spectrum transforms into the double-hump structure similarly to other X-ray pulsars accreting at low luminosities (X Persei, GX 304–1, and A 0535+262) reinforcing the conclusion that this spectral shape is typical for these objects.