SRG/ART-XC and NuSTAR observations of the X-ray pulsar GRO J1008-57 in the lowest luminosity state

TL;DR

This study presents the first broadband observations of the X-ray pulsar GRO J1008-57 in its lowest luminosity state, revealing pulsations and a characteristic spectral transformation at low accretion levels.

Contribution

It provides new insights into the pulsar's behavior at very low luminosities, including detection of pulsations and spectral shape changes, expanding understanding of accretion processes.

Findings

Pulsations detected at the lowest luminosity state.

Pulsed fraction is about 20%, lower than in brighter states.

Spectrum transforms into a double-hump structure at low luminosity.

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 the Mikhail Pavlinsky ART-XC telescope on board SRG and NuSTAR right before the beginning of a Type I outburst. GRO J1008-57 was detected in the state with the lowest observed luminosity around several $\times 10^{34}$ erg s$^{-1}$ and consequently accreting from the cold disk. Timing analysis allowed to significantly 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, A0535+262) reinforcing conclusion that this spectral shape is typical for these objects.