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

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.

8 pages, 5 figures, accepted to ApJ

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Cited by in corpus (13)

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• X-ray emission from magnetized neutron star atmospheres at low mass accretion rates. I. Phase-averaged spectrum
• X-ray pulsar GRO J1008$-$57 as an orthogonal rotator
• RX J0440.9+4431: another supercritical X-ray pulsar
• SRGA J124404.1-632232/SRGU J124403.8-632231: a new X-ray pulsar discovered in the all-sky survey by SRG
• The unaltered pulsar: GRO J1750-27, a super-critical X-ray neutron star that does not blink an eye
• SRG/ART-XC, Swift, NICER and NuSTAR study of different states of the transient X-ray pulsar MAXI J0903-531
• Discovery of a bimodal luminosity distribution in persistent Be/X-ray pulsar 2RXP J130159.6-635806
• Double-hump spectrum, pulse profile dip, and pulsed fraction spectra from the low-accretion regime in the X-ray pulsar MAXI J0655-013
• Spectral evolution of X-ray pulsar 4U 1901+03 during the 2019 outburst based on Insight-HXMT and NuSTAR observations
• Detection of a cyclotron line in the Be X-ray pulsar IGR J06074$+$2205
• Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA NuSTAR and X-shooter
• Losing a minute every two years: SRG X-ray view on the rapidly accelerating X-ray pulsar SXP1323