Evidence for different accretion regimes in GRO J1008-57

Abstract

We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a luminosity range of three orders of magnitude using NuSTAR, Suzaku and RXTE data. We find significant evolution of the spectral parameters with luminosity. In particular the photon index hardens with increasing luminosity at intermediate luminosities between $10^{36}$ $-$ $10^{37}$ erg s${}^{-1}$. This evolution is stable and repeatedly observed over different outbursts. However, at the extreme ends of the observed luminosity range, we find that the correlation breaks down, with a significance level of at least $3.7\sigma$. We conclude that these changes indicate transitions to different accretion regimes, which are characterized by different deceleration processes, such as Coulomb or radiation breaking. We compare our observed luminosity levels of these transitions to theoretical predications and discuss the variation of those theoretical luminosity values with fundamental neutron star parameters. Finally, we present detailed spectroscopy of the unique "triple peaked" outburst in 2014/15 which does not fit in the general parameter evolution with luminosity. The pulse profile on the other hand is consistent with what is expected at this luminosity level, arguing against a change in accretion geometry. In summary, GRO J1008-57 is an ideal target to study different accretion regimes due to the well constrained evolution of its broad-band spectral continuum over several orders of magnitude in luminosity.

13 pages, 7 figures, 3 tables. Accepted for publication in A&A

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Cited by in corpus (6)

• Looking at A 0535+26 at low luminosities with NuSTAR
• Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1
• Insight-HXMT firm detection of the highest energy fundamental cyclotron resonance scattering feature in the spectrum of GRO J1008-57
• NuSTAR view of Be/X-ray binary pulsar 2S 1417-624 during 2018 giant outburst
• The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE
• Fast time optical variability in Be/X-ray binaries. Pulsation and rotation