Is the plateau state in GRS 1915+105 equivalent to canonical hard states?

Pieter Van Oers,Dipankar Maitra,Sera Markoff,Emilios Harlaftis,Vivek Dhawan,Farid Rahoui,Jörn Wilms,Alberto J Castro-Tirado,Michael Nowak,Jerome Rodriguez

doi:10.1111/j.1365-2966.2010.17339.x

Abstract

GRS1915+105 is a very peculiar black hole binary that exhibits accretion-related states that are not observed in any other stellar-mass black hole system. One of these states, however -- referred to as the plateau state -- may be related to the canonical hard state of black hole X-ray binaries. Both the plateau and hard state are associated with steady, relatively lower X-ray emission and flat/inverted radio emission, that is sometimes resolved into compact, self-absorbed jets. However, while generally black hole binaries quench their jets when the luminosity becomes too high, GRS1915+105 seems to sustain them despite the fact that it accretes at near- or super-Eddington rates. In order to investigate the relationship between the plateau and the hard state, we fit two multi-wavelength observations using a steady-state outflow-dominated model, developed for hard state black hole binaries. The data sets consist of quasi-simultaneous observations in radio, near-infrared and X-ray bands. Interestingly, we find both significant differences between the two plateau states, as well as between the best-fit model parameters and those representative of the hard state. We discuss our interpretation of these results, and the possible implications for GRS 1915+105's relationship to canonical black hole candidates.

Highlights

The microquasar GRS 1915+105 was discovered on 15 August 1992, by the WATCH all-sky monitor, aboard the Russian GRANAT satellite (Castro-Tirado, Brandt, & Lund 1992; Castro-Tirado et al 1994)
As the focus of our paper is on modelling the nonthermal spectrum, and the exact stellar model needed for GRS 1915+105 is uncertain, we use a simple blackbody to model the NIR, with the temperature fixed as discussed in the previous Section
Despite the fact that the MNW05 model was originally intended for application to hard states in canonical blackIn Memoriam hole binary (BHB) only, it appears to well approximate the plateau state in GRS 1915+105

Summary

Introduction

The microquasar GRS 1915+105 was discovered on 15 August 1992, by the WATCH all-sky monitor, aboard the Russian GRANAT satellite (Castro-Tirado, Brandt, & Lund 1992; Castro-Tirado et al 1994). It is a hard X-ray transient located in the constellation of Aquila, at l = 45.37◦, b = −0.22◦, and was the first stellar mass accreting black. In Memoriam hole binary (BHB) discovered to exhibit superluminal velocities in its radio emitting-ejecta (Mirabel & Rodriguez 1994). Subsequent observations with instruments onboard the Rossi X-ray Timing Explorer (RXTE ) have revealed a richness in variability, distinguishing GRS 1915+105 from every other known BHB, over which astronomers are still puzzling to this day. Other models have interpreted the spectral changes as resulting from the disappearance of the corona (Chaty 1998; Rodriguez et al 2008a,b), or from the dissipation of magnetic energy via magneto-hydrodynamical plasma processes (e.g. Tagger et al 2004)

Results

Discussion

Conclusion