Spectral and timing evolution of GRO J1655-40 during its outburst of 2005

In a recent outburst which lasted for 260 days, the black hole candidate GRO J1655-40 exhibited a behaviour similar to its last outburst observed almost eight years ago. We analyze a total of 150 observational spells in 122 days of data spreaded over the entire outburst phase of Feb. 2005 to Oct. 2005. From our study, a comprehensive understanding of the detailed behaviour of this black hole candidate has emerged. Based on the degree of importance of the black body and the power-law components we divide the entire episode in four spectral states, namely, hard, soft, very soft and intermediate. Quasi-Periodic oscillations (QPOs) were found in two out of these four states, namely, in the hard and the intermediate states. In the hard state, at the rising phase of the outburst, QPO frequency ranged from 0.034 - 17.78Hz and the spectra was fitted by a disk black body, power-law and iron emission line at 6.2 - 6.5 keV. In the intermediate state, QPOs vary from 13.17Hz to 19.04Hz and the QPO frequency modulation in this state was not significant. The spectra in this state are well fitted by the disk black body and the power-law components. In the hard state of the declining phase of the outburst, we found QPOs of decreasing frequency from 13.14 Hz to 0.034 Hz. The spectra of this state were fitted by a disk black body and power-law components, but in the initial few days a cooler Comptonized component was required for a better fit. In the soft and very soft states, the spectral states are mostly dominated by the strong disk black body component.

💡 Research Summary

The paper presents a comprehensive timing and spectral study of the 2005 outburst of the black‑hole candidate GRO J1655‑40, which lasted roughly 260 days from February to October. Using the Rossi X‑ray Timing Explorer (RXTE), the authors assembled 150 observational intervals covering 122 days of the outburst. Each interval was fitted with a combination of a multicolour disk‑blackbody (diskbb), a power‑law component, and, when required, a Gaussian iron‑line at 6.2–6.5 keV and a cool Comptonisation component (compTT). Based on the relative contributions of the thermal (disk) and non‑thermal (power‑law) components, the authors defined four distinct spectral states: hard, soft, very soft, and intermediate.

In the hard state, which appears both at the rise and the decline of the outburst, quasi‑periodic oscillations (QPOs) are detected. During the rising hard phase the QPO centroid frequency drifts from a very low 0.034 Hz up to 17.78 Hz, while the spectrum is characterised by a cool disk (kT_in≈0.6–1.0 keV), a hard power‑law (photon index Γ≈1.5–2.0), and a prominent Fe Kα line. The large frequency sweep suggests a rapid inward movement of the truncation radius of the inner accretion flow, possibly driven by an increase in the mass‑accretion rate. In the declining hard phase the trend reverses: the QPO frequency decreases from 13.14 Hz back to 0.034 Hz. Early in this decay a cool Comptonising component (kT_e≈5 keV, optical depth τ≈10) is required, indicating a transient, optically thick corona that gradually disappears as the source returns to a more standard disk‑plus‑corona configuration.

The intermediate state is identified by a relatively strong disk (kT_in≈1.2–1.5 keV) together with a softer power‑law (Γ≈2.2–2.5). QPOs are present but confined to a narrow high‑frequency band of 13.17–19.04 Hz, and their amplitude and centroid frequency show little variability, implying a stable resonant mode in the inner flow. Spectrally, the intermediate state is well described by just the diskbb and power‑law components; no additional iron line or Comptonisation is needed.

In the soft and very soft states the spectrum is dominated almost entirely by the thermal disk component, with temperatures reaching 1.5–2.2 keV and the power‑law contribution becoming negligible. No QPOs are detected in these states, and the iron line weakens considerably, consistent with a geometrically thin, optically thick accretion disk extending close to the innermost stable circular orbit.

Comparing the 2005 outburst with the previous 1996–1997 event, the authors find strikingly similar state transitions, QPO behaviour, and spectral evolution, suggesting that the same underlying physical mechanisms are at work. The study supports a picture in which variations in the mass‑accretion rate drive the truncation radius of the inner hot flow, the formation and disappearance of a compact corona, and the launching or quenching of a relativistic jet. The confinement of QPOs to the hard and intermediate states points to a strong dependence of the oscillation mechanism on the geometry and optical depth of the inner flow, possibly involving Lense‑Thirring precession or magneto‑acoustic resonances. Overall, the paper provides a detailed observational benchmark for theoretical models of disk‑corona‑jet coupling in black‑hole X‑ray binaries.