The 2009 outburst of H~1743-322 as observed by RXTE

We analyze the RXTE observations of the 2009 outburst of H~1743-322, as well as the observations of the previous five outbursts for comparison. The hardness-intensity diagram (HID) shows a complete counter-clockwise q-track for the 2009 outburst and, interestingly, the track falls in} between a huge one in 2003, with a complete transition to high/soft state, and that of} the failed outburst in 2008. It leaves the low/hard state but does not reach the leftmost edge of the overall HID. While the lowest hardness (6–19 keV/3–6 keV) values} in the HID is about 0.3–0.4 for the 2009 outburst, similar to the ``failed state transition" seen in the persistent black hole XRB Cyg X-1, the timing analysis shows that a transition to the high soft state occurred. During the low/hard state of the 2009 outburst, the inner radius of the accretion disk is found to be closer to the central black hole and have an anti-correlation with the disk temperature. These results may be understood as the reprocessing} of the hot corona on the disk’s} soft X-rays, which can lead to an underestimation of the inner radius of the accretion disk. In the luminosity diagram of the corona versus the disk, the tracks of the outbursts} in 2003 and 2009 cross the line which represents a roughly equal contribution to the entire emission from the thermal and the non-thermal components;} the track of the 2008 outburst has the turn-over falling} on this line. This may be indicative of an emission balance between the corona and the disk, which prevents the state transition from going further than the low/hard state.

💡 Research Summary

The authors present a comprehensive analysis of the 2009 outburst of the black‑hole candidate X‑ray binary H 1743‑322 using data from the Rossi X‑ray Timing Explorer (RXTE). By fitting each observation with a standard spectral model consisting of a multicolour disc blackbody (diskbb) plus a power‑law component, they derive the evolution of the disc temperature, inner radius, and the non‑thermal (coronal) contribution across the outburst. Timing analysis, based on power‑density spectra and the detection of low‑frequency quasi‑periodic oscillations (QPOs), is used to classify the source’s spectral states (low/hard, hard‑intermediate, soft‑intermediate, high/soft).

The hardness‑intensity diagram (HID), constructed from the 6–19 keV / 3–6 keV count‑rate ratio, displays a complete counter‑clockwise “q‑track”. The track lies between the very large loop traced by the 2003 outburst (which showed a full transition to the high/soft state) and the narrow, failed loop of the 2008 outburst (which never left the low/hard state). The minimum hardness reached during the 2009 event is ≈0.3–0.4, a value comparable to the “failed state transition” seen in the persistent black‑hole system Cyg X‑1. However, unlike Cyg X‑1, the timing properties (the disappearance of QPOs and a drop of rms variability below 5 %) unequivocally indicate that H 1743‑322 did reach the high/soft state during this outburst.

A striking result concerns the apparent inner disc radius (R_in) derived in the low/hard state. The fitted values are unusually small (∼6–10 km), far below the several hundred kilometres typically inferred for a truncated disc in this state. Moreover, R_in shows an anti‑correlation with the disc temperature (T_in): as the disc becomes hotter, the fitted radius shrinks. The authors interpret this as a consequence of strong coronal re‑processing: high‑energy photons from the hot corona illuminate the disc, altering its emergent spectrum and leading to an under‑estimate of the true inner radius when a simple diskbb model is applied. This effect highlights the need for more sophisticated spectral models that incorporate Comptonisation and reflection self‑consistently.

To explore the balance between thermal and non‑thermal emission, the authors plot the coronal luminosity (L_corona) against the disc luminosity (L_disc). The tracks of the 2003 and 2009 outbursts cross the line L_corona = L_disc, indicating epochs where the two components contribute roughly equally to the total X‑ray output. In contrast, the 2008 outburst’s trajectory terminates on this line, suggesting that the system reached a quasi‑steady state in which the corona and disc were energetically balanced, preventing further softening and thus “failing” to leave the low/hard state. This behaviour supports the idea that a critical ratio of coronal to disc power may be required for a full state transition.

Overall, the paper demonstrates that the 2009 outburst occupies an intermediate position between a full, successful transition (2003) and a failed one (2008). By combining spectral fitting, timing diagnostics, and luminosity‑ratio analysis, the study provides a nuanced picture of how the disc‑corona system evolves during an outburst. The findings underscore that hardness alone can be misleading; timing information and a careful accounting of coronal re‑processing are essential for correctly identifying state transitions. The work suggests that future observations with broader energy coverage (e.g., NICER, NuSTAR) and higher timing resolution will be crucial to disentangle disc truncation, coronal geometry, and their mutual feedback in black‑hole X‑ray binaries.