Terzan 5 transient IGR J17480-2446: variation of burst and spectral properties with spectral states

We study the spectral state evolution of the Terzan 5 transient neutron star low-mass X-ray binary IGR J17480-2446, and how the best-fit spectral parameters and burst properties evolved with these states, using the Rossi X-ray Timing Explorer data. As reported by other authors, this is the second source which showed transitions between atoll state and Z' state. We find large scale hysteresis in the almost C’-like hardness-intensity track of the source in the atoll state. This discovery is likely to provide a missing piece of the jigsaw puzzle involving various types of hardness-intensity tracks from q'-shaped for Aquila X-1, 4U 1608-52, and many black holes to C’-shaped for many atoll sources. Furthermore, the regular pulsations, a diagonal transition between soft and hard states, and the large scale hysteresis observed from IGR J17480-2446 argue against some of the previous suggestions involving magnetic field about atolls and millisecond pulsars. Our results also suggest that the nature of spectral evolution throughout an outburst does not, at least entirely, depend on the peak luminosity of the outburst. Besides, the source took at least a month to trace the softer banana state, as opposed to a few hours to a day, which is typical for an atoll source. In addition, while the soft colour usually increases with intensity in the softer portion of an atoll source, IGR J17480-2446 showed an opposite behaviour. From the detailed spectral fitting we conclude that a blackbody+powerlaw model is the simplest one, which describes the source continuum spectra well throughout the outburst. We find that these two spectral components were plausibly connected with each other, and they worked together to cause the source state evolution. (Truncated).

💡 Research Summary

This work presents a comprehensive analysis of the transient neutron‑star low‑mass X‑ray binary IGR J17480‑2446 located in the globular cluster Terzan 5, using the extensive Rossi X‑ray Timing Explorer (RXTE) data set obtained between 13 October and 19 November 2010 (46 observations, total exposure ≈ 2.97 Ms). The authors construct colour‑colour (CD) and hardness‑intensity (HID) diagrams by defining hard colour as the count‑rate ratio (9.2–18.9 keV)/(5.7–9.2 keV) and soft colour as (3.9–5.7 keV)/(2.6–3.9 keV). The data are divided into eleven temporal segments, each tracing a distinct portion of the HID track, which enables a clear view of the source’s evolution.

In the earliest segment the source resides in a low‑intensity, high‑hard‑colour state (≈ 185 counts s⁻¹ PCU⁻¹, hard colour ≈ 0.70), consistent with an extreme‑island (or island) state typical of atoll sources. Within a day the intensity rises dramatically (≈ 690 counts s⁻¹, hard colour ≈ 0.53) and the source moves smoothly toward lower hard colour values. Segments 2–4 display a nearly linear hard‑to‑soft transition that forms an almost “C‑shaped” HID track. Crucially, the hard‑to‑soft branch lies significantly above the soft‑to‑hard branch (segments 6–11), producing a large‑scale hysteresis that is far more pronounced than the modest hysteresis usually seen in atoll sources and more reminiscent of the “q‑shaped” loops observed in many black‑hole transients. This hysteresis persists despite the source spending most of the outburst in an atoll‑like state, suggesting that the shape of the HID track does not depend solely on the peak luminosity.

During the interval 17–21 October (segment 5) the CD and HID reveal clear “Z‑like” tracks. The authors further split this interval into sub‑segments (5a–5e) and show that the source traverses both harder and softer portions of the Z‑track, with evidence of a brief return to the harder branch in sub‑segment 5b. The Z‑track appears most prominently when the average intensity reaches its maximum (≈ 898 counts s⁻¹), confirming that the source attains a high mass‑accretion rate during this phase.

Spectral analysis is performed on non‑burst (persistent) PCU‑2 data in the 3–15 keV band. After fixing the interstellar absorption at N_H = 3.8 × 10²² cm⁻², the authors test several models and find that a two‑component continuum consisting of a blackbody (bbodyrad) plus a power‑law, together with a broad Gaussian emission line near 6.4 keV (FWHM ≈ 1.0 keV, equivalent width ≈ 187 eV), provides the best description of all spectra. The blackbody temperature varies between ≈ 0.6 and 1.2 keV, while the power‑law photon index ranges from ≈ 1.5 to 2.2. The Gaussian is interpreted as a broadened Fe Kα line arising from the inner accretion disc. Importantly, the relative contribution of the two continuum components evolves systematically: the power‑law dominates in the hard, low‑soft‑colour state, whereas the blackbody fraction grows as the source softens. This correlated evolution suggests a physical coupling, possibly with the blackbody representing emission from the boundary layer and the power‑law arising from Comptonisation in a corona or the base of a jet.

Burst properties are examined in parallel with the spectral evolution. In the atoll‑like segments (1–4) the source exhibits regular, short‑interval (≈ 10–30 s) thermonuclear (type‑I) bursts. As the persistent intensity increases, burst peak fluxes and fluences gradually decline, consistent with a reduced fuel column due to more rapid accumulation. When the source enters the Z‑like state (segments 5a–5d), bursts essentially disappear and millihertz quasi‑periodic oscillations (mHz QPOs) emerge. The appearance of mHz QPOs aligns with the marginally stable nuclear burning model, indicating that the burning regime shifts from unstable (bursting) to stable (oscillatory) as the accretion rate approaches a critical value. The unusually short burst recurrence times, together with the 11 Hz pulsation, imply that the relatively strong magnetic field of this slow pulsar influences fuel confinement and burning, making it an analogue of the clock‑like burster GS 1826‑238 but with markedly shorter intervals.

The authors draw several key conclusions. First, IGR J17480‑2446 provides a rare example of a source that transitions from an atoll to a Z state, and its HID exhibits a pronounced “C‑shaped” hysteresis that bridges the gap between the classic “q‑shaped” loops of black‑hole transients and the more modest atoll tracks. Second, the evolution of spectral parameters (blackbody temperature, power‑law index, Fe line strength) is tightly coupled to the source’s position in the CD/HID, indicating that changes in the accretion geometry and corona/disc coupling drive the state transitions. Third, the interplay between burst activity and mHz QPOs demonstrates that spin, magnetic field strength, and mass‑accretion rate jointly determine the nuclear burning regime, challenging models that attribute atoll/Z behaviour solely to luminosity or spin. Overall, this study enriches our understanding of the phenomenological continuum between neutron‑star atoll, Z, and black‑hole spectral states, and highlights IGR J17480‑2446 as a valuable laboratory for probing accretion physics, disc‑corona interactions, and thermonuclear burning on neutron stars.