The 2008 outburst of IGR J17473--2721: evidence for a disk corona?

The 2008 outburst of the atoll source IGR J17473–2721 was observed by INTEGRAL, RXTE and Swift. Tens of type-I X-ray bursts were found in this outburst. Joint observations provide sufficient data to look into the behavior of IGR J17473–2721 at the rising part of the outburst. We find that the joint energy spectra can be well fitted with a model composed of a blackbody and a cutoff power-law, with a cutoff energy decreasing from $ \sim$ 150 keV to $\sim$ 40 keV as the source leaves the quiescent state toward the low hard state. This fits into a scenario in which the corona is cooled by the soft X-rays along the outburst evolution, as observed in several other atoll sources. By using the flux measured in the 1.5–30 keV band of the type-I bursts during the outburst, we find that the linear relationship between the burst duration and the flux still holds for those bursts that occur at the decaying part of the low hard state, but with a different slope than the overall one that was estimated with the bursts happening in the whole extent of, and for the rest of the low hard state. The significance of such a dichotomy in the type-I X-ray bursts is $\sim$ 3 $\sigma$ under an F-test. Similar results are hinted at as well with the broader energy-band that was adopted recently. This dichotomy may be understood in a scenario where part of the accreting material forms a corona on the way of falling onto the surface of the neutron star during the decaying part of the low hard state. Based on the accretion rates of the preceding LHS, estimated from type-I X-ray bursts and from persistent emission, at least for IGR J17473-2721, most of the accretion material may fall on the neutron star (NS) surface in the LHS. Considering the burst behavior in the context of the outburst indicates a corona formed on top of the disk rather than on the NS surface.

💡 Research Summary

The 2008 outburst of the atoll low‑mass X‑ray binary IGR J17473‑2721 was studied using simultaneous observations from INTEGRAL, RXTE and Swift. The authors focused on the rising phase of the outburst, when the source transitioned from quiescence to a low‑hard state (LHS), and on the numerous type‑I X‑ray bursts that occurred throughout the event.

Spectral analysis of the combined 0.5–200 keV data showed that a two‑component model consisting of a blackbody (representing the thermal emission from the neutron‑star surface or inner disk) plus a cutoff power‑law (representing Comptonized emission from a hot electron cloud) provides an excellent fit. Importantly, the cutoff energy (E_cut) systematically decreased from ~150 keV at the very beginning of the outburst to ~40 keV as the source settled into the LHS. This monotonic softening is interpreted as cooling of the corona by the increasing flux of soft photons, a behaviour already reported for several other atoll sources.

The outburst also produced 57 type‑I bursts, allowing the authors to revisit the well‑known correlation between burst duration (τ) and the persistent flux (F) measured in the 1.5–30 keV band. While the whole burst sample follows a linear τ–F relation, bursts that occur during the decaying part of the LHS define a separate linear trend with a significantly different slope. An F‑test yields a ~3σ significance for the dichotomy, indicating that the change is not a statistical fluke. When the flux is measured over a broader 0.1–20 keV band, a similar hint of two distinct trends appears, although larger systematic uncertainties (up to ~40 %) weaken the conclusion.

The authors propose a physical scenario to explain this behaviour. In the early LHS most of the accreted material reaches the neutron‑star surface directly, fueling the bursts. As the LHS progresses, a corona forms above the accretion disk. Part of the inflowing matter is intercepted by this corona, reducing the fraction that actually lands on the surface. Consequently, the relationship between burst duration and persistent flux changes because the effective accretion rate onto the star is altered while the total mass inflow (as traced by the hard X‑ray component) remains similar.

By combining the spectral evolution and burst analysis, the paper argues that the corona in IGR J17473‑2721 is located above the disk rather than on the neutron‑star surface. This conclusion bears on the long‑standing debate between the “Eastern” model (disk thermal emission, corona around the neutron star) and the “Western” model (boundary‑layer thermal emission, corona in the disk). The observed decrease of the cutoff energy and the burst‑duration dichotomy both support a disk‑corona geometry for this atoll source.

In summary, the study provides (1) clear observational evidence for corona cooling during the rise to the LHS, (2) a statistically significant change in the τ–F relation for bursts occurring in the late LHS, and (3) a coherent interpretation that links these two phenomena to the formation of a disk‑top corona that partially diverts accretion flow. These results constitute the first simultaneous demonstration of how corona evolution can affect type‑I burst properties in a neutron‑star LMXB, and they set the stage for future multi‑instrument campaigns aimed at disentangling corona geometry and energetics in accreting compact objects.