Slow and Fast Transitions in the Rising Phase of Outbursts from NS-LMXB transients, AqlX-1 and 4U1608-52

We analyzed the initial rising behaviors of X-ray outbursts from two transient low-mass X-ray binaries (LMXBs) containing a neutron-star (NS), Aql X-1 and 4U 1608-52, which are continuously being monitored by MAXI/GSC in 2–20 keV, RXTE/ASM in 2–10 keV, and Swift/BAT in 15–50 keV. We found that the observed ten outbursts are classified into two types by the patterns of the relative intensity evolutions in the two energy bands below/above 15 keV. One type behaves as the 15–50 keV intensity achieves the maximum during the initial hard-state period and drops greatly at the hard-to-soft state transition. On the other hand, the other type does as both the 2–15 keV and the 15–50 keV intensities achieve the maximums after the transition. The former have the longer initial hard-state ($\gtrsim$ 9 d) than the latter’s ($\ltsim$5 d). Therefore, we named them as slow-type (S-type) and fast-type (F-type), respectively. These two types also show the differences in the luminosity at the hard-to-soft state transition as well as in the average luminosity before the outburst started, where the S-type are higher than the F-type in the both. These results suggest that the X-ray radiation during the pre-outburst period, which heats up the accretion disk and delays the disk transition (i.e., from a geometrically thick disk to a thin one), would determine whether the following outburst becomes S-type or F-type. The luminosity when the hard-to-soft state transition occurs is higher than $\sim 8 \times10^{36}$ erg s$^{-1}$ in the S-type, which corresponds to 4% of the Eddington luminosity for a 1.4 \Mo NS.

💡 Research Summary

The authors investigated the early rise phases of X‑ray outbursts from two well‑studied transient neutron‑star low‑mass X‑ray binaries (NS‑LMXBs), Aql X‑1 and 4U 1608‑52. Using continuous monitoring data from three all‑sky instruments—MAXI/GSC (2–20 keV), RXTE/ASM (2–10 keV), and Swift/BAT (15–50 keV)—they assembled light curves for ten outbursts that occurred between 2009 and 2015. For each outburst they identified the hard‑to‑soft state transition (H‑S transition) as the moment when the soft‑band (2–15 keV) flux began a rapid rise while the hard‑band (15–50 keV) flux started to decline.

The key observational result is that the ten outbursts separate naturally into two groups based on the relative evolution of the soft and hard fluxes around the transition. In the “slow‑type” (S‑type) events the hard‑band intensity climbs to a maximum during the initial hard state, then drops sharply at the H‑S transition; the soft‑band flux reaches its peak only after the transition. These outbursts display a relatively long initial hard state (≥ 9 days) and a higher transition luminosity, typically > 8 × 10³⁶ erg s⁻¹ (≈ 4 % of the Eddington limit for a 1.4 M⊙ neutron star). In contrast, the “fast‑type” (F‑type) events show both hard and soft bands peaking after the transition, have a short initial hard state (≤ 5 days), and exhibit lower transition luminosities.

A striking difference between the two groups is the average pre‑outburst luminosity measured in the days preceding the rise. S‑type outbursts are preceded by a higher baseline flux in the 2–15 keV band, indicating that the accretion disc is already partially heated by X‑ray irradiation before the main mass‑transfer episode begins. The authors argue that this pre‑outburst heating delays the geometrical transition of the disc from a hot, geometrically thick configuration (hard state) to a cooler, thin disc (soft state). Consequently, a higher mass‑accretion rate (Ṁ) is required to trigger the H‑S transition, which explains both the longer hard‑state duration and the higher transition luminosity of S‑type events.

The paper therefore proposes a unified picture: the nature of an outburst (slow vs. fast) is set by the thermal state of the disc prior to the outburst. If the disc is already warm (high pre‑outburst X‑ray flux), the system follows the S‑type evolution; if the disc is cold (low pre‑outburst flux), it follows the F‑type evolution. This interpretation extends earlier findings for black‑hole X‑ray binaries, where similar “hard‑state duration” and “transition‑luminosity” dichotomies have been reported, and suggests that irradiation‑driven disc heating is a universal regulator of state transitions in accreting compact objects.

Methodologically, the study demonstrates the power of combining soft‑ and hard‑band all‑sky monitoring to disentangle state‑transition physics. However, the analysis remains phenomenological; the authors do not present detailed spectral modeling or hydrodynamic simulations that would quantify the disc temperature, viscosity, or irradiation feedback. Future work could incorporate multi‑wavelength (optical/IR) monitoring to trace the outer disc response, and numerical simulations to test how pre‑outburst irradiation modifies the critical Ṁ for the H‑S transition.

In summary, the paper identifies two distinct classes of outburst rise behavior in NS‑LMXBs, links them to pre‑outburst disc heating, and shows that the hard‑to‑soft transition luminosity is a discriminant: S‑type transitions occur above ≈8 × 10³⁶ erg s⁻¹, while F‑type transitions happen at lower luminosities. These findings provide a new observational constraint for theoretical models of accretion‑disc state changes and highlight the importance of the disc’s thermal history in shaping the evolution of transient X‑ray binaries.