X-ray softening in the new X-ray transient XTE J1719-291 during its 2008 outburst decay

The X-ray transient XTE J1719-291 was discovered with RXTE/PCA during its outburst in 2008 March, which lasted at least 46 days. Its 2-10 keV peak luminosity is 7E35 erg/s assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM-Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM-Newton spectrum can not be fitted with a simple one-component model, but it can be fitted with a thermal component (black body or disc black body) plus power-law model affected by absorption. Therefore, the softening of the X-ray spectrum with decreasing X-ray luminosity might be due to a change in photon index or alternatively it might be due to a change in the properties of the soft component. Assuming that the system is an X-ray binary, we estimated a long-term time-averaged mass accretion rate of ~ 7.7E-13 M_sun/yr for a neutron star as compact object and ~ 3.7E10-13 M_sun/yr in the case of a black hole. Although no conclusive evidence is available about the nature of the accretor, based on the X-ray/optical luminosity ratio we tentatively suggest that a neutron star is present in this system.

💡 Research Summary

The paper presents a multi‑instrument study of the 2008 outburst of the newly discovered X‑ray transient XTE J1719‑291, classifying it as a very faint X‑ray transient (VFXT). The outburst was first detected by RXTE/PCA on 21 March 2008 and was followed for ~46 days with RXTE, Swift/XRT, XMM‑Newton/EPIC and Chandra/HRC‑I. Assuming a distance of 8 kpc, the 2–10 keV peak luminosity is ≈7 × 10³⁵ erg s⁻¹, well below the typical bright transient range (10³⁷–10³⁹ erg s⁻¹).

Spectral analysis was performed with XSPEC. All observations can be described by an absorbed power‑law, but the photon index (Γ) evolves: as the source fades, Γ increases from ~2.0 to ~2.7, indicating a softening of the spectrum. To avoid instrument‑dependent biases, the authors compute a hardness ratio (HR = 2–10 keV / 0.5–2 keV) using only Swift/XRT data; HR follows the same trend, confirming genuine spectral softening rather than calibration or pile‑up effects.

The XMM‑Newton EPIC spectrum, thanks to its higher signal‑to‑noise and coverage down to 0.5 keV, cannot be fitted adequately with a single power‑law (χ²ν≈1.2). Adding a soft thermal component (either a blackbody with kT≈0.32 keV or a multicolour disc blackbody with Tin≈0.45 keV) plus a harder power‑law (Γ≈1.6–1.7) yields a statistically acceptable fit (χ²ν≈1.06). The thermal component contributes roughly 30 % of the 0.5–10 keV flux, demonstrating that even at low luminosities a soft excess is present. Swift and RXTE, whose low‑energy response is limited, cannot constrain this component directly.

To explore the evolution of the thermal component, the authors fix N_H and Γ to the XMM‑Newton values and fit the two brightest Swift observations (Obs 5 and 6) with a blackbody plus power‑law, allowing only the blackbody temperature to vary. The resulting temperatures are 0.46 ± 0.06 keV and 0.56 +0.05/‑0.09 keV, respectively. Although the change is not highly significant, it suggests that the soft component may become hotter as the overall luminosity declines, opposite to the behaviour seen in many brighter transients where the disc cools during decay.

The authors estimate the long‑term, time‑averaged mass accretion rate from the outburst fluence. Using an efficiency η≈0.2 for a neutron star (NS) and η≈0.1 for a black hole (BH), they obtain \dot{M}_NS≈7.7 × 10⁻¹³ M⊙ yr⁻¹ and \dot{M}_BH≈3.7 × 10⁻¹³ M⊙ yr⁻¹. These very low rates are typical for VFXTs and imply that the system spends most of its life in a low‑accretion, quiescent state.

An optical counterpart was identified at the precise Chandra position with the MPI/ESO 2.2 m telescope. The source was not detected in a later observation, leading to an absolute V magnitude limit of >5.8 (assuming 8 kpc), consistent with a K0V or later low‑mass companion. The X‑ray‑to‑optical flux ratio (L_X/L_opt≈10³) is higher than typical BH low‑mass X‑ray binaries, favouring a neutron‑star primary. No Type‑I X‑ray bursts or coherent pulsations were observed, so the nature of the compact object remains ambiguous.

In summary, XTE J1719‑291 provides a rare, well‑sampled example of a VFXT that exhibits clear spectral softening during decay, with a detectable soft thermal component even at luminosities below 10³⁶ erg s⁻¹. The results support the idea that low‑luminosity accretion flows can retain a disc‑like or boundary‑layer emission component, and that the spectral evolution may be driven either by changes in the power‑law index, variations in the thermal component, or a combination of both. The paper highlights the importance of broad‑band, low‑energy X‑ray coverage for understanding the physics of faint transients and suggests that future deep observations (e.g., with NICER or Athena) combined with optical/IR monitoring will be essential to definitively determine whether the compact object is a neutron star or a black hole.