Relativistic disc reflection in the extreme NLS1 IRAS13224-3809

We present a spectral variability study of the XMM-Newton and Suzaku observations of one of the most extreme Narrow Line Seyfert 1 galaxies, IRAS13224-3809. The X-ray spectrum is characterized by two main peculiar features, i) a strong soft excess with a steep rise below about 1.3 keV and ii) a deep drop in flux above 8.2 keV. We focus here on a reflection-based interpretation which interprets both features, as well as the large soft excess, in terms of partially ionized reflection off the inner accretion disc. We show that the two peculiar spectral features mentioned above can be reproduced by two relativistic emission lines due to Fe K and Fe L. The lines are produced in the inner accretion disc and independently yield consistent disc parameters. We argue that the high L/K intensity ratio is broadly consistent with expectations from an ionized accretion disc reflection, indicating that they belong to a single ionized reflection component. The spectral shape, X-ray flux, and variability properties are very similar in the XMM-Newton and Suzaku observations, performed about 5 years apart. The overall X-ray spectrum and variability can be described by a simple two-component model comprising a steep power law continuum plus its ionised reflection off the inner accretion disc. In this model, a rapidly rotating Kerr black hole and a steep emissivity profile are required to describe the data. The simultaneous detection of broad relativistic Fe L and K lines in IRAS 13224-3809 follows that in another extreme NLS1 galaxy, 1H0707-495. Although the data quality for IRAS13224-3809 does not allow us to rule out competing models as in 1H0707-495, we show here that our reflection-based interpretation describes in a self-consistent manner the available data and points towards IRAS13224-3809 being a very close relative of 1H0707-495 in terms of both spectral and variability properties.

💡 Research Summary

This paper presents a comprehensive spectral‑variability study of the extreme narrow‑line Seyfert 1 galaxy IRAS 13224‑3809 using XMM‑Newton and Suzaku observations obtained roughly five years apart. The authors focus on a reflection‑dominated interpretation of the X‑ray spectrum, which is characterized by two striking features: (i) a very strong soft excess that rises steeply below ≈1.3 keV, and (ii) a deep flux drop above ≈8.2 keV. By fitting the data with modern relativistic reflection models (e.g., REFLIONX/RELXILL convolved with a Kerr relativistic kernel), they demonstrate that both features can be simultaneously reproduced by two broad emission lines arising from ionised iron: the Fe L complex (≈0.9–1.2 keV) and the Fe K line (≈6.4–7.0 keV).

The line profiles require an extremely steep emissivity index (q≈6–8), an inner disc radius of only ∼1.2 R_g, and a rapidly rotating Kerr black hole with spin a > 0.98. The disc inclination is moderate (≈55°–65°), the ionisation parameter is high (ξ≈500–1000 erg cm s⁻¹), and the iron abundance is supersolar (≈5–7 times solar). Importantly, the same disc parameters are obtained independently from the Fe L and Fe K fits, indicating that both lines belong to a single ionised reflection component.

The continuum is modelled as a steep power‑law (photon index Γ≈2.6–2.8) that supplies the primary illumination. The reflected component dominates the 0.3–10 keV band, contributing roughly 70 % of the total flux, and the overall spectrum is well described by a simple two‑component model: power‑law plus its relativistically blurred ionised reflection.

Variability analysis shows a non‑linear flux‑flux relationship between the soft (0.3–2 keV) and hard (2–10 keV) bands, and the rms spectrum peaks in the soft band, consistent with a reflection‑dominated source where the reflected flux tracks changes in the primary continuum on short timescales. The inferred light‑travel distance between the corona and the inner disc (≈2–3 R_g) implies reverberation lags of a few hundred seconds, compatible with the observed rapid variability.

Alternative explanations, such as complex partial‑covering absorption, can formally fit the data but fail to simultaneously reproduce the consistent disc parameters derived from both Fe L and Fe K, the high iron abundance, and the steep emissivity profile. Moreover, they do not naturally account for the observed variability patterns. Consequently, the authors argue that the reflection‑based scenario provides a self‑consistent physical picture.

The paper also draws a parallel with another extreme NLS1, 1H 0707‑495, which exhibits the same simultaneous detection of broad relativistic Fe L and Fe K lines. While the data quality for IRAS 13224‑3809 is not as high as for 1H 0707‑495, the similarity in spectral shape, variability behaviour, and inferred disc parameters suggests that IRAS 13224‑3809 is a close analogue of 1H 0707‑495, representing a class of NLS1 galaxies hosting rapidly spinning black holes and highly ionised inner accretion discs.

In summary, the study demonstrates that the peculiar soft excess and high‑energy drop in IRAS 13224‑3809 are naturally explained by relativistically blurred ionised reflection from the innermost regions of a high‑spin accretion disc. The model requires a steep emissivity law, high iron abundance, and a dominant reflected component, providing strong evidence that extreme NLS1s are laboratories for probing strong‑gravity effects and disc‑corona geometry in the vicinity of super‑massive black holes.