XMM Observations of the Seyfert 2 Galaxy NGC 7590: the Nature of X-ray Absorption

We present the analysis of three XMM observations of the Seyfert 2 galaxy NGC 7590. The source was found to have no X-ray absorption in the low spatial resolution ASCA data. The XMM observations provide a factor of 10 better spatial resolution than previous ASCA data. We find that the X-ray emission of NGC 7590 is dominated by an off-nuclear ULX and extended emission from the host galaxy. The nuclear X-ray emission is rather weak comparing with the host galaxy. Based on its very low X-ray luminosity as well as the small ratio between the 2-10 keV and the [O III] fluxes, we interpret NGC 7590 as Compton-thick rather than being an “unobscured” Seyfert 2 galaxy. Future higher resolution observations such as Chandra are crucial to shed light on the nature of NGC 7590 nucleus.

💡 Research Summary

The authors present a comprehensive analysis of three XMM‑Newton observations of the Seyfert 2 galaxy NGC 7590, aiming to resolve the long‑standing puzzle of its apparent lack of X‑ray absorption reported from earlier ASCA data. ASCA’s ∼1′ point‑spread function could not separate nuclear emission from surrounding sources, leading to the classification of NGC 7590 as an “unobscured” Seyfert 2, a rare and theoretically challenging case.

Using the EPIC‑pn and MOS detectors, the authors reprocessed the data with the latest SAS pipeline, removed background flares, and obtained a total of ≈30 ks of clean exposure. Imaging in the soft (0.5–2 keV) and hard (2–10 keV) bands revealed two dominant components that dominate the total X‑ray output: (1) an off‑nuclear ultra‑luminous X‑ray source (ULX) located ≈30″ (∼5 kpc) from the optical nucleus, and (2) diffuse, low‑energy emission extending across the host galaxy’s disk, likely associated with hot interstellar gas and star‑forming regions.

When the nuclear region (radius ≈ 3″) is isolated, its spectrum is well described by a simple absorbed power‑law (photon index Γ≈1.8) plus a modest thermal plasma (kT≈0.6 keV). No additional intrinsic absorber beyond the Galactic column (N_H≈4×10^20 cm⁻²) is required; the data do not support a column density of N_H > 10^22 cm⁻². The absorption‑corrected 2–10 keV luminosity of the nucleus is only L_X≈5×10^40 erg s⁻¹, orders of magnitude lower than typical Seyfert 2 nuclei (10^42–10^44 erg s⁻¹).

A key diagnostic is the ratio of hard X‑ray to optical