An XMM-Newton observation of the extreme narrow-line Seyfert 1 galaxy Mrk 359

We present XMM-Newton observations of Mrk 359, the first narrow-line Seyfert 1 galaxy (NLS1) discovered. Even among NLS1s, Mrk 359 is an extreme case with extraordinarily narrow optical emission lines. The XMM-Newton data show that Mrk 359 has a significant soft X-ray excess which displays only weak absorption and emission features. The (2-10)keV continuum, including reflection, is flatter than that of the typical NLS1, with Γ~1.84. A strong emission line of equivalent width ~200eV is also observed, centred near 6.4keV. We fit this emission with two line components of approximately equal strength: a broad iron line from an accretion disc and a narrow, unresolved core. The unresolved line core has an equivalent width of ~120eV and is consistent with fluorescence from neutral iron in distant reprocessing gas, possibly in the form of a `molecular torus'. Comparison of the narrow-line strengths in Mrk 359 and other low-moderate luminosity Seyfert 1 galaxies with those in QSOs suggests that the solid angle subtended by the distant reprocessing gas decreases with increasing active galactic nucleus luminosity.