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 Seyfer t I galaxy (NLS1) discovered. Even among NLS1s, Mrk 359 is an extreme case with extraordinarily narrow optical emission lines. The XMM-Newton data sho w that Mrk 359 has a significant soft X-ray excess which displays only weak absorption and emission features. The (2-10)keV continuum, including refle ction, is flatter than that of the typical NLS1, with Gamma approximate to 1.84. A strong emission line of equivalent width approximate to 200 eV is a lso observed, centred near 6.4 keV. We fit this emission with two line comp onents of approximately equal strength: a broad iron line from an accretion disc and a narrow, unresolved core, The unresolved line core has an equiva lent width of approximate to 120 eV and is consistent with fluorescence fro m neutral iron in distant reprocessing gas, possibly in the form of a 'mole cular torus'. Comparison of the narrow-line strengths in Mrk 359 and other low-moderate luminosity Seyfert I galaxies with those in QSOs suggests that the solid angle subtended by the distant reprocessing gas decreases with i ncreasing active galactic nucleus luminosity.