The broad-band spectrum of the narrow-line Seyfert 1 NGC 4748: from UV to hard X-ray

Abstract

We report the results obtained by a broad-band (0.5–500 keV) data analysis of narrow-line Seyfert 1 galaxy NGC 4748 observed with an XMM-Newton/PN, INTEGRAL/ISGRI and SWIFT/BAT telescopes. This galaxy has a soft X-ray excess that is typical for the class of narrow-line Seyfert 1. The question of the origin of soft excess in such objects is still unclear. We tested and compared two spectral models for the soft X-ray spectra based on the different physical scenarios. The first one is based on the Done and Nayakshin model of two-phase accretion disc in a vertical direction, which includes two reflection zones with different ionization levels. According to this model, we found that a highly ionized reflection has the value of ionization ξ∼3000ergs−1cm and is mostly responsible for the soft excess. This reflection becomes comparable with a low ionized one (ξ∼30ergs−1cm) in moderate X-ray range. However, this model requires also an additional component at soft energies with kT∼ 300 eV. The second model is an energetically self-consistent model and assumes that a soft excess arises from optically thick thermal Comptonization of the disc emission. Combination of the UV (from XMM/Optical monitor) and X-ray data in the latter model allowed us to determine a mass of the central black hole of 6.9 × 10 6M⊙ and Eddington ratio logL/LEdd≃−0.57. Also, we were not able to rule out one of competing models using only X-ray spectra of NGC 4748.