Chandra LETGS observation of the variable NLS1 galaxy Ark 564^⋆

Centro de Física, Instituto Venezolano de Investigaciones Científicas (IVIC), PO Box 025304, Caracas 1020A, Venezuela
e-mail: josem@ivic.gob.ve

Received: 21 September 2012
Accepted: 18 January 2013

Abstract

We present an analysis of the 100 ks X-ray spectrum of the narrow-line Seyfert 1 Galaxy Ark 564, taken with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra. Using χ² statistics, several continuum models of the time-averaged spectrum of this object are compared, obtaining a semi-empirical solution for the description of the intrinsic emission continuum and a physical solution for the intrinsic absorption of the system. We find that the 0.1−10 keV spectrum can be well described by a power law plus two thermal components that account for the soft step. We are also able to detect and measure several narrow, unresolved absorption lines arising from highly ionized species of C, N, O, and Fe. The material seems to have a velocity consistent with the systemic velocity of the galaxy. This piece of evidence, in addition to the very low observed column density of N_H ~ 10²⁰ cm^-2, is in good agreement with the scenario of a transverse biconical outflow with a gas density of n ≲ 3 × 10¹² cm^-3 at distances beyond the broad-line region r ≳ 10 lt-days, but a dust torus origin cannot be ruled out.