The size of the X-ray emitting region in SWIFT J2127.4+5654 via a broad line region cloud X-ray eclipse

Abstract

We present results obtained from the time-resolved X-ray spectral analysis of the Narrow- Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ~130 ks XMM-Newton observation. We reveal large spectral variations, especially during the first ~90 ks of the XMM- Newton exposure. The spectral variability can be attributed to a partial eclipse of the X-ray source by an intervening low-ionization/cold absorbing structure (cloud) with column density NH = 2.0+0.2-0.3 × 1022 cm-2 which gradually covers and then uncovers the X-ray emitting region with covering fraction ranging from zero to ~43 per cent. Our analysis enables us to constrain the size, number density and location of the absorbing cloud with good accuracy. We infer a cloud size (diameter) of Dc ≤ 1.5 × 1013 cm, corresponding to a density of nc ≥ 1.5 × 109 cm-3 at a distance of Rc ≥ 4.3 × 1016 cm from the central black hole. All of the inferred quantities concur to identify the absorbing structure with one single cloud associated with the broad line region of SWIFT J2127.4+5654. We are also able to constrain the X-ray emitting region size (diameter) to be Ds ≤ 2.3 × 1013 cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5 × 107Mo), translates into Ds ≤ 10.5 gravitational radii (rg) with larger sizes (in rg) being associated with smaller black hole masses, and vice versa. We also confirm the presence of a relativistically distorted reflection component off the inner accretion disc giving rise to a broad relativistic Fe K emission line and small soft excess (small because of the high Galactic column density), supporting the measurement of an intermediate black hole spin in SWIFT J2127.4+5654 that was obtained from a previous Suzaku observation. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.