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

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 N[SUB]H[/SUB] = 2.0^{+0.2}_{-0.3}× 10^{22} cm[SUP]-2[/SUP] 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 D[SUB]c[/SUB] ≤ 1.5 × 10[SUP]13[/SUP] cm, corresponding to a density of n[SUB]c[/SUB] ≥ 1.5 × 10[SUP]9[/SUP] cm[SUP]-3[/SUP] at a distance of R[SUB]c[/SUB] ≥ 4.3 × 10[SUP]16[/SUP] 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 D[SUB]s[/SUB] ≤ 2.3 × 10[SUP]13[/SUP] cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5 × 10[SUP]7[/SUP] M[SUB]⊙[/SUB]), translates into D[SUB]s[/SUB] ≤ 10.5 gravitational radii (r[SUB]g[/SUB]) with larger sizes (in r[SUB]g[/SUB]) 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.