The Effects of Relativistic Bulk Motion of X‐Ray Flares in the Corona on the Iron Kα Line in Seyfert 1 Galaxies

Abstract

It is likely that dramatic magnetic flares in a corona above a black hole accretion disk dominate the X-ray emission in Seyfert 1 galaxies. Such flares are likely to move with mildly relativistic bulk velocity. We study the effects of the bulk motion of X-ray flares on the shape and equivalent width of the iron Kα line from an untruncated cold disk around a Kerr black hole using fully relativistic calculations. The flares are located above a cold accretion disk, either on or off the rotation axis. The upward/outward bulk motion of flares causes a reduction of the iron Kα line width, while the downward/inward bulk motion of flares causes an increase of the iron Kα line width. To a distant observer with a low inclination angle (θo 30°), larger upward/outward bulk velocities decrease the extension of the red wing, with little change in the location of the blue edge. In contrast, an observer at a large inclination angle (e.g., θo = 60°) sees both the red wing and the blue edge change with the bulk velocity. The equivalent width of the iron Kα line decreases rapidly with increasing bulk velocity of flares. However, the narrower line profiles observed in some objects (e.g., IC 4329A and NGC 4593) are difficult to produce using the outflowing magnetic flare model with an appropriate equivalent width unless the X-ray emission is concentrated in an outer region with a of several tens of rg = GM/c2 or more. This suggests that other parameters, such as the truncation radius due to disruption or ionization of the inner disk, may still be needed. An important result is that the iron Kα line intensity is found to be constant even though the continuum flux varies significantly, which is true for outflowing magnetic flares with different bulk velocities but similar intrinsic luminosities when located close to the central black hole. This is caused by the combination of the effects of relativistic beaming and gravitational lensing. We find that fluctuations in the bulk velocities of outflowing low-height flares located at the inner region (r 15rg) can account for a constant iron Kα line and significant continuum variation as observed in MCG -6-30-15 and NGC 5548. This is especially interesting for MCG -6-30-15, whose behavior is difficult to explain using the ionization model.