Active galactic nuclei (AGN) are thought to be powered by accretion of
material onto a supermassive black hole. An AGN radiates brightly almost
over the entire electromagnetic spectrum. According to the standard model,
the optical/UV emission is due to the accretion disc, while the X-ray
emission is produced by Compton up-scattering of the disc photons in a
hot plasma, the so-called corona. Moreover, AGN often expel large amounts
of matter through powerful winds of ionized gas. Such winds are normally
seen as "warm absorbers", which cause narrow absorption lines in the UV
and soft X-ray band.
NGC 5548 is a well-studied Seyfert galaxy hosting an AGN. It was the
object of an observing campaign from May 2013 to February 2014, with six
satellites (see Kaastra et al. 2014). INTEGRAL observed the source four
times in summer 2013, simultaneously with XMM-Newton, and two times in
January 2014. The primary goals of the campaign were the study of the
ionized outflow and the determination of the nature and origin of the
accretion-powered emission.
The results of the campaign were quite surprising. The nucleus was found
to be obscured by a long-lasting, clumpy stream of gas never seen before
in this source. It blocks 90% of the soft X-ray emission and causes
simultaneous deep, broad UV absorption troughs. The gas is outflowing
with a velocity of a few thousand km/s, and it is located a few light
days away from the central source. The physical and geometrical properties
of this newly-observed obscurer are consistent with those of a wind launched
from the accretion disc.
The broad-band monitoring allowed to determine the spectral energy
distribution from the near-infrared to the X-ray band (Mehdipour et al. 2015;
see the average multiwavelength data in Fig. 1). The high-energy spectrum
was also studied in detail (Ursini et al. 2015; see the data and best-fit
model for one observation in Fig. 2). The X-ray spectrum is dominated by a
primary power law with a variable high-energy cut-off. The data agree with a
thermal Comptonization spectrum produced by a corona with a mean temperature
of 40(+40,-10) keV and an optical depth of 2.7(+0.7,−1.2), assuming a spherical
geometry. Comparing with previous observations, the physical parameters of
the X-ray corona are found to vary significantly, implying that the disc-corona
system undergoes significant changes over time.
