The
Soft X-ray Excess.
The hard X-ray
spectrum (2-50keV) of AGN overall is well-described by a power-law spectrum
(in some cases with additional iron line emission and a reflection `hump').
Moving down to lower energies (below about 1keV), we find that in many
objects there is an excess of emission above this power-law. This is known
as the `soft X-ray excess' and is believed to come from the inner edge
of the accretion
disk.
AGN which have very strong soft X-ray excesses, relative to the hard X-ray emission, are termed `Ultra-soft'.
An
optical to soft X-ray bump.
There is a similar, strong
excess often seen in the optical/UV spectra of AGN and this is known as
the `big blue bump'. It has often been suggested that the soft X-ray excess
and the big blue bump are part of the same component: an optical to soft
X-ray `bump', which we refer to as `the big bump'. The optical/UV big blue
bump may be emitted by the outer parts of the accretion disc, which are
not as hot as those producing the soft X-rays.
Our Research on Ultra-soft X-ray AGN
Here at MSSL, we have been studying ultra-soft X-ray excesses themselves and their effects at other wavelengths.
One important property of ultra-soft AGN is that they all have relatively narrow Balmer line emission. This shows that there is a fundamental relationship between the production of soft X-rays and the velocity of the Balmer line gas along our line of sight to the nucleus.
The
Narrow-line Seyfert 1 galaxy REJ1034+396.
This ultra-soft AGN was
discovered by the ROSAT Wide Field Camera and has one of the hottest big
bumps identified to date; so hot that it appears not to be emitting at
all in the UV, where normally it is very strong. Recent high resolution
optical spectroscopy (shown opposite; the data are plotted as a blue histogram)
has revealed intermediate velocity components in the [OIII]4959,5007 lines
with full widths at half maximum (FWHM) of about 1000 km/s (shown red),
as well as the usual low-velocity components (FWHM of about 400 km/s) from
the narrow
line region(the low-velocity
components are plotted green). A similar component is also seen in Hbeta;
this suggests that an intermediate velocity, intermediate-density region
exists in REJ1034+396. Emission from this region dominates the Balmer line
fluxes, making the Balmer line profiles relatively narrow. Note the presence
of a broad component in Hbeta as well (shown orange), suggesting the presence
of a `normal' broad
line region albeit weak. This work has demonstrated
a probable link between the presence (or the visibility) of a region which
is intermediate between the usual broad and narrow line clouds, and the
ultra-soft X-ray excess. It is published in Puchnarewicz et al. (1995),
Monthly Notices of the Royal Astronomical Society, volume 276 page 20;
and Mason, Puchnarewicz and Jones (1996), Monthly Notices of the Royal
Astronomical Society, volume 283, page L26.
E1346+266:
a high redshift narrow-line quasar.
At a redshift of 0.92,
E1346+266 is the most distant ultra-soft AGN identified so far. It also
has a very hot big bump and, with a luminosity of about 1e47 erg/s, one
of the strongest as well. Its spectrum, combining infra-red, optical, soft
X-ray data taken by the ROSAT satellite and X-ray upper limits from the
ASCA space telescope, are shown opposite. Despite enormous differences
in scale between E1346+266 and REJ1034+396 (E1346+266 is about ten thousand
times more luminous, and its black hole mass is a thousand times greater),
this quasar also has narrow Balmer lines with a FWHM similar to that of
REJ1034+396. The optical to X-ray spectrum is not well-represented by current
accretion
disc models, but is very similar to that of a
7e6K simple thermal Bremsstrahlung spectrum (shown in red on the diagram)
- perhaps the big bump in this object is emitted by a region filled with
optically-thin cloudlets. This work was published in Puchnarewicz, Mason
and Cordova (1994), Monthly Notices of the Royal Astronomical Society,
volume 270, page 663.
Other Extragalactic Astronomy pages:
MSSL home pages:
This page written by Liz Puchnarewicz (emp@mssl.ucl.ac.uk)
and Mat Page (mjp@mssl.ucl.ac.uk).
Last modified 17th March 1998