Transcript Document

Irradiated accretion disk
emission from an ultrasoft
AGN?
Elizabeth Puchnarewicz and Roberto Soria
Mullard Space Science Laboratory
University College, London
The spectrum of the NLS1 REJ1034+396 from the near-IR to X-rays. Also
plotted as a red curve is the model of an irradiated optically-thick,
geometrically-thin accretion disk. The high energy tail includes the effects of
Comptonization.
Soria R. & Puchnarewicz E. M., MNRAS, in press
astro-ph/0108492
The unusually hot ‘big blue bump’
The optical/UV continua of most
AGN rise towards the blue with a
slope a~0.4 while the soft X-ray
spectrum falls towards high energies
with a~2 (e.g. Laor et al. 1997). The
spectrum seems to peak in the
unobservable
EUV
and
this
continuous, optical to soft X-ray
feature, known as the ‘big blue
bump’, is believed to represent the
emission from a geometrically thin,
optically thick accretion disk.
However, the optical to X-ray
continuum of
REJ1034+396 is
highly unusual for a Seyfert 1 galaxy.
The optical/UV continuum is flat
(a~1) with no sign of the big blue
bump down to Lya. At ~0.1keV, the
soft X-ray spectrum is very strong
above the extrapolated level of the
optical/UV continuum, peaking at
~0.3keV then falling steeply towards
higher energies (Puchnarewicz et al.
1998). Puchnarewicz et al. (2001)
showed that the AD component is
one of the hottest observed in
AGN, so that its high-energy
turnover is measurable in the soft Xrays rather than the EUV. They
inferred a high accretion rate
(L~0.3-0.7LEdd), a small black hole
mass, M~106 Msun and a viewing
angle of 60-70degrees. Thus they
concluded that REJ1034+396 had a
low-mass black hole accreting close
to the Eddington limit. However,
the AD model preferred an edge-on
view but this line of sight would
pass through a co-planar torus.
When Beppo-SAX measured the 0.1 to 12keV
spectrum of REJ1034+396, observations in
the optical (WHT), UV (HST) and EUV
(EUVE) were also taken within a few weeks.
This study (Puchnarewicz et al. 2001) placed
very strong constraints on its unusually hot big
blue bump component, which has been
attributed to a high rate (~ Eddington) of
accretion onto a disk surrounding a low mass
black hole (Pounds et al 1996). The optical to
UV continuum is flat with no evidence for
host galaxy or dust contamination. This
A warped, or flared,
accretion disk in the
NLS1 REJ1034+396
residual emission is not consistent with the
cool tail of an accretion disk and its origin is
not clear. We propose that the optical/UV
flux is produced when the outer parts of the
warped accretion disk are irradiated by the
central UV/X-ray source. The X-ray
spectrum is consistent with Comptonization
in a surrounding, hot corona. The narrow
permitted line profiles, which typify ultrasoft
AGN in general (Puchnarewicz et al 1992),
are consistent with emission from an
irradiated accretion disk wind.
permitted
line
emission
hot corona
In a geometrically-thin, optically-thick accretion
disk, at a given radius R, the temperature Tn is
proportional to R-3/4. However, if the disk is
irradiated, then the temperature, Tirr, is proportional
to R-1/2 and irradiative heating will dominate over
viscous heating at large radii. If the disk is flared or
warped then emission due to irradiation will be
relatively strong in the optical/UV, where it
intercepts more of the ionizing continuum. The
spectrum will then flatten at these wavelengths. This
provides a straightforward solution to the origin of
the flat optical/UV continuum.
Emission lines from the disk?
The spectrum of REJ1034+396 is best represented
by a flared disk with inner radius, Rinn~6x1011cm
and outer radius, Rout~5x1016cm ( = 3x105 GM/c2 ;
see top left). The region of the disk which is
dominated by irradiative heating is at R > 80Rinn.
The black hole mass, M ~ 106 Msun. At this mass,
the value of the innermost stable orbit from this fit,
Rinn, is consistent with a Kerr black hole.
The Balmer lines and UV lines in this NLS1 have full widths at
OM
half maximum (FWHM) of
1500-1800km/s and exhibit broad
and narrow components. The disk-wind model of Murray &
Chiang (1997) predicts a broad emission line component emitted
from an irradiated disk, close to the surface at small radii, while a
narrow component is emitted from the photoionized wind. Thus
an irradiated disk may predict the permitted line profiles as well
as the optical to X-ray continuum.
The fits allow a range of viewing angles so that the
disk may be viewed close to the disk axis (up to
~20degrees away). This removes the problem of
having to view the AD through a co-planar
molecular torus and is consistent with suggestions
that ultrasoft AGN are seen relatively face-on
(Puchnarewicz 1992).
5x1016cm
An illustration (not drawn to scale!) of a flared accretion disk
in REJ1034+396.
References
Murray & Chiang 1997, ApJ, 474, 91
Pounds et al. 1995, MNRAS, 277, L5
Puchnarewicz et al 1992, MNRAS, 256, 589
Puchnarewicz et al 1995, MNRAS, 276, 20
Puchnarewicz et al 2001, ApJ, 500, 644
Soria & Puchnarewicz 2001, MNRAS, in press
www.mssl.ucl.ac.uk