Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

mechanics community, for non-rotating shear flows. Based on this new understanding, the question has been
reinvestigated in rotating flows (such as the keplerian flows), through i/ a phenomenological analysis (Longaretti
2002); ii/ a reinvestigation of all the relevant laboratory experimental data (Longaretti and Dauchot 2005;
Longaretti and Dauchot, in preparation; Dubrulle et al. 2005); and iii/ numerical simulations (Lesur and
Longaretti 2005). This work has lead to the conclusion that a stabilizing rotation does not quench the transition
to turbulence, but nevertheless considerably reduces the efficiency of the subcritical turbulent transport, thereby
settling this old dispute.
Geoffroy Lesur has started a PhD in September 2004, under the supervision of Pierre-Yves Longaretti, with
the aim to investigate in a more systematic way the question of turbulent transport in disks and jets, both from
analytic and numerical points of view.
15.5 Physics of high energy sources
15.5.1 Seyfert galaxies
Seyfert galaxies are strong X-ray emitters with a characteristic X-ray spectra. It is roughly power-law like
above 2 keV, with the presence of a high energy cut-off near 100 keV (e.g. Petrucci et al. 2001, ApJ, 556,
716; Zdziarski et al. 2000, ApJ, 542, 703). Secondary components, like a fluorescent iron line near 6.4 keV
and a bump in the 10-50 keV range are also generally present. The X-ray emission is generally supposed to
be produced by a hot and optically thin thermal plasma (a corona) localized above the accretion disk and
comptonizing the disk photons. The secondary components are thought to result from the Compton reflection
of the X-rays on the disk surface. Due to the lack of sensitivity in the hard X-ray/soft gamma ray energy range
(0.1-1 MeV) of the detectors, the real nature of the high energy continuum of Seyfert galaxies remains unclear.
We have recently tested two different geometries of the disk-corona configuration using the most well adapted
data for the study of the Seyfert high energy continuum i.e. the BeppoSAX 1 brightest Seyfert sample (Petrucci
& Dadina 2005 in preparation). This subsample contains 28 objects. It appears that both geometries agree
with the data but more importantly, we show that the behaviors of the physical parameters of the models
(mainly the temperature and optical depth of the X-ray corona) are strongly geometry dependent, resulting in
completely different physical interpretations. This work is an extension of previous works done on a smaller
sample (Petrucci et al, 2000, ApJ, 540, 131; Petrucci et al. 2001, ApJ, 556, 716) and underlines the weakness
of the actual constraints.
Stronger and less ambiguous constraints on the nature of the coronal plasma can be obtained from variability
studies. For example, thermal models, where hot and cold phases are in radiative equilibrium, predict that the
X-ray spectrum of the sources should harden when the corona temperature increases (Haardt et al. 1997, ApJ,
476, 620) while non-thermal models predict the reverse (Petrucci et al. 2001, A&A, 374, 719). The analysis of
the ∼1 month simultaneous IUE/RXTE monitoring campaign on NGC 7469, performed in 1996, appears to be
in agreement with thermal comptonization emission (Nandra et al. 2000, ApJ, 544, 734). We have performed
a more recent reanalysis of these data with realistic comptonization codes that gives another support to this
interpretation (Petrucci et al. 2004). Missions with X-ray/γ-ray broad band capabilities and higher sensitivity,
like the future ASTROE-2 mission, are required to progress in this field.
On the other hand, the highly sensitive instruments of the XMM-Newton satellite enable very new and
exciting results concerning the fluorescent iron line. Broad iron lines are clearly present in some Seyferts and in
objects like MCG-6-30-15, their broad profiles suggest the presence of spinning black hole, close to the extreme
Kerr value (Fabian et al., 2000, PASP, 112, 1145) and require very steep disk emissivity laws (Wilms et al. 2001,
MNRAS, 328, L27; Fabian et al. 2002, MNRAS, 335, L1), steeper than standard accretion disk one. This is
in agreement with the model we proposed some years ago where the irradiating X-ray source was concentrated
on the disk axis, above the black hole (Henri & Petrucci 1997, A&A, 326, 87; Petrucci & Henri 1997, 326, 99
but see also Martocchia & Matt 1996, A&A, 282, L53; Martocchia et al. 2002, A&A, 383, L23). More recently,
redshifted narrow iron lines have been also observed in an increase number of objects (e.g. Yaqoob et al. 2003,
ApJ, 596, 85; Longinotti et al. 2004; Porquet et al. 2004, A&A, 427, 101; Turner et al. 2004, ApJ, 603, 62;
Iwasawa et al. 2004, MNRAS, 335, 1073). These lines can be interpreted as signatures of small magnetic flares
illuminating small part of the accretion disk. We are investigating the presence of such a line in the Seyfert 1
1 BeppoSAX was an Italian-Dutch satellite covering the 0.1-200 keV energy range. It stopped observing in 2003.
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