46 research outputs found
Nuclear obscuration in the high-ionization Seyfert 2 galaxy Tol 0109-383
We report the BeppoSAX detection of a hard X-ray excess in the X-ray spectrum
of the classical high-ionization Seyfert 2 galaxy Tol0109-383. The X-ray
emission of this source observed below 7 keV is dominated by reflection from
both cold and ionized gas, as seen in the ASCA data. The excess hard X-ray
emission is presumably due to the central source absorbed by an optically thick
obscuring torus with N(H)~2e24 cm-2. The strong cold X-ray reflection, if it is
produced at the inner surface of the torus, is consistent with the picture
where much of the inner nucleus of Tol0109-383 is exposed to direct view, as
indicated by optical and infrared properties. However, the X-ray absorption
must occur at small radii in order to hide the central X-ray source but leave
the optical high-ionization emission line region unobscured. This may also be
the case for objects like the Seyfert 1 galaxy Mrk231.Comment: 7 pages, MNRAS in pres
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
X-ray Reflection By Photoionized Accretion Discs
We present the results of reflection calculations that treat the relevant
physics with a minimum of assumptions. The temperature and ionization structure
of the top five Thomson depths of an illuminated disc are calculated while also
demanding that the atmosphere is in hydrostatic equilibrium. In agreement with
Nayakshin, Kazanas & Kallman, we find that there is a rapid transition from hot
to cold material in the illuminated layer. However, the transition is usually
not sharp so that often we find a small but finite region in Thomson depth
where there is a stable temperature zone at T \sim 2 x 10^{6} K due to
photoelectric heating from recombining ions. As a result, the reflection
spectra often exhibit strong features from partially-ionized material,
including helium-like Fe K lines and edges. We find that due to the highly
ionized features in the spectra these models have difficulty correctly
parameterizing the new reflection spectra. There is evidence for a spurious
correlation in the ASCA energy range, where is the reflection
fraction for a power-law continuum of index , confirming the suggestion
of Done & Nayakshin that at least part of the R-Gamma correlation reported by
Zdziarski, Lubinski & Smith for Seyfert galaxies and X-ray binaries might be
due to ionization effects. Although many of the reflection spectra show strong
ionized features, these are not typically observed in most Seyfert and quasar
X-ray spectra.Comment: 16 pages, accepted by MNRAS, Fig. 8 is in colour Figures and tables
changed by a code update. Conclusions unchange
A long hard look at MCG-6-30-15 with XMM-Newton and BeppoSAX
We summarise the primary results from a 320 ks observation of the bright
Seyfert 1 galaxy MCG-6-30-15 with XMM-Newton and Beppo-SAX.Comment: 4 pages, 6 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
The X-ray nebula around the Seyfert 2 galaxy NGC4388
We report on X-ray emission from the Seyfert 2 galaxy NGC4388 observed with
the Chandra X-ray Observatory. A hard X-ray peak is found at the position of
the active nucleus suggested by optical and radio observations. Extended soft
X-ray emission correlates well with the ionization cone found in optical line
emission. A large soft X-ray extension is found up to 16 kpc to the north of
the galaxy. Photoionized gas with low ionization parameters (xi<3) appears to
be the likely explanation of this emission. The same ionized gas clouds could
be responsible for the optical [OIII] emission. Fe K line emission from cold
material is found to be extended by a few kpc.Comment: 12 pages, one colour figure included, MNRAS in pres
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
The Fragmentation of Pre-enriched Primordial Objects
Recent theoretical investigations have suggested that the formation of the
very first stars, forming out of metal-free gas, was fundamentally different
from the present-day case. In this paper, we study the effect of metallicity on
the evolution of the gas in a collapsing dark matter mini-halo. We model such a
system as an isolated 3\sigma peak of mass 2x10^6 M_sun that collapses at
z_coll=30, using smoothed particle hydrodynamics. The gas has a supposed level
of pre-enrichment of either 10^-4 Z_sun or 10^-3 Z_sun. We find that the
evolution proceeds very differently for the two cases. The gas in the lower
metallicity simulation fails to undergo continued collapse and fragmentation,
whereas the gas in the higher metallicity case dissipatively settles into the
center of the dark matter halo. The central gas, characterized by densities n >
10^4 cm^-3, and a temperature, T \sim 90 K, which closely follows that of the
CMB, is gravitationally unstable and undergoes vigorous fragmentation. We
discuss the physical reason for the existence of a critical metallicity, Z_crit
\sim 5x10^-4 Z_sun, and its possible dependence on redshift. Compared to the
pure H/He case, the fragmentation of the 10^-3 Z_sun gas leads to a larger
relative number of low-mass clumps.Comment: Minor revisions, 7 pages, 6 figures, MNRAS in pres
Observing Supermassive Black Holes across cosmic time: from phenomenology to physics
In the last decade, a combination of high sensitivity, high spatial
resolution observations and of coordinated multi-wavelength surveys has
revolutionized our view of extra-galactic black hole (BH) astrophysics. We now
know that supermassive black holes reside in the nuclei of almost every galaxy,
grow over cosmological times by accreting matter, interact and merge with each
other, and in the process liberate enormous amounts of energy that influence
dramatically the evolution of the surrounding gas and stars, providing a
powerful self-regulatory mechanism for galaxy formation. The different
energetic phenomena associated to growing black holes and Active Galactic
Nuclei (AGN), their cosmological evolution and the observational techniques
used to unveil them, are the subject of this chapter. In particular, I will
focus my attention on the connection between the theory of high-energy
astrophysical processes giving rise to the observed emission in AGN, the
observable imprints they leave at different wavelengths, and the methods used
to uncover them in a statistically robust way. I will show how such a combined
effort of theorists and observers have led us to unveil most of the SMBH growth
over a large fraction of the age of the Universe, but that nagging
uncertainties remain, preventing us from fully understating the exact role of
black holes in the complex process of galaxy and large-scale structure
formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the
book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and
Treves A. (Eds), 2015, Springer International Publishing AG, Cha
Gravitational Lensing by Black Holes
We review the theoretical aspects of gravitational lensing by black holes,
and discuss the perspectives for realistic observations. We will first treat
lensing by spherically symmetric black holes, in which the formation of
infinite sequences of higher order images emerges in the clearest way. We will
then consider the effects of the spin of the black hole, with the formation of
giant higher order caustics and multiple images. Finally, we will consider the
perspectives for observations of black hole lensing, from the detection of
secondary images of stellar sources and spots on the accretion disk to the
interpretation of iron K-lines and direct imaging of the shadow of the black
hole.Comment: Invited article for the GRG special issue on lensing (P. Jetzer, Y.
Mellier and V. Perlick Eds.). 31 pages, 12 figure
Chandra X-ray observations of the 3C295 cluster core
We examine the properties of the X-ray gas in the central regions of the
distant (z=0.46), X-ray luminous cluster of galaxies surrounding the powerful
radio source 3C 295, using observations made with the Chandra Observatory.
Between radii of 50-500 kpc, the cluster gas is approximately isothermal with
an emission-weighted temperature, kT ~5 keV. Within the central 50 kpc radius
this value drops to kT ~3.7 keV. The spectral and imaging Chandra data indicate
the presence of a cooling flow within the central 50 kpc radius of the cluster,
with a mass deposition rate of approximately 280 solar masses per year. We
estimate an age for the cooling flow of 1-2 Gyr, which is approximately one
thousand times older than the central radio source. We find no evidence in the
X-ray spectra or images for significant heating of the X-ray gas by the radio
source. We report the detection of an edge-like absorption feature in the
spectrum for the central 50 kpc region, which may be due to oxygen-enriched
dust grains. The implied mass in metals seen in absorption could have been
accumulated by the cooling flow over its lifetime. Combining the results on the
X-ray gas density profile with radio measurements of the Faraday rotation
measure in 3C295, we estimate the magnetic field strength in the region of the
cluster core to be B ~12 \muG.Comment: 27 pages, 16 figs, 5 tables. Accepted for publication in MNRA