212 research outputs found
The Ionizing Source of the Nucleus of NGC1097
We present new observations in X-ray and optical/ultraviolet of the nucleus
of NGC1097, known for the abrupt appearance of broad, double-peaked Balmer
lines in its spectrum in 1991. These new observations are used to construct the
spectral energy distribution (SED) of the central engine. From the SED we infer
that this AGN is radio-loud and has a bolometric luminosity L_Bol ~ 10^42
erg/s, implying a low Eddington ratio of L_Bol/L_Edd ~ 10^{-4}. These results
suggest that the central ionizing source is an advection-dominated accretion
flow (ADAF) in the form of an ellevated structure which photoionizes an outer
thin disk. We fit a simplified ADAF model to the SED and obtain limits on the
values of the mass accretion rate Mdot and accretion efficiency \eta, namely
Mdot/Mdot_Edd >= 10^{-3} and \eta <= 10^{-2}. We identify an energy budget
problem: if the central photoionizing source is isotropic, the covering factor
of the line-emitting portion of the thin accretion disk is ~ 6, i. e. the
central source accounts for only 20% of the energy emitted in the double-peaked
Balmer lines.Comment: 4 pages, 3 figures, to appear in the proceedings of "The Interplay
among Black Holes, Stars and ISM in Galactic Nuclei", IAU 222, eds. Th.
Storchi Bergmann, L.C. Ho, and H.R. Schmit
Unification of X-ray winds in Seyfert galaxies: from ultra-fast outflows to warm absorbers
The existence of ionized X-ray absorbing layers of gas along the line of
sight to the nuclei of Seyfert galaxies is a well established observational
fact. This material is systematically outflowing and shows a large range in
parameters. However, its actual nature and dynamics are still not clear. In
order to gain insights into these important issues we performed a literature
search for papers reporting the parameters of the soft X-ray warm absorbers
(WAs) in 35 type 1 Seyferts and compared their properties to those of the
ultra-fast outflows (UFOs) detected in the same sample. The fraction of sources
with WAs is >60%, consistent with previous studies. The fraction of sources
with UFOs is >34%, >67% of which also show WAs. The large dynamic range
obtained when considering all the absorbers together allows us, for the first
time, to investigate general relations among them. In particular, we find
significant correlations indicating that the closer the absorber is to the
central black hole, the higher the ionization, column, outflow velocity and
consequently the mechanical power. The absorbers continuously populate the
whole parameter space, with the WAs and the UFOs lying always at the two ends
of the distribution. This strongly suggest that these absorbers, often
considered of different types, could actually represent parts of a single
large-scale stratified outflow observed at different locations from the black
hole. The observed parameters and correlations are consistent with both
radiation pressure through Compton scattering and MHD processes contributing to
the outflow acceleration, the latter playing a major role. Most of the
absorbers, especially the UFOs, have a sufficiently high mechanical power to
significantly contribute to AGN feedback.Comment: Manuscript updated to match the MNRAS published version. Link to the
related INAF news: http://www.media.inaf.it/2013/02/05/warm-absorbers
Optical Emission Model for Binary Black Hole Merger Remnants Travelling through Discs of Active Galactic Nucleus
Active galactic nuclei (AGNs) have been proposed as plausible sites hosting a
sizable fraction of the binary black hole (BBH) mergers measured through
gravitational waves (GWs) by the LIGO-Virgo-Kagra (LVK) experiment. These GWs
could be accompanied by radiation feedback due to the interaction of the BBH
merger remnant with the AGN disc. We present a new predicted radiation
signature driven by the passage of a kicked BBH remnant throughout a thin AGN
disc. We analyse the situation of a merger occurring outside the thin disc,
where the merger is of second or higher generation in a merging hierarchical
sequence. The coalescence produces a kicked BH remnant that eventually plunges
into the disc, accretes material, and inflates jet cocoons. We consider the
case of a jet cocoon propagating quasi-parallel to the disc plane and study the
outflow that results when the cocoon emerges from the disc. Here we focus on
the long time-scale emission produced after the disc outflow expands and
becomes optically thin. The bolometric luminosity of such disc outflow evolves
as . Depending on the parameter configuration, the flare
produced by the disc outflow could be comparable to or exceed the AGN
background emission at near-infrared, optical, and extreme ultraviolet
wavelengths appearing [20-500] days after the GW event and lasting for
[1-200] days, accordingly.Comment: 11 pages, 8 figures. Submitted to MNRA
The role of the accretion disk, dust, and jets in the ir emission of low-luminosity active galactic nuclei
We use recent high-resolution infrared (IR; 1-20 ÎŒm) photometry to examine the origin of the IR emission in low-luminosity active galactic nuclei (LLAGN). The data are compared with published model fits that describe the spectral energy distribution (SED) of LLAGN in terms of an advection-dominated accretion flow, truncated thin accretion disk, and jet. The truncated disk in these models is usually not luminous enough to explain the observed IR emission, and in all cases its spectral shape is much narrower than the broad IR peaks in the data. Synchrotron radiation from the jet appears to be important in very radio-loud nuclei, but the detection of strong silicate emission features in many objects indicates that dust must also contribute. We investigate this point by fitting the IR SED of NGC 3998 using dusty torus and optically thin (Ïmid-IR ⌠1) dust shell models. While more detailed modeling is necessary, these initial results suggest that dust may account for the nuclear mid-IR emission of many LLAGN. © 2013. The American Astronomical Society. All rights reserved..A.A.-H. acknowledges support from the Spanish Plan Nacional de AstronomĂa y AstrofĂsica under grant AYA2009-05705-E, CRAfrom PN-AYA2010-21887-C04.04. R.N.was supported by an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA.Peer Reviewe
Systematic search for gamma-ray periodicity in active galactic nuclei detected by the Fermi Large Area Telescope
We use nine years of gamma-ray data provided by the Fermi Large Area
Telescope (LAT) to systematically study the light curves of more than two
thousand active galactic nuclei (AGN) included in recent Fermi-LAT catalogs.
Ten different techniques are used, which are organized in an automatic
periodicity-search pipeline, in order to search for evidence of periodic
emission in gamma rays. Understanding the processes behind this puzzling
phenomenon will provide a better view about the astrophysical nature of these
extragalactic sources. However, the observation of temporal patterns in
gamma-ray light curves of AGN is still challenging. Despite the fact that there
have been efforts on characterizing the temporal emission of some individual
sources, a systematic search for periodicities by means of a full likelihood
analysis applied to large samples of sources was missing. Our analysis finds 11
AGN, of which 9 are identified for the first time, showing periodicity at more
than 4sigma in at least four algorithms. These findings will help in solving
questions related to the astrophysical origin of this periodic behavior.Comment: 16 pages, 5 figures, 4 tables. Accepted by Ap
Evidence for a Starburst within 9 pc of the Active Nucleus of NGC 1097
We report evidence for a recent burst of star formation located within 9 pc
of the active nucleus of NGC 1097. The observational signatures of the
starburst include UV absorption lines and continuum emission from young stars
observed in a small-aperture HST spectrum. The burst is a few times 10^6 yr
old, has a mass of approximately 10^6 solar masses, an observed luminosity of
1.5 x 10^7 solar luminosities and is obscured by approximately 3 visual
magnitudes. The importance of this finding is two-fold: (1) the proximity of
the starburst to the active nucleus and thus possible association with it; (2)
its obscuration by and apparent association with a dusty absorbing medium,
while the broad emission lines appear unobscured, suggesting that the starburst
could be embedded in a circumnuclear torus as predicted in the Unified Model of
active galactic nuclei.Comment: 12 pages, 3 eps figures, accepted for publication in ApJ Letter
Constraining Sources of Ultra High Energy Cosmic Rays Using High Energy Observations with the Fermi Satellite
We analyze the conditions that enable acceleration of particles to ultra-high
energies, ~10^{20} eV (UHECRs). We show that broad band photon data recently
provided by WMAP, ISOCAM, Swift and Fermi satellites, yield constraints on the
ability of active galactic nuclei (AGN) to produce UHECRs. The high energy (MeV
- GeV) photons are produced by Compton scattering of the emitted low energy
photons and the cosmic microwave background or extra-galactic background light.
The ratio of the luminosities at high and low photon energies can therefore be
used as a probe of the physical conditions in the acceleration site. We find
that existing data excludes core regions of nearby radio-loud AGN as possible
acceleration sites of UHECR protons. However, we show that giant radio lobes
are not excluded. We apply our method to Cen A, and show that acceleration of
protons to ~10^{20} eV can only occur at distances >~ 100 kpc from the core.Comment: Extended discussion on former results; Accepted for publication in
JCA
Properties of Accretion Flows Around Coalescing Supermassive Black Holes
What are the properties of accretion flows in the vicinity of coalescing
supermassive black holes (SBHs)? The answer to this question has direct
implications for the feasibility of coincident detections of electromagnetic
(EM) and gravitational wave (GW) signals from coalescences. Such detections are
considered to be the next observational grand challenge that will enable
testing general relativity in the strong, nonlinear regime and improve our
understanding of evolution and growth of these massive compact objects. In this
paper we review the properties of the environment of coalescing binaries in the
context of the circumbinary disk and hot, radiatively inefficient accretion
flow models and use them to mark the extent of the parameter space spanned by
this problem. We report the results from an ongoing, general relativistic,
hydrodynamical study of the inspiral and merger of black holes, motivated by
the latter scenario. We find that correlated EM+GW oscillations can arise
during the inspiral phase followed by the gradual rise and subsequent drop-off
in the light curve at the time of coalescence. While there are indications that
the latter EM signature is a more robust one, a detection of either signal
coincidentally with GWs would be a convincing evidence for an impending SBH
binary coalescence. The observability of an EM counterpart in the hot accretion
flow scenario depends on the details of a model. In the case of the most
massive binaries observable by the Laser Interferometer Space Antenna, upper
limits on luminosity imply that they may be identified by EM searches out to
z~0.1-1. However, given the radiatively inefficient nature of the gas flow, we
speculate that a majority of massive binaries may appear as low luminosity AGN
in the local universe.Comment: Revised version accepted to Class. Quantum Grav. for proceedings of
8th LISA Symposium. 15 pages, 3 figures, includes changes suggested in
referee report
The impact of mergers on relaxed X-ray clusters - III. Effects on compact cool cores
(Abridged) We use the simulations presented in Poole et al. 2006 to examine
the effects of mergers on compact cool cores in X-ray clusters. We propose a
scheme for classifying the morphology of clusters based on their surface
brightness and entropy profiles. Three dominant morphologies emerge: two
hosting compact cores and central temperatures which are cool (CCC systems) or
warm (CWC systems) and one hosting extended cores which are warm (EWC systems).
We find that CCC states are disrupted only after direct collisions between
cluster cores in head-on collisions or during second pericentric passage in
off-axis mergers. By the time they relax, our remnant cores have generally been
heated to warm core (CWC or EWC) states but subsequently recover CCC states.
The only case resulting in a long-lived EWC state is a slightly off-axis 3:1
merger for which the majority of shock heating occurs during the accretion of a
low-entropy stream formed from the disruption of the secondary's core.
Compression prevents core temperatures from falling until after relaxation thus
explaining the observed population of relaxed CWC systems with no need to
invoke AGN feedback. The morphological segregation observed in the L_x-T_x and
beta-r_c scaling relations is reflected in our simulations as well. However,
none of the cases we have studied produce sufficiently high remnant central
entropies to account for the most under-luminous EWC systems observed. Lastly,
systems which initially host central metallicity gradients do not yield merger
remnants with flat metallicity profiles. Taken together, these results suggest
that once formed, compact core systems are remarkably stable against disruption
from mergers. It remains to be demonstrated exactly how the sizable observed
population of extended core systems was formed.Comment: 19 pages, 8 figures, submitted for publication in MNRA
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