46 research outputs found
The Far-infrared Continuum of Quasars
ISO provides a key new far-infrared window through which to observe the
multi-wavelength spectral energy distributions (SEDs) of quasars and active
galactic nuclei (AGN). It allows us, for the first time, to observe a
substantial fraction of the quasar population in the far-IR, and to obtain
simultaneous, multi-wavelength observations from 5--200 microns. With these
data we can study the behavior of the IR continuum in comparison with
expectations from competing thermal and non-thermal models. A key to
determining which mechanism dominates, is the measurement of the peak
wavelength of the emission and the shape of the far-IR--mm turnover. Turnovers
which are steeper than frequency^2.5 indicate thermal dust emission in the
far-IR.
Preliminary results from our ISO data show broad, fairly smooth, IR continuum
emission with far-IR turnovers generally too steep to be explained by
non-thermal synchrotron emission. Assuming thermal emission throughout leads to
a wide inferred temperature range of 50-1000 K. The hotter material, often
called the AGN component, probably originates in dust close to and heated by
the central source, e.g. the ubiquitous molecular torus. The cooler emission is
too strong to be due purely to cool, host galaxy dust, and so indicates either
the presence of a starburst in addition to the AGN or AGN-heated dust covering
a wider range of temperatures than present in the standard, optically thick
torus models.Comment: 4 pages, to be published in the proceedings of "The Universe as Seen
by ISO," ed. M. Kessler. This and related papers can be found at
http://hea-www.harvard.edu/~ehooper/ISOkp/ISOkp.htm
Infrared Properties of High Redshift and X-ray Selected AGN Samples
The NASA/ISO Key Project on active galactic nuclei (AGN) seeks to better
understand the broad-band spectral energy distributions (SEDs) of these sources
from radio to X-rays, with particular emphasis on infrared properties. The ISO
sample includes a wide variety of AGN types and spans a large redshift range.
Two subsamples are considered herein: 8 high-redshift (1 < z < 4.7) quasars;
and 22 hard X-ray selected sources.
The X-ray selected AGN show a wide range of IR continuum shapes, extending to
cooler colors than the optical/radio sample of Elvis et al. (1994). Where a
far-IR turnover is clearly observed, the slopes are < 2.5 in all but one case
so that non-thermal emission remains a possibility. The highest redshift
quasars show extremely strong, hot IR continua requiring ~ 100 solar masses of
500 - 1000 Kelvin dust with ~ 100 times weaker optical emission. Possible
explanations for these unusual properties include: reflection of the optical
light from material above/below a torus; strong obscuration of the optical
continuum; or an intrinsic deficit of optical emission.Comment: 8 pages, 3 figures (2 color), to be published in the Springer Lecture
Notes of Physics Series as part of the proceedings for "ISO Surveys of a
Dusty Universe," a workshop held at Ringberg Castle, Germany, November 8 -
12, 1999. Requires latex style files for this series: cl2emult.cls,
cropmark.sty, lnp.sty, sprmindx.sty, subeqnar.sty (included with submission
Chandra and XMM-Newton observations of Tololo 0109-383
We present and discuss Chandra and XMM-Newton observations of the Seyfert 2
galaxy and Compton-thick absorbed source, Tololo~0109-383. The hard X-ray
emission (i.e. above 2 keV), is dominated by a spatially unresolved
reflection component, as already discovered by previous ASCA and BeppoSAX
observ ations. The soft X-ray emission is partly (15%) extended over
about 1 kpc. Below 2 keV, the spectrum is very steep and two emission lines,
probably due to recombination to He-like ions of oxygen and neon, are clearly
present. Combining X-rays and optical information taken from the literature, we
propose an overall scenario for the nuclear regions of this source.Comment: 6 pages, 6 figures, A&A 399, 51
Luminosity-variation independent location of the circum-nuclear, hot dust in NGC 4151
After recent sensitivity upgrades at the Keck Interferometer (KI), systematic
interferometric 2um studies of the innermost dust in nearby Seyfert nuclei are
within observational reach. Here, we present the analysis of new
interferometric data of NGC 4151, discussed in context of the results from
recent dust reverberation, spectro-photometric and interferometric campaigns.
The complete data set gives a complex picture, in particular the measured
visibilities from now three different nights appear to be rather insensitive to
the variation of the nuclear luminosity. KI data alone indicate two scenarios:
the K-band emission is either dominated to ~90% by size scales smaller than
30mpc, which falls short of any dust reverberation measurement in NGC 4151 and
of theoretical models of circum-nuclear dust distributions. Or contrary, and
more likely, the K-band continuum emission is dominated by hot dust (>= 1300K)
at linear scales of about 50mpc. The linear size estimate varies by a few tens
of percent depending on the exact morphology observed. Our interferometric,
deprojected centro-nuclear dust radius estimate of 55+-5mpc is roughly
consistent with the earlier published expectations from circum-nuclear, dusty
radiative transfer models, and spectro-photometric modeling. However, our data
do not support the notion that the dust emission size scale follows the nuclear
variability of NGC 4151 as a R_dust \propto L_nuc^0.5 scaling relation. Instead
variable nuclear activity, lagging, and variable dust response to illumination
changes need to be combined to explain the observations.Comment: 19 pages, 3 figures, 3 tables, accepted for publication in Ap
A model for the X-ray absorption in Compton--thin AGN
The fraction of AGN with photoelectric absorption in the X-rays ranging from
NH of 10^{22} up to about 10^{24} cm^{-2} (Compton-thin) appears
observationally to be anticorrelated to their luminosity Lx. This recently
found evidence is used to investigate the location of the absorbing gas. The
molecular torus invoked in the unified picture of AGN, while it can be regarded
as confirmed on several grounds to explain the Compton-thick objects, do not
conform to this new constraint, at least in its physical models as developed so
far. In the frame of observationally based evidence that in Compton-thin
sources the absorbing gas might be located far away from the X-ray source, it
is shown that the gravitational effects of the black hole (BH) on the molecular
gas in a disk, within 25-450 pc (depending on the BH mass, from 10^6 to 10^9
M_solar, leads naturally to the observed anticorrelation, under the assumption
of a statistical correlation between the BH mass and Lx. Its normalization is
also reproduced provided that the surface density, Sigma, of this gas is larger
than about 150-200 M_solar pc^{-2}, and assuming that the bolometric luminosity
is one tenth of the Eddington limit. Interestingly, the required values are
consistent with the value of the 300 pc molecular disk in our own galaxy,
namely 500 M_solar pc^{-2}. In a sample of nearby galaxies from the BIMA SONG
survey, it is found that half of the objects have central Sigma larger than 150
M_solar pc${-2}. Given the simplicity of the proposed model, this finding is
very encouraging, waiting for future higher resolution surveys in CO on more
distant galaxies.Comment: Astronomy and Astrophysics, in pres
Unveiling the broad band X-ray continuum and iron line complex in Mkr 841
Mkr 841 is a bright Seyfert 1 galaxy known to harbor a strong soft excess and
a variable K iron line. It has been observed during 3 different periods
by XMM for a total cumulated exposure time of 108 ks. We present in this
paper a broad band spectral analysis of the complete EPIC-pn data sets. We were
able to test two different models for the soft excess, a relativistically
blurred photoionized reflection (\r model) and a relativistically smeared
ionized absorption (\a model). The continuum is modeled by a simple cut-off
power law and we also add a neutral reflection. These observations reveal the
extreme and puzzling spectral and temporal behaviors of the soft excess and
iron line. The 0.5-3 keV soft X-ray flux decreases by a factor 3 between 2001
and 2005 and the line shape appears to be a mixture of broad and narrow
components. We succeed in describing this complex broad-band 0.5-10 keV
spectral variability using either \r or \a to fit the soft excess. Both models
give statistically equivalent results even including simultaneous BeppoSAX data
up to 200 keV. Both models are consistent with the presence of remote
reflection characterized by a constant narrow component in the data. However
they differ in the presence of a broad line component present in \r but not
needed in \a. This study also reveals the sporadic presence of relativistically
redshifted narrow iron lines.Comment: Accepted in A&A. 17 pages and 21 figure
New constraints on the continuum-emission mechanism of AGN: Intensive monitoring of NGC 7469 in the X-ray and ultraviolet
We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC
7469 in the X-ray with RXTE over a ~30d baseline. The source shows strong
variability with a root-mean-square (rms) amplitude of ~16 per cent, and
peak-to-peak variations of a factor of order 2. Simultaneous data over this
period were obtained in the ultraviolet (UV) using IUE, making this the most
intensive X-ray UV/X-ray variability campaign performed for any active galaxy.
Comparison of the continuum light curves reveals very similar amplitudes of
variability, but different variability characteristics, with the X-rays showing
much more rapid variations. The data are not strongly correlated at zero lag.
The largest absolute value of the correlation coefficient occurs for an
anticorrelation between the two bands, with the X-ray variations leading the UV
by ~4d. The largest positive correlation is for the ultraviolet to lead the
X-rays by ~4d. Neither option appears to be compatible with any simple
interband transfer function. The peak positive correlation at ~4d occurs
because the more prominent peaks in the UV light curve appear to lead those in
the X-rays by this amount. However, the minima of the light curves are
near-simultaneous. These observations provide new constraints on theoretical
models of the central regions of active galactic nuclei. Models in which the
observed UV emission is produced solely by re-radiation of absorber X-rays are
ruled out by our data, as are those in which the X-rays are produced solely by
Compton upscattering of the observed UV component by a constant distribution of
particles.Comment: 33 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journal. Also available via
http://lheawww.gsfc.nasa.gov/users/nandra/pubs/7469/abstract.htm
The HELLAS2XMM survey: XI. Unveiling the nature of X-ray Bright Optically Normal Galaxies
X-ray Bright Optically Normal Galaxies (XBONGs) constitute a small but not
negligible fraction of hard X-ray selected sources in recent Chandra and
XMM-Newton surveys. Even though several possibilities were proposed to explain
why a relatively luminous hard X-ray source does not leave any significant
signature of its presence in terms of optical emission lines, the nature of
XBONGs is still subject of debate. We aim to a better understanding of their
nature by means of a multiwavelength and morphological analysis of a small
sample of these sources. Good-quality photometric near-infrared data
(ISAAC/VLT) of four low-redshift (z=0.1-0.3) XBONGs, selected from the
HELLAS2XMM survey, have been used to search for the presence of the putative
nucleus, applying the surface-brightness decomposition technique through the
least-squares fitting program GALFIT. The surface brightness decomposition
allows us to reveal a nuclear point-like source, likely to be responsible of
the X-ray emission, in two out of the four sources. The results indicate that
moderate amounts of gas and dust, covering a large solid angle (possibly 4pi)
at the nuclear source, combined with the low nuclear activity, may explain the
lack of optical emission lines. The third XBONG is associated with an X-ray
extended source and no nuclear excess is detected in the near infrared at the
limits of our observations. The last source is associated to a close (d< 1
arcsec) double system and the fitting procedure cannot achieve a firm
conclusion.Comment: 20 pages, 12 figures, A&A in pres
The Far-Infrared Spectral Energy Distributions of X-ray-selected Active Galaxies
[Abridged] We present ISO far-infrared (IR) observations of 21 hard X-ray
selected AGN from the HEAO-1 A2 sample. We compare the far-IR to X-ray spectral
energy distributions (SEDs) of this sample with various radio and optically
selected AGN samples. The hard-X-ray selected sample shows a wider range of
optical/UV shapes extending to redder near-IR colors. The bluer objects are
Seyfert 1s, while the redder AGN are mostly intermediate or type 2 Seyferts.
This is consistent with a modified unification model in which the amount of
obscuring material increases with viewing angle and may be clumpy. Such a
scenario, already suggested by differing optical/near-IR spectroscopic and
X-ray AGN classifications, allows for different amounts of obscuration of the
continuum emission in different wavebands and of the broad emission line region
which results in a mixture of behaviors for AGN with similar optical emission
line classifications. The resulting limits on the column density of obscuring
material through which we are viewing the redder AGN are 100 times lower than
for the standard optically thick torus models. The resulting decrease in
optical depth of the obscuring material allows the AGN to heat more dust at
larger radial distances. We show that an AGN-heated, flared, dusty disk with
mass 10^9 solar and size of few hundred pc is able to generate optical-far-IR
SEDs which reproduce the wide range of SEDs present in our sample with no need
for an additional starburst component to generate the long-wavelength, cooler
part of the IR continuum.Comment: 40 pages, 14 figures, accepted for publication in Astrophysical
Journal, V. 590, June 10, 200
The Science Case for an Extended Spitzer Mission
Although the final observations of the Spitzer Warm Mission are currently
scheduled for March 2019, it can continue operations through the end of the
decade with no loss of photometric precision. As we will show, there is a
strong science case for extending the current Warm Mission to December 2020.
Spitzer has already made major impacts in the fields of exoplanets (including
microlensing events), characterizing near Earth objects, enhancing our
knowledge of nearby stars and brown dwarfs, understanding the properties and
structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to
study galaxy birth and evolution. By extending Spitzer through 2020, it can
continue to make ground-breaking discoveries in those fields, and provide
crucial support to the NASA flagship missions JWST and WFIRST, as well as the
upcoming TESS mission, and it will complement ground-based observations by LSST
and the new large telescopes of the next decade. This scientific program
addresses NASA's Science Mission Directive's objectives in astrophysics, which
include discovering how the universe works, exploring how it began and evolved,
and searching for life on planets around other stars.Comment: 75 pages. See page 3 for Table of Contents and page 4 for Executive
Summar