2,344 research outputs found
Relativistic ionized accretion disc models of MCG--6-30-15
We present results from fitting ionized accretion disc models to three long
ASCA observations of the Seyfert 1 galaxy MCG--6-30-15. All three datasets can
be fit by a model consisting of ionized reflection from the inner region of the
accretion disc (with twice solar Fe abundance) and a separate diskline
component from farther out on the the disc. The diskline is required to fit the
height of the observed Fe K line profile. However, we show that a much simpler
model of reflection from a very weakly ionized constant density disc also fits
the data. In this case only a single cold Fe K line at 6.4 keV is required to
fit the observed line. The ionized disc models predict that O VIII K, C VI K,
Fe XVII L, and Fe XVIII L lines will appear in the soft X-ray region of the
reflection spectrum, but are greatly blurred due to Compton scattering. The
equivalent width (EW) of O VIII K is estimated to be about 10 eV and seems to
be as strong as the blend of the Fe L lines. This result creates difficulty for
the claim of a strong relativistic O VIII line in the XMM-Newton grating
spectrum of MCG--6-30-15, although we can not strictly rule it out since
MCG--6-30-15 was in an anomalously low state during that observation. We find
that increasing the O abundance or breaking the continuum below 2 keV will not
significantly strengthen the line. The second Fe K line component in the
ionized disc model may arise from neutral reflection from a flared disc, or
from a second illumination event. The data cannot distinguish between the two
cases, and we conclude that single zone ionized disc models have difficulty
fitting these hard X-ray data of MCG--6-30-15.Comment: 6 pp., accepted by MNRAS Letter
Lifting the Veil on Obscured Accretion: Active Galactic Nuclei Number Counts and Survey Strategies for Imaging Hard X-Ray Missions
Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope which, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV, and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a greater than tenfold increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population shows enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions
Constraining Radiatively Inefficient Accretion Flows with Polarization
The low-luminosity black hole Sgr A* provides a testbed for models of
Radiatively Inefficient Accretion Flows (RIAFs). Recent sub-millimeter linear
polarization measurements of Sgr A* have provided evidence that the electrons
in the accretion flow are relativistic over a large range of radii. Here, we
show that these high temperatures result in elliptical plasma normal modes.
Thus, polarized millimeter and sub-millimeter radiation emitted within RIAFs
will undergo generalized Faraday rotation, a cyclic conversion between linear
and circular polarization. This effect will not depolarize the radiation even
if the rotation measure is extremely high. Rather, the beam will take on the
linear and circular polarization properties of the plasma normal modes. As a
result, polarization measurements of Sgr A* in this frequency regime will
constrain the temperature, density and magnetic profiles of RIAF models.Comment: 4 pages, 3 figures, accepted by ApJ Letter
The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?
We report the discovery of an X-ray counterpart to the southern radio hotspot
of the largest-known radio quasar 4C 74.26 (whose redshift is z=0.104). Both
XMM-Newton and Chandra images reveal the same significant (10arcsec, i.e.
19kpc) offset between the X-ray hotspot and the radio hotspot imaged with
MERLIN. The peak of the X-ray emission may be due to synchrotron or
inverse-Compton emission. If synchrotron emission, the hotspot represents the
site of particle acceleration and the offset arises from either the jet
exhibiting Scheuer's `dentist's drill' effect or a fast spine having less
momentum than the sheath surrounding it, which creates the radio hotspot. If
the emission arises from the inverse-Compton process, it must be
inverse-Compton scattering of the CMB in a decelerating relativistic flow,
implying that the jet is relativistic (Gamma >= 2) out to a distance of at
least 800kpc. Our analysis, including optical data from the Liverpool
Telescope, rules out a background AGN for the X-ray emission and confirms its
nature as a hotspot, making it the most X-ray luminous hotspot yet detected.Comment: 9 pages, 9 figures, definitive version published by MNRA
An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies
We present an XMM-Newton observation of the bright, narrow-line, ultrasoft
Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved,
showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless
excess of emission at lower energies. The spectrum can be adequately
parameterised using a simple double power-law model. The source is strongly
variable over the course of the observation but shows only weak spectral
variability, with the fractional variability amplitude remaining approximately
constant over more than a decade in energy. The curved continuum shape and weak
spectral variability are discussed in terms of various physical models for the
soft X-ray excess emission, including reflection off the surface of an ionised
accretion disc, inverse-Compton scattering of soft disc photons by thermal
electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal
distribution. We emphasise the possibility that the strong soft excess may be
produced by dissipation of accretion energy in the hot, upper atmosphere of the
putative accretion disc.Comment: 9 pages, accepted for publication in MNRA
Fe K\alpha emission from photoionized slabs: the impact of the iron abundance
Iron K\alpha emission from photoionized and optically thick material is
observed in a variety of astrophysical environments including X-ray binaries,
active galactic nuclei, and possibly gamma-ray bursts. This paper presents
calculations showing how the equivalent width (EW) of the Fe K line depends on
the iron abundance of the illuminated gas and its ionization state -- two
variables subject to significant cosmic scatter. Reflection spectra from a
constant density slab which is illuminated with a power-law spectrum with
photon-index \Gamma are computed using the code of Ross & Fabian. When the Fe K
EW is measured from the reflection spectra alone, we find that it can reach
values greater than 6 keV if the Fe abundance is about 10 times solar and the
illuminated gas is neutral. EWs of about 1 keV are obtained when the gas is
ionized. In contrast, when the EW is measured from the incident+reflected
spectrum, the largest EWs are ~800 keV and are found when the gas is ionized.
When \Gamma is increased, the Fe K line generally weakens, but significant
emission can persist to larger ionization parameters. The iron abundance has
its greatest impact on the EW when it is less than 5 times solar. When the
abundance is further increased, the line strengthens only marginally.
Therefore, we conclude that Fe K lines with EWs much greater than 800 eV are
unlikely to be produced by gas with a supersolar Fe abundance. These results
should be useful in interpreting Fe K emission whenever it arises from
optically thick fluorescence.Comment: 5 pages, 5 figures, accepted by MNRAS Letter
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