11 research outputs found
On the Prospect of Constraining Black-Hole Spin Through X-ray Spectroscopy of Hotspots
Future X-ray instrumentation is expected to allow us to significantly improve
the constraints derivedfrom the Fe K lines in AGN, such as the black-hole
angular momentum (spin) and the inclination angle of the putative accretion
disk. We consider the possibility that measurements of the persistent,
time-averaged Fe K line emission from the disk could be supplemented by the
observation of a localized flare, or "hotspot", orbiting close to the black
hole. Although observationally challenging, such measurements would recover
some of the information loss that is inherent to the radially-integrated line
profiles. We present calculations for this scenario to assess the extent to
which, in principle, black-hole spin may be measured. We quantify the
feasibility of this approach using realistic assumptions about likely
measurement uncertainties.Comment: 7 pages, 7 figures. Accepted for publication in Ap
Monte Carlo simulations of the Nickel K fluorescent emission line in a toroidal geometry
We present new results from Monte Carlo calculations of the flux and
equivalent width (EW) of the Ni Kalpha fluorescent emission line in the
toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549).
In the Compton-thin regime, the EW of the Ni Kalpha line is a factor of ~22
less than that of the Fe Kalpha line but this factor can be as low as ~6 in the
Compton-thick regime. We show that the optically-thin limit for this ratio
depends only on the Fe to Ni abundance ratio, it being independent of the
geometry and covering factor of the reprocessor, and also independent of the
shape of the incident X-ray continuum. We give some useful analytic expressions
for the absolute flux and the EW of the Ni Kalpha line in the optically-thin
limit. When the reprocessor is Compton-thick and the incident continuum is a
power-law with a photon index of 1.9, the Ni Kalpha line EW has a maximum value
of ~3 eV and ~250 eV for non-intercepting and intercepting lines-of-sight
respectively. Larger EWs are obtained for flatter continua. We have also
studied the Compton shoulder of the Ni Ka line and find that the ratio of
scattered to unscattered flux in the line has a maximum value of 0.26, less
than the corresponding maximum for the Fe Kalpha line. However, we find that
the shape of the Compton shoulder profile for a given column density and
inclination angle of the torus is similar to the corresponding profile for the
Fe Ka line. Our results will be useful for interpreting X-ray spectra of active
galactic nuclei (AGNs) and X-ray binary systems in which the system parameters
are favorable for the Ni Kalpha line to be detected.Comment: Accepted for publication in MNRAS. 8 pages, 5 figure
Precision Fe Kalpha and Fe Kbeta Line Spectroscopy of the Seyfert 1.9 Galaxy NGC 2992 with Suzaku
We present detailed time-averaged X-ray spectroscopy in the 0.5--10 keV band
of the Seyfert~1.9 galaxy NGC 2992 with the Suzaku X-ray Imaging Spectrometers
(XIS). We model the complex continuum in detail. There is an Fe K line emission
complex that we model with broad and narrow lines and we show that the
intensities of the two components are decoupled at a confidence level >3sigma.
The broad Fe K line has an EW of 118 (+32,-61) eV and could originate in an
accretion disk (with inclination angle greater than ~30 degrees). The narrow Fe
Kalpha line has an EW of 163 (+47,-26) eV and is unresolved FWHM <4090 km/s)
and likely originates in distant matter. The absolute flux in the narrow line
implies that the column density out of the line-of-sight could be much higher
than measured in the line-of-sight, and that the mean (historically-averaged)
continuum luminosity responsible for forming the line could be a factor of
several higher than that measured from the data. We also detect the narrow Fe
Kbeta line with a high signal-to-noise ratio and describe a new robust method
to constrain the ionization state of Fe responsible for the Fe Kalpha and Fe
Kbeta lines that does not require any knowledge of possible gravitational and
Doppler energy shifts affecting the line energies. For the distant
line-emitting matter (e.g. the putative obscuring torus) we deduce that the
predominant ionization state is lower than Fe VIII (at 99% confidence),
conservatively taking into account residual calibration uncertainties in the
XIS energy scale and theoretical and experimental uncertainties in the Fe K
fluorescent line energies. From the limits on a possible Compton-reflection
continuum it is likely that the narrow Fe Kalpha and Fe Kbeta lines originate
in a Compton-thin structure.Comment: Abstract is abridged. Accepted for publication in the Suzaku special
issue of PASJ (November 2006). 18 pages, 6 figure
The energy budget for X-ray to infrared reprocessing in Compton-thin and Compton-thick active galaxies
Heavily obscured active galactic nuclei (AGNs) play an important role in
contributing to the cosmic X-ray background (CXRB). However, the AGNs found in
deep X-ray surveys are often too weak to allow direct measurement of the column
density of obscuring matter. One method adopted in recent years to identify
heavily obscured, Compton-thick AGNs under such circumstances is to use the
observed mid-infrared to X-ray luminosity ratio as a proxy for the column
density. This is based on the supposition that the amount of energy lost by the
illuminating X-ray continuum to the obscuring matter and reprocessed into
infrared emission is directly related to the column density and that the proxy
is not sensitive to other physical parameters of the system (aside from
contamination by dust emission from, for example, star-forming regions). Using
Monte Carlo simulations, we find that the energy losses experienced by the
illuminating X-ray continuum in the obscuring matter are far more sensitive to
the shape of the X-ray continuum and to the covering factor of the X-ray
reprocessor than they are to the column density of the material. Specifically
we find that it is possible for the infrared to X-ray luminosity ratio for a
Compton-thin source to be just as large as that for a Compton-thick source even
without any contamination from dust. Since the intrinsic X-ray continuum and
covering factor of the reprocessor are poorly constrained from deep X-ray
survey data, we conclude that the mid-infrared to X-ray luminosity ratio is not
a reliable proxy for the column density of obscuring matter in AGNs even when
there is no other contribution to the mid-infrared luminosity aside from X-ray
reprocessing. This conclusion is independent of the geometry of the obscuring
matter.Comment: Accepted for publication in MNRAS. 12 pages, 7 figure
The Compton shoulder of the Fe Kalpha fluorescent emission line in active galactic nuclei
We present new, high signal-to-noise ratio results from a Monte Carlo study
of the properties of the Compton shoulder of the Fe Kalpha emission line in the
toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549).
The model comprehensively covers the Compton-thin to Compton-thick regimes and
we find that the variety of Compton shoulder profiles is greater than that for
both (centrally-illuminated) spherical and disk geometries. Our Monte Carlo
simulations were done with a statistical accuracy that is high enough to
reveal, for the case of an edge-on, Compton-thick torus, a new type of Compton
shoulder that is not present in the spherical or disk geometries. Such a
Compton shoulder is dominated by a narrow back-scattering feature at ~6.24 keV.
Our results also reveal a dependence of the shape of the Compton shoulder (and
its magnitude relative to the Fe Kalpha line core) on the spectral shape of the
incident X-ray continuum. We also show the effects of velocity broadening on
the Fe Kalpha line profile and find that if either the velocity width or
instrument resolution is greater than a FWHM of ~2000 km/s, the Compton
shoulder begins to become blended with the line core and the characteristic
features of the Compton shoulder become harder to resolve. In particular, at a
FWHM of ~7000 km/s the Compton shoulder is NOT resolved at all, its only
signature being a weak asymmetry in the blended line profile. Thus, CCD X-ray
detectors cannot unambiguously resolve the Compton shoulder. Our results are
freely available in a format that is suitable for direct spectral-fitting of
the continuum and line model to real data.Comment: Accepted for publication in MNRAS. 12 pages, 6 figures. Abstract is
abridge
An X-ray Spectral Model for Compton-Thick Toroidal Reprocessors
The central engines of both type 1 and type 2 AGNs are thought to harbor a
toroidal structure that absorbs and reprocesses high-energy photons from the
central X-ray source. If the reprocessor is Compton-thick, the calculation of
emission-line and continuum spectra that are suitable for direct fitting to
X-ray data is challenging because the reprocessed emission depends on the
spectral shape of the incident continuum, which may not be directly observable.
We present new Monte-Carlo calculations of Green's functions for a toroidal
reprocessor that provide significant improvements over currently available
models. The Green's function approach enables the construction of X-ray
spectral fitting models that allow arbitrary incident spectra as part of the
fitting process. The calculations are fully relativistic and have been
performed for column densities that cover the Compton-thin to Compton-thick
regime, for incident photon energies up to 500 keV. The reprocessed continuum
and fluorescent line emission due to Fe Ka, Fe Kb, and Ni Ka are treated
self-consistently, eliminating the need for ad hoc modeling that is currently
common practice. We find that the spectral shape of the Compton-thick
reflection spectrum in both the soft and hard X-ray bands in our toroidal
geometry is different compared to that obtained from disk models. A key result
of our study is that a Compton-thick toroidal structure that subtends the same
solid angle at the X-ray source as a disk can produce a reflection spectrum
that is ~6 times weaker than that from a disk. This highlights the widespread
and erroneous interpretation of the so-called "reflection-fraction" as a solid
angle, obtained from fitting disk-reflection models to Compton-thick sources
without regard for proper consideration of geometry. (Abridged)Comment: 19 pages, 11 figures, 1 table. Accepted for publication in MNRA
Precision Fe Kα and Fe Kβ Line Spectroscopy of the Seyfert 1.9 NGC 2992 with Suzaku
We present detailed time-averaged X-ray spectroscopy in the 0.5–10 keV band of the Seyfert 1.9 galaxy NGC 2992 with the Suzaku X-ray Imaging Spectrometer (XIS). There is an Fe K line emission complex that we model with broad and narrow lines and we show that the intensities of the two components are decoupled at a confidence level > 3σ . The broad line (EW = 118+32 −61 eV) could originate in an accretion disk (inclined at > 30◦). The narrow Fe Kα line (EW = 163+47 −26 eV) is unresolved by the XIS at 99% confidence and likely originates in distant matter. A significant (narrow) Fe Kβ line is also detected and we describe a new robust method to constrain the ionization state of Fe in the distant line emitter (e.g. the putative obscuring torus). The method does not require any knowledge of possible gravitational and Doppler energy shifts and we deduce that the predominant ionization state of Fe in the distant matter is lower than Fe VIII (at 99% confidence), conservatively taking into account residual calibration uncertainties and theoretical and experimental uncertainties in the Fe K fluorescent line energies. We argue that the narrow Fe Kα and Fe Kβ lines likely originate in a Compton-thin structure.</p