52 research outputs found
A Systematic Study of The Optical to X-ray Spectral Properties of Active Galactic Nuclei
In this thesis I present a detailed study of the optical to X-ray spectral properties of Active Galactic Nuclei (AGN). We propose a new broadband SED model which combines the standard accretion disc emission, low and high temperature Comptonisation components by introducing a corona radius. Applying the new models to broadband spectral data, we found that RX J0136.9-3510 and RE J0134+396 have similar {\it rms} spectra and broadband SEDs, representing a distinct spectral state which can only be attained by super Eddington flows.
A detailed optical and X-ray spectral analysis is then carried out for a big sample of 51 unobscured nearby type 1 AGNs. We find that NLS1s tend to have softer 2-10 keV spectra, lower black hole mass, higher Eddington ratio, higher index and smaller coronal radius. The edge of Balmer continuum is shifted redwards and smoothed by more than predicted by the FWHM of the Balmer emission lines. A new method called `Correlation Spectra Technique (CST)' is proposed, which is powerful for multi-waveband spectral analysis. We find that among the three Balmer line components, the broad component has the best correlation with hard X-ray emission. Optical oxygen forbidden lines all well correlate with the hard X-ray emission.
We conducted a systematic cross-correlation among the 9 key SED parameters: , , , , FWHM, M, , L and L, and found the driven parameters to be M, and L (or equivalently ). AGN's intrinsic SEDs exhibit strong diversity and changes similarly with , , , , FWHM and M. However, the SED shape is not sensitive to L
Probing the Interstellar Dust towards the Galactic Centre: Dust Scattering Halo around AX J1745.6-2901
AX J1745.6-2901 is an X-ray binary located at only 1.45 arcmin from Sgr A*,
showcasing a strong X-ray dust scattering halo. We combine Chandra and
XMM-Newton observations to study the halo around this X-ray binary. Our study
shows two major thick dust layers along the line of sight (LOS) towards AX
J1745.6-2901. The LOS position and of these two layers depend on the
dust grain models with different grain size distribution and abundances. But
for all the 19 dust grain models considered, dust Layer-1 is consistently found
to be within a fractional distance of 0.11 (mean value: 0.05) to AX
J1745.6-2901 and contains only (19-34)% (mean value: 26%) of the total LOS
dust. The remaining dust is contained in Layer-2, which is distributed from the
Earth up to a mean fractional distance of 0.64. A significant separation
between the two layers is found for all the dust grain models, with a mean
fractional distance of 0.31. Besides, an extended wing component is discovered
in the halo, which implies a higher fraction of dust grains with typical sizes
590 \AA\ than considered in current dust grain models. Assuming AX
J1745.6-2901 is 8 kpc away, dust Layer-2 would be located in the Galactic disk
several kpc away from the Galactic Centre (GC). The dust scattering halo biases
the observed spectrum of AX J1745.6-2901 severely in both spectral shape and
flux, and also introduces a strong dependence on the size of the instrumental
point spread function and the source extraction region. We build Xspec models
to account for this spectral bias, which allow us to recover the intrinsic
spectrum of AX J1745.6-2901 free from dust scattering opacity. If dust Layer-2
also intervenes along the LOS to Sgr A* and other nearby GC sources, a
significant spectral correction for the dust scattering opacity would be
necessary for all these GC sources.Comment: 20 pages, published by MNRAS; revised values in Table-1 and Table-B
Effects of Interstellar Dust Scattering on the X-ray Eclipses of the LMXB AX J1745.6-2901 in the Galactic Center
AX J1745.6-2901 is an eclipsing low mass X-ray binary (LMXB) in the Galactic
Centre (GC). It shows significant X-ray excess emission during the eclipse
phase, and its eclipse light curve shows an asymmetric shape. We use archival
XMM-Newton and Chandra observations to study the origin of these peculiar X-ray
eclipsing phenomena. We find that the shape of the observed X-ray eclipse light
curves depends on both photon energy and the shape of the source extraction
region, and also shows differences between the two instruments. By performing
detailed simulations for the time-dependent X-ray dust scattering halo, as well
as directly modelling the observed eclipse and non-eclipse halo profiles of AX
J1745.6-2901, we obtained solid evidence that its peculiar eclipse phenomena
are indeed caused by the X-ray dust scattering in multiple foreground dust
layers along the line-of-sight (LOS). The apparent dependence on the
instruments is caused by different instrumental point-spread-functions. Our
results can be used to assess the influence of dust scattering in other
eclipsing X-ray sources, and raise the importance of considering the timing
effects of dust scattering halo when studying the variability of other X-ray
sources in the GC, such as Sgr A*. Moreover, our study of halo eclipse
reinforces the existence of a dust layer local to AX J1745.6-2901 as reported
by Jin et al. (2017), as well as identifying another dust layer within a few
hundred parsecs to Earth, containing up to several tens of percent LOS dust,
which is likely to be associated with the molecular clouds in the Solar
neighbourhood. The remaining LOS dust is likely to be associated with the
molecular clouds located in the Galactic disk in-between.Comment: 25 pages, 18 figures, 5 tables, accepted by MNRA
Exploring the Interstellar Medium Using an Asymmetric X-ray Dust Scattering Halo
SWIFT J1658.2-4242 is an X-ray transient discovered recently in the Galactic
plane, with severe X-ray absorption corresponding to an equivalent hydrogen
column density of cm. Using new
Chandra and XMM-Newton data, we discover a strong X-ray dust scattering halo
around it. The halo profile can be well fitted by the scattering from at least
three separated dust layers. During the persistent emission phase of SWIFT
J1658.2-4242, the best-fit dust scattering based on the
COMP-AC-S dust grain model is consistent with . The best-fit
halo models show that 85-90 percent of the intervening gas and dust along the
line of sight of SWIFT J1658.2-4242 are located in the foreground ISM in the
Galactic disk. The dust scattering halo also shows significant azimuthal
asymmetry, which appears consistent with the inhomogeneous distribution of
foreground molecular clouds. By matching the different dust layers to the
distribution of molecular clouds along the line of sight, we estimate the
source distance to be 10 kpc, which is also consistent with the results
given by several other independent methods of distance estimation. The dust
scattering opacity and the existence of a halo can introduce a significant
spectral bias, the level of which depends on the shape of the instrumental
point spread function and the source extraction region. We create the Xspec
dscor model to correct for this spectral bias for different X-ray instruments.
Our study reenforces the importance of considering the spectral effects of dust
scattering in other absorbed X-ray sources.Comment: 21 pages, 13 figures, 6 tables, accepted for publication in Ap
Super-Eddington QSO RX J0439.6-5311 – II. Multiwavelength constraints on the global structure of the accretion flow
We present a detailed multiwavelength study of an unobscured, highly super-Eddington Type-1 QSO RX J0439.6-5311. We combine the latest XMM–Newton observation with all archival data from infrared to hard X-rays. The optical spectrum is very similar to that of 1H 0707-495 in having extremely weak [O III] and strong Fe II emission lines, although the black hole mass is probably slightly higher at 5-10 × 106 M⊙. The broad-band spectral energy distribution is uniquely well defined due to the extremely low Galactic and intrinsic absorption, so the bolometric luminosity is tightly constrained. The optical/UV accretion disc continuum is seen down to 900 Å, showing that there is a standard thin disc structure down to R ≥ 190–380 Rg and determining the mass accretion rate through the outer disc. This predicts a much higher bolometric luminosity than observed, indicating that there must be strong wind and/or advective energy losses from the inner disc, as expected for a highly super-Eddington accretion flow. Significant outflows are detected in both the narrow-line region (NLR) and broad-line region (BLR) emission lines, confirming the presence of a wind. We propose a global picture for the structure of a super-Eddington accretion flow where the inner disc puffs up, shielding much of the potential NLR material, and show how inclination angle with respect to this and the wind can explain very different X-ray properties of RX J0439.6-5311 and 1H 0707-495. Therefore, this source provides strong supporting evidence that ‘simple’ and ‘complex’ super-Eddington NLS1s can be unified within the same accretion flow scenario but with different inclination angles. We also propose that these extreme NLS1s could be the low-redshift analogues of weak emission-line quasars
Wavelength Dependences of the Optical/UV and X-ray Luminosity Correlations of Quasars
The inter-band correlations between optical/UV and X-ray luminosities of
active galactic nuclei (AGN) are important for understanding the disc-coronal
connection, as well as using AGN as standard candles for cosmology. It is
conventional to measure the X-ray luminosity at rest frame 2 keV and compare to
the UV luminosity at the rest-frame 2500 \AA, but the wavelength-dependence was
never well explored. In this work, we adopt a well-defined sample of 1169
unobscured quasars in the redshift range 0.13 - 4.51, and apply the
direct-correlation method to explore how the correlation with the 2 keV
luminosity changes at different optical/UV wavelengths, from 1280 - 5550 \AA\
where the spectral quality is high. We find that the luminosity at all UV
continuum wavelengths correlates with the X-ray luminosity similarly to that at
2500 \AA, and that these correlations are better than at the optical
wavelengths. Strong self-correlation is also found in the broadband optical/UV
continuum, supporting the scenario that it is dominated by the disc emission.
Correlations of various emission lines are also investigated (e.g. C IV, C
III], Mg II, H, [O III]), including the
Baldwin effect and correlations involving line-widths. We find the forms of
these line correlations are different, and they are also different from their
underlying continua, suggesting various complexities in the line-generation
process. We discuss these results in the disc-wind scenario. Our study confirms
that the rest-frame 2500 \AA\ is a good wavelength to represent the optical/UV
continual properties of quasars, and shows the advantages of the
direct-correlation method.Comment: 18 pages, 15 figures, accepted for publication in MNRA
A physically-based model of the ionizing radiation from active galaxies for photoionization modeling
We present a simplified model of Active Galactic Nucleus (AGN) continuum
emission designed for photoionization modeling. The new model {\sc oxaf}
reproduces the diversity of spectral shapes that arise in physically-based
models. We identify and explain degeneracies in the effects of AGN parameters
on model spectral shapes, with a focus on the complete degeneracy between the
black hole mass and AGN luminosity. Our re-parametrized model {\sc oxaf}
removes these degeneracies and accepts three parameters which directly describe
the output spectral shape: the energy of the peak of the accretion disk
emission , the photon power-law index of the non-thermal
emission , and the proportion of the total flux which is emitted in the
non-thermal component . The parameter is
presented as a function of the black hole mass, AGN luminosity, and `coronal
radius' of the {\sc optxagnf} model upon which {\sc oxaf} is based. We show
that the soft X-ray excess does not significantly affect photoionization
modeling predictions of strong emission lines in Seyfert narrow-line regions.
Despite its simplicity, {\sc oxaf} accounts for opacity effects where the
accretion disk is ionized because it inherits the `color correction' of {\sc
optxagnf}. We use a grid of {\sc mappings} photoionization models with {\sc
oxaf} ionizing spectra to demonstrate how predicted emission-line ratios on
standard optical diagnostic diagrams are sensitive to each of the three {\sc
oxaf} parameters. The {\sc oxaf} code is publicly available in the Astrophysics
Source Code Library.Comment: 14 pages, 9 figures, 1 table. Accepted for publication in Ap
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