193 research outputs found
AGN spectral states from simultaneous UV and X-ray observations by XMM-Newton
The supermassive black holes in active galactic nuclei (AGN) and stellar-mass
black holes in X-ray binaries (XRBs) are believed to work in a similar way.
While XRBs evolve rapidly and several sources have undergone a few complete
cycles from quiescence to an outburst and back, most AGN remain in the same
state over periods of decades, due to their longer characteristic timescale
proportional to their size. However, the study of the AGN spectral states is
still possible with a large sample of sources. Multi-wavelength observations
are needed for this purpose since the AGN thermal disc emission dominates in
the ultraviolet energy range, while the up-scattered hot-corona emission is
detected in X-rays. We compared simultaneous UV and X-ray measurements of AGN
obtained by the XMM-Newton satellite. The non-thermal flux was constrained from
the 2-12 keV X-ray luminosity, while the thermal disc component was estimated
from the UV flux at 2900A. The hardness (ratio between the X-ray and UV plus
X-ray luminosity) and the total luminosity were used to construct the AGN state
diagrams. For sources with reliable mass measurements, the Eddington ratio was
used instead of the total luminosity. The state diagrams show that the
radio-loud sources have on average higher hardness, due to the lack of the
thermal disc emission in the UV band, and have flatter intrinsic X-ray spectra.
In contrast, the sources with high luminosity and low hardness are radio-quiet
AGN. The hardness-Eddington ratio diagram reveals that the average
radio-loudness is stronger for low-accreting sources, while it decreases when
the accretion rate is close to the Eddington limit. Our results indicate that
the general properties of AGN accretion states are similar to those of X-ray
binaries. This suggests that the AGN radio dichotomy of radio-loud and
radio-quiet sources can be explained by the evolution of the accretion states.Comment: 13 pages, 12 figures, accepted in A&
An X-ray variable absorber within the Broad Line Region in Fairall 51
Fairall 51 is a polar-scattered Seyfert 1 galaxy, a type of active galaxies
believed to represent a bridge between unobscured type-1 and obscured type-2
objects. Fairall 51 has shown complex and variable X-ray absorption but only
little is known about its origin. In our research, we observed Fairall 51 with
the X-ray satellite Suzaku in order to constrain a characteristic time-scale of
its variability. We performed timing and spectral analysis of four observations
separated by 1.5, 2 and 5.5 day intervals. We found that the 0.5-50 keV
broadband X-ray spectra are dominated by a primary power-law emission (with the
photon index ~ 2). This emission is affected by at least three absorbers with
different ionisations (log(xi) ~ 1-4). The spectrum is further shaped by a
reprocessed emission, possibly coming from two regions -- the accretion disc
and a more distant scattering region. The accretion disc emission is smeared by
the relativistic effects, from which we measured the spin of the black hole as
a ~ 0.8 (+-0.2). We found that most of the spectral variability can be
attributed to the least ionised absorber whose column density changed by a
factor of two between the first (highest-flux) and the last (lowest-flux)
observation. A week-long scale of the variability indicates that the absorber
is located at the distance ~ 0.05 pc from the centre, i.e., in the Broad Line
Region.Comment: 12 pages, 9 figures, accepted to A&
Active galaxy 4U 1344-60: did the relativistic line disappear?
X-ray bright active galactic nuclei represent a unique astrophysical
laboratory for studying accretion physics around super-massive black holes. 4U
1344-60 is a bright Seyfert galaxy which revealed relativistic reflection
features in the archival XMM-Newton observation. We present the spectroscopic
results of new data obtained with the Suzaku satellite and compare them with
the previous XMM-Newton observation. The X-ray continuum of 4U 1344-60 can be
well described by a power-law component with the photon index ~ 1.7 modified by
a fully and a partially covering local absorbers. We measured a substantial
decrease of the fraction of the partially absorbed radiation from around 45% in
the XMM-Newton observation to less than 10% in the Suzaku observation while the
power-law slope remains constant within uncertainties. The iron line in the
Suzaku spectrum is relatively narrow, keV, without any
suggestion for relativistic broadening. Regarding this, we interpret the iron
line in the archival XMM-Newton spectrum as a narrow line of the same width
plus an additional red-shifted emission around 6.1 keV. No evidence of the
relativistic reflection is present in the Suzaku spectra. The detected
red-shifted iron line during the XMM-Newton observation could be a temporary
feature either due to locally enhanced emission or decreased ionisation in the
innermost accretion flow.Comment: 10 pages, 11 figures, accepted to A&
Star-formation rate and stellar mass calibrations based on infrared photometry and their dependence on stellar population age and extinction
The stellar mass () and the star-formation rate (SFR) are among the
most important features that characterize galaxies. Measuring these fundamental
properties accurately is critical for understanding the present state of
galaxies, and their history. This work explores the dependence of the IR
emission of galaxies on their extinction, and the age of their stellar
populations (SPs). It aims at providing accurate IR SFR and
calibrations that account for SP age and extinction while quantifying their
scatter. We use the CIGALE spectral energy distribution (SED) fitting code to
create models of galaxies with a wide range of star-formation histories, dust
content, and interstellar medium properties. We fit the relations between
and SFR with IR and optical photometry of the model-galaxy SEDs with
the MCMC method, and perform a machine-learning random forest analysis on the
same data set in order to validate the latter. This work provides calibrations
for the SFR using a combination of the WISE bands 1 and 3, or the JWST F200W
and F2100W bands. It also provides mass-to-light ratio calibrations based on
the WISE band-1, or the JWST band F200W, along with the optical or
colors. These calibrations account for the biases attributed to the SP age,
while they are given in the form of extinction-dependent and
extinction-independent relations. They show robust estimations while minimizing
the scatter and biases throughout a wide range of SFRs and stellar masses. The
SFR calibration offers better results, especially in dust-free or passive
galaxies where the contributions of old SPs or biases from the lack of dust are
significant. Similarly, the calibration yields significantly better
results for dusty/high-SFR galaxies where dust emission can otherwise bias the
estimations.Comment: 18 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysics on 16 March 202
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