512 research outputs found
Evidence for a Variable Ultrafast Outflow in the Newly Discovered Ultraluminous Pulsar NGC 300 ULX-1
Ultraluminous pulsars are a definite proof that persistent super-Eddington
accretion occurs in nature. They support the scenario according to which most
Ultraluminous X-ray Sources (ULXs) are super-Eddington accretors of stellar
mass rather than sub-Eddington intermediate mass black holes. An important
prediction of theories of supercritical accretion is the existence of powerful
outflows of moderately ionized gas at mildly relativistic speeds. In practice,
the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is
required to resolve their observational signatures in ULXs. Using RGS, outflows
have been discovered in the spectra of 3 ULXs (none of which are currently
known to be pulsars). Most recently, the fourth ultraluminous pulsar was
discovered in NGC 300. Here we report detection of an ultrafast outflow (UFO)
in the X-ray spectrum of the object, with a significance of more than
3{\sigma}, during one of the two simultaneous observations of the source by
XMM-Newton and NuSTAR in December 2016. The outflow has a projected velocity of
65000 km/s (0.22c) and a high ionisation factor with a log value of 3.9. This
is the first direct evidence for a UFO in a neutron star ULX and also the first
time that this its evidence in a ULX spectrum is seen in both soft and hard
X-ray data simultaneously. We find no evidence of the UFO during the other
observation of the object, which could be explained by either clumpy nature of
the absorber or a slight change in our viewing angle of the accretion flow.Comment: 10 pages, 4 figures. Accepted to MNRA
A growth-rate indicator for Compton-thick active galactic nuclei
Due to their heavily obscured central engines, the growth rate of
Compton-thick (CT) active galactic nuclei (AGN) is difficult to measure. A
statistically significant correlation between the Eddington ratio,
{\lambda}, and the X-ray power-law index, {\Gamma}, observed in
unobscured AGN offers an estimate of their growth rate from X-ray spectroscopy
(albeit with large scatter). However, since X-rays undergo reprocessing by
Compton scattering and photoelectric absorption when the line-of-sight to the
central engine is heavily obscured, the recovery of the intrinsic {\Gamma} is
challenging. Here we study a sample of local, predominantly Compton-thick
megamaser AGN, where the black hole mass, and thus Eddington luminosity, are
well known. We compile results on X-ray spectral fitting of these sources with
sensitive high-energy (E> 10 keV) NuSTAR data, where X-ray torus models which
take into account the reprocessing effects have been used to recover the
intrinsic {\Gamma} values and X-ray luminosities, L. With a simple
bolometric correction to L to calculate {\lambda}, we find a
statistically significant correlation between {\Gamma} and {\lambda} (p
= 0.007). A linear fit to the data yields {\Gamma} =
(0.410.18)log{\lambda}+(2.38 0.20), which is
statistically consistent with results for unobscured AGN. This result implies
that torus modeling successfully recovers the intrinsic AGN parameters. Since
the megamasers have low-mass black holes (M M)
and are highly inclined, our results extend the {\Gamma}-{\lambda}
relationship to lower masses and argue against strong orientation effects in
the corona, in support of AGN unification. Finally this result supports the use
of {\Gamma} as a growth-rate indicator for accreting black holes, even for
Compton-thick AGN.Comment: Accepted for publication in Ap
Heavy X-ray obscuration in the most-luminous galaxies discovered by WISE
Hot Dust-Obscured Galaxies (Hot DOGs) are hyperluminous
() infrared galaxies with
extremely high (up to hundreds of K) dust temperatures. The sources powering
both their extremely high luminosities and dust temperatures are thought to be
deeply buried and rapidly accreting supermassive black holes (SMBHs). Hot DOGs
could therefore represent a key evolutionary phase in which the SMBH growth
peaks. X-ray observations can be used to study their obscuration levels and
luminosities. In this work, we present the X-ray properties of the 20
most-luminous () known Hot DOGs at
. Five of them are covered by long-exposure ( ks) Chandra and
XMM-Newton observations, with three being X-ray detected, and we study their
individual properties. One of these sources (W01160505) is a Compton-thick
candidate, with column density
derived from X-ray spectral fitting. The remaining 15 Hot DOGs have been
targeted by a Chandra snapshot (3.1 ks) survey. None of these 15 is
individually detected; therefore we applied a stacking analysis to investigate
their average emission. From hardness-ratio analysis, we constrained the
average obscuring column density and intrinsic luminosity to be
log and
, which are consistent with
results for individually detected sources. We also investigated the
and relations, finding hints that Hot
DOGs are typically X-ray weaker than expected, although larger samples of
luminous obscured QSOs are needed to derive solid conclusions.Comment: MNRAS, accepted 2017 November 29 . Received 2017 November 29 ; in
original form 2017 October 11. 15 pages, 6 figure
The 2-79 keV X-ray Spectrum of the Circinus Galaxy with NuSTAR, XMM-Newton and Chandra: a Fully Compton-Thick AGN
The Circinus galaxy is one of the nearest obscured AGN, making it an ideal
target for detailed study. Combining archival Chandra and XMM-Newton data with
new NuSTAR observations, we model the 2-79 keV spectrum to constrain the
primary AGN continuum and to derive physical parameters for the obscuring
material. Chandra's high angular resolution allows a separation of nuclear and
off-nuclear galactic emission. In the off-nuclear diffuse emission we find
signatures of strong cold reflection, including high equivalent-width neutral
Fe lines. This Compton-scattered off-nuclear emission amounts to 18% of the
nuclear flux in the Fe line region, but becomes comparable to the nuclear
emission above 30 keV. The new analysis no longer supports a prominent
transmitted AGN component in the observed band. We find that the nuclear
spectrum is consistent with Compton-scattering by an optically-thick torus,
where the intrinsic spectrum is a powerlaw of photon index ,
the torus has an equatorial column density of cm and the intrinsic AGN keV luminosity is
erg/s. These values place Circinus along the same
relations as unobscured AGN in accretion rate-vs- and -vs-
phase space. NuSTAR's high sensitivity and low background allow us to study the
short time-scale variability of Circinus at X-ray energies above 10 keV for the
first time. The lack of detected variability favors a Compton-thick absorber,
in line with the the spectral fitting results.Comment: Accepted for publication in Ap
A model checking approach to the parameter estimation of biochemical pathways
Model checking has historically been an important tool to
verify models of a wide variety of systems. Typically a model has to exhibit
certain properties to be classed āacceptableā. In this work we use
model checking in a new setting; parameter estimation. We characterise
the desired behaviour of a model in a temporal logic property and alter
the model to make it conform to the property (determined through
model checking). We have implemented a computational system called
MC2(GA) which pairs a model checker with a genetic algorithm. To
drive parameter estimation, the fitness of set of parameters in a model is
the inverse of the distance between its actual behaviour and the desired
behaviour. The model checker used is the simulation-based Monte Carlo
Model Checker for Probabilistic Linear-time Temporal Logic with numerical
constraints, MC2(PLTLc). Numerical constraints as well as the
overall probability of the behaviour expressed in temporal logic are used
to minimise the behavioural distance. We define the theory underlying
our parameter estimation approach in both the stochastic and continuous
worlds. We apply our approach to biochemical systems and present
an illustrative example where we estimate the kinetic rate constants in
a continuous model of a signalling pathway
The nature of the torus in the heavily obscured AGN Markarian 3: an X-ray study
In this paper we report the results of an X-ray monitoring campaign on the
heavily obscured Seyfert galaxy Markarian 3 carried out between the fall of
2014 and the spring of 2015 with NuSTAR, Suzaku and XMM-Newton. The hard X-ray
spectrum of Markarian 3 is variable on all the time scales probed by our
campaign, down to a few days. The observed continuum variability is due to an
intrinsically variable primary continuum seen in transmission through a large,
but still Compton-thin column density (N_H~0.8-1.110 cm).
If arranged in a spherical-toroidal geometry, the Compton scattering matter has
an opening angle ~66 degrees and is seen at a grazing angle through its upper
rim (inclination angle ~70 degrees). We report a possible occultation event
during the 2014 campaign. If the torus is constituted by a system of clouds
sharing the same column density, this event allows us to constrain their number
(175) and individual column density, [~(4.91.5)10
cm]. The comparison of IR and X-ray spectroscopic results with
state-of-the art "torus" models suggests that at least two thirds of the X-ray
obscuring gas volume might be located within the dust sublimation radius. We
report also the discovery of an ionized absorber, characterised by variable
resonant absorption lines due to He- and H-like iron. This discovery lends
support to the idea that moderate column density absorbers could be due to
clouds evaporated at the outer surface of the torus, possibly accelerated by
the radiation pressure due to the central AGN emission leaking through the
patchy absorber.Comment: Accepted for publication in MNRAS, 17 pages, 11 figures, 5 table
X-ray bolometric corrections for Compton-thick active galactic nuclei
We present X-ray bolometric correction factors, (), for Compton-thick (CT) active galactic nuclei (AGN) with the aim
of testing AGN torus models, probing orientation effects, and estimating the
bolometric output of the most obscured AGN. We adopt bolometric luminosities,
, from literature infrared (IR) torus modeling and compile published
intrinsic 2--10 keV X-ray luminosities, , from X-ray torus modeling of
NuSTAR data. Our sample consists of 10 local CT AGN where both of these
estimates are available. We test for systematic differences in
values produced when using two widely used IR torus models and two widely used
X-ray torus models, finding consistency within the uncertainties. We find that
the mean of our sample in the range
erg/s is log
with an intrinsic scatter of dex, and that our derived
values are consistent with previously established relationships between
and and and Eddington ratio. We
investigate if is dependent on by comparing our results on
CT AGN to published results on less-obscured AGN, finding no significant
dependence. Since many of our sample are megamaser AGN, known to be viewed
edge-on, and furthermore under the assumptions of AGN unification whereby
unobscured AGN are viewed face-on, our result implies that the X-ray emitting
corona is not strongly anisotropic. Finally, we present values
for CT AGN identified in X-ray surveys as a function of their observed ,
where an estimate of their intrinsic is not available, and redshift,
useful for estimating the bolometric output of the most obscured AGN across
cosmic time.Comment: Accepted for publication in Ap
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