2,643 research outputs found
Weighing the black holes in ultraluminous X-ray sources through timing
We describe a new method to estimate the mass of black holes in Ultraluminous
X-ray Sources (ULXs). The method is based on the recently discovered
``variability plane'', populated by Galactic stellar-mass black-hole candidates
(BHCs) and supermassive active galactic nuclei (AGNs), in the parameter space
defined by the black-hole mass, accretion rate and characteristic frequency. We
apply this method to the two ULXs from which low-frequency quasi-periodic
oscillations have been discovered, M82 X-1 and NGC 5408 X-1. For both sources
we obtain a black-hole mass in the range 100~1300 Msun, thus providing evidence
for these two sources to host an intermediate-mass black hole.Comment: 5 pages, 2 figures, Accepted by MNRA
Regioselectivity of aryl azide cycloaddition to methyl propiolate in aqueous media: Experimental evidence versus local DFT HSAB principle
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XMM-Newton study of the complex and variable spectrum of NGC 4051
We study the X-ray spectral variability of the Narrow Line Seyfert 1 galaxy
NGC 4051 as observed during two XMM-Newton observations. The data show evidence
for a neutral and constant reflection component and for constant emission from
photoionized gas, which are included in all spectral models. The nuclear
emission can be modelled both in terms of a ``standard model'' (pivoting power
law plus a black body component for the soft excess) and of a two--component
one (power law plus ionized reflection from the accretion disc). The standard
model results indicate that the soft excess does not follow the standard black
body law. Moreover, although the spectral slope is correlated with flux, which
is consistent with spectral pivoting, the hardest photon indexes are so flat as
to require rather unusual scenarios. These problems can be solved in terms of
the two-component model in which the soft excess is not thermal, but due to the
ionized reflection component. The variability of the reflection component from
the inner disc closely follows the predictions of the light bending model,
suggesting that most of the primary nuclear emission is produced in the very
innermost regions, only a few gravitational radii from the central black hole.
(abridged)Comment: accepted for publication in MNRA
Error Cascades in Observational Learning: An Experiment on the Chinos Game
The paper reports an experimental study based on a variant of the popular Chinos game, which is used as a simple but paradigmatic instance of observational learning. There are three players, arranged in sequence, each of whom wins a fixed price if she manages to guess the total number of coins lying in everybodyâs hands. Our evidence shows that, despite the remarkable frequency of equilibrium outcomes, deviations from optimal play are also significant. And when such deviations occur, we find that, for any given player position, the probability of a mistake is increasing in the probability of a mistake of her predecessors. This is what we call an error cascade, which we rationalize by way of a simple model of ânoisy equilibriumâ.positional learning, error cascades
Probing 3D Density and Velocity Fields of ISM in Centers of Galaxies with Future X-Ray Observations
Observations of bright and variable "reflected" X-ray emission from molecular
clouds located within inner hundred parsec of our Galaxy have demonstrated that
the central supermassive black hole, Sgr A*, experienced short and powerful
flares in the past few hundred years. These flares offer a truly unique
opportunity to determine 3D location of the illuminated clouds (with ~10 pc
accuracy) and to reveal their internal structure (down to 0.1 pc scales). Short
duration of the flare(s), combined with X-rays high penetration power and
insensitivity of the reflection signal to thermo- and chemo-dynamical state of
the gas, ensures that the provided diagnostics of the density and velocity
fields is unbiased and almost free of the projection and opacity effects. Sharp
and sensitive snapshots of molecular gas accessible with aid of future X-ray
observatories featuring large collecting area and high angular (arcsec-level)
and spectral (eV-level) resolution cryogenic bolometers will present invaluable
information on properties of the supersonic turbulence inside the illuminated
clouds, map their shear velocity field and allow cross-matching between X-ray
data and velocity-resolved emission of various molecular species provided by
ALMA and other ground-based facilities. This will highlight large and
small-scale dynamics of the dense gas and help uncovering specifics of the ISM
lifecycle and high-mass star formation under very extreme conditions of
galactic centers. While the former is of particular importance for the SMBH
feeding and triggering AGN feedback, the latter might be an excellent test case
for star formation taking place in high-redshift galaxies.Comment: White paper submitted to the Astro2020 Decadal Surve
Variation of the X-ray non-thermal emission in the Arches cloud
The origin of the iron fluorescent line at 6.4 keV from an extended region
surrounding the Arches cluster is debated and the non-variability of this
emission up to 2009 has favored the low-energy cosmic-ray origin over a
possible irradiation by hard X-rays. By probing the variability of the Arches
cloud non-thermal emission in the most recent years, including a deep
observation in 2012, we intend to discriminate between the two competing
scenarios. We perform a spectral fit of XMM-Newton observations collected from
2000 to 2013 in order to build the Arches cloud lightcurve corresponding to
both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a
30% flux drop in 2012, detected with more than 4 sigma significance for both
components. This implies that a large fraction of the studied non-thermal
emission is due to the reflection of an X-ray transient source.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
A tidal disruption flare in a massive galaxy? Implications for the fuelling mechanisms of nuclear black holes
We argue that the `changing look' AGN recently reported by LaMassa et al.
could be a luminous flare produced by the tidal disruption of a super-solar
mass star passing just a few gravitational radii outside the event horizon of a
nuclear black hole. This flare occurred in a massive,
star forming galaxy at redshift , robustly characterized thanks to
repeated late-time photometric and spectroscopic observations. By taking
difference-photometry of the well sampled multi-year SDSS Stripe-82
light-curve, we are able to probe the evolution of the nuclear spectrum over
the course of the outburst. The tidal disruption event (TDE) interpretation is
consistent with the very rapid rise and the decay time of the flare, which
displays an evolution consistent with the well-known behaviour (with
a clear superimposed re-brightening flare). Our analysis places constraints on
the physical properties of the TDE, such as the putative disrupted star's mass
and orbital parameters, as well as the size and temperature of the emitting
material. The properties of the broad and narrow emission lines observed in two
epochs of SDSS spectra provide further constraints on the circum-nuclear
structure, and could be indicative that the system hosted a moderate-luminosity
AGN as recently as a few years ago, and is likely undergoing residual
accretion as late as ten years after peak, as seen from the broad H
emission line. We discuss the complex interplay between tidal disruption events
and gas accretion episodes in galactic nuclei, highlighting the implications
for future TDE searches and for estimates of their intrinsic rates.Comment: 20 pages, 9 figures, 3 tables. Accepted for publication in MNRA
Evolution of the disc atmosphere in the X-ray binary MXB 1659-298, during its 2015-2017 outburst
We report on the evolution of the X-ray emission of the accreting neutron
star (NS) low mass X-ray binary (LMXB), MXB 1659-298, during its most recent
outburst in 2015-2017. We detected 60 absorption lines during the soft state
(of which 21 at more than 3 ), that disappeared in the hard state
(e.g., the Fe xxv and Fe xxvi lines). The absorbing plasma is at rest, likely
part of the accretion disc atmosphere. The bulk of the absorption features can
be reproduced by a high column density () of highly
ionised () plasma. Its disappearance during the
hard state is likely the consequence of a thermal photo-ionisation instability.
MXB 1659-298's continuum emission can be described by the sum of an absorbed
disk black body and its Comptonised emission, plus a black body component. The
observed spectral evolution with state is in line with that typically observed
in atoll and stellar mass black hole LMXB. The presence of a relativistic Fe
K disk-line is required during the soft state. We also tentatively
detect the Fe xxii doublet, whose ratio suggests an electron density of the
absorber of , hence, the absorber is likely located at
from the illuminating source, well inside the Compton and
outer disc radii. MXB 1659-298 is the third well monitored atoll LMXB
showcasing intense Fe xxv and Fe xxvi absorption during the soft state that
disappears during the hard state.Comment: MNRAS in pres
Echoes of multiple outbursts of Sagittarius A* revealed by Chandra
The relatively rapid spatial and temporal variability of the X-ray radiation
from some molecular clouds near the Galactic center shows that this emission
component is due to the reflection of X-rays generated by a source that was
luminous in the past, most likely the central supermassive black hole,
Sagittarius A*. Studying the evolution of the molecular cloud reflection
features is therefore a key element to reconstruct Sgr A*'s past activity. The
aim of the present work is to study this emission on small angular scales in
order to characterize the source outburst on short time scales. We use Chandra
high-resolution data collected from 1999 to 2011 to study the most rapid
variations detected so far, those of clouds between 5' and 20' from Sgr A*
towards positive longitudes. Our systematic spectral-imaging analysis of the
reflection emission, notably of the Fe Kalpha line at 6.4 keV and its
associated 4-8 keV continuum, allows us to characterize the variations down to
15" angular scale and 1-year time scale. We reveal for the first time abrupt
variations of few years only and in particular a short peaked emission, with a
factor of 10 increase followed by a comparable decrease, that propagates along
the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts
with the slower 10-year linear variations we reveal in all the other molecular
structures of the region. Based on column density constraints, we argue that
these two different behaviors are unlikely to be due to the same illuminating
event. The variations are likely due to a highly variable active phase of Sgr
A* sometime within the past few hundred years, characterized by at least two
luminous outbursts of a few-year time scale and during which the Sgr A*
luminosity went up to at least 10^39 erg/s.Comment: 17 pages, 16 figures, Accepted for publication in Astronomy &
Astrophysic
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