159 research outputs found
Studying the X-ray hysteresis in GX 339-4: the disc and iron line over one decade
We report on a comprehensive and consistent investigation into the X-ray
emission from GX 339-4. All public observations in the 11 year RXTE archive
were analysed. Three different types of model - single powerlaw, broken
powerlaw and a disc + powerlaw - were fitted to investigate the evolution of
the disc, along with a fixed gaussian component at 6.4 keV to investigate any
iron line in the spectrum. We show that the relative variation in flux and
X-ray colour between the two best sampled outbursts are very similar. The decay
of the disc temperature during the outburst is clearly seen in the soft state.
The expected decay is S_Disc \propto T^4; we measure T^4.75\pm0.23. This
implies that the inner disc radius is approximately constant in the soft state.
We also show a significant anti-correlation between the iron line significant
width and the X-ray flux in the soft state while in the hard state the EW is
independent of the flux. This results in hysteresis in the relation between
X-ray flux and both line flux and EW. To compare the X-ray binary outburst to
the behaviour seen in AGN, we construct a Disc Fraction Luminosity Diagram for
GX 339-4, the first for an X-ray binary. The shape qualitatively matches that
produced for AGN. Linking this with the radio emission from GX 339-4 the change
in radio spectrum between the disc and power-law dominated states is clearly
visible.Comment: Accepted for publication in MNRAS, 20 pages, 17 figures. For high-res
version see http://www.astro.soton.ac.uk/~r.j.dunn/publications.htm
Magnetic field effects on neutrino production in microquasars
We investigate the effects of magnetic fields on neutrino production in
microquasars. We calculate the steady particle distributions for the pions and
muons generated in p-gamma and p-p interactions in the jet taking the effects
of all energy losses into account. The obtained neutrino emission is
significantly modified due to the synchrotron losses suffered by secondary
pions and muons. The estimates made for neutrino fluxes arriving on the Earth
imply that detection of high-energy neutrinos from the vicinity of the compact
object can be difficult. However, in the case of windy microquasars, the
interaction of energetic protons in the jet with matter of dense clumps of the
wind could produce detectable neutrinos. This is because the pions and muons at
larger distances from the compact object will not be affected by synchrotron
losses.Comment: 12 pages, 11 figures, accepted for publication in A&
The variability plane of accreting compact objects
Recently, it has been shown that soft-state black hole X-ray binaries and
active galactic nuclei populate a plane in the space defined by the black hole
mass, accretion rate and characteristic frequency. We show that this plane can
be extended to hard-state objects if one allows a constant offset for the
frequencies in the soft and the hard state. During a state transition the
frequencies rapidly move from one scaling to the other depending on an
additional parameter, possibly the disk-fraction. The relationship between
frequency, mass and accretion rate can be further extended by including weakly
accreting neutron stars. We explore if the lower kHz QPOs of neutron stars and
the dwarf nova oscillations of white dwarfs can be included as well and discuss
the physical implications of the found correlation.Comment: Accepted for publication in MNRA
The proton low-mass microquasar: high-energy emission
A population of unidentified gamma-ray sources is forming a structure
resembling a halo around the Galactic center. These sources are highly
variable, and hence they should be associated with compact objects.
Microquasars are objects undergoing accretion with relativistic jets; if such
an object has a low-mass, evolved, donor star, it might be found in the
Galactic halo. If these low-mass microquasars can generate detectable gamma-ray
emission, then they are natural candidates to account for the halo high-energy
sources. We aim to construct models for high-energy emission of low-mass
microquasars, which could produce a significant luminosity in the gamma-ray
domain. We consider that a significant fraction of the relativistic particles
in the jets of low-mass microquasars are protons and then we study the
production of high-energy emission through proton synchrotron radiation and
photopion production. Photopair production and leptonic processes are
considered as well. We compute a number of specific models with different
parameters to explore the possibilities of this scenario.} We find that
important luminosities, in the range of erg s, can be
achieved by proton synchrotron radiation in the Gamma-Ray Large Area Space
Telescope (GLAST) energy range, and lower, but still significant luminosities
at higher energies for some models. We conclude that the "proton microquasar"
model offers a very interesting alternative to account for the halo gamma-ray
sources and presents a variety of predictions that might be tested in the near
future by instruments like GLAST, the High-Energy Stereoscopic System II (HESS
II), the Major Atmospheric Gamma-ray Imaging Cherenkov telescope II (MAGIC II),
and neutrino telescopes like IceCube.Comment: 11 pages, 7 figures, final version, accepted for publication in A&
The balance of power: accretion and feedback in stellar mass black holes
In this review we discuss the population of stellar-mass black holes in our
galaxy and beyond, which are the extreme endpoints of massive star evolution.
In particular we focus on how we can attempt to balance the available accretion
energy with feedback to the environment via radiation, jets and winds,
considering also possible contributions to the energy balance from black hole
spin and advection. We review quantitatively the methods which are used to
estimate these quantities, regardless of the details of the astrophysics close
to the black hole. Once these methods have been outlined, we work through an
outburst of a black hole X-ray binary system, estimating the flow of mass and
energy through the different accretion rates and states. While we focus on
feedback from stellar mass black holes in X-ray binary systems, we also
consider the applicability of what we have learned to supermassive black holes
in active galactic nuclei. As an important control sample we also review the
coupling between accretion and feedback in neutron stars, and show that it is
very similar to that observed in black holes, which strongly constrains how
much of the astrophysics of feedback can be unique to black holes.Comment: To be published in Haardt et al. Astrophysical Black Holes. Lecture
Notes in Physics. Springer 201
An overview of jets and outflows in stellar mass black holes
In this book chapter, we will briefly review the current empirical
understanding of the relation between accretion state and and outflows in
accreting stellar mass black holes. The focus will be on the empirical
connections between X-ray states and relativistic (`radio') jets, although we
are now also able to draw accretion disc winds into the picture in a systematic
way. We will furthermore consider the latest attempts to measure/order jet
power, and to compare it to other (potentially) measurable quantities, most
importantly black hole spin.Comment: Accepted for publication in Space Science Reviews. Also to appear in
the Space Sciences Series of ISSI - The Physics of Accretion on to Black
Holes (Springer Publisher
XMM-Newton observations of ultraluminous X-ray sources in nearby galaxies
An XMM-Newton study of ultraluminous X-ray sources (ULX) has been performed
in a sample of 10 nearby Seyfert galaxies. Eighteen ULX have been found with
positional uncertainty of about 4". The large collecting area of XMM-Newton
makes the statistics sufficient to perform spectral fitting with simple models
in 8 cases. The main results of the present minisurvey strengthen the theory
that the ULX could be accreting black holes in hard or soft state. In some
cases, the contribution of the ULX to the overall X-ray flux appears to be
dominant with respect to that of the active nucleus. In addition, 6 ULX present
probable counterparts at other wavelengths (optical/infrared, radio). A
multiwavelength observing strategy is required to better assess the nature of
these sources.Comment: 10 pages, 11 figures. Accepted for publication by Astronomy and
Astrophysics. If you want figures at full resolution, please send an email to
Foschin
Robots that can adapt like animals
As robots leave the controlled environments of factories to autonomously
function in more complex, natural environments, they will have to respond to
the inevitable fact that they will become damaged. However, while animals can
quickly adapt to a wide variety of injuries, current robots cannot "think
outside the box" to find a compensatory behavior when damaged: they are limited
to their pre-specified self-sensing abilities, can diagnose only anticipated
failure modes, and require a pre-programmed contingency plan for every type of
potential damage, an impracticality for complex robots. Here we introduce an
intelligent trial and error algorithm that allows robots to adapt to damage in
less than two minutes, without requiring self-diagnosis or pre-specified
contingency plans. Before deployment, a robot exploits a novel algorithm to
create a detailed map of the space of high-performing behaviors: This map
represents the robot's intuitions about what behaviors it can perform and their
value. If the robot is damaged, it uses these intuitions to guide a
trial-and-error learning algorithm that conducts intelligent experiments to
rapidly discover a compensatory behavior that works in spite of the damage.
Experiments reveal successful adaptations for a legged robot injured in five
different ways, including damaged, broken, and missing legs, and for a robotic
arm with joints broken in 14 different ways. This new technique will enable
more robust, effective, autonomous robots, and suggests principles that animals
may use to adapt to injury
A broadband leptonic model for gamma-ray emitting microquasars
Observational and theoretical studies point to microquasars (MQs) as possible
counterparts of a significant fraction of the unidentified gamma-ray sources
detected so far. At present, a proper scenario to explain the emission beyond
soft X-rays from these objects is not known, nor what the precise connection is
between the radio and the high-energy radiation. We develop a new model where
the MQ jet is dynamically dominated by cold protons and radiatively dominated
by relativistic leptons. The matter content and power of the jet are both
related with the accretion process. The magnetic field is assumed to be close
to equipartition, although it is attached to and dominated by the jet matter.
For the relativistic particles in the jet, their maximum energy depends on both
the acceleration efficiency and the energy losses. The model takes into account
the interaction of the relativistic jet particles with the magnetic field and
all the photon and matter fields. Such interaction produces significant amounts
of radiation from radio to very high energies through synchrotron, relativistic
Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission
produced by changes in the accretion process (e.g. via orbital eccentricity) is
also expected. The effects of the gamma-ray absorption by the external photon
fields on the gamma-ray spectrum have been taken into account, revealing clear
spectral features that might be observed. This model is consistent to the
accretion scenario, energy conservation laws, and current observational
knowledge, and can provide deeper physical information of the source when
tested against multiwavelength data.Comment: 15 pages, 12 figures, A&A, in press (text and plots improved after
minor corrections in calculations, text improved also by referee comments
Discovery of a relation between black hole mass and soft X-ray time lags in active galactic nuclei
We carried out a systematic analysis of time lags between X-ray energy bands
in a large sample (32 sources) of unabsorbed, radio quiet active galactic
nuclei (AGN), observed by XMM-Newton. The analysis of X-ray lags (up to the
highest/shortest frequencies/time-scales), is performed in the
Fourier-frequency domain, between energy bands where the soft excess (soft
band) and the primary power law (hard band) dominate the emission. We report a
total of 15 out of 32 sources displaying a high frequency soft lag in their
light curves. All 15 are at a significance level exceeding 97 per cent and 11
are at a level exceeding 99 per cent. Of these soft lags, 7 have not been
previously reported in the literature, thus this work significantly increases
the number of known sources with a soft/negative lag. The characteristic
time-scales of the soft/negative lag are relatively short (with typical
frequencies and amplitudes of \nu\sim 0.07-4 \times 10^{-3} Hz and \tau\sim
10-600 s, respectively), and show a highly significant (\gsim 4\sigma)
correlation with the black hole mass. The measured correlations indicate that
soft lags are systematically shifted to lower frequencies and higher absolute
amplitudes as the mass of the source increases. To first approximation, all the
sources in the sample are consistent with having similar mass-scaled lag
properties. These results strongly suggest the existence of a mass-scaling law
for the soft/negative lag, that holds for AGN spanning a large range of masses
(about 2.5 orders of magnitude), thus supporting the idea that soft lags
originate in the innermost regions of AGN and are powerful tools for testing
their physics and geometry.Comment: 12 pages, 6 figures. Revised version, accepted for publication in
MNRA
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