1,627 research outputs found
Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia supernovae, accretion-induced collapse, ionization, and intermediate-mass black holes?
Observations of hundreds of supersoft x-ray sources (SSSs) in external
galaxies have shed light on the diversity of the class and on the natures of
the sources. SSSs are linked to the physics of Type Ia supernovae and
accretion-induced collapse, ultraluminous x-ray sources and black holes, the
ionization of the interstellar medium, and tidal disruption by supermassive
black holes. The class of SSSs has an extension to higher luminosities:
ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an
extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with
energies above 1 eV, but few or none with energies above 2 keV. Finally, a
significant fraction of the SSSs found in external galaxies switch states
between observations, becoming either quasisoft or hard. For many systems
``supersoft'' refers to a temporary state; SSSs are sources, possibly including
a variety of fundamentally different system types, that pass through such a
state. We review those results derived from extragalactic data and related
theoretical work that are most surprising and that suggest directions for
future research.Comment: submitted to Astron.Nachr.; latex, 6 figure
Radio Foregrounds for the 21cm Tomography of the Neutral Intergalactic Medium at High Redshifts
Absorption or emission against the cosmic microwave background radiation
(CMB) may be observed in the redshifted 21cm line if the spin temperature of
the neutral intergalactic medium prior to reionization differs from the CMB
temperature. This so-called 21cm tomography should reveal important information
on the physical state of the intergalactic medium at high redshifts. The
fluctuations in the redshifted 21 cm, due to gas density inhomogeneities at
early times, should be observed at meter wavelengths by the next generation
radio telescopes such as the proposed {\it Square Kilometer Array (SKA)}. Here
we show that the extra-galactic radio sources provide a serious contamination
to the brightness temperature fluctuations expected in the redshifted 21 cm
emission from the IGM at high redshifts. Unless the radio source population
cuts off at flux levels above the planned sensitivity of SKA, its clustering
noise component will dominate the angular fluctuations in the 21 cm signal. The
integrated foreground signal is smooth in frequency space and it should
nonetheless be possible to identify the sharp spectral feature arising from the
non-uniformities in the neutral hydrogen density during the epoch when the
first UV sources reionize the intergalactic medium.Comment: 5 pages emulateapj with 1 figure, accepted to Ap
The formation of compact massive self-gravitating discs in metal-free haloes with virial temperatures of ~ 13000-30000 K
We have used the hydrodynamical AMR code ENZO to investigate the dynamical
evolution of the gas at the centre of dark matter haloes with virial velocities
of ~ 20 - 30 kms and virial temperatures of ~ 13000-30000 K at z ~ 15 in a
cosmological context. The virial temperature of the dark matter haloes is above
the threshold where atomic cooling by hydrogen allows the gas to cool and
collapse. We neglect cooling by molecular hydrogen and metals, as may be
plausible if H_2 cooling is suppressed by a meta-galactic Lyman-Werner
background or an internal source of Lyman-Werner photons, and metal enrichment
has not progressed very far. The gas in the haloes becomes gravitationally
unstable and develops turbulent velocities comparable to the virial velocities
of the dark matter haloes. Within a few dynamical times it settles into a
nearly isothermal density profile over many decades in radius losing most of
its angular momentum in the process. About 0.1 - 1 % of the baryons, at the
centre of the dark matter haloes, collapse into a self-gravitating, fat,
ellipsoidal, centrifugally supported exponential disc with scale-length of ~
0.075-0.27 pc and rotation velocities of 25-60 kms. We are able to follow the
settling of the gas into centrifugal support and the dynamical evolution of the
compact disc in each dark matter halo for a few dynamical times. The dynamical
evolution of the gas at the centre of the haloes is complex. In one of the
haloes the gas at the centre fragments into a triple system leading to strong
tidal perturbations and eventually to the in-fall of a secondary smaller clump
into the most massive primary clump. The formation of centrifugally supported
self-gravitating massive discs is likely to be an important intermediary stage
en route to the formation of a massive black hole seed.Comment: Re-submitted to MNRAS taking into account the referee's suggestions
for moderate revision. 16 pages, 11 figure
Police and thieves in the stadium: measuring the (multiple) effects of football matches on crime
Large sporting events affect criminal behaviour via three channels: fan concentration, self-incapacitation and police displacement. I exploit information on football matches for London teams linked to detailed recorded crime data at the area level to estimate these effects empirically. I find that only property crime increases in the communities hosting matches but not violent offences. There is a negative away game attendance effect on crime which is due to voluntary incapacitation of potential offenders attending a match. Police displacement during home games increases property crime by 7 percentage points for every extra 10000 supporters in areas that are left underprotecte
A Theory of Challenge and Threat States in Athletes: a revised conceptualization
The Theory of Challenge and Threat States in Athletes (TCTSA) provides a psychophysiological framework for how athletes anticipate motivated performance situations. The purpose of this review is to discuss how research has addressed the 15 predictions made by the TCTSA, to evaluate the mechanisms underpinning the TCTSA in light of the research that has emerged in the last ten years, and to inform a revised TCTSA (TCTSA-R). There was support for many of the 15 predictions in the TCTSA, with two main areas for reflection identified; to understand the physiology of challenge and to re-evaluate the concept of resource appraisals. This re-evaluation informs the TCTSA-R which elucidates the physiological changes, predispositions, and cognitive appraisals that mark challenge and threat states. First, the relative strength of the sympathetic nervous system response is outlined as a determinant of challenge and threat patterns of reactivity and we suggest that oxytocin and neuropeptide Y are also key indicators of an adaptive approach to motivated performance situations and can facilitate a challenge state. Second, although predispositions were acknowledged within the TCTSA, how these may influence challenge and threat states was not specified. In the TCTSA-R it is proposed that one’s propensity to appraise stressors as a challenge that most strongly dictates acute cognitive appraisals. Third, in the TCTSA-R a more parsimonious integration of Lazarusian ideas of cognitive appraisal and challenge and threat is proposed. Given that an athlete can make both challenge and threat primary appraisals and can have both high or low resources compared to perceived demands, a 2x2 bifurcation theory of challenge and threat is proposed. This reflects polychotomy of four parts; high challenge, low challenge, low threat, and high threat. For example, in low threat, an athlete can evince a threat state but still perform well so long as they perceive high resources. Consequently, we propose suggestions for research concerning measurement tools and a reconsideration of resources to include social support. Finally, applied recommendations are made based on adjusting demands and enhancing resources.N/
Jet-disc coupling through a common energy reservoir in the black hole XTE J1118+480
We interpret the rapid correlated UV/optical/ X-ray variability of XTE
J1118+480 as a signature of the coupling between the X-ray corona and a jet
emitting synchrotron radiation in the optical band. We propose a scenario in
which the jet and the X-ray corona are fed by the same energy reservoir where
large amounts of accretion power are stored before being channelled into either
the jet or the high energy radiation. This time dependent model reproduces the
main features of the rapid multi-wavelength variability of XTE J1118+480.
Assuming that the energy is stored in the form of magnetic field, we find that
the required values of the model parameters are compatible with both a patchy
corona atop a cold accretion disc and a hot thick inner disc geometry. The
range of variability timescales for the X-ray emitting plasma are consistent
with the dynamical times of an accretion flow between 10 and 100 Schwarzschild
radii. On the other hand, the derived range of timescales associated with the
dissipation in the jet extends to timescales more than 10 times larger,
confirming the suggestion that the generation of a powerful outflow requires
large scale coherent poloidal field structures. A strong requirement of the
model is that the total jet power should be at least a few times larger than
the observed X-ray luminosity. This would be consistent with the overall low
radiative efficiency of the source. We present independent arguments showing
that the jet probably dominates the energetic output of all accreting black
holes in the low-hard state.Comment: 14 pages, 2 figures, to appear in MNRA
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