7,077 research outputs found
Quasi-Periodic Oscillations and energy spectra from the two brightest Ultra-Luminous X-ray sources in M82
Ultra-Luminous X-ray sources are thought to be accreting black holes that
might host Intermediate Mass Black Holes (IMBH), proposed to exist by
theoretical studies, even though a firm detection (as a class) is still
missing. The brightest ULX in M82 (M82 X-1) is probably one of the best
candidates to host an IMBH. In this work we analyzed the data of the recent
release of observations obtained from M82 X-1 taken by XMM-Newton. We performed
a study of the timing and spectral properties of the source. We report on the
detection of (46+-2) mHz Quasi-Periodic Oscillations (QPOs) in the power
density spectra of two observations. A comparison of the frequency of these
high-frequency QPOs with previous detections supports the 1:2:3 frequency
distribution as suggested in other studies. We discuss the implications if the
(46+-2) mHz QPO detected in M82 X-1 is the fundamental harmonic, in analogy
with the High-Frequency QPOs observed in black hole binaries. For one of the
observations we have detected for the first time a QPO at 8 mHz (albeit at a
low significance), that coincides with a hardening of the spectrum. We suggest
that the QPO is a milli-hertz QPO originating from the close-by transient ULX
M82 X-2, with analogies to the Low-Frequency QPOs observed in black hole
binaries.Comment: 9 pages (with 4 figures and 4 tables). Accepted for publication in
MNRAS (26/09/13
Playing Quantum Physics Jeopardy with zero-energy eigenstates
We describe an example of an exact, quantitative Jeopardy-type quantum
mechanics problem. This problem type is based on the conditions in
one-dimensional quantum systems that allow an energy eigenstate for the
infinite square well to have zero curvature and zero energy when suitable Dirac
delta functions are added. This condition and its solution are not often
discussed in quantum mechanics texts and have interesting pedagogical
consequences.Comment: 8 pages, 3 figures, requires graphicx and epsfig packages. Additional
information, including individual files containing the Worksheet and a
Worksheet template, are available at
http://webphysics.davidson.edu/mjb/jeopardy
A Unified Description of the Timing Features of Accreting X-ray Binaries
We study an empirical model for a unified description of the power spectra of
accreting neutron stars and black holes. This description is based on a
superposition of multiple Lorentzians and offers the advantage that all QPO and
noise components are dealt with in the same way, without the need of deciding
in advance the nature of each component. This approach also allows us to
compare frequencies of features with high and low coherences in a consistent
manner and greatly facilitates comparison of power spectra across a wide range
of source types and states. We apply the model to six sources, the
low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269 and GS 1826-24, the
high-latitude transient XTE J1118+480, the bright system Cir X-1, and the Z
source GX 17+2. We find that it provides a good description of the observed
spectra, without the need for a scale-free (1/f) component. We update
previously reported correlations between characteristic frequencies of timing
features in the light of this new approach and discuss similarities between
different types of systems which may point towards similar underlying physics.Comment: 13 pages, to appear in The Astrophysical Journa
A Disk--Jet interaction model for the X--Ray Variability in Microquasars
We propose a simple dynamical model that may account for the observed
spectral and temporal properties of GRS 1915+105 and XTE J1550-5634. The model
is based on the assumption that a fraction of the radiation emitted by a hot
spot lying on the accreting disk is dynamically Comptonized by the relativistic
jet that typically accompanies the microquasar phenomenon. We show that
scattering by the jet produces a detectable modulation of the observed flux. In
particular, we found that the phase lag between hard and soft photons depends
on the radial position of the hot spot and, if the angle between the jet and
the line of sight is sufficiently large, the lags of the fundamental and its
harmonics may be either positive or negative.Comment: 14 pages, 4 figures, accepted for publication in ApJ Part
The ABC of low-frequency quasi-periodic oscillations in Black-Hole Candidates: analogies with Z-sources
Three main types of low-frequency quasi-periodic oscillations (LFQPOs) have
been observed in Black Hole Candidates. We re-analyzed RXTE data of the bright
systems XTE J1859+226, XTE J1550-564 and GX 339-4, which show all three of
them. We review the main properties of these LFQPOs and show that they follow a
well-defined correlation in a fractional rms vs. softness diagram. We show that
the frequency behavior through this correlation presents clear analogies with
that of Horizontal-, Normal- and Flaring-Branch Oscillations in Z sources, with
the inverse of the fractional rms being the equivalent of the curvilinear
coordinate Sz through the Z track.Comment: 5 pages, 4 figures, accepted by ApJ, uses emulateap
The impact of working conditions on mental health: Novel evidence from the UK
This paper investigates the causal impact of working conditions on mental health in the UK, combining new longitudinal data on working conditions from the European Working Conditions Survey with microdata from the UK Household Longitudinal Survey (Understanding Society). Our empirical strategy accounts for the endogenous sorting of individuals into occupations by including individual fixed effects. We address the potential endogeneity of occupational change over time by focusing only on individuals who remain in the same occupation (ISCO 3-digit), exploiting the variation in working conditions within each occupation over time. This variation, determined primarily by general macroeconomic conditions, is likely to be exogenous from the individual point of view. Our results indicate that, for female workers, improvements in working conditions such as skills and discretion, working time quality, and work intensity improve mental health outcomes such as loss of confidence, anxiety, social dysfunction, and risk of clinical depression. These effects are clinically relevant and substantial for younger and older female workers and larger for workers in occupations characterised by an inherently higher level of job strain. We detail how different dimensions of job quality impact different mental health outcomes for different age groups. Our results have important implications for public policies and firms which aim to improve workers’ wellbeing and productivity through workplace interventions focused on mental health
The 1998 outburst of the X-ray transient XTE J2012+381 as observed with BeppoSAX
We report on the results of a series of X-ray observations of the transient
black hole candidate XTE J2012+381 during the 1998 outburst performed with the
BeppoSAX satellite. The observed broad-band energy spectrum can be described
with the superposition of an absorbed disk black body, an iron line plus a high
energy component, modelled with either a power law or a Comptonisation tail.
The source showed pronounced spectral variability between our five
observations. While the soft component in the spectrum remained almost
unchanged throughout our campaign, we detected a hard spectral tail which
extended to 200 keV in the first two observations, but became barely detectable
up to 50 keV in the following two. A further re-hardening is observed in the
final observation. The transition from a hard to a soft and then back to a hard
state occurred around an unabsorbed 0.1-200 keV luminosity of 10^38 erg/s (at
10 kpc). This indicates that state transitions in XTE 2012+281 are probably not
driven only by mass accretion rate, but additional physical parameters must
play a role in the evolution of the outburst.Comment: Paper accepted for publication on A&A (macro included, 9 pages, 5
figures
Coupling Between Periodic and Aperiodic Variability in SAX J1808.4-3658
We detect a significant broadening in the wings of the 401 Hz peak in the
power spectrum of the accreting millisecond binary pulsar SAX J1808.4-3658.
This feature is consistent with the convolution of the red noise present in the
power spectrum with the harmonic line. We conclude that the flux modulated by
the spin period shows aperiodic variability similar to the red noise in the
overall flux, suggesting such variability also originates at the magnetic caps
close to the neutron star surface. This is analogous to the results found in
some longer period, higher magnetic field pulsators in high mass X-ray
binaries.Comment: 16 pages, 3 figures, to be published in The Astrophysical Journa
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