8,082 research outputs found
Time Dilation and Quasar Variability
The timescale of quasar variability is widely expected to show the effects of
time dilation. In this paper we analyse the Fourier power spectra of a large
sample of quasar light curves to look for such an effect. We find that the
timescale of quasar variation does not increase with redshift as required by
time dilation. Possible explanations of this result all conflict with widely
held consensus in the scientific community.Comment: 6 pages including 3 figures. Accepted for publication in ApJ Letter
Time, institutional support and quality of decision making in child protection:A cross-country analysis
This paper examines perceptions of time and institutional support for decision making and staff confidence in child welfare staffs ultimate decisions – examining differences and similarities between and within the service-oriented Nordic countries (Norway and Finland) and the risk-oriented Anglo-American countries (England and California). The study identifies a high degree of work pressure across all the countries, lines of predominantly vertical institutional support and relatively high confidence in decisions. Finland stands out with higher perceived work pressure and with a horizontal support line, whereas England stands out with workers having a lower degree of confidence in their own and others’ decisions
Colour changes in quasar light curves
The study of quasar variability has long been seen as a way to understanding
the structure of the central engine of active galactic nuclei, and as a means
of verifying the morphology of the standard model. Much work has already been
done on the statistical properties of light curves monitored in one colour, and
it is now possible to use these observations to test predictions of theoretical
models. The addition of a second colour can add enormously to the power of such
tests, and put tight constraints on the nature of the variations. In this paper
a yearly monitoring programme of several hundred quasars in blue and red
passbands covering 21 years is presented. The statistics of colour changes are
examined for a 15 year period of homogeneous data with Fourier power spectrum
analysis, in a form suitable for testing against theoretical predictions. The
results of the Fourier analysis show that there is more power in blue light on
all timescales than in the red. Examination of the light curves shows several
different modes of colour change. However, if allowance is made for the effects
of the underlying host galaxy, the variations become close to achromatie. There
are however structural differences between red and blue light curves which
cannot be accounted for in this way, and various modes of variability including
disc instability and microlensing are examined to provide explanations for
these features.Comment: 11 pages including 6 figures. Published in MNRA
M Dwarfs from Hubble Space Telescope Star Counts. IV
We study a sample of about 1400 disk M dwarfs that are found in 148 fields
observed with the Wide Field Camera 2 (WFC2) on the Hubble Space Telescope and
162 fields observed with pre-repair Planetary Camera 1 (PC1), of which 95 of
the WFC2 fields are newly analyzed. The method of maximum likelihood is applied
to derive the luminosity function and the Galactic disk parameters. At first,
we use a local color-magnitude relation and a locally determined
mass-luminosity relation in our analysis. The results are consistent with those
of previous work but with considerably reduced statistical errors. These small
statistical errors motivate us to investigate the systematic uncertainties.
Considering the metallicity gradient above the Galactic plane, we introduce a
modified color-magnitude relation that is a function of Galactic height. The
resultant M dwarf luminosity function has a shape similar to that derived using
the local color-magnitude relation but with a higher peak value. The peak
occurs at and the luminosity function drops sharply toward . We then apply a height-dependent mass-luminosity function
interpolated from theoretical models with different metallicities to calculate
the mass function. Unlike the mass function obtained using local relations,
which has a power-law index , the one derived from the
height-dependent relations tends to be flat (). The resultant
local surface density of disk M dwarfs (12.2 +/- 1.6 M_sun pc^{-2}) is somewhat
smaller than the one obtained using local relations (14.3 +/- 1.3 M_sun
pc^{-2}). Our measurement favors a short disk scale length, H = 2.75 +/- 0.16
(statistical) +/- 0.25 (systematic) kpc.Comment: 20 pages, 10 ps figures, accepted for publication in Ap
On the relationship between sigma models and spin chains
We consider the two-dimensional non-linear sigma model with
topological term using a lattice regularization introduced by Shankar and Read
[Nucl.Phys. B336 (1990), 457], that is suitable for studying the strong
coupling regime. When this lattice model is quantized, the coefficient
of the topological term is quantized as , with integer or
half-integer. We study in detail the relationship between the low energy
behaviour of this theory and the one-dimensional spin- Heisenberg model. We
generalize the analysis to sigma models with other symmetries.Comment: To appear in Int. J. MOd. Phys.
Long Term Variability of SDSS Quasars
We use a sample of 3791 quasars from the Sloan Digital Sky Survey (SDSS)
Early Data Release (EDR), and compare their photometry to historic plate
material for the same set of quasars in order to study their variability
properties. The time base-line we attain this way ranges from a few months to
up to 50 years. In contrast to monitoring programs, where relatively few
quasars are photometrically measured over shorter time periods, we utilize
existing databases to extend this base-line as much as possible, at the cost of
sampling per quasar. Our method, however, can easily be extended to much larger
samples. We construct variability Structure Functions and compare these to the
literature and model functions. From our modeling we conclude that 1) quasars
are more variable toward shorter wavelengths, 2) their variability is
consistent with an exponentially decaying light-curve with a typical time-scale
of ~2 years, 3) these outbursts occur on typical time-scales of ~200 years.
With the upcoming first data release of the SDSS, a much larger quasar sample
can be used to put these conclusions on a more secure footing.Comment: 16 pages, accepted for publication in AJ, Sept issu
Behaviours preceding suicides at railway and underground locations: a multimethodological qualitative approach
Background: Suicides by train have devastating consequences for families, the rail industry, staff dealing with the aftermath of such incidents, and potential witnesses. To reduce suicides and suicide attempts by rail it is important to learn how safe interventions can be made. However, very little is known about how to identify someone who may be about to make a suicide attempt at a railway location (including underground/subways). The current research employed a novel way of understanding what behaviours might immediately precede a suicide or suicide attempt at these locations.
Design and Methods: A qualitative thematic approach was used for three parallel studies. Data were gathered from several sources including: interviews with individuals who survived a suicide attempt at a railway or underground location (N=9); CCTV footage of individuals who died by suicide at a rail or underground location (N=16); and qualitative survey data providing views from rail staff (N=79).
Results: Our research suggests there are several behaviours that people may carry out before a suicide or suicide attempt at a rail location including: station hopping and platform switching; limiting contact with others; positioning themselves at the end of the track where the train/tube approaches; allowing trains to pass by; and carrying out repetitive behaviours.
Interpretation: There are several behaviours that may be identifiable in the moments leading up to a suicide or suicide attempt on the railways, which may present opportunities for intervention. These findings have implications for several stakeholders including rail providers, transport police and other organisations focused on suicide prevention
Lifestyle interventions and prevention of suicide
Over the past years, there has been a growing interest in the association between lifestyle psychosocial interventions, severe mental illness, and suicide risk. Patients with severe mental disorders have higher mortality rates, poor health states, and higher suicide risk compared to the general population. Lifestyle behaviors are amenable to change through the adoption of specific psychosocial interventions, and several approaches have been promoted. The current article provides a comprehensive review of the literature on lifestyle interventions, mental health, and suicide risk in the general population and in patients with psychiatric disorders. For this purpose, we investigated lifestyle behaviors and lifestyle interventions in three different age groups: adolescents, young adults, and the elderly. Several lifestyle behaviors including cigarette smoking, alcohol use, and sedentary lifestyle are associated with suicide risk in all age groups. In adolescents, growing attention has emerged on the association between suicide risk and internet addiction, cyberbullying and scholastic and family difficulties. In adults, psychiatric symptoms, substance and alcohol abuse, weight, and occupational difficulties seems to have a significant role in suicide risk. Finally, in the elderly, the presence of an organic disease and poor social support are associated with an increased risk of suicide attempt. Several factors may explain the association between lifestyle behaviors and suicide. First, many studies have reported that some lifestyle behaviors and its consequences (sedentary lifestyle, cigarette smoking underweight, obesity) are associated with cardiometabolic risk factors and with poor mental health. Second, several lifestyle behaviors may encourage social isolation, limiting the development of social networks, and remove individuals from social interactions; increasing their risk of mental health problems and suicide
Photometric Monitoring of the Gravitationally Lensed Ultraluminous BAL Quasar APM08279+5255
We report on one year of photometric monitoring of the ultraluminous BAL
quasar APM 08279+5255. The temporal sampling reveals that this gravitationally
lensed system has brightened by ~0.2 mag in 100 days. Two potential causes
present themselves; either the variability is intrinsic to the quasar, or it is
the result of microlensing by stars in a foreground system. The data is
consistent with both hypotheses and further monitoring is required before
either case can be conclusively confirmed. We demonstrate, however, that
gravitational microlensing can not play a dominant role in explaining the
phenomenal properties exhibited by APM 08279+5255. The identification of
intrinsic variability, coupled with the simple gravitational lensing
configuration, would suggest that APM 08279+5255 is a potential golden lens
from which the cosmological parameters can be derived and is worthy of a
monitoring program at high spatial resolution.Comment: 17 pages, with 2 figures. Accepted for publication in P.A.S.
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