1,407 research outputs found
Ultraviolet and X-ray variability of NGC 4051 over 45 days with XMM-Newton and Swift
We analyse 15 XMM-Newton observations of the Seyfert galaxy NGC 4051 obtained
over 45 days to determine the ultraviolet (UV) light curve variability
characteristics and search for correlated UV/X-ray emission. The UV light curve
shows variability on all time scales, however with lower fractional rms than
the 0.2-10 keV X-rays. On days-weeks timescales the fractional variability of
the UV is Fvar ~ 8%, and on short (~ hours) timescales Fvar ~ 2%. The
within-observation excess variance in 4 of the 15 UV observations was found be
much higher than the remaining 11. This was caused by large systematic
uncertainties in the count rate masking the intrinsic source variance. For the
four "good" observations we fit an unbroken power-law model to the UV power
spectra with slope -2.0 +/- 0.5. We compute the UV/X-ray Cross-correlation
function for the "good" observations and find a correlation of ~ 0.5 at time
lag of ~ 3 ks, where the UV lags the X-rays. We also compute for the first time
the UV/X-ray Cross-spectrum in the range 0-28.5 ks, and find a low coherence
and an average time lag of ~ 3 ks. Combining the 15 XMM-Newton and the Swift
observations we compute the DCF over +/-40 days but are unable to recover a
significant correlation. The magnitude and direction of the lag estimate from
the 4 "good" observations indicates a scenario where ~ 25 % of the UV variance
is caused by thermal reprocessing of the incident X-ray emission.Comment: 11 pages, 8 figures. Accepted for publication in MNRA
The flux-dependent X-ray time lags in NGC 4051
We present an analysis of the X-ray time lags for the highly variable Seyfert
1 galaxy NGC 4051, based on a series of XMM-Newton observations taken in 2009.
We investigate the Fourier frequency dependent time lags in the light curves
between the 0.3--1.0 keV and 2.0--5.0 keV energy bands as a function of source
flux, including simultaneous modelling of the resulting lag-frequency spectra.
We find the shape of the lag-frequency spectra to vary significantly and
systematically with source flux. We model the lag-frequency spectra using
simple transfer functions, and find that two time lag components are required,
one in each energy band. The simplest acceptable fits have only the relative
contribution of the lagged component in the hard band varying with flux level,
which can be associated with changes in the energy spectrum. We discuss the
interpretation of these results in terms of the currently popular models for
X-ray time lags.Comment: 9 pages, 4 figures. Accepted for publication in MNRA
On characterising the variability properties of X-ray light curves from active galaxies
We review some practical aspects of measuring the amplitude of variability in
`red noise' light curves typical of those from Active Galactic Nuclei (AGN).
The quantities commonly used to estimate the variability amplitude in AGN light
curves, such as the fractional rms variability amplitude, F_var, and excess
variance, sigma_XS^2, are examined. Their statistical properties, relationship
to the power spectrum, and uses for investigating the nature of the variability
processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var)
shows large changes from one part of the light curve to the next, even when the
variability is produced by a stationary process. This limits the usefulness of
these estimators for quantifying differences in variability amplitude between
different sources or from epoch to epoch in one source. Some examples of the
expected scatter in the variance are tabulated for various typical power
spectral shapes, based on Monte Carlo simulations. The excess variance can be
useful for comparing the variability amplitudes of light curves in different
energy bands from the same observation. Monte Carlo simulations are used to
derive a description of the uncertainty in the amplitude expected between
different energy bands (due to measurement errors). Finally, these estimators
are used to demonstrate some variability properties of the bright Seyfert 1
galaxy Markarian 766. The source is found to show a strong, linear correlation
between rms amplitude and flux, and to show significant spectral variability.Comment: 14 pages. 12 figures. Accepted for publication in MNRA
The Ubiquity of the rms-flux relation in Black Hole X-ray Binaries
We have investigated the short term linear relation between the rms
variability and the flux in 1,961 observations of 9 black hole X-ray binaries.
The rms-flux relation for the 1-10 Hz range is ubiquitously observed in any
observation with good variability signal to noise (> 3 % 1-10 Hz fractional
rms). This concurs with results from a previous study of Cygnus X-1 (Gleissner
et. al. 2004), and extends detection of the rms-flux relation to a wider range
of states. We find a strong dependence of the flux intercept of the rms-flux
relation on source state; as the source transitions from the hard state into
the hard intermediate state the intercept becomes strongly positive. We find
little evidence for flux dependence of the broad-band noise within the PSD
shape, excepting a small subset of observations from one object in an anomalous
soft-state. We speculate that the ubiquitous linear rms-flux relation in the
broad band noise of this sample, representing a range of different states and
objects, indicates that its formation mechanism is an essential property of the
luminous accretion flow around black holes.Comment: 12 pages, 6 figures, accepted for publication in MNRA
Using the daylight savings clock change to show ambient light conditions significantly influence active travel
This article reports a novel procedure used to investigate whether ambient light conditions affect the number of people who choose to walk or cycle. Pedestrian and cyclist count data were analysed using the biannual daylight-saving clock changes to compare daylight and after-dark conditions whilst keeping seasonal and time-of-day factors constant. Changes in frequencies during a one-hour case period before and after a clock change, when light conditions varied significantly between daylight and darkness, were compared against control periods when the light condition did not change. Odds ratios indicated the numbers of pedestrians and cyclists during the case period were significantly higher during daylight conditions than after-dark, resulting in a 62% increase in pedestrians and a 38% increase in cyclists. These results show the importance of light conditions on the numbers of pedestrian and cyclists, and highlight the potential of road lighting as a policy measure to encourage active travel after-dark
Comparison of time/phase lags in the hard state and plateau state of GRS 1915+105
We investigate the complex behavior of energy- and frequency-dependent
time/phase lags in the plateau state and the radio-quiet hard state of GRS
1915+105. In our timing analysis, we find that when the source is faint in the
radio, QPOs are observed above 2 Hz and typically exhibit soft lags (soft
photons lag hard photons), whereas QPOs in the radio-bright plateau state are
found below 2.2 Hz and consistently show hard lags. The phase lag at the QPO
frequency is strongly anti-correlated with the QPO frequency, changing sign at
2.2 Hz. However, the phase lag at the frequency of the first harmonic is
positive and nearly independent of frequency at at ~0.172 rad, regardless of
the radio emission. The lag-energy dependence at the first harmonic is also
independent of radio flux. However, the lags at the QPO frequency are negative
at all energies during the radio-quiet state, but lags at the QPO frequency
during the plateau state are positive at all energies and show a
'reflection-type' evolution of the lag-energy spectra with respect to the
radio-quiet state. The lag-energy dependence is roughly logarithmic, but there
is some evidence for a break around 4-6 keV. Finally, the Fourier
frequency-dependent phase lag spectra are fairly flat during the plateau state,
but increase from negative to positive during the radio-quiet state. We discuss
the implications of our results in the light of some generic models.Comment: 9 pages, 7 figures, accepted for publication in Ap
Studying accreting black holes and neutron stars with time series: beyond the power spectrum
The fluctuating brightness of cosmic X-ray sources, particularly accreting
black holes and neutron star systems, has enabled enormous progress in
understanding the physics of turbulent accretion flows, the behaviour of matter
on the surfaces of neutron stars and improving the evidence for black holes.
Most of this progress has been made by analysing and modelling time series data
in terms of their power and cross spectra, as will be discussed in other
articles in this volume. Recently, attempts have been made to make use of other
aspects of the data, by testing for non-linearity, non-Gaussianity, time
asymmetry and by examination of higher order Fourier spectra. These projects,
which have been made possible by the vast increase in data quality and quantity
over the past decade, are the subject of this article.Comment: 13 pages, 6 figures, in "Noise and Fluctuations" Proc. SPIE vol. 660
A whole-year approach showing that ambient light level influences walking and cycling
Many studies have used surveys to investigate the reactions to changes in lighting from people who walk or cycle. An alternative approach is to use objective data, specifically the number of pedestrians and cyclists present under different lighting conditions, which has been reported previously using a daylight savings transition approach. This article presents a different method for analysing the effect of ambient light conditions in which data from the whole year are examined, rather than only the two weeks either side of the biannual daylight savings clock changes. The results confirm that ambient light has a significant impact: for a given time of day, more people walk or cycle when it is daylight than after dark. While both methods use an odds ratio approach, which should account for environmental changes other than lighting, the results suggest the daylight savings method of analysis better isolates changes in weather from the effects of ambient light on travel choice than does the whole-year method
Road lighting density and brightness linked with increased cycling rates after-dark
Cycling has a range of benefits as is recognised by national and international policies aiming to increase cycling rates. Darkness acts as a barrier to people cycling, with fewer people cycling after-dark when seasonal and time-of-day factors are accounted for. This paper explores whether road lighting can reduce the negative impact of darkness on cycling rates. Changes in cycling rates between daylight and after-dark were quantified for 48 locations in Birmingham, United Kingdom, by calculating an odds ratio. These odds ratios were compared against two measures of road lighting at each location: 1) Density of road lighting lanterns; 2) Relative brightness as estimated from night-time aerial images. Locations with no road lighting showed a significantly greater reduction in cycling after-dark compared with locations that had some lighting. A nonlinear relationship was found between relative brightness at a location at night and the reduction in cyclists after-dark. Small initial increases in brightness resulted in large reductions in the difference between cyclist numbers in daylight and after-dark, but this effect reached a plateau as brightness increased. These results suggest only a minimal amount of lighting can promote cycling after-dark, making it an attractive mode of transport year-round
A comparison of approaches for investigating the impact of ambient light on road traffic collisions
A recent paper proposed a more precise approach for investigating the impact of ambient light (daylight versus after dark) on road traffic collisions. The present paper first repeated that analysis of road traffic collisions in the UK to test reproducibility; it then extended the analysis to determine whether the greater precision affected the outcome of road traffic collision analyses. Results of the previous analysis were reproduced in terms of the direction of the effect, but the repeated analysis found greater differences between daylight and darkness. The odds ratio determined using the new method led to higher odds ratios than the analyses used in some past studies, suggesting that past studies may have underestimated the detrimental effect of darkness on road traffic collision risk
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