4,926 research outputs found
The glitch-induced identity changes of PSR J1119-6127
We demonstrate that the high-magnetic field pulsar J1119-6127 exhibits three
different types of behaviour in the radio band. Trailing the "normal" profile
peak there is an "intermittent" peak and these components are flanked by two
additional components showing very erratic "RRAT-like" emission. Both the
intermittent and RRAT-like events are extremely rare and are preceded by a
large amplitude glitch in the spin-down parameters. The post-glitch spin-down
rate is smaller than the pre-glitch rate. This type of relaxation is very
unusual for the pulsar population as a whole, but is observed in the glitch
recovery of a RRAT. The abnormal emission behaviour in PSR J1119-6127 was
observed up to three months after the epoch of the large glitch, suggestive of
changes in the magnetospheric conditions during the fast part of the recovery
process. We argue that both the anomalous recoveries and the emission changes
could be related to reconfigurations of the magnetic field. Apart from the
glitches, the spin-down of PSR J1119-6127 is relatively stable, allowing us to
refine the measurement of the braking index (n=2.684\pm0.002) using more than
12 years of timing data. The properties of this pulsar are discussed in light
of the growing evidence that RRATs do not form a distinct class of pulsar, but
rather are a combination of different extreme emission types seen in other
neutron stars. Different sub-classes of the RRATs can potentially be separated
by calculating the lower limit on the modulation index of their emission. We
speculate that if the abnormal behaviour in PSR J1119-6127 is indeed glitch
induced then there might exist a population of neutron stars which only become
visible in the radio band for a short duration in the immediate aftermath of
glitch activity. These neutron stars will be visible in the radio band as
sources that only emit some clustered pulses every so many years.Comment: 20 pages, 10 figures, Accepted for publication in MNRA
BK Lyncis: The Oldest Old Nova?... And a Bellwether for Cataclysmic-Variable Evolution
We summarize the results of a 20-year campaign to study the light curves of
BK Lyncis, a nova-like star strangely located below the 2-3 hour orbital period
gap in the family of cataclysmic variables. Two apparent "superhumps" dominate
the nightly light curves - with periods 4.6% longer, and 3.0% shorter, than
P_orb. The first appears to be associated with the star's brighter states
(V~14), while the second appears to be present throughout and becomes very
dominant in the low state (V~15.7).
Starting in the year 2005, the star's light curve became indistinguishable
from that of a dwarf nova - in particular, that of the ER UMa subclass.
Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of
the probable nova on 30 December 101, and (b) it has been fading ever since,
but has taken ~2000 years for the accretion rate to drop sufficiently to permit
dwarf-nova eruptions. If such behavior is common, it can explain other puzzles
of CV evolution. One: why the ER UMa class even exists (because all members can
be remnants of recent novae). Two: why ER UMa stars and short-period novalikes
are rare (because their lifetimes, which are essentially cooling times, are
short). Three: why short-period novae all decline to luminosity states far
above their true quiescence (because they're just getting started in their
postnova cooling). Four: why the orbital periods, accretion rates, and
white-dwarf temperatures of short-period CVs are somewhat too large to arise
purely from the effects of gravitational radiation (because the unexpectedly
long interval of enhanced postnova brightness boosts the mean mass-transfer
rate). These are substantial rewards in return for one investment of
hypothesis: that the second parameter in CV evolution, besides P_orb, is time
since the last classical-nova eruption.Comment: PDF, 46 pages, 4 tables, 10 figures; in preparation; more info at
http://cbastro.org
Congested Traffic States in Empirical Observations and Microscopic Simulations
We present data from several German freeways showing different kinds of
congested traffic forming near road inhomogeneities, specifically lane
closings, intersections, or uphill gradients. The states are localized or
extended, homogeneous or oscillating. Combined states are observed as well,
like the coexistence of moving localized clusters and clusters pinned at road
inhomogeneities, or regions of oscillating congested traffic upstream of nearly
homogeneous congested traffic. The experimental findings are consistent with a
recently proposed theoretical phase diagram for traffic near on-ramps [D.
Helbing, A. Hennecke, and M. Treiber, Phys. Rev. Lett. {\bf 82}, 4360 (1999)].
We simulate these situations with a novel continuous microscopic single-lane
model, the ``intelligent driver model'' (IDM), using the empirical boundary
conditions. All observations, including the coexistence of states, are
qualitatively reproduced by describing inhomogeneities with local variations of
one model parameter.
We show that the results of the microscopic model can be understood by
formulating the theoretical phase diagram for bottlenecks in a more general
way. In particular, a local drop of the road capacity induced by parameter
variations has practically the same effect as an on-ramp.Comment: Now published in Phys. Rev. E. Minor changes suggested by a referee
are incorporated; full bibliographic info added. For related work see
http://www.mtreiber.de/ and http://www.helbing.org
Use of CCTV to determine road accident factors in urban areas
This paper sets out to assess whether there is a potential use for images collected through the increasingly ubiquitous use of CCTV cameras in urban areas as a means of increasing understanding of the causes of road traffic accidents. Information on causation and contributory factors is essential as a means of understanding why accidents occurred and how the occurrence of similar events may be prevented in the future. CCTV records of accidents could provide an independent perspective on an accident and have the potential to increase both the quality and quantity of information available to the safety researcher.
This study focuses on an area of central Leeds in the UK and shows that an existing CCTV camera system used for urban traffic management reasons has the potential to
1 record around a quarter of the accidents which occur in the area, based on patterns of past occurrence. Most city centres in the UK will have similar camera systems set up. By the introduction of additional strategically placed cameras and replacement of existing cameras with ones dedicated to accident recording, this figure could be increased substantially.
The paper also considers how effective cameras and video records will be as a means of identifying contributory factor information once an accident is recorded. The contributory factor classification used by a recently introduced system in Britain is assessed in terms of how visible each of the factors is likely to be on video and their relative frequency of occurrence. It is concluded that CCTV has a high potential to provide corroborative evidence about many of the most commonly occurring factors, and to throw further light on accident causation
An Efficient Safety-oriented Car-following Model for Connected Automated Vehicles Considering Discrete Signals
With the rapid development of Connected and Automated Vehicle (CAV)
technology, limited self-driving vehicles have been commercially available in
certain leading intelligent transportation system countries. When formulating
the car-following model for CAVs, safety is usually the basic constraint.
Safety-oriented car-following models seek to specify a safe following distance
that can guarantee safety if the preceding vehicle were to brake hard suddenly.
The discrete signals of CAVs bring a series of phenomena, including discrete
decision-making, phase difference, and discretely distributed communication
delay. The influences of these phenomena on the car-following safety of CAVs
are rarely considered in the literature. This paper proposes an efficient
safety-oriented car-following model for CAVs considering the impact of discrete
signals. The safety constraints during both normal driving and a sudden hard
brake are incorporated into one integrated model to eliminate possible
collisions during the whole driving process. The mechanical delay information
of the preceding vehicle is used to improve car-following efficiency. Four
modules are designed to enhance driving comfort and string stability in case of
heavy packet losses. Simulations of a platoon with diversified vehicle types
demonstrate the safety, efficiency, and string stability of the proposed model.
Tests with different packet loss rates imply that the model could guarantee
safety and driving comfort in even poor communication environments
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