26,283 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
Complete trails of co-authorship network evolution
The rise and fall of a research field is the cumulative outcome of its
intrinsic scientific value and social coordination among scientists. The
structure of the social component is quantifiable by the social network of
researchers linked via co-authorship relations, which can be tracked through
digital records. Here, we use such co-authorship data in theoretical physics
and study their complete evolutionary trail since inception, with a particular
emphasis on the early transient stages. We find that the co-authorship networks
evolve through three common major processes in time: the nucleation of small
isolated components, the formation of a tree-like giant component through
cluster aggregation, and the entanglement of the network by large-scale loops.
The giant component is constantly changing yet robust upon link degradations,
forming the network's dynamic core. The observed patterns are successfully
reproducible through a new network model
A broadband radio study of the average profile and giant pulses from PSR B1821-24A
We present the results of wide-band (720-2400 MHz) study of PSR B1821-24A
(J1824-2452A, M28A), an energetic millisecond pulsar visible in radio, X-rays
and gamma-rays. In radio, the pulsar has a complex average profile which spans
>85% of the spin period and exhibits strong evolution with observing frequency.
For the first time we measure phase-resolved polarization properties and
spectral indices of radio emission throughout almost all of the on-pulse
window. We combine this knowledge with the high-energy information to compare
M28A to other known gamma-ray millisecond pulsars and to speculate that M28A's
radio emission originates in multiple regions within its magnetosphere (i.e.
both in the slot or outer gaps near the light cylinder and at lower altitudes
above the polar cap). M28A is one of the handful of pulsars which are known to
emit Giant Pulses (GPs) -- short, bright radio pulses of unknown nature. We
report a drop in the linear polarization of the average profile in both windows
of GP generation and also a `W'-shaped absorption feature (resembling a double
notch), partly overlapping with one of the GP windows. The GPs themselves have
broadband spectra consisting of multiple patches with fractional spectral width
() of about 0.07. Although our time resolution was not
sufficient to resolve the GP structure on the microsecond scale, we argue that
GPs from this pulsar most closely resemble the GPs from the main pulse of the
Crab pulsar, which consist of a series of narrowband nanoshots.Comment: 16 pages, 8 figures, accepted to Ap
Discovery of a radio relic in the low mass, merging galaxy cluster PLCK G200.9-28.2
Radio relics at the peripheries of galaxy clusters are tracers of the elusive
cluster merger shocks. We report the discovery of a single radio relic in the
galaxy cluster PLCK G200.9-28.2 (, ) using the Giant Metrewave Radio Telescope at 235 and 610 MHz and
the Karl G. Jansky Very Large Array at 1500 MHz. The relic has a size of Mpc, an arc-like morphology and is located at 0.9 Mpc from the
X-ray brightness peak in the cluster. The integrated spectral index of the
relic is . The spectral index map between 235 and 610 MHz shows
steepening from the outer to the inner edge of the relic in line with the
expectation from a cluster merger shock. Under the assumption of diffusive
shock acceleration, the radio spectral index implies a Mach number of
for the shock. The analysis of archival XMM Newton data shows that
PLCK G200.9-28.2 consists of a northern brighter sub-cluster, and a southern
sub-cluster in a state of merger. This cluster has the lowest mass among the
clusters hosting single radio relics. The position of the Planck Sunyaev
Ze'ldovich effect in this cluster is offset by 700 kpc from the X-ray peak in
the direction of the radio relic, suggests a physical origin for the offset.
Such large offsets in low mass clusters can be a useful tool to select
disturbed clusters and to study the state of merger.Comment: 10 pages, 7 figures, 4 tables. Accepted for publication in MNRA
Explosive Percolation: Unusual Transitions of a Simple Model
In this paper we review the recent advances on explosive percolation, a very
sharp phase transition first observed by Achlioptas et al. (Science, 2009).
There a simple model was proposed, which changed slightly the classical
percolation process so that the emergence of the spanning cluster is delayed.
This slight modification turns out to have a great impact on the percolation
phase transition. The resulting transition is so sharp that it was termed
explosive, and it was at first considered to be discontinuous. This surprising
fact stimulated considerable interest in "Achlioptas processes". Later work,
however, showed that the transition is continuous (at least for Achlioptas
processes on Erdos networks), but with very unusual finite size scaling. We
present a review of the field, indicate open "problems" and propose directions
for future research.Comment: 27 pages, 4 figures, Review pape
Cyclone Codes
We introduce Cyclone codes which are rateless erasure resilient codes. They
combine Pair codes with Luby Transform (LT) codes by computing a code symbol
from a random set of data symbols using bitwise XOR and cyclic shift
operations. The number of data symbols is chosen according to the Robust
Soliton distribution. XOR and cyclic shift operations establish a unitary
commutative ring if data symbols have a length of bits, for some prime
number . We consider the graph given by code symbols combining two data
symbols. If such random pairs are given for data symbols, then a
giant component appears, which can be resolved in linear time. We can extend
Cyclone codes to data symbols of arbitrary even length, provided the Goldbach
conjecture holds.
Applying results for this giant component, it follows that Cyclone codes have
the same encoding and decoding time complexity as LT codes, while the overhead
is upper-bounded by those of LT codes. Simulations indicate that Cyclone codes
significantly decreases the overhead of extra coding symbols
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