490 research outputs found
The optimal schedule for pulsar timing array observations
In order to maximize the sensitivity of pulsar timing arrays to a stochastic
gravitational wave background, we present computational techniques to optimize
observing schedules. The techniques are applicable to both single and
multi-telescope experiments. The observing schedule is optimized for each
telescope by adjusting the observing time allocated to each pulsar while
keeping the total amount of observing time constant. The optimized schedule
depends on the timing noise characteristics of each individual pulsar as well
as the performance of instrumentation. Several examples are given to illustrate
the effects of different types of noise. A method to select the most suitable
pulsars to be included in a pulsar timing array project is also presented.Comment: 16 pages, 6 figures, accepted by MNRA
Resolving the decades-long transient FIRST J141918.9+394036: an orphan long gamma-ray burst or a young magnetar nebula?
Ofek (2017) identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as
a radio source sharing similar properties and host galaxy type to the compact,
persistent radio source associated with the first known repeating fast radio
burst, FRB 121102. Law et al. (2018) showed that FIRST J1419+3940 is a
transient source decaying in brightness over the last few decades. One possible
interpretation is that FIRST J1419+3940 is a nearby analogue to FRB 121102 and
that the radio emission represents a young magnetar nebula (as several
scenarios assume for FRB 121102). Another interpretation is that FIRST
J1419+3940 is the afterglow of an `orphan' long gamma-ray burst (GRB). The
environment is similar to where most such events are produced. To distinguish
between these hypotheses, we conducted radio observations using the European
VLBI Network at 1.6 GHz to spatially resolve the emission and to search for
millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact
radio source with a flux density of (on 2018
September 18) and a source size of (i.e. given the angular diameter distance of ).
These results confirm that the radio emission is non-thermal and imply an
average expansion velocity of . Contemporaneous
high-time-resolution observations using the 100-m Effelsberg telescope detected
no millisecond-duration bursts of astrophysical origin. The source properties
and lack of short-duration bursts are consistent with a GRB jet expansion,
whereas they disfavor a magnetar birth nebula.Comment: 8 pages, 4 figures, accepted for publication in ApJ
Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810-197
After spending almost a decade in a radio-quiet state, the Anomalous X-ray
Pulsar XTE J1810-197 turned back on in early December 2018. We have observed
this radio magnetar at 1.5 GHz with ~daily cadence since the first detection of
radio re-activation on 8 December 2018. In this paper, we report on the current
timing properties of XTE J1810-197 and find that the magnitude of the spin
frequency derivative has increased by a factor of 2.6 over our 48-day data set.
We compare our results with the spin-down evolution reported during its
previous active phase in the radio band. We also present total intensity pulse
profiles at five different observing frequencies between 1.5 and 8.4 GHz,
collected with the Lovell and the Effelsberg telescopes. The profile evolution
in our data set is less erratic than what was reported during the previous
active phase, and can be seen varying smoothly between observations. Profiles
observed immediately after the outburst show the presence of at least five
cycles of a very stable ~50-ms periodicity in the main pulse component that
lasts for at least tens of days. This remarkable structure is seen across the
full range of observing frequencies.Comment: 9 pages, 7 figures, updated with additional analysis of the 50-ms
oscillation, accepted for publication in MNRA
Single pulse and profile variability study of PSR J1022+1001
Millisecond pulsars (MSPs) are known as highly stable celestial clocks.
Nevertheless, recent studies have revealed the unstable nature of their
integrated pulse profiles, which may limit the achievable pulsar timing
precision. In this paper, we present a case study on the pulse profile
variability of PSR J1022+1001. We have detected approximately 14,000 sub-pulses
(components of single pulses) in 35-hr long observations, mostly located at the
trailing component of the integrated profile. Their flux densities and
fractional polarisation suggest that they represent the bright end of the
energy distribution in ordinary emission mode and are not giant pulses. The
occurrence of sub-pulses from the leading and trailing components of the
integrated profile is shown to be correlated. For sub-pulses from the latter, a
preferred pulse width of approximately 0.25 ms has been found. Using
simultaneous observations from the Effelsberg 100-m telescope and the
Westerbork Synthesis Radio Telescope, we have found that the integrated profile
varies on a timescale of a few tens of minutes. We show that improper
polarisation calibration and diffractive scintillation cannot be the sole
reason for the observed instability. In addition, we demonstrate that timing
residuals generated from averages of the detected sub-pulses are dominated by
phase jitter, and place an upper limit of ~700 ns for jitter noise based on
continuous 1-min integrations.Comment: 13 pages, 20 figures, 3 tables, accepted for publication in MNRA
Can we see pulsars around Sgr A*? - The latest searches with the Effelsberg telescope
Radio pulsars in relativistic binary systems are unique tools to study the
curved space-time around massive compact objects. The discovery of a pulsar
closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr
A*, would provide a superb test-bed for gravitational physics. To date, the
absence of any radio pulsar discoveries within a few arc minutes of Sgr A* has
been explained by one principal factor: extreme scattering of radio waves
caused by inhomogeneities in the ionized component of the interstellar medium
in the central 100 pc around Sgr A*. Scattering, which causes temporal
broadening of pulses, can only be mitigated by observing at higher frequencies.
Here we describe recent searches of the Galactic centre region performed at a
frequency of 18.95 GHz with the Effelsberg radio telescope.Comment: 3 pages, 2 figures, Proceedings of IAUS 291 "Neutron Stars and
Pulsars: Challenges and Opportunities after 80 years", 201
Model-based asymptotically optimal dispersion measure correction for pulsar timing
In order to reach the sensitivity required to detect gravitational waves,
pulsar timing array experiments need to mitigate as much noise as possible in
timing data. A dominant amount of noise is likely due to variations in the
dispersion measure. To correct for such variations, we develop a statistical
method inspired by the maximum likelihood estimator and optimal filtering. Our
method consists of two major steps. First, the spectral index and amplitude of
dispersion measure variations are measured via a time-domain spectral analysis.
Second, the linear optimal filter is constructed based on the model parameters
found in the first step, and is used to extract the dispersion measure
variation waveforms. Compared to current existing methods, this method has
better time resolution for the study of short timescale dispersion variations,
and generally produces smaller errors in waveform estimations. This method can
process irregularly sampled data without any interpolation because of its
time-domain nature. Furthermore, it offers the possibility to interpolate or
extrapolate the waveform estimation to regions where no data is available.
Examples using simulated data sets are included for demonstration.Comment: 15 pages, 15 figures, submitted 15th Sept. 2013, accepted 2nd April
2014 by MNRAS. MNRAS, 201
Loss of chaos in combustion noise as a precursor of impending combustion instability
Combustion noise has been traditionally thought of as stochastic fluctuations present in the background of the dynamics in combustors amongst the flow, heat release and the chamber acoustics. Through a series of determinism tests, we show that these aperiodic fluctuations are in fact chaotic of moderately high dimensions (d0 ≅ 8–10). These chaotic fluctuations then transition to high amplitude combustion instability when the operating conditions are varied towards leaner equivalence ratios. Precursors to such a transition from chaos to dynamics dominated by periodic oscillations are of interest to designers and operators of combustors in estimating the boundaries of operability. We introduce a test for chaos, known as 0–1 test for chaos in the literature, as a measure of the proximity of the combustor to an impending instability. The measure is robust and shows a smooth transition for variation in flow conditions towards instability enabling thresholds to be set for operational boundaries
Structural and Electrical Studies of Boric Acid Doped Cadmium Oxide Thin Film by JNSP Technique for Optoelectronic Applications
The present work deals with preparation and characterization of boric acid (b) dopped cdo thin films by the jet nebulizer spray pyrolysis technique at optimized temperature 450°C. Boric acid doped cdo thin films were prepared by jet nebulizer spray pyrolysis technique with different wt% of boric acid (x=0, 0.5,2.5, 4). The xrd pattern of various weight percentage boric acid dopped cdo thin films show the polycrystalline nature with cubic structure. At room temperature, the electrical conductivity of the prepared films increases with 4Wt% of 2.32x10-3 S/cm. The plot of voltage versus current as a function of temperature (RT-30°C) indicates the ohmic behavior of the films. Moreover, significant optoelectronic applications are cadmium oxide dopped with boric acid thin films at room temperature electrical resistivity is in the order of 101Ω cm which is low enough to be a good supercapacitor electrode material
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