164 research outputs found
Simulated performance of an order statistic threshold strategy for detection of narrowband signals
The application of order statistics to signal detection is becoming an increasingly active area of research. This is due to the inherent robustness of rank estimators in the presence of large outliers that would significantly degrade more conventional mean-level-based detection systems. A detection strategy is presented in which the threshold estimate is obtained using order statistics. The performance of this algorithm in the presence of simulated interference and broadband noise is evaluated. In this way, the robustness of the proposed strategy in the presence of the interference can be fully assessed as a function of the interference, noise, and detector parameters
AVIRIS ground data-processing system
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) has been under development at JPL for the past four years. During this time, a dedicated ground data-processing system has been designed and implemented to store and process the large amounts of data expected. This paper reviews the objectives of this ground data-processing system and describes the hardware. An outline of the data flow through the system is given, and the software and incorporated algorithms developed specifically for the systematic processing of AVIRIS data are described
Radiation properties of extreme nulling pulsar J1502-5653
We report on radiation properties of extreme nulling pulsar J1502-5653, by
analyzing the data acquired from the Parkes 64-m telescope at 1374 MHz. The
radio emission from this pulsar exhibits sequences of several tens to several
hundreds consecutive burst pulses, separated by null pulses, and the appearance
of the emission seems quasi-periodic. The null fraction from the data is
estimated to be 93.6%. No emission is detected in the integrated profile of all
null pulses. Systematic modulations of pulse intensity and phase are found at
the beginning of burst-pulse sequences just after null. The intensity usually
rises to a maximum for the first few pulses, then declines exponentially
afterwards, and becomes stable after few tens of pulse periods. The peak phase
appears at later longitudes for the first pulse, then drifts to earlier
longitudes rapidly, and then systematic drifting gradually vanishes while the
intensity becomes stable. In this pulsar, the intensity variation and phase
modulation of pulses are correlated in a short duration after the emission
starts following a null. Observed properties of the pulsar are compared with
other nulling pulsars published previously, and the possible explanation for
phase modulation is discussed.Comment: 5 pages, 7 figures. Accepted by MNRA
PSR J1829+2456: a relativistic binary pulsar
We report the discovery of a new binary pulsar, PSR J1829+2456, found during
a mid-latitude drift-scan survey with the Arecibo telescope. Our initial timing
observations show the 41-ms pulsar to be in a 28-hr, slightly eccentric, binary
orbit. The advance of periastron, omegadot = 0.28 +/- 0.01 deg/yr is derived
from our timing observations spanning 200 days. Assuming that the advance of
periastron is purely relativistic and a reasonable range of neutron star masses
for PSR J1829+2456 we constrain the companion mass to be between 1.22 Msun and
1.38 Msun, making it likely to be another neutron star. We also place a firm
upper limit on the pulsar mass of 1.38 Msun. The expected coalescence time due
to gravitational-wave emission is long (~60 Gyr) and this system will not
significantly impact upon calculations of merger rates that are relevant to
upcoming instruments such as LIGO.Comment: Accepted MNRAS, 5 pages, 3 figure
A Modified Scalar-Tensor-Vector Gravity Theory and the Constraint on its Parameters
A gravity theory called scalar-tensor-vector gravity (STVG) has been recently
developed and succeeded in solar system, astrophysical and cosmological scales
without dark matter [J. W. Moffat, J. Cosmol. Astropart. Phys. 03, 004 (2006)].
However, two assumptions have been used: (i) , where and
are and in the Schwarzschild coordinates (static and
spherically symmetric); (ii) scalar field in the solar system. These
two assumptions actually imply that the standard parametrized post-Newtonian
parameter . In this paper, we relax these two assumptions and study
STVG further by using the post-Newtonian (PN) approximation approach. With
abandoning the assumptions, we find in general cases of STVG.
Then, a version of modified STVG (MSTVG) is proposed through introducing a
coupling function of scalar field G: . We have derived the metric
and equations of motion (EOM) in 1PN for general matter without specific
equation of state and point masses firstly. Subsequently, the secular
periastron precession of binary pulsars in harmonic coordinates
is given. After discussing two PPN parameters ( and ) and two
Yukawa parameters ( and ), we use of four
binary pulsars data (PSR B1913+16, PSR B1534+12, PSR J0737-3039 and PSR
B2127+11C) to constrain the Yukawa parameters for MSTVG:
m and if
we fix .Comment: 39 pages, 4 figures, accepted by PR
The two-hour orbit of a binary millisecond X-ray pulsar
Typical radio pulsars are magnetized neutron stars that are born rapidly
rotating and slow down as they age on time scales of 10 to 100 million years.
However, millisecond radio pulsars spin very rapidly even though many are
billions of years old. The most compelling explanation is that they have been
"spun up" by the transfer of angular momentum during accretion of material from
a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs
consist of a neutron star or black hole accreting from a companion less than
one solar mass.) The recent detection of coherent X-ray pulsations with a
millisecond period from a suspected LMXB system appears to confirm this link.
Here we report observations showing that the orbital period of this binary
system is two hours, which establishes it as an LMXB. We also find an apparent
modulation of the X-ray flux at the orbital period (at the two per cent level),
with a broad minimum when the pulsar is behind this low-mass companion star.
This system seems closely related to the "black widow" millisecond radio
pulsars, which are evaporating their companions through irradiation. It may
appear as an eclipsing radio pulsar during periods of X-ray quiescence.Comment: 4 pages with 1 figure. Style files included. Fig. 2 deleted and text
revised. To appear in Nature. Press embargo until 18:00 GMT on 1998 July 2
The Binary Pulsar PSR 1908+00 in NGC 6760
We present orbital parameters of the 3.6 ms binary pulsar 1908 +00 in the globular cluster NGC 6760. The orbital period is 3.4 hr, and the mass function is 3 x 10^6 M_⊙ , implying a minimum companion mass of 0.018 M_⊙ . The companion is probably degenerate; and if it is hydrogen, it is close to overflowing its Roche lobe. The only other millisecond binary radio pulsar systems with orbital period < 10 hr and mass function below 10-3 M_⊙ are the eclipsing pulsar 1957+20 and 1744-24A, and the very low mass binary 0021-721.
These pulsars are ablating their companions and may be the progenitors of isolated millisecond pulsars. PSR 1908+00 shows no evidence for long-duration eclipses as are seen in 1744-24A, but short-duration eclipses
as in 1957 + 20 are not excluded
The Parkes multibeam pulsar survey: IV. Discovery of 180 pulsars and parameters for 281 previously known pulsars
The Parkes multibeam pulsar survey has led to the discovery of more than 700
pulsars. In this paper, we provide timing solutions, flux densities and pulse
profiles for 180 of these new discoveries. Two pulsars, PSRs J1736-2843 and
J1847-0130 have rotational periods P > 6s and are therefore among the slowest
rotating radio pulsars known. Conversely, with P = 1.8ms, PSR J1843-1113 has
the third shortest period of pulsars currently known. This pulsar and PSR
J1905+0400 (P = 3.8ms) are both solitary. We also provide orbital parameters
for a new binary system, PSR J1420-5625, which has P = 34ms, an orbital period
of 40 days and a minimum companion mass of 0.4 solar masses. The 10 degree-wide
strip along the Galactic plane that was surveyed is known to contain 264 radio
pulsars that were discovered prior to the multibeam pulsar survey. We have
redetected almost all of these pulsars and provide new dispersion measure
values and flux densities at 20cm for the redetected pulsars.Comment: 35 pages, accepted for publication in MNRAS, a high quality image of
the figure on page 32 is available from
http://www.atnf.csiro.au/research/pulsar/images/pmsurvey_fig.p
The subpulse modulation properties of pulsars at 92 cm and the frequency dependence of subpulse modulation
A large sample of pulsars has been observed to study their subpulse
modulation at an observing wavelength (when achievable) of both 21 and 92 cm
using the Westerbork Synthesis Radio Telescope. In this paper we present the
92-cm data and a comparison is made with the already published 21-cm results.
We analysed 191 pulsars at 92 cm using fluctuation spectra. The sample of
pulsars is as unbiased as possible towards any particular pulsar
characteristics. For 15 pulsars drifting subpulses are discovered for the first
time and 26 of the new drifters found in the 21-cm data are confirmed. We
discovered nulling for 8 sources and 8 pulsars are found to intermittently emit
single pulses that have pulse energies similar to giant pulses. It is estimated
that at least half of the total population of pulsars have drifting subpulses
when observations with a high enough signal-to-noise ratio would be available.
It could well be that the drifting subpulse mechanism is an intrinsic property
of the emission mechanism itself, although for some pulsars it is difficult or
impossible to detect. Drifting subpulses are in general found at both
frequencies, although the chance of detecting drifting subpulses is possibly
slightly higher at 92 cm. It appears that the youngest pulsars have the most
disordered subpulses and the subpulses become more and more organized into
drifting subpulses as the pulsar ages. The correlations with the modulation
indices are argued to be consistent with the picture in which the radio
emission can be divided in a drifting subpulse signal plus a quasi-steady
signal which becomes, on average, stronger at high observing frequencies. The
measured values of P3 at the two frequencies are highly correlated, but there
is no evidence for a correlation with other pulsar parameters.Comment: 30 pages, 10 figures, accepted for publication in A&A, astro-ph
version is missing 191 figures due to file size restrictions. Please download
the appendix from
http://www.astron.nl/~stappers/wiki/doku.php?id=resources:publication
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