21 research outputs found
Autonomous Spacecraft Navigation With Pulsars
An external reference system suitable for deep space navigation can be
defined by fast spinning and strongly magnetized neutron stars, called pulsars.
Their beamed periodic signals have timing stabilities comparable to atomic
clocks and provide characteristic temporal signatures that can be used as
natural navigation beacons, quite similar to the use of GPS satellites for
navigation on Earth. By comparing pulse arrival times measured on-board a
spacecraft with predicted pulse arrivals at a reference location, the
spacecraft position can be determined autonomously and with high accuracy
everywhere in the solar system and beyond. The unique properties of pulsars
make clear already today that such a navigation system will have its
application in future astronautics. In this paper we describe the basic
principle of spacecraft navigation using pulsars and report on the current
development status of this novel technology.Comment: 22 pages, 12 figures, 2 tables, to be published in the proceedings of
the workshop "Relativistic Positioning Systems and their Scientific
Applications", held on 19-21 Sept. 2012, Brdo near Kranj, Sloveni
Linear acceleration emission of pulsar relativistic streaming instability and interacting plasma bunches
Linear acceleration emission is one of the mechanisms that might explain
intense coherent radio emissions of radio pulsars. This mechanism is, however,
not well understood because the effects of collective plasma response and
nonlinear plasma evolution on the resulting emission power must be taken into
account. In addition, details of the radio emission properties of this
mechanism are unknown, which limits the observational verification of the
emission model.
By including collective and nonlinear plasma effects, we calculate radio
emission power properties by the linear acceleration emission mechanism that
occurs via the antenna principle for two instabilities in neutron star
magnetospheres: 1) a relativistic streaming instability and 2) interactions of
plasma bunches/clouds.
We utilize 1D electrostatic relativistic particle-in-cell simulations to
evolve the instabilities self-consistently. From the simulations, the power
properties of coherent emission are obtained by novel post-processing of
electric currents.
We found that the total radio power by plasma bunch interactions exceeds the
power of the streaming instability by eight orders of magnitude. The wave power
generated by a plasma bunch interaction can be as large as
~W. Therefore, simultaneously
interacting plasma bunches may account for the total observed radio power of
typical pulsars (-~W). The radio spectrum of the plasma bunch
is characterized by a flatter profile for lower frequencies and a power-law
index up to for higher frequencies. The plasma bunches
radiate in a wide range of frequencies simultaneously, fulfilling no specific
relation between emission frequency and height in the magnetosphere. The power
of the streaming instability is more narrowband than that of the interacting
bunches.Comment: 17 pages, 13 figures, 1 tabl
A multiwavlength study of PSR B0628-28: The first overluminous rotation-powered pulsar?
The ROSAT source RX J0630.8-2834 was suggested by positional coincidence to
be the X-ray counterpart of the old field pulsar PSR B0628-28. This
association, however, was regarded to be unlikely based on the computed
energetics of the putative X-ray counterpart. In this paper we report on
multiwavelength observations of PSR B0628-28 made with the ESO/NTT observatory
in La Silla, the Jodrell Bank radio observatory and XMM-Newton. Although the
optical observations do not detect any counterpart of RX J0630.8-2834 down to a
limiting magnitude of V=26.1 mag and B=26.3 mag, XMM-Newton observations
finally confirmed it to be the pulsar's X-ray counterpart by detecting X-ray
pulses with the radio pulsar's spin-period. The X-ray pulse profile is
characterized by a single broad peak with a second smaller peak leading the
main pulse component by ~144 degree. The fraction of pulsed photons is (38 +-
7)% with no strong energy dependence in the XMM-Newton bandpass. The pulsar's
X-ray spectrum is well described by a single component power law with photon
index 2.63^{+0.23}_{-0.15}, indicating that the pulsar's X radiation is
dominated by non-thermal emission processes. A low level contribution of
thermal emission from residual cooling or from heated polar caps, cannot be
excluded. The pulsar's spin-down to X-ray energy conversion efficiency is
obtained to be ~16% for the radio dispersion measure inferred pulsar distance.
If confirmed, PSR B0628-28 would be the first X-ray overluminous
rotation-powered pulsar identified among all ~1400 radio pulsars known today.Comment: Accepted for publication in ApJ. Find a paper copy with higher
resolution images at
ftp://ftp.xray.mpe.mpg.de/people/web/astro-ph-0505488_rev2.pd
On Pair Production in the Crab Pulsar
We consider the widespread assumption that coherent pulsar radio emission is
based on extended pair production leading to plasma densities highly exceeding
the Goldreich-Julian density. We show as an example that the observed low
frequency (160 MHz) emission of the Crab pulsar is incompatible to the model of
extended pair production. Our results rule out significant pair production if a
plasma process is responsible for coherence and the radio emission originates
from inside the light cylinder.Comment: accepted for publication in ApJ Letters; 4 pages, no figure
Precision timing of PSR J1012+5307 and strong-field GR tests
We report on the high precision timing analysis of the pulsar-white dwarf
binary PSR J1012+5307. Using 15 years of multi-telescope data from the European
Pulsar Timing Array (EPTA) network, a significant measurement of the variation
of the orbital period is obtained. Using this ideal strong-field gravity
laboratory we derive theory independent limits for both the dipole radiation
and the variation of the gravitational constant.Comment: 3 pages, Proceedings of the 12th Marcel Grossmann Meeting on General
Relativity (MG 12
Radio emission from a pulsarâs magnetic pole revealed by general relativity
International audienceBinary pulsars are affected by general relativity (GR), causing the spin axis of each pulsar to precess. We present polarimetric radio observations of the pulsar PSR J1906+0746 that demonstrate the validity of the geometrical model of pulsar polarization. We reconstruct the (sky-projected) polarization emission map over the pulsarâs magnetic pole and predict the disappearance of the detectable emission by 2028. Two tests of GR are performed using this system, including the spin precession for strongly self-gravitating bodies. We constrain the relativistic treatment of the pulsar polarization model and measure the pulsar beaming fraction, with implications for the population of neutron stars and the expected rate of neutron star mergers
A 2.1 Solar Mass Pulsar Measured by Relativistic Orbital Decay
PSR J0751+1807 is a millisecond pulsar in a circular 6 hr binary system with
a helium white dwarf secondary. Through high precision pulse timing
measurements with the Arecibo and Effelsberg radio telescopes, we have detected
the decay of its orbit due to emission of gravitational radiation. This is the
first detection of the relativistic orbital decay of a low-mass, circular
binary pulsar system. The measured rate of change in orbital period, corrected
for acceleration biases, is dP_b/dt=(-6.4+-0.9)x10^-14. Interpreted in the
context of general relativity, and combined with measurement of Shapiro delay,
it implies a pulsar mass of 2.1+-0.2 solar masses, the most massive pulsar
measured. This adds to the emerging trend toward relatively high neutron star
masses in neutron star--white dwarf binaries. Additionally, there is some
evidence for an inverse correlation between pulsar mass and orbital period in
these systems. We consider alternatives to the general relativistic analysis of
the data, and we use the pulsar timing data to place limits on violations of
the strong equivalence principle.Comment: 9 pages, Submitted to Ap
Revealing the X-ray emission processes of old rotation-powered pulsars: XMM-Newton Observations of PSR B0950+08,PSR B0823+26 and PSR J2043+2740
We have completed part of a program to study the X-ray emission properties of
old rotation-powered pulsars with XMM-Newton in order to probe and identify the
origin of their X-radiation. The X-ray emission from these old pulsars is
largely dominated by non-thermal processes. None of the observed spectra
required adding a thermal component consisting of either a hot polar cap or
surface cooling emission to model the data. The X-ray spectrum of PSR 0950+08
is best described by a single power law of photon-index
1.93^{+0.14}_{-0.12}.Taking optical data from the VLT FORS1 into account a
broken power law model is found to describe the pulsar's broadband spectrum
from the optical to the X-ray band. Temperature upper limits for possible
contributions from a heated polar cap or the whole neutron star surface are
T_{pc} < 0.87 x10^6 K and T_s < 0.48 x 10^6 K, respectively. We also find that
the X-ray emission from PSR 0950+08 is pulsed with two peaks per rotation
period. The phase separation between the two X-ray peaks is ~144 degree. The
main radio peak and the trailing X-ray peak are almost phase aligned. The
fraction of X-ray pulsed photons is ~30%. A phase-resolved spectral analysis
confirms the non-thermal nature of the pulsed emission. Detailed pulse profile
simulations constrain the pulsar's emission geometry to be that of an almost
orthogonal rotator. The spectral emission properties observed for PSR 0823+26
are similar to those of PSR 0950+08. For PSR J2043+2740 we report the first
detection of X-ray emission. A power law spectrum, or a combination of a
thermal and a power law spectrum all yield acceptable descriptions of its X-ray
spectrum. No X-ray pulses are detected from PSR J2043+2740. A pulsed fraction
upper limit is 57%.Comment: Accepted for publication in ApJ on July 15, 2004. The paper with
higher resolution images can be obtained form
ftp://ftp.xray.mpe.mpg.de/people/web/X-ray-emission-from-old-pulsar
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap