48 research outputs found
A Search for Cold Dust around Neutron Stars
We present observations of nine radio pulsars using the
Heinrich-Hertz-Telescope at \lambda 0.87mm and the IRAM 30-m telescope at
\lambda 1.2mm in search for a cold dust around these sources. Five of the
program pulsars have been observed for the first time at the mm-wavelengths.
The results are consistent with the absence of circumpulsar disks that would be
massive enough () to support planet formation according to
the scenarios envisioned for solar-type stars, but they do not exclude lower
mass () disks for a wide range of grain sizes. These
conclusions confirm the previously published results and, together with the
current lack of further detections of pulsar planets, they suggest that planet
formation around neutron stars is not a common phenomenon.Comment: 5 pages, 1 figure, accepted for publication in A&
The binary pulsar PSR J1811-1736: evidence of a low amplitude supernova kick
Aims: The binary pulsar PSR J1811-1736 has been identified, since its
discovery, as a member of a double neutron star system. Observations of such
binary pulsars allow the measurement of general relativistic effects, which in
turn lead to information about the orbiting objects and, in a few cases, to
tests of theories of gravity. Methods: Regular timing observations have been
carried out with three of the largest European radio telescopes involved in
pulsar research. The prospects of continued observations were studied with
simulated timing data. Pulse scattering times were measured using dedicated
observations at 1.4 GHz and at 3.1 GHz, and the corresponding spectral index
has also been determined. The possibility of detecting the yet unseen companion
as a radio pulsar was also investigated. A study of the natal kick received by
the younger neutron star at birth was performed. Results: We present an up to
date and improved timing solution for the binary pulsar PSR J1811-1736. One
post-Keplerian parameter, the relativistic periastron advance, is measured and
leads to the determination of the total mass of this binary system. The pulse
profile at 1.4 GHz is heavily broadened by interstellar scattering, limiting
the timing precision achievable at this frequency and the measurability of
other post-keplerian parameters. Interstellar scattering is unlikely to be the
reason for the continued failure to detect radio pulsations from the companion
of PSR J1811-1736. The probability distribution that we derive for the
amplitude of the kick imparted on the companion neutron star at its birth
indicates that the kick has been of low amplitude.Comment: 8 pages, 4 figures, accepted for publication on A&A. (Abridged
abstract
Observations of pulsars at 9 millimetres
The behaviour of the pulsar spectrum at high radio frequencies can provide
decisive information about the nature of the radio emission mechanism. We
report recent observations of a selected sample of pulsars at lambda=9mm (32
GHz) with the 100-m Effelsberg radio telescope.Three pulsars, PSR B0144+59, PSR
B0823+26, and PSR B2022+50, were detected for the first time at this frequency.
We confirm the earlier flux density measurements for a sample of six pulsars,
and we are able to place upper flux density limits for another 12 pulsars. We
find that all pulsar spectra have a simple form that can be described using
only three parameters, one of which is the lifetime of short nano-pulses in the
emission region.The study of the transition region from coherent to incoherent
emission needs further and more sensitive observations at even higher radio
frequencies.Comment: to appear in A&A (in press), 7 pages, 3 figure
The Frequency Evolution of Interstellar Pulse Broadening from Radio Pulsars
In this paper we report multi-frequency measurements of pulse broadening
times (tau_d) for nine medium dispersion measure (DM pc
cm) pulsars observed over a wide frequency range. The low frequency data
at 243, 325 and 610 MHz are new observations done with the Giant Metrewave
Radio Telescope (GMRT). The frequency dependence of tau_d for all but one (PSR
B1933+16) of our sources is consistent with the Kolmogorov spectrum of electron
density fluctuations in a turbulent medium. PSR B1933+16, however, shows a very
flat spectrum as previously observed for high DM pulsars. Our observations
combined with earlier published results enable us to study the spectral index
of tau_d over the whole observed DM range. While the spectral properties are
generally consistent with a Kolmogorov spectrum, pulsars seen along
line-of-sights towards the inner Galaxy or complex regions often show
deviations from this expected behaviour.Comment: 7 pages, 5 figures, accepted for publication in A&
Anomalous scattering of highly dispersed pulsars
We report multifrequency measurements of scatter broadening times for nine
highly dispersed pulsars over a wide frequency range (0.6 -- 4.9 GHz). We find
the scatter broadening times to be larger than expected and to scale with
frequency with an average power-law index of , i.e. significantly
less than that expected from standard theories. Such possible discrepancies
have been predicted very recently by Cordes & Lazio.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter
XMM-Newton spectral and timing analysis of the faint millisecond pulsars PSR J0751+1807 and PSR J1012+5307
We present XMM-Newton MOS imaging and PN timing data of the faint millisecond
pulsars PSR J0751+1807 and PSR J1012+5307. We find 46 sources in the MOS field
of view of PSR J0751+1807 searching down to an unabsorbed flux limit of 3 x
10^-15 ergs cm^-2 s^-1 (0.2-10.0 keV). We present, for the first time, the
X-ray spectra of these two faint millisecond pulsars. We find that a power law
model best fits the spectrum of PSR J0751+1807, Gamma=1.59+/-0.20, with an
unabsorbed flux of 4.4 x 10^-14 ergs cm^-2 s^-1 (0.2-10.0 keV). A power law is
also a good description of the spectrum of PSR J1012+5307, Gamma=1.78+/-0.36,
with an unabsorbed flux of 1.2 x 10^-13 ergs cm^-2 s^-1 (0.2-10.0 keV).
However, a blackbody model can not be excluded as the best fit to this data. We
present some evidence to suggest that both of these millisecond pulsars show
pulsations in this X-ray band. We find some evidence for a single broad X-ray
pulse for PSR J0751+1807 and we discuss the possibility that there are two
pulses per spin period for PSR J1012+5307.Comment: 8 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
BeppoSAX observation of PSR B1937+21
We present the results of a BeppoSAX observation of the fastest rotating
pulsar known: PSR B1937+21. The ~200 ks observation (78.5 ks MECS/34 ks LECS
on-source time) allowed us to investigate with high statistical significance
both the spectral properties and the pulse profile shape. The pulse profile is
clearly double peaked at energies > ~4 keV. Peak widths are compatible with the
instrumental time resolution and the second pulse lags the main pulse 0.52 in
phase, like is the case in the radio. In the 1.3-4 keV band we detect a ~45% DC
component; conversely the 4-10 keV pulsed fraction is consistent with 100%. The
on-pulse spectrum is fitted with an absorbed power-law of spectral index ~1.2,
harder than that of the total flux which is ~1.9. The total unabsorbed (2-10
keV) flux is F_{2-10} = 4.1 10^-13 cgs, implying a luminosity of L_X = 5.0
10^31 \Theta (d/3.6 kpc)^2 erg s^-1 and a X-ray efficiency of \eta = 4.5 10^-5
\Theta, where \Theta is the solid angle spanned by the emission beam. These
results are in agreement with those obtained by ASCA and a more recent
Rossi-XTE observation. The hydrogen column density N_H ~2 10^22 cm^-2 is ~10
times higher than expected from the radio dispersion measure and average
Galactic density of e-. Though it is compatible (within 2\sigma) with the
Galactic (HI derived) value of ~1 10^22 cm^-2, inspection of dust extinction
maps reveal that the pulsar falls in a highly absorbed region. In addition, 1.4
GHz radio map shows that the nearby (likely unrelated) HII source 4C21.53W is
part of a circular emission region ~4' across.Comment: 8 pages, 5 figures; accepted for publication in A&
The Parallax, Mass and Age of the PSR J2145-0750 binary system
We present results of timing measurements of the binary millisecond pulsar
PSR J2145-0750. Combining timing data obtained with the Effelsberg and Lovell
radio telescopes we measure a significant timing parallax of 2.0(6) mas placing
the system at 500 pc distance to the solar system. The detected secular change
of the projected semi-major axis of the orbit
lt-s s, where , is caused by the proper motion of
the system. With this measurement we can constrain the orbital inclination
angle to i<61\degr, with a median likelihood value of 46\degr which is
consistent with results from polarimetric studies of the pulsar magnetosphere.
This constraint together with the non-detection of Shapiro delay rules out
certain combinations of the companion mass, , and the inclination, .
For typical neutron star masses and using optical observations of the
carbon/oxygen-core white dwarf we derive a mass range for the companion of . We apply evolutionary white dwarf cooling
models to revisit the cooling age of the companion. Our analysis reveals that
the companion has an effective temperature of K and a
cooling age of Gyr, which is roughly a factor of three
lower than the pulsar's characteristic age of 10.4 Gyr. The cooling age implies
an initial spin period of ms, which is very close to the current
period.Comment: 11 pages, 5 figures, accepted for publication in A&