1,509 research outputs found
RFI Identification and Mitigation Using Simultaneous Dual Station Observations
RFI mitigation is a critically important issue in radio astronomy using
existing instruments as well as in the development of next-generation radio
telescopes, such as the Square Kilometer Array (SKA). Most designs for the SKA
involve multiple stations with spacings of up to a few thousands of kilometers
and thus can exploit the drastically different RFI environments at different
stations. As demonstrator observations and analysis for SKA-like instruments,
and to develop RFI mitigation schemes that will be useful in the near term, we
recently conducted simultaneous observations with Arecibo Observatory and the
Green Bank Telescope (GBT). The observations were aimed at diagnosing RFI and
using the mostly uncorrelated RFI between the two sites to excise RFI from
several generic kinds of measurements such as giant pulses from Crab-like
pulsars and weak HI emission from galaxies in bands heavily contaminated by
RFI. This paper presents observations, analysis, and RFI identification and
excision procedures that are effective for both time series and spectroscopy
applications using multi-station data.Comment: 12 pages, 9 figures (4 in ps and 5 in jpg formats), Accepted for
publication in Radio Scienc
VLA Observations of Single Pulses from the Galactic Center Magnetar
We present the results of a 7-12 GHz phased-array study of the Galactic
center magnetar J1745-2900 with the Karl G. Jansky Very Large Array (VLA).
Using data from two 6.5 hour observations from September 2014, we find that the
average profile is comprised of several distinct components at these epochs and
is stable over day timescales and GHz frequencies. Comparison with
additional phased VLA data at 8.7 GHz shows significant profile changes on
longer timescales. The average profile at 7-12 GHz is dominated by the jitter
of relatively narrow pulses. The pulses in each of the four main profile
components seen in September 2014 are uncorrelated in phase and amplitude,
though there is a small but significant correlation in the occurrence of pulses
in two of the profile components. Using the brightest pulses, we measure the
dispersion and scattering parameters of J1745-2900. A joint fit of 38 pulses
gives a 10 GHz pulse broadening time of and a dispersion measure of . Both of these results are consistent with previous measurements,
which suggests that the scattering and dispersion measure of J1745-2900 may be
stable on timescales of several years.Comment: 20 pages, 10 figures, published in Ap
VLBI astrometry of PSR J2222-0137: a pulsar distance measured to 0.4% accuracy
The binary pulsar J2222-0137 is an enigmatic system containing a partially
recycled millisecond pulsar and a companion of unknown nature. Whilst the low
eccentricity of the system favors a white dwarf companion, an unusual double
neutron star system is also a possibility, and optical observations will be
able to distinguish between these possibilities. In order to allow the absolute
luminosity (or upper limit) of the companion object to be properly calibrated,
we undertook astrometric observations with the Very Long Baseline Array to
constrain the system distance via a measurement of annual geometric parallax.
With these observations, we measure the parallax of the J2222-0137 system to be
3.742 +0.013 -0.016 milliarcseconds, yielding a distance of 267.3 +1.2 -0.9 pc,
and measure the transverse velocity to be 57.1 +0.3 -0.2 km/s. Fixing these
parameters in the pulsar timing model made it possible to obtain a measurement
of Shapiro delay and hence the system inclination, which shows that the system
is nearly edge-on (sin i = 0.9985 +/- 0.0005). Furthermore, we were able to
detect the orbital motion of J2222-0137 in our VLBI observations and measure
the longitude of ascending node. The VLBI astrometry yields the most accurate
distance obtained for a radio pulsar to date, and is furthermore the most
accurate parallax for any radio source obtained at "low" radio frequencies
(below ~5 GHz, where the ionosphere dominates the error budget). Using the
astrometric results, we show the companion to J2222-0137 will be easily
detectable in deep optical observations if it is a white dwarf. Finally, we
discuss the implications of this measurement for future ultra-high-precision
astrometry, in particular in support of pulsar timing arrays.Comment: 22 pages, 7 figures, accepted for publication in Ap
A Survey of 56 Mid-latitude EGRET Error Boxes for Radio Pulsars
We have conducted a radio pulsar survey of 56 unidentified gamma-ray sources
from the 3rd EGRET catalog which are at intermediate Galactic latitudes (5 deg.
< |b| < 73 deg.). For each source, four interleaved 35-minute pointings were
made with the 13-beam, 1400-MHz multibeam receiver on the Parkes 64-m radio
telescope. This covered the 95% error box of each source at a limiting
sensitivity of about 0.2 mJy to pulsed radio emission for periods P > 10 ms and
dispersion measures < 50 pc cm-3. Roughly half of the unidentified gamma-ray
sources at |b| > 5 deg. with no proposed active galactic nucleus counterpart
were covered in this survey. We detected nine isolated pulsars and four
recycled binary pulsars, with three from each class being new. Timing
observations suggest that only one of the pulsars has a spin-down luminosity
which is even marginally consistent with the inferred luminosity of its
coincident EGRET source. Our results suggest that population models, which
include the Gould belt as a component, overestimate the number of isolated
pulsars among the mid-latitude Galactic gamma-ray sources and that it is
unlikely that Gould belt pulsars make up the majority of these sources.
However, the possibility of steep pulsar radio spectra and the confusion of
terrestrial radio interference with long-period pulsars (P > 200 ms) having
very low dispersion measures (< 10 pc cm-3, expected for sources at a distance
of less than about 1 kpc) prevent us from strongly ruling out this hypothesis.
Our results also do not support the hypothesis that millisecond pulsars make up
the majority of these sources. Non-pulsar source classes should therefore be
further investigated as possible counterparts to the unidentified EGRET sources
at intermediate Galactic latitudes.Comment: 24 pages, including 4 figures and 3 tables. Accepted for publication
in Ap
Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2-216
We present 1420 MHz polarization images of a 2.5 X 2.5 degree region around
the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian
Galactic Plane Survey (CGPS). An arc of low polarized intensity appears
prominently in the north-east portion of the visible disk of Sh 2-216,
coincident with the optically identified interaction region between the PN and
the interstellar medium (ISM). The arc contains structural variations down to
the ~1 arcminute resolution limit in both polarized intensity and polarization
angle. Several polarization-angle "knots" appear along the arc. By comparison
of the polarization angles at the centers of the knots and the mean
polarization angle outside Sh 2-216, we estimate the rotation measure (RM)
through the knots to be -43 +/- 10 rad/m^2. Using this estimate for the RM and
an estimate of the electron density in the shell of Sh 2-216, we derive a
line-of-sight magnetic field in the interaction region of 5.0 +/- 2.0 microG.
We believe it more likely the observed magnetic field is interstellar than
stellar, though we cannot completely dismiss the latter possibility. We
interpret our observations via a simple model which describes the ISM magnetic
field around Sh 2-216, and comment on the potential use of old PNe as probes of
the magnetized ISM.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Radio timing and optical photometry of the black widow system PSR J1518+0204C in the globular cluster M5
We report on the determination of astrometric, spin and orbital parameters
for PSR J1518+0204C, a "black widow" binary millisecond pulsar in the globular
cluster M5. The accurate position and orbital parameters obtained from radio
timing allowed us to search for the optical companion. By using WFC3/HST images
we identified a very faint variable star (m_F390W > 24.8, m_F606W > 24.3,
m_F814W > 23.1) located at only 0.25" from the pulsar's timing position. Due to
its strong variability, this star is visible only in a sub-sample of images.
However, the light curve obtained folding the available data with the orbital
parameters of the pulsar shows a maximum at the pulsar inferior conjunction and
a possible minimum at the pulsar superior conjunction. Furthermore, the shape
of the optical modulation indicates a heating process possibly due to the
pulsar wind. This is the first identification of an optical companion to a
black widow pulsar in the dense stellar environment of a globular cluster.Comment: Accepted for publication by ApJ; 24 Pages, 5 Figures, 1 Tabl
Observing Radio Pulsars in the Galactic Centre with the Square Kilometre Array
The discovery and timing of radio pulsars within the Galactic centre is a
fundamental aspect of the SKA Science Case, responding to the topic of "Strong
Field Tests of Gravity with Pulsars and Black Holes" (Kramer et al. 2004;
Cordes et al. 2004). Pulsars have in many ways proven to be excellent tools for
testing the General theory of Relativity and alternative gravity theories (see
Wex (2014) for a recent review). Timing a pulsar in orbit around a companion,
provides a unique way of probing the relativistic dynamics and spacetime of
such a system. The strictest tests of gravity, in strong field conditions, are
expected to come from a pulsar orbiting a black hole. In this sense, a pulsar
in a close orbit ( < 1 yr) around our nearest supermassive black
hole candidate, Sagittarius A* - at a distance of ~8.3 kpc in the Galactic
centre (Gillessen et al. 2009a) - would be the ideal tool. Given the size of
the orbit and the relativistic effects associated with it, even a slowly
spinning pulsar would allow the black hole spacetime to be explored in great
detail (Liu et al. 2012). For example, measurement of the frame dragging caused
by the rotation of the supermassive black hole, would allow a test of the
"cosmic censorship conjecture." The "no-hair theorem" can be tested by
measuring the quadrupole moment of the black hole. These are two of the prime
examples for the fundamental studies of gravity one could do with a pulsar
around Sagittarius A*. As will be shown here, SKA1-MID and ultimately the SKA
will provide the opportunity to begin to find and time the pulsars in this
extreme environment.Comment: 14 pages, 5 figures, to be published in: "Advancing Astrophysics with
the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)04
Discovery of three new millisecond pulsars in Terzan 5
We report on the discovery of three new millisecond pulsars (namely
J1748-2446aj, J1748-2446ak and J1748-2446al) in the inner regions of the dense
stellar system Terzan 5. These pulsars have been discovered thanks to a method,
alternative to the classical search routines, that exploited the large set of
archival observations of Terzan 5 acquired with the Green Bank Telescope over 5
years (from 2010 to 2015). This technique allowed the analysis of stacked power
spectra obtained by combining ~206 hours of observation. J1748-2446aj has a
spin period of ~2.96 ms, J1748-2446ak of ~1.89 ms (thus it is the fourth
fastest pulsar in the cluster) and J1748-2446al of ~5.95 ms. All the three
millisecond pulsars are isolated and currently we have timing solutions only
for J1748-2446aj and J1748-2446ak. For these two systems, we evaluated the
contribution to the measured spin-down rate of the acceleration due to the
cluster potential field, thus estimating the intrinsic spin-down rates, which
are in agreement with those typically measured for millisecond pulsars in
globular clusters. Our results increase to 37 the number of pulsars known in
Terzan 5, which now hosts 25% of the entire pulsar population identified, so
far, in globular clusters.Comment: 10 pages, 6 figures. ApJ (accepted
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