2,533 research outputs found
Circulating Subbeam Systems and the Physics of Pulsar Emission
The purpose of this paper is to suggest how detailed single-pulse
observations of ``slow'' radio pulsars may be utilized to construct an
empirical model for their emission. It links the observational synthesis
developed in a series of papers by Rankin starting in the 1980s to the more
recent empirical feedback model of Wright (2003a) by regarding the entire
pulsar magnetosphere as a non-steady, non-linear interactive system with a
natural built-in delay. It is argued that the enhanced role of the outer gap in
such a system indicates an evolutionary link to younger pulsars, in which this
region is thought to be highly active, and that pulsar magnetospheres should no
longer be seen as being ``driven'' by events on the neutron star's polar cap,
but as having more in common with planetary magnetospheres and auroral
phenomena.Comment: 15 pages, 3 figure
The `Periodic Nulls' of Radio Pulsar J1819+1305
We present a single-pulse study of the four-component pulsar J1819+1305,
whose ``null'' pulses bunch at periodic intervals of around 57 times the
rotation period. The emission bursts between the null bunches exhibit
characteristic modulations at two shorter periodicities of approximately 6.2
and 3 times the rotation period, the former found largely in the two outer
components, and the latter only in the first component. Many bursts commence
with bright emission in second component, exhibit positive six-period drift
across the full profile width, and end with 3-period modulation in the leading
component. The 57-period cycle can be modelled geometrically as a sparsely
filled subbeam carousel with nulls appearing whenever our line of sight
intersects a circulating empty region. This interpretation is compatible with
other recent evidence for periodic, carousel-related nulling and appears to
support the physics of a polar-gap emission model for ``drifting'' subpulses,
but the subtle structure of the emission bursts defies an easy explanation.Comment: 8 pages, 9 figure
Drifting, moding, and nulling: another look at pulsar B1918+19
Arecibo observations of the conal triple pulsar B1918+19 at 0.327- and
1.4-GHz are used to analyse its subpulse behaviour in detail. We confirm the
presence of three distinct drift modes (A,B,C) plus a disordered mode (N) and
show that they follow one another in specific cycles. Interpreting the pulsar's
profile as resulting from a sightline traverse which cuts across an outer cone
and tangentially grazes an inner cone, we demonstrate that the phase modulation
of the inner cone is locked to the amplitude modulation of the outer cone in
all the drift modes. The 9% nulls are found to be largely confined to the
dominant B and N modes, and, in the N mode, create alternating bunches of nulls
and emission in a quasi-periodic manner with an averaged fluctuation rate of
about 12 rotation periods (). We explore the assumption that the apparent
drift is the first alias of a faster drift of subbeams equally spaced around
the cones. This is shown to imply that all modes A, B and C have a common
circulation time of 12 and differ only in the number of subbeams. This
timescale is on the same order as predicted by the classic {\bf E}{\bf
B} drift and also coincides with the N-mode modulation. We therefore arrive at
a picture where the circulation speed remains roughly invariant while the
subbeams progressively diminish in number from modes A to B to C, and are then
re-established during the N mode. We suggest that aliasing combined with
subbeam loss may be responsible for apparently dramatic changes in drift rates
in other pulsars
The NASA-UC Eta-Earth Program: I. A Super-Earth Orbiting HD 7924
We report the discovery of the first low-mass planet to emerge from the
NASA-UC Eta-Earth Program, a super-Earth orbiting the K0 dwarf HD 7924.
Keplerian modeling of precise Doppler radial velocities reveals a planet with
minimum mass M_P sin i = 9.26 M_Earth in a P = 5.398 d orbit. Based on
Keck-HIRES measurements from 2001 to 2008, the planet is robustly detected with
an estimated false alarm probability of less than 0.001. Photometric
observations using the Automated Photometric Telescopes at Fairborn Observatory
show that HD 7924 is photometrically constant over the radial velocity period
to 0.19 mmag, supporting the existence of the planetary companion. No transits
were detected down to a photometric limit of ~0.5 mmag, eliminating transiting
planets with a variety of compositions. HD 7924b is one of only eight planets
known with M_P sin i < 10 M_Earth and as such is a member of an emerging family
of low-mass planets that together constrain theories of planet formation.Comment: ApJ accepted, 10 pages, 10 figures, 4 table
Simultaneous Observations of Comet C/2002 T7 (LINEAR) with the Berkeley-Illinois-Maryland Association and Owens Valley Radio Observatory Interferometers: HCN and CH_3OH
We present observations of HCN J = 1-0 and CH_3OH J(K_a, K_c) = 3(1, 3)-4(0, 4) A+ emission from comet C/2002 T7 (LINEAR) obtained simultaneously with the Owens Valley Radio Observatory (OVRO) and Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers. We combined the data from both arrays to increase the (u, v) sampling and signal to noise of the detected line emission. We also report the detection of CH_3OH J(K_a, K_c) = 8(0, 8)-7(1, 7) A^+ with OVRO data alone. Using a molecular excitation code that includes the effects of collisions with water and electrons, as well as pumping by the Solar infrared photons (for HCN alone), we find a production rate of HCN of 2.9 × 10^(26) s^(–1) and for CH_3OH of 2.2 × 10^(27) s^(–1). Compared to the adopted water production rate of 3 × 10^(29) s^(–1), this corresponds to an HCN/H_2O ratio of 0.1% and a CH_3OH/H_2O ratio of 0.7%. We critically assess the uncertainty of these values due to the noise (~10%), the uncertainties in the adopted comet model (~50%), and the uncertainties in the adopted collisional excitation rates (up to a factor of 2). Pumping by Solar infrared photons is found to be a minor effect for HCN, because our 15" synthesized beam is dominated by the region in the coma where collisions dominate. Since the uncertainties in the derived production rates are at least as large as one-third of the differences found between comets, we conclude that reliable collision rates and an accurate comet model are essential. Because the collisionally dominated region critically depends on the water production rate, using the same approximate method for different comets may introduce biases in the derived production rates. Multiline observations that directly constrain the molecular excitation provide much more reliable production rates
The NASA-UC Eta-Earth Program: II. A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses
We report the discovery of HD 156668b, an extrasolar planet with a minimum
mass of M_P sin i = 4.15 M_Earth. This planet was discovered through Keplerian
modeling of precise radial velocities from Keck-HIRES and is the second
super-Earth to emerge from the NASA-UC Eta-Earth Survey. The best-fit orbit is
consistent with circular and has a period of P = 4.6455 d. The Doppler
semi-amplitude of this planet, K = 1.89 m/s, is among the lowest ever detected,
on par with the detection of GJ 581e using HARPS. A longer period (P ~ 2.3 yr),
low-amplitude signal of unknown origin was also detected in the radial
velocities and was filtered out of the data while fitting the short-period
planet. Additional data are required to determine if the long-period signal is
due to a second planet, stellar activity, or another source. Photometric
observations using the Automated Photometric Telescopes at Fairborn Observatory
show that HD 156668 (an old, quiet K3 dwarf) is photometrically constant over
the radial velocity period to 0.1 mmag, supporting the existence of the planet.
No transits were detected down to a photometric limit of ~3 mmag, ruling out
transiting planets dominated by extremely bloated atmospheres, but not
precluding a transiting solid/liquid planet with a modest atmosphere.Comment: This planet was announced at the 2010 AAS meeting in Wash. DC; 12
pages, 8 figures, 3 tables, submitted to Ap
The NASA-UC Eta-Earth Program: III. A Super-Earth orbiting HD 97658 and a Neptune-mass planet orbiting Gl 785
We report the discovery of planets orbiting two bright, nearby early K dwarf
stars, HD 97658 and Gl 785. These planets were detected by Keplerian modelling
of radial velocities measured with Keck-HIRES for the NASA-UC Eta-Earth Survey.
HD 97658 b is a close-in super-Earth with minimum mass Msini = 8.2 +/- 1.2
M_Earth, orbital period P = 9.494 +/- 0.005 d, and an orbit that is consistent
with circular. Gl 785 b is a Neptune-mass planet with Msini = 21.6 +/- 2.0
M_Earth, P = 74.39 +/- 0.12 d, and orbital eccentricity 0.30 +/- 0.09.
Photometric observations with the T12 0.8 m automatic photometric telescope at
Fairborn Observatory show that HD 97658 is photometrically constant at the
radial velocity period to 0.09 mmag, supporting the existence of the planet.Comment: Submitted to ApJ, 7 pages, 6 figures, 5 table
An Idealized Pulsar Magnetosphere: the Relativistic Force-Free Approximation
The non-dissipative relativistic force-free condition should be a good
approximation to describe the electromagnetic field in much of the pulsar
magnetosphere, but we may plausibly expect it to break down in singular
domains. Self-consistent magnetospheric solutions are found with field lines
closing both at and within the light-cylinder. In general, the detailed
properties of the solutions may be affected critically by the physics
determining the appropriate choice of equatorial boundary condition beyond the
light-cylinder.Comment: 31 pages, 3 figure
Five Intermediate-Period Planets from the N2K Sample
We report the detection of five Jovian mass planets orbiting high metallicity
stars. Four of these stars were first observed as part of the N2K program and
exhibited low RMS velocity scatter after three consecutive observations.
However, follow-up observations over the last three years now reveal the
presence of longer period planets with orbital periods ranging from 21 days to
a few years. HD 11506 is a G0V star with a planet of \msini = 4.74 \mjup in a
3.85 year orbit. HD 17156 is a G0V star with a 3.12 \mjup planet in a 21.2 day
orbit. The eccentricity of this orbit is 0.67, one of the highest known for a
planet with a relatively short period. The orbital period for this planet
places it in a region of parameter space where relatively few planets have been
detected. HD 125612 is a G3V star with a planet of \msini = 3.5 \mjup in a 1.4
year orbit. HD 170469 is a G5IV star with a planet of \msini = 0.67 \mjup in a
3.13 year orbit. HD 231701 is an F8V star with planet of 1.08 \mjup in a 142
day orbit. All of these stars have supersolar metallicity. Three of the five
stars were observed photometrically but showed no evidence of brightness
variability. A transit search conducted for HD 17156 was negative but covered
only 25% of the search space and so is not conclusive.Comment: 13 pages, 9 figures, accepted ApJ Resubmitted here with some
additional data, modified Keplerian orbit
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