286 research outputs found
Representations of Time Coordinates in FITS
In a series of three previous papers, formulation and specifics of the
representation of World Coordinate Transformations in FITS data have been
presented. This fourth paper deals with encoding time. Time on all scales and
precisions known in astronomical datasets is to be described in an unambiguous,
complete, and self-consistent manner. Employing the well--established World
Coordinate System (WCS) framework, and maintaining compatibility with the FITS
conventions that are currently in use to specify time, the standard is extended
to describe rigorously the time coordinate. World coordinate functions are
defined for temporal axes sampled linearly and as specified by a lookup table.
The resulting standard is consistent with the existing FITS WCS standards and
specifies a metadata set that achieves the aims enunciated above.Comment: FITS WCS Paper IV: Time. 27 pages, 11 table
Detection and localization of continuous gravitational waves with pulsar timing arrays: the role of pulsar terms
A pulsar timing array is a Galactic-scale detector of nanohertz gravitational
waves (GWs). Its target signals contain two components: the `Earth term' and
the `pulsar term' corresponding to GWs incident on the Earth and pulsar
respectively. In this work we present a Frequentist method for the detection
and localization of continuous waves that takes into account the pulsar term
and is significantly faster than existing methods. We investigate the role of
pulsar terms by comparing a full-signal search with an Earth-term-only search
for non-evolving black hole binaries. By applying the method to synthetic data
sets, we find that (i) a full-signal search can slightly improve the detection
probability (by about five percent); (ii) sky localization is biased if only
Earth terms are searched for and the inclusion of pulsar terms is critical to
remove such a bias; (iii) in the case of strong detections (with
signal-to-noise ratio 30), it may be possible to improve pulsar
distance estimation through GW measurements.Comment: 12 pages, 9 figures, typos corrected. To match the published version.
Code implementing this method is available at the PPTA Wiki pag
Search for the optical counterpart of the nearby pulsar J0108-1431
We present the results of first deep optical observations of the field of the
old ( yr), nearby, isolated pulsar J0108-1431, in an attempt to
detect its optical counterpart. The observations were performed using the FORS1
instrument at the focus of the European Southern Observatory Antu Telescope of
the VLT. Observations with the Australia Telescope Compact Array (ATCA) were
made to determine an accurate position for the radio pulsar at the current
epoch. The imaging data, obtained in the V, B, and U passbands reveal no
counterpart at the revised radio position down to ,
and . For a distance of 130 pc, estimated from the pulsar's
dispersion measure, our constraints on the optical flux put an upper limit of
K for the surface temperature of the neutron star, assuming
a stellar radius km. Our new radio position allows us to place an
upper limit on the pulsar proper motion of 82 mas yr which, for d=130
pc, implies a transverse velocity \la 50 km sec.Comment: 15 pages, 4 figures, accepted for publication in Ap
The WHI Corona from Differential Emission Measure Tomography
A three dimensional (3D) tomographic reconstruction of the local differential
emission measure (LDEM) of the global solar corona during the whole heliosphere
interval (WHI, Carrington rotation CR-2068) is presented, based on STEREO/EUVI
images. We determine the 3D distribution of the electron density, mean
temperature, and temperature spread, in the range of heliocentric heights 1.03
to 1.23 Rsun. The reconstruction is complemented with a potential field source
surface (PFSS) magnetic-field model. The streamer core, streamer legs, and
subpolar regions are analyzed and compared to a similar analysis previously
performed for CR-2077, very near the absolute minimum of the Solar Cycle 23. In
each region, the typical values of density and temperature are similar in both
periods. The WHI corona exhibits a streamer structure of relatively smaller
volume and latitudinal extension than during CR-2077, with a global
closed-to-open density contrast about 6% lower, and a somewhat more complex
morphology. The average basal electron density is found to be about 2.23 and
1.08 x 10^8 cm^-3, in the streamer core and subpolar regions, respectively. The
electron temperature is quite uniform over the analyzed height range, with
average values of about 1.13 and 0.93 MK, in the streamer core and subpolar
regions, respectively. Within the streamer closed region, both periods show
higher temperatures at mid-latitudes and lower temperatures near the equator.
Both periods show beta>1 in the streamer core and beta<1 in the surrounding
open regions, with CR-2077 exhibiting a stronger contrast. Hydrostatic fits to
the electron density are performed, and the scale height is compared to the
LDEM mean electron temperature. Within the streamer core, the results are
consistent with an isothermal hydrostatic plasma regime, with the temperatures
of ions and electrons differing by up to about 10% .. (continues)..Comment: 13 Figure
Coronal Mass Ejection Reconstruction from Three Viewpoints via Simulation Morphing. I. Theory and Examples
The problem of reconstructing the three-dimensional (3D) density distribution of a coronal mass ejection (CME) from three simultaneous coronagraph observations is timely in that the COR1 and COR2 coronagraphs on the dual-spacecraft STEREO mission complement the LASCO coronagraphs on the SOHO satellite and the Mk4 on Mauna Loa. While the separation angle between the STEREO spacecraft and the Earth depends on the time since the launch in 2006, the reconstruction problem is always severely underinformed. So far, all 3D reconstruction efforts have made use of relatively simple parameterized models in order to determine the 3D structure of the CME. Such approaches do not utilize the power of 3D MHD simulation to inform the reconstruction. This paper considers the situation in which a specific CME event observed in coronagraphs from three viewpoints is later simulated by solving MHD equations. The reconstruction is then subjected to an invertible morphological operator chosen so that morphed MHD simulation is most consistent with the three-viewpoint coronagraph data. The morphological operations are explained mathematically and synthetic examples are given. The practical application to reconstructing CMEs from STEREO and SOHO data is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98585/1/0004-637X_761_1_24.pd
Morphology and characteristics of radio pulsars
This review describes the observational properties of radio pulsars, fast
rotating neutron stars, emitting radio waves. After the introduction we give a
list of milestones in pulsar research. The following chapters concentrate on
pulsar morphology: the characteristic pulsar parameters such as pulse shape,
pulsar spectrum, polarization and time dependence. We give information on the
evolution of pulsars with frequency since this has a direct connection with the
emission heights, as postulated in the radius to frequency mapping (RFM)
concept. We deal successively with the properties of normal (slow) pulsars and
of millisecond (fast-recycled) pulsars. The final chapters give the
distribution characteristics of the presently catalogued 1300 objects.Comment: 33 pages, PDF with 30 PostScript figures, see
http://springerlink.metapress.com/link.asp?id=d6k3a6wunb138dpl Accepted by
Astronomy & Astrophysivs Review
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