Geostationary satellites are unique among orbital spacecraft in that they experience no appreciable atmospheric drag. After concluding their respective missions, geostationary spacecraft remain in orbit virtually in perpetuity. As such, they represen
t some of human civilizations longest lasting artifacts. With this in mind, the EchoStar XVI satellite, to be launched in fall 2012, will play host to a time capsule intended as a message for the deep future. Inspired in part by the Pioneer Plaque and Voyager Golden Records, the EchoStar XVI Artifact is a pair of gold-plated aluminum jackets housing a small silicon disc containing one hundred photographs. The Cover Etching, the subject of this paper, is etched onto one of the two jackets. It is a temporal map consisting of a star chart, pulsar timings, and other information describing the epoch from which EchoStar XVI came. The pulsar sample consists of 13 rapidly rotating objects, 5 of which are especially stable, having spin periods < 10 ms and extremely small spindown rates. In this paper, we discuss our approach to the time map etched onto the cover and the scientific data shown on it; and we speculate on the uses that future scientists may have for its data. The other portions of the EchoStar XVI Artifact will be discussed elsewhere.
Accurate distances to pulsars can be used for a variety of studies of the Galaxy and its electron content. However, most distance measures to pulsars have been derived from the absorption (or lack thereof) of pulsar emission by Galactic HI gas, which
typically implies that only upper or lower limits on the pulsar distance are available. We present a critical analysis of all measured HI distance limits to pulsars and other neutron stars, and translate these limits into actual distance estimates through a likelihood analysis that simultaneously corrects for statistical biases. We also apply this analysis to parallax measurements of pulsars in order to obtain accurate distance estimates and find that the parallax and HI distance measurements are biased in different ways, because of differences in the sampled populations. Parallax measurements typically underestimate a pulsars distance because of the limited distance to which this technique works and the consequential strong effect of the Galactic pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading to overestimated distances because the bright pulsars on which this technique is applicable are more likely to be nearby given their brightness.
We present results from multi-epoch neutral hydrogen (HI) absorption observations of six bright pulsars with the Arecibo telescope. Moving through the interstellar medium (ISM) with transverse velocities of 10--150 AU/yr, these pulsars have swept acr
oss 1--200 AU over the course of our experiment, allowing us to probe the existence and properties of the tiny scale atomic structure (TSAS) in the cold neutral medium (CNM). While most of the observed pulsars show no significant change in their HI absorption spectra, we have identified at least two clear TSAS-induced opacity variations in the direction of B1929+10. These observations require strong spatial inhomogeneities in either the TSAS clouds physical properties themselves or else in the clouds galactic distribution. While TSAS is occasionally detected on spatial scales down to 10 AU, it is too rare to be characterized by a spectrum of turbulent CNM fluctuations on scales of 10-1000 AU, as previously suggested by some work. In the direction of B1929+10, an apparent correlation between TSAS and interstellar clouds inside the warm Local Bubble (LB) indicates that TSAS may be tracing the fragmentation of the LB wall via hydrodynamic instabilities. While similar fragmentation events occur frequently throughout the ISM, the warm medium surrounding these cold cloudlets induces a natural selection effect wherein small TSAS clouds evaporate quickly and are rare, while large clouds survive longer and become a general property of the ISM.
We have used the Arecibo telescope to measure the HI absorption spectra of eight pulsars. We show how kinematic distance measurements depend upon the values of the galactic constants R_o and Theta_o, and we select our preferred current values from th
e literature. We then derive kinematic distances for the low-latitude pulsars in our sample and electron densities along their lines of sight. We combine these measurements with all others in the inner galactic plane visible from Arecibo to study the electron density in this region. The electron density in the interarm range 48 degrees < l < 70 degrees is [0.017 (-0.007,+0.012) (68% c.l.)] cm^(-3). This is 0.75 (-0.22,+0.49) (68% c.l.) of the value calculated by the Cordes & Lazio (2002) galactic electron density model. The model agrees more closely with electron density measurements toward Arecibo pulsars lying closer to the galactic center, at 30 degrees<l<48 degrees. Our analysis leads to the best current estimate of the distance of the relativistic binary pulsar B1913+16: d=(9.0 +/- 3) kpc. We use the high-latitude pulsars to search for small-scale structure in the interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19 exhibited significant changes in its absorption spectrum over 22 yr, indicating HI structure on a ~500 AU scale.
