Received (to be inserted by publisher); Revised (to be inserted by publisher); Accepted (to be inserted by publisher); X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, neutron stars, and gamma-ray bursts. We designed, built and tested a X-ray polarimeter, X-Calibur, to be used in the focal plane of the balloon-borne InFOCµS grazing incidence X-ray telescope. X-Calibur combines a low-Z scatterer with a CZT detector assembly to measure the polarization of 20 − 80 keV X-rays making use of the fact that polarized photons scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of 80%. The X-Calibur detector assembly is completed, tested, and fully calibrated. The response to a polarized X-ray beam was measured successfully at the Cornell High Energy Synchrotron Source. This paper describes the design, calibration and performance of the X-Calibur polarimeter. In principle, a similar space-borne scattering polarimeter could operate over the broader 2 − 100 keV energy band.

We present a pilot survey of 21 lines of sight towards ultracompact H II (UCH II) regions and three towards continuum-free lines of sight in the formaldehyde (H 2 CO) 1 10 − 1 11 (6 cm) and 2 11 − 2 12 (2 cm) transitions, using the H 2 CO centimeter lines as a molecular gas densitometer. Using Arecibo and Green Bank beam-matched observations, we measure the density of 51 detected H 2 CO line pairs and present upper limits on density for an additional 24 detected 1 10 − 1 11 lines. We analyze the systematic uncertainties in the H 2 CO densitometer, achieving H 2 density measurements with accuracies ∼ 0.1 − 0.3 dex. The densities measured are not correlated with distance, implying that it is possible to make accurate density measurements throughout the galaxy without a distance bias. We confirm that ultracompact HII regions are associated with, and possibly embedded in, gas at densities n(H 2 ) 10 5 cm −3 . The densities measured in line-of-sight molecular clouds suggest that they consist of low volume filling factor (f ∼ 10 −2 ) gas at high (n(H 2 ) > 10 4 cm −3 ) density, which is inconsistent with purely supersonic turbulence and requires high-density clumping greater than typically observed in gravo-turbulent simulations. We observe complex line morphologies that indicate density variations with velocity around UCH II regions, and we classify a subset of the UCH II molecular envelopes as collapsing or expanding. We compare these measurements to Bolocam Galactic Plane Survey 1.1 mm observations, and note that most UCH II regions have 1.1 mm emission consisting of significant (5-70%) free-free emission and are therefore not necessarily dominated by optically thin dust emission as is often assumed when computing clump masses. A comparison of our data with the Mangum et al.(2008) starburst sample shows that the area filling factor of dense (n(H 2 ) ∼ 10 5 cm −3 ) molecular gas in typical starburst galaxies is 0.01, but in extreme starburst galaxies like Arp 220, is ∼ 0.1, suggesting that Arp 220 is physically similar to an oversized UCH II region.

We report new observations of the 1 10 -1 11 (6 cm) and 2 11 -2 12 (2 cm) transitions of ortho-formaldehyde (o-H 2 CO) in absorption at z = 0.68466 toward the gravitational lens system B0218 + 357. Radiative transfer modeling indicates that both transitions are anti-inverted relative to the 4.6 K microwave background, regardless of the source covering factor, with excitation temperatures of ∼1 K and 1.5-2 K for the 1 10 -1 11 and 2 11 -2 12 lines, respectively. Using these observations and a large velocity gradient radiative transfer model that assumes a gradient of 1 km s −1 pc −1 , we obtain a molecular hydrogen number density of 2 × 10 3 cm −3 < n(H 2 ) < 1 × 10 4 cm −3 and a column density of 2.5 × 10 13 cm −2 < N(o-H 2 CO) < 8.9 × 10 13 cm −2 , where the allowed ranges conservatively include the range of possible source covering factors in both lines. The measurements suggest that H 2 CO excitation in the absorbing clouds in the B0218 + 357 lens is analogous to that in Galactic molecular clouds: it would show H 2 CO absorption against the cosmic microwave background if it were not illuminated by the background quasar or if it were viewed from another direction.

Over the last decade, considerable effort has been made to measure the proper motions of the pulsars B1757−24 and B1951+32 in order to establish or refute associations with nearby supernova remnants and to understand better the complicated geometries of their surrounding nebulae. We present proper motion measurements of both pulsars with the Very Large Array, increasing the time baselines of the measurements from 3.9 yr to 6.5 yr and from 12.0 yr to 14.5 yr, respectively, compared to previous observations. We confirm the nondetection of proper motion of PSR B1757−24, and our measurement of (µ α , µ δ ) = (−11±9, −1±15) mas yr −1 confirms that the association of PSR B1757−24 with SNR G5.4−1.2 is unlikely for the pulsar characteristic age of 15.5 kyr, although an association cannot be excluded for a significantly larger age. For PSR B1951+32, we measure a proper motion of (µ α , µ δ ) = (−28.8 ± 0.9, −14.7 ± 0.9) mas yr −1 , reducing the uncertainty in the proper motion by a factor of 2 compared to previous results. After correcting to the local standard of rest, the proper motion indicates a kinetic age of ∼51 kyr for the pulsar, assuming it was born near the geometric center of the supernova remnant. The radio-bright arc of emission along the pulsar proper motion vector shows time-variable structure, but moves with the pulsar at an approximately constant separation ∼2.5 ′′ , lending weight to its interpretation as a shock structure driven by the pulsar.