57 research outputs found
Exploring Zwitterionic Coronas as Alternatives to Traditional PEG Architectures for Improved siRNA Polyplex Pharmacokinetics and Tumor Delivery
Chandra Phase-Resolved X-ray Spectroscopy of the Crab Pulsar II
We present a new study of the X-ray spectral properties of the Crab Pulsar.
The superb angular resolution of the Chandra X-ray Observatory enables
distinguishing the pulsar from the surrounding nebulosity. Analysis of the
spectrum as a function of pulse phase allows the least-biased measure of
interstellar X-ray extinction due primarily to photoelectric absorption and
secondarily to scattering by dust grains in the direction of the Crab Nebula.
We modify previous findings that the line-of-sight to the Crab is
under-abundant in oxygen and provide measurements with improved accuracy and
less bias. Using the abundances and cross sections from Wilms, Allen & McCray
(2000) we find [O/H] = ( is
solar abundance). We also measure for the first time the impact of scattering
of flux out of the image by interstellar grains. We find . Analysis of the spectrum as a function of pulse phase also
measures the X-ray spectral index even at pulse minimum --- albeit with
increasing statistical uncertainty. The spectral variations are, by and large,
consistent with a sinusoidal variation. The only significant variation from the
sinusoid occurs over the same phase range as some rather abrupt behavior in the
optical polarization magnitude and position angle. We compare these spectral
variations to those observed in Gamma-rays and conclude that our measurements
are both a challenge and a guide to future modeling and will thus eventually
help us understand pair cascade processes in pulsar magnetospheres. The data
were also used to set new, and less biased, upper limits to the surface
temperature of the neutron star for different models of the neutron star
atmosphere.Comment: 32 pages, 6 figures submitted to the Astrophysical journa
The Identification of the X-ray Counterpart to PSR J2021+4026
We report the probable identification of the X-ray counterpart to the
gamma-ray pulsar PSR J2021+4026 using imaging with the Chandra X-ray
Observatory ACIS and timing analysis with the Fermi satellite. Given the
statistical and systematic errors, the positions determined by both satellites
are coincident. The X-ray source position is R.A. 20h21m30.733s, Decl. +40 deg
26 min 46.04sec (J2000) with an estimated uncertainty of 1.3 arsec combined
statistical and systematic error. Moreover, both the X-ray to gamma-ray and the
X-ray to optical flux ratios are sensible assuming a neutron star origin for
the X-ray flux. The X-ray source has no cataloged infrared-to-visible
counterpart and, through new observations, we set upper limits to its optical
emission of i' >23.0 mag and r' > 25.2mag. The source exhibits an X-ray
spectrum with most likely both a powerlaw and a thermal component. We also
report on the X-ray and visible light properties of the 43 other sources
detected in our Chandra observation.Comment: Accepted for publication in the Astrophysical Journa
Fighting for Tenure: The Jenny Harrison Case Opens Pandora\u27s Box of Issues About Tenure, Discrimination, and the Law
- …