1,168 research outputs found
ON THE MEASUREMENT OF A COSMOLOGICAL DIPOLE IN THE PHOTON NUMBER COUNTS OF GAMMA-RAY BURSTS
If gamma-ray bursts are cosmological or in a halo distribution their
properties are expected to be isotropic (at least to 1st order). However, our
motion with respect to the burst parent population (whose proper frame is
expected to be that of the Cosmic Microwave Background (CMB), or that of a
static halo) will cause a dipole effect in the distribution of bursts and in
their photon number counts (together termed a Compton-Getting effect). We argue
that the photon number count information is necessary to distinguish a genuine
Compton-Getting effect from some other anisotropy and to fully test the proper
frame isotropy of the bursts. Using the 2B burst catalogue and the dipole
determined from the CMB, we find the surprising result that although the number
weighted distribution is consistent with isotropy, the fluence weighted dipole
has a correlation with the CMB dipole that has a probability of occuring only
10% of the time for an isotropic photon distribution. Furthermore, the photon
and number dipoles are inconsistent under the hypothesis of isotropy, at the
2-sigma level. This could be an indication that a non-negligible fraction of
gamma-ray bursts originate in the local, anisotropic universe. (shortened
Abstract)Comment: Accepted by ApJ. Self-unpacking (use csh), uuencoded, compressed
Postscript, 16 pages + 4 Figures (5 files
The structure of galactic HI in directions of low total column density
A detailed 21 cm study of areas of that have the smallest known amount of HI in the northern sky was performed. These observations were corrected for stray radiation. The region of main interest, around alpha = 10(h)45(m), delta = 57 deg 20', has a minimium N(HI) of 4.5 x 10 to the 19th power/sq cm. Spectra taken at 21' resolution over a field 4 x 3 deg in this direction show up to four HI line components. Two, near 0 and -50 km/s, are ubiquitous. There is also a narrow component at -10 km/s attributable to a diffuse cloud covering half of the field, and scattered patches of HI at v -100 km/s. the low and intermediate velocity components have a broad line width and are so smoothly distributed across the region that it is unlikely that they contain significant unresolved angular structure. Eight other low column density directions were also observed. Their spectra typically have several components, but the total column density is always 7 x 10 to the 19th power/sq cm and changes smoothly along a 2 deg strip. Half of the directions show narrow lines arising from weak diffuse HI clouds that contain 0.5 to 3.0 x 10 to the 19th power/sq cm
Small scale H I structure and the soft X-ray background
The observed anticorrelation between diffuse soft X-ray flux and H I column density has been explained as absorption of soft X-rays produced in a hot galactic halo, assuming that the neutral interstellar material is sufficiently clumped to reduce the soft X-ray absorption cross section by a factor of two to three. A 21 cm emission line study of H I column density variations at intermediate and high galactic latitudes to 10' spatial resolution has been done. The results confirm conclusions from preliminary work at coarser resolution, and in combination with other data appear to rule out the hypothesis that clumping of neutral interstellar matter on any angular scale significantly reduces X-ray absorption cross sections in the 0.13 - 0.28 keV energy range. It is concluded therefore that the observed anticorrelation is not primarily a consequence of absorption of soft X-rays produced in a hot galactic halo
Gas Gain Measurements from a Negative Ion TPC X-ray Polarimeter
Gas-based time projection chambers (TPCs) have been shown to be highly
sensitive X-ray polarimeters having excellent quantum efficiency while at the
same time achieving large modulation factors. To observe polarization of the
prompt X-ray emission of a Gamma-ray burst (GRB), a large area detector is
needed. Diffusion of the electron cloud in a standard TPC could be prohibitive
to measuring good modulation when the drift distance is large. Therefore, we
propose using a negative ion TPC (NITPC) with Nitromethane (CH3NO2) as the
electron capture agent. The diffusion of negative ions is reduced over that of
electrons due to the thermal coupling of the negative ions to the surrounding
gas. This allows for larger area detectors as the drift distance can be
increased without degrading polarimeter modulation. Negative ions also travel
~200 times slower than electrons, allowing the readout electronics to operate
slower, resulting in a reduction of instrument power. To optimize the NITPC
design, we have measured gas gain with SciEnergy gas electron multipliers
(GEMs) in single and double GEM configurations. Each setup was tested with
different gas combinations, concentrations and pressures: P10 700 Torr, Ne+CO2
700 Torr at varying concentrations of CO2 and Ne+CO2+CH3NO2 700 Torr. We report
gain as a function of total voltage, measured from top to bottom of the GEM
stack, and as a function of drift field strength for the gas concentrations
listed above. Examples of photoelectron tracks at 5.9 keV are also presented.Comment: 6 pages, 6 figures, accepted for publication in IEEE Trans Nucl Sc
Detecting solar axions using Earth's magnetic field
We show that solar axion conversion to photons in the Earth's magnetosphere
can produce an x-ray flux, with average energy \sim 4 keV, which is measurable
on the dark side of the Earth. The smallness of the Earth's magnetic field is
compensated by a large magnetized volume. For axion masses < 10^{-4} eV, a
low-Earth-orbit x-ray detector with an effective area of 10^4 cm^2, pointed at
the solar core, can probe the photon-axion coupling down to 10^{-11} GeV^{-1},
in one year. Thus, the sensitivity of this new approach will be an order of
magnitude beyond current laboratory limits.Comment: 3 pages, 1 figure, typos corrected, references adde
The 2-10 keV XRB dipole and its cosmological implications
The hard X-ray (>2 keV) emission of the local and distant Universe as
observed with the HEAO1-A2 experiment is reconsidered in the context of large
scale cosmic structure. Using all-sky X-ray samples of AGN and galaxy clusters
we remove the dominant local X-ray flux from within a redshift of ~ 0.02. We
evaluate the dipolar and higher order harmonic structure in 4 X-ray colours.
The estimated dipole anisotropy of the unresolved flux appears to be consistent
with a combination of the Compton-Getting effect due to the Local Group motion
(dipole amplitude Delta = 0.0042) and remaining large scale structure (0.0023
<~ Delta <~ 0.0085), in good agreement with the expectations of Cold Dark
Matter models. The observed anisotropy does however also suggest a
non-negligible Galactic contribution which is more complex than current, simple
models of >2 keV Galactic X-ray emission. Comparison of the soft and hard
colour maps with a harmonic analysis of the 1.5 keV ROSAT all-sky data
qualitatively suggests that at least a third of the faint, unresolved ~ 18 deg
scale structure in the HEAO1-A2 data may be Galactic in origin. However, the
effect on measured flux dipoles is small (<~3%).
We derive an expression for dipole anisotropy and acceleration and
demonstrate how the dipole anisotropy of the distant X-ray frame can constrain
the amplitude of bulk motions of the universe. From observed bulk motions over
a local ~ 50 Mpc/h radius volume we determine 0.14 <~ Omega^0.6/b_x(0) <~ 0.59.Comment: 39 pages, Revised version accepted ApJ Main Journal, 3 new Figures +
additional tex
- …