1,689 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 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
Absolute Timing of the Crab Pulsar with RXTE
We have monitored the phase of the main X-ray pulse of the Crab pulsar with
the Rossi X-ray Timing Explorer (RXTE) for almost eight years, since the start
of the mission in January 1996. The absolute time of RXTE's clock is
sufficiently accurate to allow this phase to be compared directly with the
radio profile. Our monitoring observations of the pulsar took place bi-weekly
(during the periods when it was at least 30 degrees from the Sun) and we
correlated the data with radio timing ephemerides derived from observations
made at Jodrell Bank. We have determined the phase of the X-ray main pulse for
each observation with a typical error in the individual data points of 50 us.
The total ensemble is consistent with a phase that is constant over the
monitoring period, with the X-ray pulse leading the radio pulse by
0.0102+/-0.0012 period in phase, or 344+/-40 us in time. The error estimate is
dominated by a systematic error of 40 us in the radio data, arising from
uncertainties in the variable amount of pulse delay due to interstellar
scattering and instrumental calibration. The statistical error is 0.00015
period, or 5 us. The separation of the main pulse and interpulse appears to be
unchanging at time scales of a year or less, with an average value of
0.4001+/-0.0002 period. There is no apparent variation in these values with
energy over the 2-30 keV range. The lag between the radio and X-ray pulses may
be constant in phase (rotational) or constant in time (linear pathlength). We
are not (yet) able to distinguish between these two interpretations.Comment: 11 pages, 2 figure
Discovery of a New 89 Second X-ray Pulsar XTE J1906+09
We report on the discovery of a new pulsating X-ray source during Rossi X-ray
Timing Explorer observations of a low galactic latitude field centered at RA
(J2000) = 19 hr 05 m 43 s and Dec (J2000) = +08 deg 58 arcmin 48 arcsec.
Significant pulsations were detected by both the PCA and HEXTE instruments
aboard RXTE at a fundamental period of 89.17 +/- 0.02 seconds, with higher
harmonics also visible in the 2-10 keV power spectrum. The folded lightcurve
from the source is multiply peaked at lower energies, and changes to single
peaked morphology above ~20 keV. The phase averaged spectrum from the source is
well fit by strongly absorbed power law or thermal bremsstrahlung spectral
models of photon index 1.9 +/- 0.1 or temperature 19.5 +/- 4.6 keV,
respectively. The mean neutral hydrogen column density is approximately 10^23
cm^-2, suggesting a distance of >10 kpc to the source and a minimum 2-10 keV
X-ray luminosity of 2*10^{35} ergs s^{-1}. By comparison with other pulsars
with similar periods and luminosities, we suggest that XTE J1906+09 has a
supergiant companion with an underfilled Roche lobe. We speculate further that
one of the M stars in a peculiar M star binary system may be the companion.Comment: 14 pages, 2 figures. Accepted by ApJ Letter
Comprehensive Spectral Analysis of Cyg X-1 using RXTE Data
We analyse a large number () pointed RXTE observations of Cyg X-1 and
model the spectrum of each one. A subset of the observations for which there is
simultaneous reliable measure of the hardness ratio by the All Sky Monitor,
shows that the sample covers nearly all the spectral shapes of Cyg X-1. The
relative strength, width of the Iron line and the reflection parameter are in
general correlated with the high energy photon spectral index . This is
broadly consistent with a geometry where for the hard state (low ) there is a hot inner Comptonizing region surrounded by a truncated cold
disk. The inner edge of the disk moves inwards as the source becomes softer
till finally in the soft state (high ) the disk fills the inner
region and active regions above the disk produce the Comptonized component.
However, the reflection parameter shows non-monotonic behaviour near the
transition region (), suggestive of a more complex geometry or
physical state of the reflector. Additionally, the inner disk temperature,
during the hard state, is on the average higher than in the soft one, albeit
with large scatter. These inconsistencies could be due to limitations in the
data and the empirical model used to fit them. The flux of each spectral
component is well correlated with which shows that unlike some other
black hole systems, Cyg X-1 does not show any hysteresis behaviour. In the soft
state, the flux of the Comptonized component is always similar to the disk one,
which confirms that the ultra-soft state (seen in other brighter black hole
systems) is not exhibited by Cyg X-1. The rapid variation of the Compton
Amplification factor with , naturally explains the absence of spectra
with , despite a large number having .Comment: 12 pages, 8 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
Human Hair Follicle Germinative Epidermal Cell Culture
Isolated human hair follicle germinative epidermal cells were observed in vitro for the first time. When cultured alone, this small, round, novel cell type did not grow, divide, take on an outer root sheath-type appearance, or display any obvious signs of epidermal differentiation. We have previously described comparable cells from rat vibrissa follicles. However, in combination with human hair follicle dermal papilla populations, the germinative epidermal cells were stimulated into proliferative and complex interactive behaviors. This included the formation of composite organotypic structures containing not only impressively intact basement membrane, but also the hair-specific form, glassy membrane
Large Scale Fluctuations in the X-Ray Background
We present an attempt to measure the large angular scale fluctuations in the
X-Ray Background (XRB) from the HEAO1-A2 data, expressed in terms of spherical
harmonics. We model the harmonic coefficients assuming a power spectrum and an
epoch-dependent bias parameter, and using a phenomenological scenario
describing the evolution of the X-ray sources. From the few low-order
multipoles detected above shot noise, we estimate the power-spectrum
normalization on scales intermediate between those explored by local galaxy
redshift surveys (~ 100 Mpc) and by the COBE Microwave Background measurements
(~ 1000 Mpc). We find that the HEAO1 harmonics are consistent with present
epoch rms fluctuations of the X-ray sources bx(0)sigma8 ~ 1-2 in 8 Mpc spheres.
Therefore the observed fluctuations in the XRB are roughly as expected from
interpolating between the local galaxy surveys and the COBE CMB experiment. We
predict that an X-ray all-sky surface brightness survey resolving sources a
factor of 10 fainter than HEAO1, may reveal fluctuations to significantly
larger angular scales and therefore more strongly constrain the large scale
structure of the Universe on scales of hundreds of Mpcs.Comment: 14 pages, 3 Postscript figures, uses aaspp4.sty and psfig. Revised
following referee's report. Accepted for publication in Ap
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