New spectral classification technique for X-ray sources: quantile analysis

We present a new technique called "quantile analysis" to classify spectral properties of X-ray sources with limited statistics. The quantile analysis is superior to the conventional approaches such as X-ray hardness ratio or X-ray color analysis to study relatively faint sources or to investigate a certain phase or state of a source in detail, where poor statistics does not allow spectral fitting using a model. Instead of working with predetermined energy bands, we determine the energy values that divide the detected photons into predetermined fractions of the total counts such as median (50%), tercile (33% & 67%), and quartile (25% & 75%). We use these quantiles as an indicator of the X-ray hardness or color of the source. We show that the median is an improved substitute for the conventional X-ray hardness ratio. The median and other quantiles form a phase space, similar to the conventional X-ray color-color diagrams. The quantile-based phase space is more evenly sensitive over various spectral shapes than the conventional color-color diagrams, and it is naturally arranged to properly represent the statistical similarity of various spectral shapes. We demonstrate the new technique in the 0.3-8 keV energy range using Chandra ACIS-S detector response function and a typical aperture photometry involving background subtraction. The technique can be applied in any energy band, provided the energy distribution of photons can be obtained.Comment: 11 pages, 9 figures, accepted for publication in Ap

CMB dipoles and other low-order multipoles in the quasispherical Szekeres model

Several authors have previously shown that a Gpc-scale void based on the spherically symmetric Lemaıˆtre-Tolman-Bondi (LTB) model can provide a good fit to certain cosmological data, including the SNIa data, but it is only consistent with the observed CMB dipole if we are located very close to the center, in violation of the Copernican principle. In this work we investigate the more general quasispherical Szekeres model, which does not include spherical symmetry, in order to determine whether this option may be less constricting. We find that the observer is still constrained to a small region, but it is not as geometrically ''special'' as the center of a LTB void. Furthermore, whereas the quadrupole and octupole near the center of a LTB void are necessarily small, certain Szekeres models can include a significant quadrupole while still being consistent with the observed dipole, hinting that Szekeres models may be able to give an explanation for the observed quadrupole/octupole anomalies

Far Ultraviolet Spectroscopic Explorer Spectroscopy of the Nova-like BB Doradus

We present an analysis of the Far Ultraviolet Spectroscopic Explorer ({\it{FUSE}}) spectra of the little-known southern nova-like BB Doradus. The spectrum was obtained as part of our Cycle 8 {\it FUSE} survey of high declination nova-like stars. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10deg). Assuming an average WD mass of 0.8 solar leads to a mass accretion rate of 1.E-9 Solar mass/year and a distance of the order of 650 pc, consistent with the extremely low galactic reddening in its direction. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are over-abundant by 3 and 20 times solar, respectively