107 research outputs found
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
Discovery of a Significant Magnetic CV Population in the Limiting Window
[Abridged] We have discovered 10 periodic X-ray sources from the 1 Ms Chandra
ACIS observation of the Limiting Window (LW), a low extinction region (A_V~3.9)
at 1.4 Deg south of the Galactic center. The observed periods (~1.3 to 3.4
hours) and the X-ray luminosities (10^{31.8-32.9} erg s^-1 at 8 kpc) of the 10
periodic sources, combined with the lack of bright optical counterparts and
thus high X-ray-to-optical flux ratios, suggest that they are likely accreting
binaries, in particular, magnetic cataclysmic variables (MCVs). All of the 10
sources exhibit a relatively hard X-ray spectrum (PLI<2 for a power law model)
and X-ray spectra of at least five show an extinction larger than the field
average expected from the interstellar medium in the region. The discovery of
these periodic X-ray sources in the LW further supports the current view that
MCVs constitute the majority of low luminosity hard X-ray sources (~10^{30-33}
erg s^-1) in the Bulge. The period distribution of these sources resembles
those of polars, whereas the relatively hard spectra suggest that they could be
intermediate polars (IPs). These puzzling properties can be explained by
unusual polars with buried magnetic fields or a rare sub-class of MCVs, nearly
synchronous MCVs. These unusual MCVs may provide important clues in the
evolutionary path of MCVs from IPs to polars. The completeness simulation
indicates >~40% of the hard X-ray sources in the LW are periodic. Therefore,
this discovery provides a first direct evidence of a large MCV population in
the Bulge.Comment: 15 pages, 10 figures, 5 tables, submitted to ApJ, revised in response
to the referee's revie
Initial Optical Results for the ChaMPlane Survey
We provide a brief description of the optical survey being conducted under
the NOAO Long Term Surveys program in support of the Chandra Multiwavelength
Plane (ChaMPlane) Survey (see paper by Grindlay et al. in this Volume). A
representative photometry result is shown, along with spectroscopic followup.Comment: 1 page, 2 figures (in 3 files). Astronomische Nachrichten, in press
(Feb 2003). Proceedings of "X-ray Surveys, in the Light of New
Observatories", 4-6 September, Santander, Spai
Deep Chandra Survey of the Small Magellanic Cloud. II. Timing Analysis of X-ray Pulsars
We report the timing analysis results of X-ray pulsars from a recent deep
Chandra survey of the Small Magellanic Cloud (SMC). We have analyzed a total
exposure of 1.4 Ms from 31 observations over a 1.2 deg region in the SMC
under a Chandra X-ray Visionary Program. Using the Lomb-Scargle and epoch
folding techniques, we have detected periodic modulations from 20 pulsars and a
new candidate pulsar. The survey also covers 11 other pulsars with no clear
sign of periodic modulation. The 0.5-8 keV X-ray luminosity () of the
pulsars ranges from to erg s at 60 kpc. All the
Chandra sources with erg s exhibit
X-ray pulsations. The X-ray spectra of the SMC pulsars (and high mass X-ray
binaries) are in general harder than those of the SMC field population. All but
SXP~8.02 can be fitted by an absorbed power-law model with a photon index of
1.5. The X-ray spectrum of the known magnetar SXP~8.02 is
better fitted with a two-temperature blackbody model. Newly measured pulsation
periods of SXP~51.0, SXP~214 and SXP~701 are significantly different from the
previous XMM-Newton and RXTE measurements. This survey provides a rich data set
for energy-dependent pulse profile modeling. Six pulsars show an almost
eclipse-like dip in the pulse profile. Phase-resolved spectral analysis reveals
diverse spectral variation during pulsation cycle: e.g., for an absorbed
power-law model, some exhibit an (anti)-correlation between absorption and
X-ray flux, while others show more intrinsic spectral variation.Comment: 24 pages, 19 figures, 11 tables, submitted to Ap
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