Retrieve Spectral Metrics for the X-Atlas Dataset


Overview

Last Update: Tuesday, 10-Jul-2007 16:49:05 EDT by Owen Westbrook

Synopsis:

This thread will guide a user through the X-Atlas web interface in the task of retrieving the hardness ratios, quantiles, and high-resolution spectral metrics for any and all observations in X-Atlas database.

Purpose:

Retrieve, sort, and filter the X-ray hardness ratios and quantiles for the entire X-Atlas dataset.



Contents


Spectral Metrics

Spectral metrics offer a means of extracting key information about a source's temperature, absorption, or metallicity without performing a full analysis on the high-resolution gratings spectrum. We calculate three types of spectral metrics for each observation in X-Atlas. The first two, hardness ratios and quantiles, are methods of calculating X-ray color and are computed from the predicted ACIS spectra. The third is a system developed by Vinay Kashyap for the characterization of high-resolution X-ray spectra by line-line, line-continuum, and continuum-continuum ratios.

1. Hardness Ratios

Hardness ratios are a way of measuring X-ray color with pre-determined energy passbands. X-Atlas uses the following passbands:

  1. Soft: 0.3-0.9 keV
  2. Medium: 0.9-1.5 keV
  3. Hard: 1.5-8.0 keV
We will let S, M, and H represent the total counts in each passband. S, M, and H are used to calculate the following hardness ratios:
HR1 = (H-M)/(H+M)
HR2 = (M-S)/(M+S)
HR3 = (H-S)/(H+S)
Hardness ratios for multiple sources may be plotted with HR1 vs. HR2.

2. Quantiles

Quantiles are another method of measuring X-ray color; for a complete description, see Hong et al., 2004. Unlike hardness ratios, quantiles do not use predetermined energy passbands. Instead, quantiles represent the energies that would divide a spectrum into predetermined fractions of the total counts. We calculate energy quantiles of 25%, 50%, and 75%. From these, normalized quantiles are computed as:

Qx = (Ex%-Elo)/(Ehi-Elo),
where
Elo = 0.3 keV; Ehi = 8.0 keV

Quantile values may be plotted on a quantile-based color-color diagram, with a measure of the broadness of the emission on the y-axis and a form of the median emission energy on the x-axis.

3. Spectral Summarizer Metrics

High-resolution X-ray spectra generally require time-consuming and painstaking emission measure analyses to extract parameters such as temperature structure and composition. Vinay Kashyap has been developing methods for summarizing the information contained within these spectra in easily measureable quantities. These metrics include line-line, line-continuum, and continuum-continuum ratios.


Retrieving Metrics for a Particular Observation

To retrieve these spectral metrics for a specific observation or target, first search for the observation using the X-Atlas Search Page. From the listings retrieved by the search, click on the target name of the source of interest to view the target's preview page. About halfway down this page, the hardness ratios, quantiles, and high-resolution spectral metrics are listed for the target.


Retrieving Metrics for a Subset of Observations in X-Atlas

X-ray hardness and quantiles values for each target in X-Atlas are listed here. Spectral summarizer metrics are available here. Both pages are filterable and sortable by any of the metrics that they list.


Return to ThreadsX-Atlas Home