An Online Archive of Chandra's Stellar
High Energy Transmission Gratings
Observations
Owen W. Westbrook1, Nancy Remage Evans1, Scott J. Wolk1,
Vinay L. Kashyap1, Joy Nichols1, Peter J. Mendygral2,1,
Jonathan D. Slavin1, and Wayne L. Waldron3
1
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA
2University of Minnesota, Minneapolis, MN
3
Eureka Scientific, Inc., Oakland, CA
Image credit: HETG observation of  Pup, observation ID 640, Chandra X-ray Observatory
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Introduction
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X-Atlas is an online database of every publicly available point-source observation made
with the Chandra High Energy Transmission Grating (HETG)
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Intended to facilitate the rapid comparison and characterization of stellar X-ray emission
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X-Atlas features:
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Interactive web interface with searching and plotting capabilities
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131 uniformly processed HETG observations of ~25 high-mass stars (O, B, and
Wolf-Rayet) and ~40 low-mass stars (F through M)
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Predictions of the low-resolution ACIS-S and ACIS-I spectra convolved from the
HETG data for each target
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Observation metadata, X-ray color analyses, variability analyses, and spectral fits
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Spectral montages and X-ray line profiles
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Predicted Low-Resolution Spectra
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For comparison with stellar sources in archival ACIS images, we create
predictions of the low-resolution ACIS-S and ACIS-I spectra
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We coadd the high-resolution HEG and MEG HETG flux spectra, multiply by
the ACIS effective area and then convolve the result with the ACIS response
Counts s-1 bin-1
 Per, ObsID 4512
Blue: Actual Extracted Zeroth-Order
Spectrum
Red: Predicted Zeroth-Order Spectrum
Wavelength (Å)
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For verification, we compare the predicted HETG zeroth-order spectrum to
the actual extracted zeroth-order spectrum
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Searching the X-Atlas Database
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Search database on target
and observation parameters
such as:
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Observation ID
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Target name
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Science Category
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Spectral type
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Object type
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Observation and public date
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Cone search by RA and Dec.
Order results by desired
search parameter
X-ray Grating Spectroscopy 2007
X-Atlas Search Interface
Owen Westbrook
cxc.harvard.edu/XATLAS
X-Atlas Query Results
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Displays basic observation data
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Provides links to:
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Plotting interface
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Target preview pages
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HETG spectra previews
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Zeroth-order image
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X-Atlas Search Query Results
X-ray Grating Spectroscopy 2007
Owen Westbrook
Hardness, quantiles, and
spectral metrics (see poster by
Vinay Kashyap)
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Spectral Fits
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Light curves and variability
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Chandra instrumental configuration
for the observation
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Publications associated with the
ObsID
cxc.harvard.edu/XATLAS
Spectral Plot Interface
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Plot up to three targets simultaneously
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Choose spectral grating arm and order for each target
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Select from multiple datasets:
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High-Resolution HETG/ACIS-S
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Low-Res. HETG/ACIS-S Zeroth Order
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Predicted Low-Res. ACIS-S or ACIS-I
Aimpoint
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Predicted Low-Res. Zeroth Order
Three plotting options
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Select pre-produced plot
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Customize plot range and axes
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Compare up to six spectral lines
X-ray Grating Spectroscopy 2007
X-Atlas Spectral Plot Interface
Owen Westbrook
cxc.harvard.edu/XATLAS
X-Atlas Plots and Data Products
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Plots generate within a few
seconds
Automatic line identification
Plots available for download
in GIF and Postscript formats
Original HEG and MEG data
offered in Pulse Height
Amplitude (PHA) format
along with appropriate gARFs
All five plot datasets available
X-Atlas Plot and Data Retrieval Page
in FITS and text formats
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Later Spectral Type
Later Spectral Type
Spectral Montages
Decreasing Energy
Increasing Energy
Montages of high-resolution HETG/ACIS-S MEG spectra (left) and simulated low-resolution ACIS-S Aimpoint
spectra (right) for six OB supergiants, ordered by spectral class. From top to bottom: HD 93129AB ((2x)O2If*+...),
 Pup (O4I(n)f), HD 150136 (O3.5If*+...),  Per (O7.5III(n)((f)) ),  Ori (O9.5II+...),  Ori (O9.7Ib),  Ori (B0Ia).
MEG spectra are in counts s-1 Å-1 vs. wavelength (Å), predicted ACIS-S spectra are in counts s-1 keV-1 vs. energy
(keV). (see Walborn, “Multiwavelength Systematics of OB Spectra,” 26th IAU Meeting, 2006)
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Line Profiles
Mg XII 8.42 Å
Ne X 12.13 Å Fe XVII 15.01 Å Fe XVII 16.78 Å O VIII 18.97 Å
N VII 24.78 Å
 Pup
 Sco
1 Ori C
X-ray line profiles for selected X-Atlas targets, extracted from the HETG MEG spectra. y-
axis is counts s-1 Å-1; x-axis is Doppler velocity (4000 km/s).
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Early Science Results: Hardness Ratios
 Sco
B0.2 V
Capella
HD 68273
G5 IIIe+...
WC8 + O7.5 III-V
 Ori
 Cas
O9.7 Ib
B0IVpe
 Cru
B0.5 III
TW Hya
1 Ori C
K8 Ve
O5.5 V
X-ray hardness ratios (HR1 vs. HR2 ) derived from the predicted ACIS-I aimpoint spectra for the stars in X-Atlas:
HR1 = (H-M)/(H+M) and HR2 = (M-S)/(M+S), where S, M, and H are the total predicted counts in the following
passbands: S: 0.3-0.9 keV; M: 0.9-1.5 keV; H: 1.5-8.0 keV).
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Early Science Results: Quantiles
- Use the predicted ACIS-S or ACIS-I data to calculate normalized quantiles of 25%, 50%, and 75%:
Qx = (Ex% - Elo)/(Ehi – Elo)
Elo = 0.3 keV; Ehi = 8.0 keV; Ex% = Energy that x% of the total predicted counts fall below
- Plot each star on a quantile-based color-color diagram overlaid with a model grid of H column
depth (NH) and temperature (kT) (Hong et al., 2004)
ACIS-I QCCD (Solar Metallicity): High-Mass Stars
Two distinct physical states in high-mass stars?
ACIS-I QCCD (Metallicity = 0.1 Solar) Low-Mass Stars
F = normal F star G = normal G star K = normal K star
- Normal OB stars: <1 keV
M = normal M star R = RS CVn star T = T Tauri star
- Peculiar OB stars: >3 keV
W = W UMa star
X-ray Grating Spectroscopy 2007
Owen Westbrook
A = Algol
* = FK Com
cxc.harvard.edu/XATLAS
Early Science Results: Variability
Gregory-Loredo (1992) test for variability
Number of Stars Variable Probably Variable Not Variable
High-Mass Stars
24
10
1
13
Low-Mass Stars
40
34
1
5
Total
64
44
2
18
Right: Light curve of Algol (ObsID 604, binned at
intervals of 1200 s (blue) and 4000 s (red)
Variability of Capella
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Capella observations taken ~1 yr. apart
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Not variable over the period of an
observation (~30 ks)
Conclusively variable over larger
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timescales (cf. Rassen and Kaastra (2007))
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Acknowledgments
This work was made possible by:
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Chandra Grants GO5-6006E and GO5-6006A
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Chandra X-ray Center NASA Contract NAS8-39073
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Chandra Archival Project AR5-6002A (ANCHORS - cxc.harvard.edu/ANCHORS)
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Chandra Grant GO5-6006A (supporting Wayne L. Waldron)
http://cxc.harvard.edu/XATLAS
Additional thanks to Norbert S.
Schulz, Dave P. Huenemoerder,
and Paola Testa of the MIT Kavli
Institute for their feedback during
testing.
All-sky coverage map of X-Atlas observations in galactic coordinates (O,
B, and Wolf-Rayet stars in blue, F through M stars in yellow) overlaid on a
RASS 1.5 keV image from SkyView.
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Target Preview Pages
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Multiple Extractions
Cyg OB2 (ObsID 2572)
Cyg OB2 8A
X-ray Grating Spectroscopy 2007
Cyg OB2 7
Owen Westbrook
cxc.harvard.edu/XATLAS
Coadding Observations
HD 93129AB
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Spectral Fitting
1. Estimate column density (NH = 5.9*10^21*E(B-V)) if E(B-V) is known
2. Fit the ungrouped spectrum (CSTAT statistics, Powell optimization, kT1 [0.01-1 keV] and kT2 [1-10 kev],
fixed Abund = 0.3 Solar)
3. Fit the grouped spectrum (Chi-squared data-variance statistics, Levenberg-Marquardt optimization, kT1
[0.01-2] and kT2 [1-10] keV
4. If E(B-V) is unknown, fit again with kT, NH floating
5. Freeze kT and NH; fit for abundances
6. If two temperatures have not been found or the chi-squared statistic is > 2:
A. Fit again, fixing abundance to the best-fit value
ObsID 3; 1 Ori C
B. Fit with kT, NH, and abundance as free parameters
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS
Creating the Predicted ACIS Spectra
1. Adaptively smooth the HEG and MEG cts/s/bin spectra
- units: cts/bin
2. Multiply the HEG and MEG counts/s/bin spectra by the appropriate gAR
- units: ph/bin/cm^2
3. Rebin the MEG data to the HEG wavelength grid
- units: ph/bin/cm^2
4. Coadd the HEG and MEG spectra, weighting by the errors on each bin
- units: ph/bin/cm^2
5. Multiply by the ACIS effective area (Aimpoint ARF)
- units: cts/bin
6. Convolve with the ACIS Aimpoint RMF
- units: cts/bin
7. Divide by the exposure time, wavelength (Å) per bin
- units: cts/s/Å
X-ray Grating Spectroscopy 2007
Owen Westbrook
cxc.harvard.edu/XATLAS