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Chapter 8
HETG: Chandra High Energy Transmission Grating

8.1  Instrument Overview

The HETG itself consists of two sets of gratings, each with different period. One set, the Medium Energy Grating (MEG), intercepts rays from the outer HRMA shells and is optimized for medium energies. The second set, the High Energy Gratings (HEG), intercepts rays from the two inner shells and is optimized for high energies. Both gratings are mounted on a single support structure and therefore are used concurrently. The two sets of gratings are mounted with their rulings at different angles so that the dispersed images from the HEG and MEG will form a shallow X centered at the undispersed (zeroth order) position; one leg of the X is from the HEG, and the other from the MEG. The HETG is designed for use with the spectroscopic array of the Chandra CCD Advanced Imaging Spectrometer (ACIS-S) although other detectors may be used for particular applications. A summary of characteristics is given in Table 8.1.

The Instrument Principal Investigator for the HETG is Dr. Claude Canizares of the MIT Center for Space Research. See Canizares et al. (2005) for a thorough description of the instrument and its performance.

8.1.1  Examples of Observations with the HETGS

An example of an HETGS observation is presented in Figure 8.1 using data from an observation of Capella, Obsid 1318. The top panel shows an image of detected events on the ACIS-S detector with the image color indicating the ACIS-determined X-ray energy (see WWW version if this is not in color.) In this detector coordinate image (TDETX, TDETY), the features are broad due to the nominal dither motion which serves to average over detector non-uniformities. The ACIS-S chips are numbered S0 to S5 from left to right, with the aim point in S3 where the bright zeroth-order image is visible and includes a vertical frame-transfer streak (a trailed image). The HRMA optical axis passes through S3 approximately 6 mm from the S2-S3 chip gap. For further information see Figure 6.1 and related text.

HETG-diffracted photons are visible in Figure 8.1 forming a shallow "X" pattern; the full opening angle between the HEG and MEG spectra is 9.96^°. The back illuminated (BI) chips are S1 and S3. The S1 location was chosen to enhance the first order MEG spectrum since back illumination provides higher efficiency below 1 keV. The location of the zeroth-order for any particular observation, however, may be adjusted by offset pointing in order to select the energies of the photons that will be placed in the gaps between the chips. Details on gaps are presented in Section 8.2.1.

The middle panel of Figure 8.1 shows an image after the data have been aspect corrected and data filters applied to include only valid zeroth and first-order events. Note that this image was created using Sky coordinates that were rotated and had their y-axis sign "flipped" in order to match the detector coordinates view in the top panel. The lower set of panels shows an expanded view of the MEG minus-first-order spectrum with emission lines clearly visible. Wavelengths are assigned based on the diffraction angle of the events, that is, how far the events are from the zeroth-order image. Using the grating equation in Section 8.1.3, absolute wavelengths can be assigned based on the dispersion angle. A spectrum of the source is then created by binning the events into energy or wavelength bins; the spectrum from another Capella observation is shown in Figure 8.2.

Note: The dispersion distance on the detector is essentially linear in wavelength. Thus, wavelength is the natural unit for this high-resolution x-ray spectrometer. The conversion between energy and wavelength is provided by the relation: E ×λ = hc = 12.39852  keVÅ.

Each of the "arms" of the HETGS diffracted X pattern yields a first-order spectrum identified by type (HEG or MEG) and sign of the order (plus or minus.) Using ARF's (ancillary response files) and RMF's (response matrix files) these spectra can be analyzed in an XSPEC-like framework. Additionally, the CXC software package "Interactive Spectral Interpretation System" (ISIS, http://space.mit.edu/ASC/ISIS/) can be used to identify spectral lines, e.g., as seen in Figure 8.2.

8.1.2  Scientific Objectives and Grating Heritage

Although gratings have flown on Einstein and EXOSAT, the HETGS shares only the basic operating principles with these. Advanced grating technology has enabled achievement of greater efficiency and increased dispersion. The Rowland geometry of the grating plate and spectroscopic arrays maintains the telescope focal properties in the dispersion direction by minimizing dispersed image aberrations and hence contributes to improved spectral resolution.

8.1.3  HETGS Operating Principles

where m is the integer order number, λ is the photon wavelength in angstroms, p is the spatial period of the grating lines, and β is the dispersion angle. A "normal" undispersed image is formed by the zeroth-order events, m=0, and dispersed images are formed by the higher orders, primarily the first-order, m=1.

The HETGS-faceted Rowland design is shown in Figure 8.4. The "Rowland circle" is a circle whose diameter is simply the distance from the grating that would lie on the optical axis to the point in the focal plane where the zeroth order image is placed. The "Rowland torus" is formed by rotating the circle about the line in the dispersion direction going through the on-axis focus. Individual grating facets are mounted such that their centers lie on the torus . In the figure, the axis of the torus is perpendicular to the page for the side view and lies in the plane of the top view. Ideally, the detector is shaped to follow the counterpart Rowland torus in the image plane. The result is that the telescope focal properties in the dispersion direction are maintained for a large range of diffraction angles, β, thereby minimizing any grating-added optical aberrations.

An important parameter of the HETGS is the Rowland spacing, the distance from the outer intersection of the HETG axis and Rowland Circle to the HRMA focus. This Rowland spacing is what determines the value of β in the grating equation. The value of the Rowland spacing is listed in Table 8.1.

Order overlap and source confusion can be discriminated by the intrinsic energy resolution of the CCD detector (ACIS-S is the preferred detector for HETG spectroscopy since it has intrinsic energy resolution and so can separate orders; the HRC may be used if high time resolution is desired, but this choice is at the price of using the focal plane detector's energy resolution to aid in order separation). The form of a spectral image on the ACIS-S array is shown in Figure 8.1. The spectroscopic array spans about 8 arc minutes × 48 arc minutes of the sky, though image quality and resolving power degrade rapidly for sources more than about 4 arcmin off-axis. For an on-axis source, the detector edge in the dispersion direction causes a low energy cutoff of the spectrum at about 0.4 keV for the MEG and 0.8 keV for the HEG. Order selection and chip gaps are described more fully in Section 8.2.1.

8.1.4  HETG Physical Configuration

The gratings cover the annular regions through which the X-rays pass. The 192 grating facets on the outer two annuli (MEG) have a period of 4001.95 Å.  Tiling the inner two annuli are 144 (HEG) gratings, which have a period of 2000.81 Å (see Table 8.1). The two sets of gratings are mounted with their rulings at different angles so that the dispersed images from the HEG and MEG will form a shallow X centered at the undispersed (zeroth order) position; one leg of the X is from the HEG, and the other from the MEG. See Figure 8.1 for an example.

The HETG grating facets are composed of electro-plated gold bars supported on a polyimide substrate, as shown schematically in Figure 8.6. The grating bar design parameters, height and width, are nominally chosen to reduce zeroth-order and maximize first-order intensities. Choosing to have the bar width one-half of the grating period suppresses even orders and provides maximum 1st order efficiency for a rectangular profile; this is closely achieved for the MEG gratings. For the HEG gratings, the bar is wider and results in a higher 2nd order efficiency and reduced 3rd order efficiency. The bar height choice "tunes" the efficiency peak in energy by allowing X-rays to constructively interfere in first order in the region where the gold is partially transparent, primarily above 1.2 keV.

8.2  Instrument Characteristics

In the sections that follow, information on the HETGS Effective Area (ARFs) and Resolution (RMFs/LRFs) are given with an examination of the components and effects that contribute to them. In addition two other characteristics of the HETGS are briefly presented: the Background event rate for an extracted spectrum and the Absolute Wavelength accuracy.

8.2.1  HETGS Effective Area

Nominal HETGS ARFs

The nominal plots shown here are for qualitative reference only; because the fullgarf tool also accounts for a variety of other effects, e.g., dither motion, bad pixels, QE non-uniformity, etc., grating ARFs are custom made for a given observation. The details of the calibration of the ARF are discussed in Section 8.3.

Since first-order photons from both the HEG and MEG gratings provide information, to compare the HETGS with other instruments, it is useful to plot the total HETGS effective area (the combined plus and minus first-order areas of both the HEG and MEG); this is shown by the solid curve in Figure 8.11. During an observation the zeroth-order photons from HEG and MEG form a single zeroth-order image; the effective area for this zeroth-order image is also plotted on this figure (dotted line).

Note the dips caused by the gaps between chips in these figures. The observatory is dithered in order to spread the signal across a large number of pixels. For HETGS observations, sinusoidal motions with 8 arc second amplitude in spacecraft Y and Z axes are used with periods of 1000.0 and 707.0 seconds, respectively, creating a Lissajous pattern (see Section 5.8.2). When the combination of the chip gaps and dither are accounted for, a "pitch fork" dip occurs at each gap region in the ARFs. Although effects of this motion are removed in on-ground processing, observers are advised to avoid placing spectral features of interest near the gaps. More information concerning gaps are in the next section. The effective areas shown in Figure 8.7-8.11 are based on an integration over the full LSF. Most of the flux in a line will be contained within a circle of diameter 4 arcsec. The user might wish to note that in data processing, the pipeline software keeps only events that are in a spatial window that lies within 3 arcsec of the dispersion axis. This aperture guarantees that a high fraction, 97-99%, of the signal flux is retained while minimizing the contribution of the background. Further discussion of the spatial distribution of events can be found for the HRMA PSF in Chapter 4 and for the HETGS in Section 8.2.2 below.

HETG Grating Efficiency

A number of systematic effects at the 10% level have now been accounted for in the HETGS effective area via adjustments to the grating efficiencies. Observations of blazars (Marshall 2012) have been used to verify these corrections. Relative systematic errors from curved power-law spectral fits are generally less than 3% (0.5 - 8.0 keV) on small scales (see Figure 8.12). Preliminary results from cross-calibration with XMM-Newton indicate that measured fluxes agree to better than 10% (Smith & Marshall 2012).

ACIS-S Order Sorting Effects

During data analysis, this filtering is accomplished by utilizing two of the data columns supplied in the level 1.5 (or 2.0) FITS data file: the ACIS-determined energy, ENERGY, and the dispersion distance, mλ =  TG_MLAM. Ideally this order sorting would have perfect efficiency, that is, all first-order events would be correctly identified. In practice, a high sorting efficiency is achieved by accurately calibrating the ACIS ENERGY values and by accepting events in a large ENERGY range. The slight efficiency corrections that do arise are included in the ARF through values in an order sorting integrated probability (osip) file.

ACIS-S Pileup Effects

ACIS-S BI / FI QE Effects

ACIS-S Chip Gap Effects

The values in the table are based on an effective gap size of 0.502 mm, corresponding to 10^′′ on the sky. It is "effective" in the sense that the gap includes columns 1 and 1024 of the devices from which no events are reported. This value for gap size is approximate and accurate to about 2 pixels. The actual gap sizes vary slightly; more accurate values of the ACIS-S chip geometry are given in a CXCDS CALDB file `telD1999-07-23geomN006.fits' (and higher versions) and incorporated in MARX version 3.0 and higher. Relative to S3, where zeroth order is normally placed, the ACIS-S chip locations are calibrated to better than 0.2 pixels allowing accurate relative wavelengths.

8.2.2  HETGS Line Response Function

Of course the HETGS LRF is not simply a Gaussian and, as for other spectrometers, the response can be encoded at a higher level of fidelity through the use of response matrix files, RMF's. As explained below, the LRF (RMF) of the HETGS depends on all system components as well as the source spatial properties. Thus, LRF creation is carried out using a system model, e.g., the MARX ray trace software. A set of RMF's for a point source and nominal telescope properties can be created based on the latest LRF library in the Chandra CALDB which includes two Gaussian and two Lorentzian components to describe the LRF as derived from realistic MARX simulations; for examples, see the fitted LRF models in Figures 8.16 and 8.17. Note that "canned" grating RMFs are provided for observation planning but should not be used to analyze real data. They can be found at: http://cxc.harvard.edu/caldb/prop_plan/index.html

The line response function can be decoupled approximately into three contributing components: the telescope PSF, the HETG effects in the dispersion direction, and HETG effects in the cross-dispersion direction. These are described below. With the exception of "HEG scatter", all effects described here are included in MARX version 3.0 (and higher) ray trace software.

LRF: Telescope PSF and Zeroth Order

Image quality depends on many factors, and so, while a nominal LRF can be modeled, the detailed LRF will be observation dependent at some level. Factors in the telescope PSF performance include: source size and spectrum, HRMA properties, focus setting, detector effects (e.g., pixel quantization), aspect solution and reconstruction effects, and data analysis operations (e.g., pixel randomization.) While all of these effects can be modeled, the "proof of the pudding" is in the as-observed zeroth order image.

As an example, we show the results of the observation of Capella (Obsid 1318, Figure 8.1) in Figure 8.18. Both the zeroth-order event distribution and its 1-D projection indicate that the zeroth-order is heavily piled up with an unpiled event rate of order 10 events per frame time (per few square ACIS pixels). The wings of the PSF are visible but the core shape and intensity have been severely distorted. However, because the ACIS-S CCDs have their columns perpendicular to the (average) dispersion axis, the "frame-transfer streak" events or "trailed image" (see Section 6.12.1 in Chapter 6) can be used to create an accurate zeroth order LRF that is not affected by pileup, as shown. For point-sources such as Capella, measurements of the FWHM of the zeroth-order streak events for selected observations over the first two years of HETGS operation show FWHM values generally in the range 1.46 to 1.67 ACIS pixels with an average of 1.57 ACIS pixels. Thus, by appropriately examining the zeroth-order image and its LRF, one can get a good idea of the expected width of a truly monochromatic spectral line, and determine whether or not any broadening seen in a dispersed order is a spectral property of the X-ray source.

LRF: Dispersion Direction

From the grating equation, m ∆λ = p ∆βcos β+ ∆p sin β ≈ y ∆p/R_s + p ∆y / R_s , where p is the grating period, β is the dispersion angle, y is the dispersion distance and R_s is the (fixed) Rowland spacing . The two terms of interest are on the right side of λ/ ∆λ = (∆p / p + ∆y / y)⁻¹. The grating fabrication process produced tightly distributed grating periods (∆p / p < 2.5×10⁻⁴) so that the first term is important in the spectral resolution only at very high dispersion (low energy). The mirror point response function has a nearly constant size ∆y and dominates the resolution over most of the HETGS band, as shown in Figure 8.19. At very low energies there is a contribution from variation in the grating periods. These variations are taken into account in the MARX simulator.

During ground testing, we discovered that there is a low level of incoherent dispersion (or "scattering") in HEG spectra. This scattering effect distributes a small amount of the flux along the dispersion direction. The total power involved is only 1.0% of the total in first order but the light is irregularly distributed between the coherently dispersed orders. Assuming that the power distribution scales with the first order dispersion distance, there is no more than 0.02% of the first order flux in any bin of width 0.01 λ. There has been no scattering detected in the MEG spectra to a level of order 100 times fainter than in the HEG. See the HETG Ground Calibration Report listed at the end of this chapter for further details. The effects of scattering from the grating are likely to be negligible for most observations.

LRF: Cross-Dispersion Direction

The spread of facet roll angles (defining the dispersion direction for each facet, and not to be confused with the spacecraft roll angle), ∆φ, contributes a cross-dispersion term of order y∆φ. Sub-assembly measurements predicted ∆φ= 0.42 arc minutes rms. However, analysis of ground test measurements lead to a somewhat larger and more complex roll angle distribution for the gratings. In addition, six misaligned MEG facets were discovered during ground testing. The inferred facet roll angles were misaligned from the average dispersion direction by 5-23 arc min. On average, each facet contributes only 1/192 of the flux at any given energy, so the cross dispersion profile has small deviations in the form of peaks displaced from the main distribution.

To include explicitly the MEG misaligned gratings, MARX uses "sector" files which allow the specification of grating alignment and period parameters for certain regions (sectors) of each of the four shells. Using these files, the agreement between ground calibration and flight data is very good. For the MEG the misaligned gratings are explicitly included and the rest of the gratings' ∆φ term is modeled as the sum of two Gaussian distributions centered at +1 and -1 arc minute w.r.t. the nominal axis, each with an rms value of 1.5 arc minutes. For the HEG a more pronounced bi-gaussian distribution is observed and modeled: the Gaussians are offset by -1.35 and +1.65 arc minutes, each with a 1 arc minute rms, and in a relative ratio of 55:45.

In each case, these effects are accurately included in MARX version 3.0 (and above). Flight data for the Crab pulsar (Obsid 168) are shown in Figure 8.20 for the MEG and in Figure 8.21 for the HEG. Note that these profiles are on top of a significant baseline due to the presence of the Nebula. The asymmetry in the MEG profile caused by misaligned gratings is quite clear at large dispersions.

Finally, we show in Figure 8.22 how the total observed flux depends on the width of the extraction region in the cross-dispersion direction. The figure can be used to estimate the reduction in flux if analysis using a narrow extraction window, smaller than the nominal 4 arcsec full width, is planned.

Extended and Off-Axis Targets

The discussion and plots above assumed that the source has no spatially dependent variations in the spectrum. The more general case of extended sources with spatially varying spectra is briefly discussed below in Section 8.5.3.

8.2.3  Background

Figure 8.25 shows the HEG and MEG spectra of the background for a long calibration observation of HR 1099 (observation ID 62538). The background was selected from regions 8-50 arcsec from the dispersion line in the HEG and 11-50 arcsec from the dispersion line in the MEG. The pulse height selection was simple, accepting events that satisfy the relation | E_ACIS / E_TG − 1 | < 0.30 , where E_ACIS is the energy derived from the ACIS-S pulse height and E_TG is the energy based on the dispersion distance. The background was normalized to an aperture of ± 3 arcsec (full size of 6 arcsec) and averaged at 0.1 A intervals. This plot can be used to estimate the background in a dispersed spectrum at a particular wavelength for proposal purposes.

There is considerable structure in the background. For λ < 2 Å (E > 6 keV), the background is dominated by high energy events that are included in the relatively wide pulse height selection. This background can be further reduced in data analysis because the pulse height selection can be somewhat narrower at high energies. As one would expect (see Section 6.16), the background is higher in the region near 8 Å in -1 order as this portion of the dispersed spectrum is detected with a BI chip (S1). "Streaks", short-lived events observed in the S4 detector, have been removed; otherwise, the background in +1 order would be significantly higher and would show more structure.

8.2.4  Absolute Wavelength

8.3  Calibration Status

8.4  HETG Operations

8.4.1  Flight Events and Anomalies

Because the HETG insert limit switches continue to function and because the HETG is inserted against a hard stop, these anomalies have had no effect on the HETG wavelength scale.

8.4.2  Operational Constraints

8.4.3  Output Data

8.4.4   Performance Monitoring, Health and Safety

8.4.5  Thermal Response Time

8.4.6  Observation Frequency/Duty Cycle

Currently, the longest continuous observations are limited to the time between passage through the radiation belts (Section 3.4.2).

8.4.7  Radiation Considerations

A secondary concern would be changes to the Gold grating bars (which, when in place, face the HRMA) due to sputtering by particles, particularly for the high-aspect ratio HEG gratings. Diffraction order ratios are sensitive to these changes. To date, after flight experience and laboratory radiation tests, there is no evidence that this concern is other than intellectual.

8.5  Observation Planning

8.5.1  Focal Plane Detector Considerations

• ACIS or HRC: ACIS provides energy resolution which is useful for order separation , background suppression, and discrimination between multiple sources. The HRC  is likely to be useful if high time resolution is necessary especially if the ACIS mode is not helpful for the observation in question. The HRC has not yet been used with the HETG and no calibration verification has yet been performed, so this detector is not recommended for general use.
• Operating mode of ACIS-S: The ACIS-S array can operate in many modes, giving control over e.g., the read-area, pixel-binning, and read-frequency. The selection of the appropriate operating mode and its ramifications for the experiment is one of the most important that the user faces. A careful reading of Chapter 6 is recommended. The proposer should pay special attention to the pros and cons of designating optional CCDs for their HETGS observation, as opposed to simply requesting the entire ACIS-S chip set.
• Selecting the aimpoint: The capability of moving the SIM  along the spacecraft Z-axis (the cross-dispersion direction) is useful for placing the image (dispersed spectra and zeroth order) closer to the ACIS chip readouts. This placement minimizes the effects of the row-dependent energy resolution of the FI chips. For a point source, the recommended placements, stated as offsets from the nominal position are −3 mm and −4 mm for ACIS TE and CC modes respectively. The recommended shift in TE mode allows space for background data to be obtained on the readout side of the dispersed image. The CC mode, however, integrates over the columns, and so there is no advantage gained, and thus the the image can be placed even closer to the readout. NB: Shifting the aimpoint may not be desirable if the target is extended or when there are several sources in the field.
• Orientation of multiple (or extended) sources: One may need to specify a restricted range of spacecraft roll-angles to avoid overlapping spectra from multiple targets in the field, or to arrange that the dispersed spectra from particular features of an extended target do not similarly overlap. Note that roll angle constraints usually will lead to restrictions on the dates dates of target availability. See Chapter 3.
• Offset pointing: Pointing offsets may be specified and used to include or exclude nearby sources, to keep an important spectral feature clear of the gaps between chips, to put a particular low-energy feature on the higher efficiency BI chip S1, etc. Offsets greater than one or two arc minutes will, however, degrade the image quality - Chapter 4 - which in turn broadens the LRF (Section 8.2.2).
• ACIS subarray modes: One might wish to reduce the ACIS-S frame time e.g. in order to minimize the effects of pileup. The user might consider using a subarray with the HETG, as described at
  http://space.mit.edu/ASC/calib/hetgsubarray.html.)
  If the source is point-like, then a -3 mm SIM shift can be used. In this case, there are at least 6 mm of ACIS-S rows that may not be scientifically interesting (unless the user desires data from serendipitous sources). The reduced array could have 1024 - 250 = 774 rows starting at row 1 thus reducing the frame time to 2.5 s. The size of the minimal subarray depends on the low energy cut-off, E, below which the spectra are not of interest. To understand this better, please refer to Figure 8.1. The subarray must be large enough to encompass both HEG and MEG "arms". Larger subarrays are needed at lower energies where the arms are furthest apart. The minimal subarray size is y_sub = 2*y_bg + 32 + 389/E, where E is in keV, 32 pixels allow for dither, and y_bg is the size of a background region on either side of the spectrum. The background region might be of order 70 pixels (about 10 times the spectrum's extraction width). The SIM should be shifted to center the spectra in the subarray by SIM_Z = 0.024 * (y_sub/2 − 497 + 122θ_z) mm, where 497 is the row of the ACIS-S aimpoint and θ_z is the telescope Z offset that places the target precisely on the optical axis. For E = 1 keV and θ_z = 0.25^′, then y_sub = 561 rows and the SIM shift is −5.93 mm.
• Example of a set of parameters: It is instructive to examine the obscat entry for observation ID 9703 which shows observation parameters and values.
  (See http://cda.harvard.edu/chaser/startViewer.do?menuItem=details&obsid=9703.)
  The main target is a quasar. The AGN is mildly absorbed, so there is flux of interest down to the MEG limit, hence chip S0 (where the low energy flux will fall) is designated as "yes", while S5, which is redundant with S1 and S0, is designated as "optional". A subarray is used to eliminate pile-up. The SIM has been shifted in the Z direction to place the spectrum closer to the readout and to center the spectrum in the subarray. This will minimize the impacts of the row-dependent CTI on the FI-chips. Finally, a Y offset of 0.0 arcmin is used to keep the Fe-Kα region of the spectrum out of the S2-S3 chip gap in the HEG. The telescope Z offset is set to bring the zeroth order closer to the current optical axis.
• Use of continuous clocking (CC) mode: this mode can be applied in order to mitigate pileup in very bright sources. Most high resolution features will be unaffected. However, there are some calibration issues that have not yet been resolved:
  • The Si K edge can have a distorted shape and incorrect optical depth
  • A broad feature may appear at about 5.4 Å (2.3 keV)
  • There can be significant charge losses below 3 Å ( > 4 keV)
  • The spectrum may have the wrong shape above 10 Å ( < 1.2 keV)
  For further details, see ACIS Section 6.20.4 and links therein.

8.5.2  Complications from Multiple Sources

Faint Background Sources

Two Point Sources of Comparable Intensity

A Strong Source Lying Outside the Field

8.5.3  Extended Sources and Spatial-Spectral Effects

For extended sources with multiple condensations, careful selection of the roll angle (see e.g. Section 8.5.2) might make the data easier to analyze and interpret. It may also be possible to model the spectrum given information from the zero order image and/or a short ACIS exposure with the grating retracted. The ACIS spectrum can then be used as an initial guess in modeling the dispersed HEG and MEG spectra.

The diffracted images of extended objects which lead to position-dependent spectra are complicated. The complexity indicates that information is present but extracting the information is more difficult than for a point or an extended source with a uniform spectrum. For example, the plus and minus order images may not have the same appearance. An example of this effect was seen in ground test data using the double crystal monochromator source; e.g., test image H-HAS-EA-8.003 which is schematically presented and described in Figure 8.28. For astrophysical sources, variations in temperature, abundances, Doppler velocities, cooling flows, etc. can all create spatial-spectral variations. For these complex objects general analysis techniques are not available and forward folding of the spatial-spectral model through MARX is the best way to study these effects and to plan potential observations.

8.5.4  Optimizing Detection of Isolated Emission Lines: Choice of Spectrometer

When a line is isolated and appears against a "background" due primarily to the source continuum, then the signal/noise ratio is given by:

8.6  Simulations with MARX

MARX is a suite of programs designed to simulate the on-orbit performance of Chandra. It is built around a core program or engine which performs a ray trace of photon paths through all elements of the Chandra observatory. The user specifies a file containing the spectral energy distribution of the source to be simulated and then selects a model for the spatial distribution of the source, which can be a FITS image. More complicated "user source models" allow simulation of sources with spatial-spectral variations.

Once the source has been specified, MARX traces the path of photons through a model of the HRMA. Models for the High Energy Transmission Grating (HETG) and Low Energy Transmission Grating (LETG) can also be included and, in the focal plane, the user has the choice of all four Chandra detectors. The result of the simulation is converted with marx2fits into a FITS event file which can then be processed with standard CIAO tools.

The latest capabilities and instructions for use of MARX are given in the MARX User Guide at the MARX web site, http://space.mit.edu/CXC/MARX/ and in analysis threads available at http://cxc.harvard.edu/ciao/threads/index.html. WWW resources:
http://cxc.harvard.edu/cal/- CXC Instruments and Calibration page
http://cxc.harvard.edu/caldb/ - CXC CALDB page
http://space.mit.edu/HETG/- HETG home page
http://space.mit.edu/CXC/ - CXC at MIT, focusses on gratings
http://wwwastro.msfc.nasa.gov/xray/xraycal/ - MSFC ground cal site

8.7 References

Canizares, C.R., Schattenburg, M.L. and Smith, Henry I. 1985,
"The High Energy Transmission Grating Spectrometer
for AXAF", SPIE, 597, 253. 


Canizares, C.R. et al. 2000, "Initial Results
from the Chandra High Energy Transmission Grating Spectrometer",
Atomic Data Needs for X-ray Astronomy,
M.A. Bautista, T.R. Kallman, and A.K. Pradhan, eds.
http://heasarc.gsfc.nasa.gov/docs/heasarc/atomic/


Canizares, C.R. et al. 2000,  , 539, L41.


Canizares, C.R. et al. 2005,  , 117, 1144.


Davis, J.E., H.L. Marshall, M.L. Schattenburg, 
and D. Dewey, 1998, "Analysis and Modeling of Anomalous Scattering
in the AXAF HETGS", SPIE, 3444, 76.


Dewey, D., Drake, J.J., Edgar, R.J., Michaud, K.,
and Ratzlaff, P., 1998, "AXAF Grating Efficiency Measurements with Calibrated,
Non-imaging Detectors", SPIE, 3444, 48.


Dewey, D., Humphries, D. N., McLean, G. Y., and Moschella, D. A.
1994, "Laboratory Calibration of X-Ray
Transmission Diffraction Gratings", SPIE, 2280, 257.


Edgar, R.J., 2003,
Chandra X-ray Center memo dated 6/9/2003,
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