Instruments: HETG

Status, Calibration, and Software

The High-Energy-Transmission-Grating Spectrometer (HETGS) is the combined operation of Chandra's HRMA, HETG, and ACIS-S.     There has been little change in the status of these components and their performance over the past year, and so the HETGS continues to provide high-quality data as described in the Proposers' Observatory Guide (POG).

There have been no substantial changes to the HETGS calibration products in the past year. One small item is the value listed for the ACIS physical pixel size and a corresponding change in the HETG Rowland spacing back to the expected preflight value; wavelengths have been and should continue to be accurate to 100 km/s for energies below 1.5 keV. Effective area corrections derived from front-illuminated to back-illuminated CCD QE ratios are still being worked, see the HETG user web page for up-to-date information [1].

  

Figure 7: The top panel shows the expected Chandra-measured continuum based on a fit to simultaneous RXTE of GRS 1915+105 data as the thin black line. The colored HETGS observed spectra are from grating-orders as indicated. Note that pileup has distorted these dispersed spectra. In the bottom panel the HEG 1st order data have been corrected for pileup and agree well with the RXTE spectrum. (From Lee et al., [5].)

LINK TO POSTSCRIPT FILE for Figure 7

A development of note in the HETGS data analysis domain is the recent availability of software which can help analyze moderately piled-up grating spectra. ACIS pileup (POG section 6.16, [2]) in imaging (or zeroth-order) observations can be estimated and analyzed using ``pileup'' models implemented in ISIS V0.9.77[3], XSPEC V11.1[4], and Sherpa V2.2. The extension of these methods to grating dispersed data is now available as the ``gpileup'' model in ISIS. Pileup in the dispersed images happens for bright sources, e.g., lines in sources like Capella (see POG Figure 8.14) or continuum from Galactic binaries, see Figure 7. As in the imaging case, the effect of pileup is to reduce/distort the observed count rate in high-flux regions. The ``gpileup'' model can estimate and recover the underlying spectrum for moderately piled-up dispersed sources.

  

Figure 8: Absorption edges imprinted on the continuum spectrum of the Galactic binary, ``micro-quasar'' GRS 1915+105. These edges arise from neutral (cold) atoms of Mg, Si, and S along the line of sight to the source. There are tantalizing hints of X-ray Absorption Fine Structure (XAFS), wave-like modulations in the transmission indicated by the blue arrows in the middle, Si, panel. These XAFS would indicate that some of the absorbing Si is bound in molecular form. (From Lee et al., [5].)

LINK TO POSTSCRIPT FILE for Figure 8

in

Science I: All along the line-of-sight...

Information in HETGS spectra comes not only as bright emission lines (e.g., from Capella in POG Figure 8.2) but also in the form of photoelectric absorption edges and resonance absorption features (dips) produced by material along the line of sight to the source. This material may be cold interstellar material (ISM, [6]) or hot, ionized material in a wind, jet, accretion disk or accretion disk atmosphere around the bright continuum source. Measurements of ISM edges along the line of sight to GRS 1915+105 are shown in Figure 8 and correspond to a total hydrogen column of 3-16 $\times 10^{22}$ cm^-2 depending on abundance assumptions.

Science II: The wind began to howl.

Eta Carina may be the most massive and luminous star in the Galaxy and is suspected to be a massive, colliding wind binary system. Corcoran et al. [7] used the Chandra HETGS to obtain a high-resolution X-ray spectrum of the star uncontaminated by the nearby extended soft X-ray emisssion, Figure 9. This HETGS spectrum of eta Carina is unlike recently published X-ray grating spectra of single massive stars in significant ways and is generally consistent with colliding wind emission in a massive binary.

  

Figure: HETGS MEG spectrum of $\eta$ Carina. The need for a two temperature fit to the data is demonstrated here indicating that the hot gas near the star is far from isothermal and points toward colliding wind emission in a massive binary. (From Corcoran et al., [7].)

LINK TO POSTSCRIPT FILE for Figure 9

- Dan Dewey

References and Web pages:

[1] CXC's HETG user web page:

http://space.mit.edu/ASC/calib/hetg_user.html

[2] ``Event Pileup in Charge-coupled Devices'', J.E. Davis, ApJ, 562, pp. 575-582 (2001).

[3] ``ISIS: An Interactive Spectral Interpretation System for High Resolution X-Ray Spectroscopy'', Houck, J. C. and Denicola, L. A., ADASS IX, ASP Conf. Proc., Vol. 216, p.591 (2000). Web page: http://space.mit.edu/CXC/ISIS

[4]XSPEC web page:

http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/xanadu/xspec/

[5] ``High Resolution Chandra HETG and RXTE Observations of GRS 1915+105: A Hot Disk Atmosphere & Cold Gas Enriched in Iron and Silicon'', J.C. Lee et al., astro-ph/0111132 and ApJ, 567, in press (2002).

[6] ``On the Absorption of X-Rays in the Interstellar Medium'', Wilms, J., Allen, A., and McCray, R., ApJ, 542, 914 (2000).

[7] ``The Chandra HETGS X-ray Grating Spectrum of Eta Car'', M.F. Corcoran et al., ApJ, 562, 1031 (2001).