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Instruments: LETG

The Low Energy Transmission Grating Spectrometer (LETGS) continues to perform well in flight, and at the time of writing is operating nominally.

The LETGS did, however, have a slightly disadvantaged start to the Chandra mission which complicated and hampered in-flight calibration. Firstly, at the X-ray calibration facility in Marshall Space Flight Center, the gain of the default detector (HRC-S) was found to be rather low for one of the three microchannel plate segments, necessitating last minute adjustments to the instrument prior to integration in the spacecraft. Unfortunately, these adjustments changed the quantum efficiency of the detector such that, at launch, the instrument was essentially uncalibrated for energies below carbon (E < 280 eV; > 44 Å). While this might not sound too bad to those used to working at higher energies with non-dispersive optics, the uncalibrated range corresponded to nearly 3/4 of the instrument's operating bandpass, which is from 2-170Å. To attempt to remedy this situation, we designed a program of in-flight calibration measurements of sources in whose flux vs wavelength we could hope to have some confidence in understanding. Point source (or nearly so) standard candles in the X-ray sky are not so easy to find, though hot white dwarfs are good candidates at the longer wavelengths of the LETG range. At the shorter end, extragalactic sources with (hopefully) quasi-power law spectra are the only options, though these sources are quite variable. Of these, for the LETG+HRC-S it is essential to use a source with a fairly steeply increasing flux toward low energy and with low line-of-sight ISM absorption in order to avoid the signal becoming dominated by the higher order throughput of the instrument. The primary target list decided on for effective area calibration comprised, then, the hot white dwarfs Sirius B and HZ 43, the BL Lac PKS 2155-304, and the quasar 3C 273.

The undertaking of the calibration plan for all the Chandra instruments was disrupted by the unfortunate radiation damage suffered by the ACIS detector. The LETG plan was worse hit than others due to the early and intense concentration on the ACIS problem. Toward late spring in 2000 we finally managed to finish obtaining the bare minimum set of observations and have been concentrating on obtaining a more accurate characterization of the effective area since then. It was soon discovered that, after allowance for all ISM absorption effects, the UV-Ion Shield (UVIS) ground calibration at the carbon and oxygen edges did not seem to quite take out the edge features from continuum spectra. Following some adjustments to the UVIS parameters, carried out in collaboration with Herman Marshall (MIT), an analysis of the in-flight data, coupled with a thorough re-analysis of laboratory quantum efficiency measurements obtained for the HRC-S by the HRC instrument team just prior to delivery, has succeeded in producing an instrument effective area that should be accurate to a level of 10-15% absolutely, across the entire LETGS bandpass. The relative calibration should be more accurate still. These data are being implemented in CIAO software and have also been made available in ASCII form on the LETG Observer Information page as the analysis has proceeded. In particular, we have made SHERPA- and XSPEC- compatible ARFs for the effective area at the nominal 0th order aim point to aid observers and proposers while the data are integrated into other software.

Observations with the ACIS-S spectroscopic array also turned out not to be so straightforward. At the nominal detector aimpoint near the centre of the S3 chip in the cross-dispersion direction (observatory z coordinate), the CTI-induced broadening of the CCD pulse height distributions for the lower energies accessible to the LETG+ACIS-S (< 500eV; >25Å) led to complications in assessing the effective area of the combination because a fraction of the events were falling below the lower pulse height discriminator level. This can still be a problem for nominal aimpoint observations for the longer wavelengths. To alleviate the problem (though not entirely for wavelengths $\geq$ 50Å), provided the target is not too extended, LETGS observations can use a SIM offset in the zdirection to place the aimpoint closer to the CCD readout--see the Proposer's Observatory Guide for details at:

and ``Checking Your LETG/ACIS-S Obscat Setup" links on the LETGS Observer Information page at:

A glorious LETG+ACIS-S observation of the X-ray nova black hole candidate XTE J1118+480 was obtained by J. McClintock and M. Garcia and collaborators as a target of opportunity under Director's Discretionary Time. This observation proved interesting for calibration as well as science, due largely to its brightness and quasi-power law spectral distribution. The detector image is illustrated in Figure 4, providing some details of both coarse and fine support structures in the LETG.

Figure 4: The detector image of the LETG+ACIS-S observation of the X-ray nova black hole candidate XTE J1118+480. As well as producing a beautiful spectrum, this very bright and soft source illustrates nicely the coarse support structure diffraction, seen as the star pattern around the 0th order, and the fine support structure diffraction, seen as the ``cat's whiskers'' pattern that fans out either side of the dispersed spectrum and that streaks out from 0th order perpendicular to the dispersion axis. [Figure courtesy M. Garcia.]


One further hiccup to LETG operations resulted from a failure in the HETG limit switch that telemeters information on the grating reaching the correct insertion position. As a result of the way the spacecraft implements safety procedures ensuring that the HETG and LETG can not collide during operations, the LETG now has to be inserted ``by hand'' through commands that nudge it into place during real-time ground contacts. This procedure has now been successfully implemented on many different occasions. One impact of this HETG switch problem is that grating observation planning is now more restricted, and swapping in and out of the gratings is being performed less frequently than during the earlier phase of the mission.

Improvements in Data Analysis

Work has continued in improving a method for reduction of background in the HRC-S by filtering on event pulse height. Unlike the HRC-I, the HRC-S pulse height distributions are significantly more narrow than the distribution due to background events and so the latter are more amenable to filtering. The complication arises in the very spatially non-uniform gain of the detector that varies on a size scale of a fraction of a millimeter. A filter has now been designed and will be available as part of CIAO 2.1 (scheduled release early March) utilizing a simple dmcopy command. Three levels of filtering can be chosen, corresponding to fractional x-ray losses of 0.5, 2, and 5$\%$. Background reduction ranges from roughly a factor of two to five, depending on wavelength and the selected filtering level. Please visit the LETGS Observer Information web page for details (address below.)

Background reduction by making an extraction mask for LETG+HRC-S spectra that follows more closely the spatial distribution of the cross-dispersion spread function with wavelength has also very recently been implemented in CIAO and pipeline software. The former extraction window is a simple rectangle that needs to be as wide as the broadest part of the cross-dispersion profile in order not to lose signal. Because of the astigmatic nature of the grating system, at the longest LETG+HRC-S wavelengths the image is significantly defocussed and broadened in the cross-dispersion direction, which means that much extra background was needlessly included into the extraction window at shorter wavelengths. After launch we prototyped code to employ a window that broadened toward longer wavelengths and this has now been implemented as a ``bow tie'' shaped extraction region in tgextract.

Recent papers based on LETG observations

Papers have recently begun to appear in the refereed journals based on LETGS observations. JJD would be interested to receive any preprints or reprints of papers using LETGS data.

Paerels, F., Brinkman, A. C., van der Meer, R. L. J., Kaastra, J. S., Kuulkers, E., Boggende, A. J. F. d., Predehl, P., Drake, J. J., Kahn, S. M., Savin, D. W., & McLaughlin, B. M. 2001, ApJ, Interstellar X-Ray Absorption Spectroscopy of Oxygen, Neon, and Iron with the CHANDRA LETGS Spectrum of X0614+091, 546, 338

Ness, J., Mewe, R., Schmitt, J. H. M. M., Raassen, A. J. J., Porquet, D., Kaastra, J. S., van der Meer, R. L. J., Burwitz, V., & Predehl, P. 2001, AAP, Helium-like triplet density diagnostics. Applications to CHANDRA-LETGS X-ray observations of Capella and Procyon, 367, 282

Brinkman, A. C., Gunsing, C. J. T., Kaastra, J. S., van der Meer, R. L. J., Mewe, R., Paerels, F., Raassen, A. J. J., van Rooijen, J. J., Bräuninger, H., Burkert, W., Burwitz, V., Hartner, G., Predehl, P., Ness, J., Schmitt, J. H. M. M., Drake, J. J., Johnson, O., Juda, M., Kashyap, V., Murray, S. S., Pease, D., Ratzlaff, P., & Wargelin, B. J. 2000, ApJL, First Light Measurements of Capella with the Low-Energy Transmission Grating Spectrometer aboard the Chandra X-Ray Observatory, 530, L111

Kaastra, J. S., Mewe, R., Liedahl, D. A., Komossa, S., & Brinkman, A. C. 2000, AAP, X-ray absorption lines in the Seyfert 1 galaxy NGC 5548 discovered with Chandra-LETGS, 354, L83

Observer and proposer information and news on the performance of the Chandra LETGS can be found on the instruments and calibration page:

- Jeremy J. Drake and Bradford J. Wargelin

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