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The Astrophysical Journal, 530:L111­L114, 2000 Februar y 20
2000. The American Astronomical Society. All rights reser ved. Printed in U.S.A.

FIRST LIGHT MEASUREMENTS OF CAPELLA WITH THE LOW-ENERGY TRANSMISSION GRATING SPECTROMETER ABOARD THE CHANDRA X-RAY OBSERVATORY A. C. Brinkman, C. J. T. Gunsing, J. S. Kaastra, R. L. J. van der Meer, R. Mewe, F. Paerels,1 A. J. J. Raassen,2 and J. J. van Rooijen
Space Research Organization of the Netherlands (SRON), Sorbonnelaan 2, 3584 CA Utrecht, Netherlands

H. Brauninger, W. Burkert, V. Burwitz, G. Hartner, and P. Predehl ¨
Max-Planck-Institut fur Extraterrestrische Physik, Postfach 1603, ¨ D-85740 Garching, Germany

J.-U. Ness and J. H. M. M. Schmitt
Universitat Hamburg, Gojenbergsweg 122, D-21029 Hamburg, Germany ¨

and J. J. Drake, O. Johnson, M. Juda, V. Kashyap, S. S. Murray, D. Pease, P. Ratzlaff, and B. J. Wargelin
Har vard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Received 1999 November 23; accepted 1999 December 23; published 2000 January 25

ABSTRACT We present the first X-ray spectrum obtained by the Low-Energy Transmission Grating Spectrometer (LETGS) aboard the Chandra X-Ray Observatory. The spectrum is of Capella and covers a wavelength range of 5­ ° 175 A (2.5­0.07 keV). The measured wavelength resolution, which is in good agreement with ground calibration, ° is Dl 0.06 A (FWHM). Although in-flight calibration of the LETGS is in progress, the high spectral resolution and unique wavelength coverage of the LETGS are well demonstrated by the results from Capella, a coronal source rich in spectral emission lines. While the primar y purpose of this Letter is to demonstrate the spectroscopic potential of the LETGS, we also briefly present some preliminar y astrophysical results. We discuss plasma parameters derived from line ratios in narrow spectral bands, such as the electron density diagnostics of the He-like triplets of carbon, nitrogen, and oxygen, as well as resonance scattering of the strong Fe xvii line at ° 15.014 A. Subject headings: instrumentation: spectrographs -- line: identification -- plasmas -- stars: coronae -- stars: individual (Capella) -- X-rays: stars
1. INTRODUCTION

The Low-Energy Transmission Grating Spectrometer (LETGS) consists of three components of the Chandra Observatory: the High-Resolution Mirror Assembly (Van Speybroeck et al. 1997), the Low-Energy Transmission Grating (LETG; Brinkman et al. 1987, 1997; Predehl et al. 1997), and the spectroscopic array of the High-Resolution Camera (HRCS; Murray et al. 1997). The LETG, designed and manufactured in a collaborative effort of the Space Research Organization of the Netherlands and the Max-Plank-Institut fur Extraterrestri¨ sche Physik in Germany, consists of a toroidally shaped structure that supports 180 grating modules. Each module holds three 1.5 cm diameter grating facets, which have a line density of 1008 lines mm 1. The three flat detector elements of the HRC-S, each 10 cm long and 2 cm wide, are tilted to approximate the Rowland focal surface at all wavelengths, ensuring a nearly coma-free spectral image. The detector can be moved in the cross-dispersion direction and along the optical axis to optimize the focus for spectroscopy.3 An image of the LETG spectrum is focused on the HRC-S with zeroth order at the focus position and dispersed positive and negative orders symmetric on either side of it. The dis° persion is 1.15 A mm 1 in first spectral order. The spectral
Present address: Columbia University, New York, NY. Also at Astronomical Institute "Anton Pannekoek," Kruislaan 403, 1098 SJ Amsterdam, Netherlands. 3 Further information on LETGS components is found in the AXAF Observator y Guide (http://asc.har vard.edu/udocs/) and at the Chandra X-Ray Center calibration Web site (http://asc.har vard.edu/cal/).
2 1

width in the cross-dispersion direction is minimal at zeroth order and increases at larger wavelengths due to the intrinsic astigmatism of the Rowland circle spectrograph. The extraction of the spectrum from the image is done by applying a spatial filter around the spectral image and constructing a histogram of counts versus position along the dispersion direction. The background is estimated from areas on the detector away from the spectral image and can be reduced by filtering events by pulse height.
2. FIRST LIGHT SPECTRUM

Capella is a binar y system at a distance of 12.9 pc consisting of G8 and G1 giants with an orbital period of 104 days (Hummel et al. 1994). It is the brightest quiescent coronal X-ray source in the sky after the Sun and is therefore an obvious line source candidate for first light and for instrument calibration. X-rays from Capella were discovered in 1975 (Catura, Acton, & Johnson 1975; Mewe et al. 1975), and subsequent satellite obser vations provided evidence for a multitemperature component plasma (see, e.g., Mewe 1991 for references). Recent spectra were obtained with the Extreme-Ultraviolet Explorer ° ° (EUVE) longward of 70 A with a resolution of about 0.5 A (Dupree et al. 1993; Schrijver et al. 1995). The LETG first light obser vation of Capella was performed on 1999 September 6 (00h27m­10h04m UT) with LETG and HRC-S. For the analysis we use a composite of six obser vations obtained in the week after first light, with a total obser ving time of 95 ks. The HRC-S output was processed through standard pipeline processing. For LETG/HRC-S events, only the L111


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product of the wavelength and diffraction order is known because no diffraction order information can be extracted. Preliminar y analysis of the pipeline output immediately revealed a beautiful line-rich spectrum. The complete background° subtracted, negative-order spectrum between 5 and 175 A is shown in Figure 1. Line identifications were made using previously measured and/or theoretical wavelengths from the literature. The most prominent lines are listed in Table 1. The spectral resolution Dl of the LETGS is nearly constant when expressed in wavelength units, and therefore the resolving power l/Dl is greatest at long wavelengths. With the current ° uncertainty of the LETGS wavelength scale of about 0.015 A, ° this means that the prominent lines at 150 and 171 A could be used to measure Doppler shifts as small as 30 km s 1, such as may occur during stellar flare mass ejections, once the absolute wavelength calibration of the instrument has been established. This requires, however, that line rest-frame wavelengths are accurately known and that effects such as the orbital velocity of the Earth around the Sun are taken into account. Higher ° order lines, such as the strong O viii Lya line at 18.97 A, which is seen out to sixth order, can also be used.
3. DIAGNOSTICS

A quantitative analysis of the entire spectrum by multitemperature fitting or differential emission measure modeling yields a detailed thermal structure of the corona, but this requires accurate detector efficiency calibration which has not yet been completed. However, some diagnostics based on intensity ratios of lines lying closely together can already be applied. In this Letter we consider the helium-like line diagnostic and briefly discuss the resonance scattering in the Fe xvii l15.014 line. 3.1. Electron Density and Temperature Diagnostics Electron densities ne can be measured using density-sensitive spectral lines originating from metastable levels, such as the forbidden ( f ) 2 3S r 11S line in helium-like ions. This line and the associated resonance (r) 2 1P r 11S and intercombination (i) 2 3P r 11S line make up the so-called helium-like "triplet" lines (Gabriel & Jordan 1969; Pradhan 1982; Mewe, Gronenschild, & van den Oord 1985). The intensity ratio (i f )/r varies with electron temperature T, but more importantly, the ratio i/f varies with ne due to the collisional coupling between the 2 3S and 2 3P level. The LETGS wavelength band contains the He-like triplets from C, N, O, Ne, Mg, and Si (40, 29, 22, 13.5, 9.2, and ° 6.6 A, respectively). However, the Si and Mg triplets are not sufficiently resolved and the Ne ix triplet is too heavily blended with iron and nickel lines for unambiguous density analysis. The O vii lines are clean (see Fig. 2), and the C v and N vi lines can be separated from the blends by simultaneous fitting of all lines. These triplets are suited to diagnose plasmas in the range ne = 10 8­10 11 cm 3 and T 1­ 3 MK. For the C, N, and O triplets the measured i/f ratios are 0.38 0.14, 0.52 0.15, and 0.250 0.035, respectively, which imply (Pradhan 1982) ne (in 109 cm 3) = 2.8 1.3, 6 3, and 5 (1 j upper limit), respectively, for typical temperatures as indicated by the (i f )/r ratios of 1, 1, and 3 MK, respectively. This concerns the lower temperature part of a multitemperature structure which also contains a hot (6­8 MK) and dense ( 1012 cm 3) compact plasma component (see § 3.2). The derived densities are comparable to those of active regions on the Sun with a temperature of a few MK. Figure 2 shows a fit to the O vii

Fig. 1.--Complete LETGS spectrum of Capella, split into three parts for clarity. Note the difference in x and y scale for the three parts. Indicated in the plot are the triplets discussed in the text and a selection of the Fe lines at longer wavelengths. The hundred strongest lines are listed in Table 1.

triplet measured in the 1 order. The He-like triplet diagnostic, which was first applied to the Sun (e.g., Acton et al. 1972; Wolfson, Doyle, & Phillips 1983), has now for the first time been applied to a star other than the Sun. The long-wavelength region of the LETGS between 90 and ° 150 A contains a number of density-sensitive lines from 2l­2l transitions in the Fe-L ions Fe xx­Fe xxii, which provide density diagnostics for relatively hot ( 5 MK) and dense ( 1012 cm 3) plasmas (Mewe et al. 1985; Mewe, Lemen, & Schrijver 1991; Brickhouse, Raymond, & Smith 1995). These have been applied in a few cases to EUVE spectra of late-type stars and in the case of Capella have suggested densities more than 2 orders of magnitude higher than found here for cooler plasma (Dupree et al. 1993; Schrijver et al. 1995). These diagnostics will also be applied to the LETGS spectrum as soon as the long-wavelength efficiency calibration is established. ° 3.2. The 15­17 A Region: Resonance Scattering of Fe xvii? Transitions in Ne-like Fe xvii yield the strongest emission ° lines in the range 15­17 A (see Fig. 1). In principle, the optical ° line can be obtained by applying a depth t in the 15.014 A simplified escape-factor model to the ratio of the Fe xvii l15.014 resonance line with a large oscillator strength to a presumably optically thin Fe xvii line with a small oscillator ° ° strength. We use the 15.265 A line because the 16.780 A line can be affected by radiative cascades (D. A. Liedahl 1999, private communication). Solar physicists have used this tech-


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TABLE 1 Comparison of Measured and Theoretical Values of the Strongest Lines in the Capella Spectrum as Shown in Figure 1 lobsa ° (A) 6.65 ......... 6.74 ......... 8.42 ......... 9.16 ......... 9.31 ......... 11.54b ...... 12.14b ...... 12.27b ...... 12.43 ....... 12.48b ...... 13.46 ....... 13.53b ...... 13.71 ....... 13.82b ...... 14.07 ....... 14.22 ....... 14.27 ....... 14.38b ...... 14.56b ...... 15.02 ....... 15.18b ...... 15.27 ....... 15.46 ....... 15.64 ....... 15.83 ....... 15.88 ....... 16.02b ...... 16.08b ...... 16.30b ...... 16.78 ....... 17.05 ....... 17.10 ....... 17.62 ....... 18.62b ...... 18.96 21.61 21.82 22.11 ....... ....... ....... ....... lpreda ° (A) 6.65 6.74 8.42 9.17 9.32 11.55 11.53 12.13 12.12 12.26 12.29 12.43 12.83 12.85 13.45 13.52 13.51 13.55 13.70 13.83 13.84 14.06 14.21 14.26 14.38 14.36 14.54 14.56 15.01 15.18 15.21 15.27 15.46 15.63 15.83 15.87 16.01 16.00 16.08 16.11 16.34 16.31 16.78 17.06 17.10 17.63 18.63 18.63 18.97 21.60 21.80 22.10 Ic counts s 1) 5.1 2.9 4.6 6.2 3.1 3.5 ) 16.8 ) 6.6 ) 3.5 4.9 ) 9.7 11.5 ) ) 6.6 7.5 ) 4.2 18.0 5.3 6.2 ) 5.3 ) 44.2 3.4 ) 16.7 3.1 6.2 4.3 4.4 14.6 ) 16.0 ) 2.2 ) 27.9 30.5 29.5 4.4 2.0 ) 28.7 6.5 1.1 4.5 Line Identificationd He4w He6z H1AB He4w He6z He3A F22 H1AB 4C 4D C13 Ne5 N16 N15 He4w O1-68 O1-71 He5xy He6z 3A O1-50 Ne8AB F1-56, 55 F1-52, 53 F12 F2-57, 58 F10 F9 3C H3 O4 3D 3E F7 F6 F5 H2 F4 F3 O2 E2L F3-62 3F 3G M2 F1 He6z(2) He3A H1AB He4w(r) He5xy(i) He6z(f) lobsa ° (A) 24.79 ......... 28.78 ......... 29.52 ......... 30.02 ......... 33.74 ......... 34.10 ......... 34.20 ......... 36.40b ....... 37.94 ......... 44.03b ....... 44.16 ......... 45.03 ......... 45.68 ......... 50.31 ......... 50.55b ....... 51.15 ......... 51.27 ......... 54.12 ......... 54.71 ......... 56.89 ......... 60.04 ......... 62.84 ......... 63.68 ......... 66.37 ......... 68.20 ......... 68.40 ......... 75.06 ......... 75.87 ......... 85.24 ......... 85.44 ......... 90.08 ......... 93.91 ......... 94.84 ......... 101.55 ....... 102.30 ....... 102.57 ....... 103.94 ....... 108.35 ....... 113.79 ....... 117.14 ....... 118.69 ....... 119.99 ....... 121.86 ....... 128.74 ....... 132.86b ...... 150.09 ....... 171.06 ....... lpreda ° (A) 24.78 28.79 29.53 30.03 33.74 34.10 34.20 36.37 36.40 37.95 44.02 44.05 44.17 45.04 45.68 50.35 50.53 50.56 51.17 51.30 54.14 54.73 56.92 60.06 62.88 63.72 66.37 68.22 68.40 75.07 75.89 85.28 85.50 90.08 93.92 94.87 101.55 102.33 102.60 103.94 108.37 113.84 117.17 118.66 120.00 121.83 128.74 132.85 132.85 150.10 171.08 Ic counts s 1) 4.4 1.1 0.9 1.8 4.9 1.5 1.1 1.4 ) 1.1 3.3 ) 4.9 4.2 1.9 5.3 2.2 ) 2.7 2.9 2.9 4.4 1.8 1.3 2.0 2.9 2.7 1.0 1.2 0.8 0.9 0.8 0.6 1.0 12.4 0.4 2.5 0.8 0.4 4.4 6.1 0.5 1.2 1.4 1.8 2.0 1.6 4.0 ) 0.5 2.2 Line Identificationd H1AB He4w He6z 3C(2) H1AB 3G(2) M2(2) 4C(3) B6A H1AB(2) Li6A Li2 Li6B 3C(3) Li7A Na6A B6A Na6B 3G(3) M2(3) Na7B Na7A H1AB(3) 3C(4) Na8B Na8A Na9A 3G(4) M2(4) 3C(5) H1AB(4) 3G(5) M2(5) 3C(6) F4A H1AB(5) O6B 3G(6) M2(6) F4B O6A H1AB(6) B11 N6C O6D N6B C6A N6A Be13A Li5AB A4

log (Tm) 7.00 7.00 7.00 6.80 6.80 6.60 6.85 6.80 6.75 6.75 7.00 6.70 7.00 7.00 6.60 6.90 6.90 6.90 6.60 6.70 6.90 6.70 6.80 6.80 6.80 6.80 6.80 6.80 6.70 6.60 6.90 6.70 6.70 6.80 6.80 6.80 6.60 6.80 6.80 6.90 6.70 6.80 6.70 6.70 6.70 6.80 6.80 6.30 6.50 6.30 6.30 6.30

b

(10

3

Ion Si xiii Si xiii Mg xii Mg xi Mg xi Ne ix Fe xviii Ne x Fe xvii Fe xvii Fe xxi Ni xix Fe xx Fe xx Ne ix Fe xix Fe xix Ne ix Ne ix Fe xvii Fe xix Ni xix Fe xviii Fe xviii Fe xviii Fe xviii Fe xviii Fe xviii Fe xvii O viii Fe xix Fe xvii Fe xvii Fe xviii Fe xviii Fe xviii O viii Fe xviii Fe xviii Fe xix Fe xvii Fe xviii Fe xvii Fe xvii Fe xvii Fe xviii Mg xi O vii O viii O vii O vii O vii

log (Tm)b 6.30 6.20 6.20 6.70 6.10 6.70 6.70 6.70 6.30 6.50 6.30 6.10 6.30 6.70 6.30 6.50 6.20 6.50 6.70 6.70 6.50 6.50 6.50 6.70 6.50 6.50 6.50 6.70 6.70 6.70 6.50 6.70 6.70 6.70 6.80 6.50 6.90 6.70 6.70 6.70 6.90 6.50 7.10 7.00 6.90 7.00 7.00 7.00 7.10 5.50 5.80

(10

3

Ion N vii N vi N vi Fe xvii C vi Fe xvii Fe xvii Fe xvii S xii O viii Si xii Mg x Si xii Fe xvii Si xii Fe xvi Si x Fe xvi Fe xvii Fe xvii Fe xvi Fe xvi O viii Fe xvii Fe xvi Fe xvi Fe xvi Fe xvii Fe xvii Fe xvii O viii Fe xvii Fe xvii Fe xvii Fe xviii O viii Fe xix Fe xvii Fe xvii Fe xviii Fe xix O viii Fe xxii Fe xx Fe xix Fe xx Fe xxi Fe xx Fe xxiii O vi Fe ix

° Note.--For wavelengths and line identifications, see Phillips et al. 1999 (solar and laborator y measurements between 5 and 20 A), Mason ° obser vations between 90 and 175 A), and Mewe et al. 1985 and Mewe, Kaastra, & Liedahl 1995 (complete wavelength range in the MEKAL a lobs, lpred: obser ved and predicted line wavelengths; b = blend with indication of the most prominent lines in order of estimated decreasing b Tm = temperature (in kelvins) of maximum line formation. c I = raw line intensity. Note that these are included to illustrate approximate obser ved relative line strengths and do not represent definitive d Number in parentheses (m) indicates diffraction order m 1 1.

et al. 1984 (solar spectral code). strength. measurements.

nique to derive the density in active regions on the Sun (e.g., Saba et al. 1999; Phillips et al. 1996, 1997). Various theoretical models predict 15.014/15.265 ratio values in the range 3.3­4.7 with only a slow variation ( 5%) with temperature or energy in the region 2­5 MK or 0.1­0.3 keV (Brown et al. 1998; Bhatia & Doschek 1992). The fact that most ratios obser ved in the Sun typically range from 1.5 to 2.8 (Brown et al. 1998 and references above), significantly lower than the theoretical ratios, supports claims that in solar ° active regions the 15.014 A line is affected by resonant scat-

tering. The 15.014/15.265 ratio, which was recently measured in the Livermore electron beam ion trap (EBIT; Brown et al. 1998) and ranges from 2.77 to 3.15 (with individual uncertainties of about 0.2) at energies between 0.85 and 1.3 keV, is significantly lower than calculated values. Although the EBIT results do not include probably minor contributions from processes such as dielectronic recombination satellites and resonant excitation, this may imply that the amount of solar scattering has been overestimated in past analyses. Our measured ratio Fe xviii ll16.078/15.265 gives a temperature of 6 MK,


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About 150 lines have been identified, of which the brightest hundred are presented in Table 1. The high-resolution spectra of the Chandra grating spectrometers allow us to carr y out direct density diagnostics using the He-like triplets of the most abundant elements in the LETGS band, which were previously only possible for the Sun. Density estimates based on C, N, and O He-like complexes indicate densities typical of solar active regions and some 2 or more orders of magnitude lower than density estimates for the hotter (15 MK) plasma obtained from EUVE spectra. A preliminar y investigation into the effect ° of resonance scattering in the Fe xvii line at 15.014 A showed no clear evidence for opacity effects. After further LETGS inflight calibration, it is expected that relative Doppler velocities of the order of 30 km s 1 will be detectable at the longest wavelengths. The LETGS data as presented here could only be produced after dedicated efforts of many people for many years. Our special gratitude goes to the technical and scientific colleagues at SRON, MPE, and their subcontractors for making such a superb LETG and to the colleagues at many institutes for building the payload. Special thanks goes to the many teams who made Chandra a success, particularly the project scientist team, headed by Dr. Weisskopf, the MSFC project team, headed by Mr. Wojtalik, the TRW industrial teams and their subcontractors, the Chandra obser vator y team, headed by Dr. Tananbaum, and the crew of Space Shuttle flight STS-93. J. J. D., O. J., M. J., V. K., S. S. M., D. P., P. R., and B. J. W. were supported by Chandra X-Ray Center NASA contract NAS8-39073 during the course of this research.

Fig. 2.--O vii triplet in the LETGS 1 order spectrum with the resonance (r), the forbidden ( f ), and the intercombination (i) line. The measured ratios of these lines (from the fitted cur ve) are given in the text.

and the photon flux ratio 15.014/15.265 is measured to be 2.64 0.10. If we compare this to the recent EBIT results, we conclude that there is little or no evidence for opacity effects ° in the 15.014 A line seen in our Capella spectrum.
4. CONCLUSION

The Capella measurements with LETGS show a rich spec° trum with excellent spectral resolution (Dl 0.06 A, FWHM).

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