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Identification of Three Weak Lines and Their Influences on Diagnostics of Electron Temperature and Density in Astrophysical Plasmas

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CHIN.PHYS.LETT.

Vol. 21, No. 10 (2004) 2063

Identi cation of Three Weak Lines and Their In uences on Diagnostics of Electron Temperature and Density in Astrophysical Plasmas  LIANG Gui-Yun(

¥¢ª)1 , ZHAO Gang(¬¡)1, SHI Jian-Rong( ¨£§)1 , ZENG Jiao-Long(«¤¦)1 2 , 1 BIAN Xia( ©) ;

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 2 Department of Applied Physics, National University of Defense Technology, Changsha 410073 (Received 17 February 2004)

By comparing of the spectra from the Chandra LETGs observation with the synthetic spectra of argon, we identify two weak emission lines, that is, the Ar XV 27.470  A line with transition 2s3p 1P 1 2s2 1S 0 and the Ar XVI 27.872  A line with transition 3d 2D3=2 2p 2P 1=2 , which blend with the N VII Ly emission line. Several secondorder spectral lines are also identi ed, one of which is the second-order Fe XVIII 14.534  A line with transition 43 2 52 2p ( P )3d F 5=2 2p P 3=2 , which blends with the inter-combination line of He-like N VI. In order to study the eects of these new identi ed weak lines on the electron temperature and density, we diagnose the two parameters using the intensity ratio of triplets of the He-like N VI and that of resonance lines of H- and He-like N. In the literature, there is a discrepancy of the temperatures deduced from N VI from other He-like ions, such as C V and O VII. The discrepancy disappears after the contribution of these weak lines is taken into account. PACS:

95. 30. Ky, 97. 10. Ex, 97. 80. Jp

In recent years, a large number of high-resolution spectra with moderately high eective areas in soft xray region have been obtained by Chandra and XMM[1 10] For example, Mewe et al.[1] analysed the Newton. x-ray spectrum of Capella from 6  A to 175  A obtained with a low-energy transmission grating spectrometer (LETGS) on board of Chandra. Canizares et al.[2] presented the initial results from the Chandra highenergy transmission grating spectrometer (HETGS). Audard et al.[3] presented some results observed with the XMM-Newton satellite. These works indicate that successful and reliable interpretation of these x-ray spectra requires an adequate understanding of the atomic data. Under the Iron Project,[11] a great deal of atomic data of iron has already been obtained. For those non-iron elements, we recently calculated the atomic data of argon. The comparison between synthetic spectra and experimental spectra measured by Lepson [12] can demonstrate that our theoretical atomic et al. data are reliable. Therefore these atomic data can be used to identify emission lines and diagnose the properties of the astrophysical plasma. Capella star is a steady and strong solar-like star, therefore it is the most appropriate object for line identi cation of new lines and its potential application. Recent works[1;2;5] have shown that many lines from Fe and several lines from argon have been identi ed,[1;3;4] and we rstly identi ed two of Ar previously.[13] However, there are still many unidenti ed weak emission lines of S, Ar, and Ca. Some blend

with electron temperature- and density-sensitive lines, which would increase the uncertainty of the diagnosed temperature and density of the plasma. In addition, it is noted that the diagnosed temperature from N VI appears to be somewhat discrepant in comparison to those derived for C and O, and Mewe et al.[1] emphasized that it might have been aected by line blending. In this Letter, we identify two weak emission lines of argon, which blend with the N VII Ly emission line. Moreover, we identify several second-order spectral lines, one of which is identi ed as the second-order spectral line of Fe XVIII and blends with the intercombination line of N VI. The eects of these new identi ed weak lines on the electron temperature and density is discussed. Accurate atomic data is very important for the reliable interpretation of the astrophysical plasmas. We rstly calculate atomic data of Ar XIII{Ar XVI,[13;14] and then construct theoretical spectra in non-local thermal dynamic equilibrium[13 15] at logarithmic electron temperature log Te (K ) = 6:8. The reason for choosing this temperature is owing to emission measure distribution of the Capella having a peak near log Te (K ) = 6:8: For the detailed description of the calculation of the atomic data, see our previous paper.[13] In Fig. 1, we overlap the synthetic spectra on the Capella spectra of Chandra LETGs observation in the wavelength range 23.4{25.8  A. For the detailed description of the observation and data analysis, see Refs. [1,7]. In order to increase the signal-to-noise ratio, we add the

 Supported by the National Natural Science Foundation of China under Grant No 10373014, and the Chinese Academy of Sciences (No KJCX2-W2).  Email: [email protected] c 2004 Chinese Physical Society and IOP Publishing Ltd

LIANG Gui-Yun

2064

positive- and negative-order source spectra. This gure shows that there are two weak lines blending with the N VII Ly line. Present analysis shows that the two lines are from the transitions 1s2 3d 2D3=2 ! 1s2 2p 2P 1 2 1=2 of Ar XVI ( = 24:872) and 2s3p P 1 ! 2s 1S of Ar XV ( = 24:740). Therefore, we tted the 0 emission lines around 24.789  A with three Gaussian functions as shown by the dotted line in Fig. 2.

Vol.21

et al.

phot cm 2 s 1 . For the two new identi ed lines, the

uxes are (0:585  0:134)  10 4 and (0:473  0:135)  10 4 phot cm 2 s 1 , respectively. The relative intensity ratios to that of the line around 25  A are consistent with theoretical calculation. Since the HRC-S does not have suÆcient energy resolution to allow sorting of overlapping spectral orders, the dierent diraction spectra are added together. Therefore we modelled emission lines by determining their rst-order contributions and then transforming the tted results to second order. In the transformation the rst-order intensities are reduced by a factor of 11.374. This reduction factor is obtained by comparison of rst- and second-order uxes of the isolated line (15.013  A) of Fe XVII.

Comparison of the LETGs observation with the synthetic spectra of argon in wavelength 23.4{ 25.8 A. The synthetic spectra (dotted line) are constructed at the logarithmic electron temperature log T (K ) = 6:8. Fig. 1.

Chandra

e

When the contribution of the two weak lines has been considered, the value of 2 increases from 0.886 to 0.957, and degrees of freedom decreases by number 2. Figure 2 clearly shows that the multi-Gaussian tting (solid line) is better than the single-Gaussian tting (dashed line). The ux of N VII Ly decreases from (6:109  0:249)  10 4 to (5:761  0:242)  10 4

Observational spectra and tted curves around the N VII Ly line for Capella. The solid line is the tted curve using three Gaussian functions indicated by the dotted line, while the dashed line is the tting using a single Gaussian function. Fig. 2.

Table 1. Some new identi ed second-order spectral lines. First order Ion Transition obs theo Flux (10 4 (A) (A) photon cm 2 s 1 ) Ne IX (r) 1s2p 1P 1 ! 1s2 1S0 13.456 13.450 6:965  0:209 Fe XIX 2p3 (23D)3d 3D32 !1 2p4 3P 2 13.521 13.518 7:157  0:214 Ne IX (f) 1s2s S 1 ! 1s S 0 13.701 13.699 2:627  0:156 Fe XIX 2p3 (2 D)3d 1F 3 ! 2p4 1D2 13.741 13.740 1:270  0:134 Fe XIX 2p3 (46S )31d 3D3 ! 22p463P1 2 13.797 13.793 2:893  0:165 Fe XVII 2s2p 3p P 1 ! 2s 2p S 0 13.837 13.825 0:297  0:164 Fe XVIII 2p4 (1 D)3d 2D5 2 ! 2p5 2P 3 2 14.214 14.208 10:440  0:251 Fe XVIII 2p4 (1 D)3d 2F 5 2 ! 2p5 2P 3 2 14.555 14.534 4:258  0:186 Fe XVII 2p5 3d 1P 1 ! 2p6 1S 0 15.021 15.014 31:086  0:383 a This line blended with emission line of Ar XIV at 27.468  A. =

=

=

=

In Table 1, we list the uxes of rst-order line, the observed uxes and reduced contributions from rstorder lines. The reduced contribution of those strong lines is consistent with the observed uxes as shown

obs

(A) 26.915 27.012 27.388 27.483 27.564 27.667 28.414 29.107 30.023

Second order Flux (10 4 (A) photon cm 2 s 1 26.900 0:503  0:112 27.036 0:666  0:112 27.398 0:272  0:101 27.480 0:484  0:112 27.586 0:237  0:102 27.650 0:293  0:100 28.416 1:297  0:229 29.110 0:360  0:147 30.028 2:733  0:232 theo

Flux1st 11 374 :

0:612  0:063 0:629  0:064 0:231  0:030 0:117  0:020a 0:254  0:033 0:261  0:033 0:918  0:089 0:374  0:044

in the last and eighth columns. Such consistency con rms that observed lines shown in the sixth column are from the second-order spectral lines of those lines shown in the rst column of Table 1. Note the discrep-

No.10

LIANG Gui-Yun

ancy between the observed ux and the reduced contribution from the rst-order line at 27.483  A, which is due to the second order spectral line of Fe XIX blending with the emission line of Ar XIV ( = 27:468  A). The second-order spectral line ( = 29:107  A) of Fe XVIII is also identi ed, which blends with the intercombination of the He-like N VI. We decompose these two lines using two Gaussian function tting as shown in Fig. 3. These new identi ed weak lines blend with the lines of H- and He- nitrogen, and will aect the diagnostics of the electron temperatures and densities. The He-like triplets of the light element such as C, N, and O, which concern the resonance line 1s2 1S 0 {1s2p 1 P1 (r), the inter-combination line 1s2 1S {1s2p 3 P (i + j ) and the forbidden line 1s2 1S { 0 1;2 0 1s2s 3 S1 (f ), are strongly temperature- and densitysensitive. Therefore this ratio has extensively been used to diagnose the electron temperature of the coronal plasma.[1;7;16] In order to obtain the electron temperature (or electron density), one needs to calculate the intensity ratio G (or R = (i + j )=f ) as function of the temperature (or density) rstly, which decreases (or increases) monotonically with increasing of temperature (or density).[7;16] Secondly, we plot the intensity ratio. On the plotted curve, we nd the observed

2065

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value G (or R), then the corresponding electron temperature (or electron density) is the diagnosed result.

Observational spectra and tted curve for the N VI triplet for Capella. Inter-combination line and secondorder Fe XVIII line blended (dotted line). In the past literature, this line was thought of as a single inter-combination line of N VI (long dashed line). Fig. 3.

Table 2. Temperature determined from the line intensity ratio G = (i + j + f )=r for dierent He-like ions C, N, and O and from the ratio of the He- and H-like resonance lines. The value in parentheses is the mean statistical 1 error. MRD01 refers to the results of Ref.[1] and AMP03 refer to the results of Ref. [8]. Intensity ratio Electron temperature T  ( A) Present MRD01 AMP03 Present MRD01 AMP03 CV 40 731+41 472 0.80 (0.26) 1:46+10 08 1:40  0:50 61 40 268 N VI 29 084+29 535 1.21 (0.26) 1.44 (0.46) 0:88+00 62 0:50+00 50 39 20 28 787

1.01 (0:24) 1:32  0:10 O VII 21 804+22 101 0.87 (0.06) 0.89 (0.09) 0.95 (0.08) 1:98+00 25 1:80  0:30 1.66 28 21 602 C V/C VI 40 268 0.50 (0.07) 0.59 (0.24) 1:46+00 12 1:10  0:15 15 33 736 N VI/N VII 28 787 0.29 (0.04) 0.27 (0.04) 0.32 (0.07) 2:45  0:14 2:50  0:14 24 781 0.31 (0.04) 2:41  0:14 O VII/O VIII 21 602 0.33 (0.02) 0.37 (0.02) 0.21 (0.02) 3:39+00 06 3:37  0:06 10 18 969
denotes that the blend eects from the second-order Fe XVIII line have been decomposed out.  denotes that the blend eects from the two weak lines of Ar have been decomposed out. [1] The temperature is derived according to the ux ratio of Mewe :

: :

:

:

:

:

: :

:

: :

:

:

:

: :

: :

a

: :

a

: :

: :

: :

a

Table 2 lists the diagnostics of the electron temperature along with other works for comparison.[1;7;8] The upper part of the table presents the intensity ratios G of the triplets of He-like C, N, and O, and the diagnosed electron temperatures. For He-like nitrogen, the intensity ratio G decreases from 1.21 to 1.01 because of blending from the second-order spectral line

et al.

of Fe XVIII 14.555  A, while the electron temperature increases about 50%. With increase of atomic number, the diagnosed electron temperature from He-like ions also increases with the exception of that derived from N VI, as shown in the sixth column of Table 2. Such a discrepancy disappears by taking into account the contribution of the second order Fe XVIII line. In

LIANG Gui-Yun

2066

the lower part of Table 2, we list intensity ratios of the resonance lines of He- and H-like C, N, and O, and the diagnosed electron temperatures. Similarly, for N, the intensity ratio of the resonance lines increases from

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0.29 to 0.31 because of the blending with the emission lines of Ar XV and Ar XVI, while the diagnosed electron temperature decreases about 2%.

Table 3. Density determination from the line intensity ratio R = (i + j )=f for dierent He-like ions C, N, and O. The value in parentheses is the mean statistical 1 error. MRD01 refers to the results of Ref.[1], and NMS01 refers to the results of Ref.[7]. Intensity ratio Electron density  ( A) Present MRD01 NMS01 Present (109 ) MRD01 (109 ) MRD01 (109 ) C40V731 0 6:4+12 3:0  2:0 4.0 31 41 472 0.48 (0.20) 0.89 (0.60) 0.68 (0.16) N29 VI084 8 6:7+10 5:0  3:0 6.0 30 29 535 0.46 (0.16) 0.49 (0.17) 0.56 (0.08)
+2 8
0.23 (0.13) 1:2 O21 VII 804 1:0  4:6