- Email: [email protected]
ASCA data analysis of the young supernova remnant G41.1-0.3
Adv. Space Res. Vol. 25, No. 3/4, pp. 563-566, 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/00 $20.00 + 0.00 PII: SO273-1177(99)00800-5
0
Pergamon www.elsevier.nl/locate/asr
ASCA NANT
DATA ANALYSIS G41.1-0.3
Yang Chen’,
Ming Sun’, Zhen-Ru
2000
OF THE
Wang’,
and Qi-Feng
YOUNG
SUPERNOVA
REM-
Yin’
1. Department of Astronomy, Nanjing University, Nanjing 210093, P.R.China 2. NRAO, 520 Edgemont Road, Charlottesville, LA 22903-2475, USA
ABSTRACT We fit the ASCA SIS spectral data of SNR G41.1-0.3 with both a two-component, NE1 model and an EI model. Hard ( 2 2 keV) X-ray emission arises essentially from the hot component, containing S and Fe K lines. The cool component, which was found to be arriving at an ionization equilibrium, generates mainly Mg, Si, Fe L lines, and continuum, contributing essentially to the soft emission. The high emission measure of the cool component suggests that the remnant evolves in a cloudy medium. We did not find powerlaw, bremsstrahlung, and blackbody components, as well as pulsed signals. The ASCA SIS images were restored and compared with the ROSAT HRI and 20cm VLA maps. The X-ray image might suggest a bipolar structure for the remnant that encounters a denser medium in the west. Relevant parameters of the remnant were derived. 0 2000 COSPAR. Published by Elsevier Science Ltd.
1
INTRODUCTION
SNR G41.1-0.3 has a clear shell structure with a small average diameter (D N 3’.6) in 20 cm radio observation (Caswell et al. 1982). The HI absorption measurements provide a distance estimate d 2 7.5 kpc (Caswell et al. 1975), and the C - D relationship infers a distance N 12-13 kpc (Caswell & Lerche 1979; Milne 1979). The significant deviations from circular symmetry would suggest a young age, and a preliminary estimation of the age is t N 600yr (Caswell et al. 1982; Becker, Markert, & Donahue 1985). The EINSTEIN observation (Becker et al., 1985) showed two concentrations of soft X-ray emission in the IPC image, and yielded roughly an estimate of the plasma temperature and the hydrogen column density. Here we present a spectral analysis of the ASCA SIS data and a comparison of the ASCA SIS, ROSAT HRI, and radio morphologies.
2
OBSERVATIONAL
DATA
ANALYSIS
We acquired the X-ray data of SNR G41.1-0.3 from the archival ASCA data. The ROSAT HRI data and VLA 20cm data are also used for comparing the remnant morphology. The SNR G41.1-0.3 was observed by the ASCA satellite on April 8, 1995, with the detectors SIS and GIS, in l-CCD clocking mode. The data were screened using the standard processing. The effective exposure times after screening were 34.1 kiloseconds for SISO and 33.7 kiloseconds for SISl, and the total events amount 33943 and 27116 for SISO and SIS 1, respectively. Spectrum
The spectrum
of the remnant
was extracted
from the SIS data within
a circular
region
of radius N 4’, centered at R.A. = 19.:07”33’, decl. = 07’08’00” (J2000). Background spectra were extracted from the archival data of a blank sky near G41.1-0.3. The SISO spectrum was fitted with two thermal component non-equilibrium ionization (NEI) model using the spectral code SPEX (Kaastra et al. 1996) in the energy range 0.5 - 8.4 keV. Two cases, in which the element abundances of the two components either vary separately (Figure 1) or are coupled, were investigated. The fitting results are 563
564
Y Chen ernl.
Table: Fitting simultaneously
results obtained by applying SPEX to the SISO spectral data, and applying XSPEC to the SISO and SISl data. The 90% confidence range (Ax2 = 2.706) are affiliated.
t Here d= Rkpc is used
separated
case
SPEX coupled
case
whole
XSPEC portion A
portion
8
0
B
hot component
n,n~V
(10" cm-3)t
T (keV) net (lOI
cme3s)
Pk/ HI WHI
1.29+;:;;
0.99:;:;: 3.84+Oco -0.04
2,63+0.43 -0.53 6.26:;';; x lo-2
5*01+,0:;; x 1o-2
1 89+'.36 ,',,+IE . -0.00
0.69+;:$,
0
0.67+0.05 -0.00
O 6O+o.32 * -0.29
1.98+;:0,;
P/HI
3 72tO.65 . -0.56
0 .g1to.00 -0.00
P/HI
194-8.24
*
-4.32
,',,+Es . -0.00
0.52f;:;;
1 .oo+;:;; 5 .157+3.u2 -1.37
3.73tl.94 -1.06
cool component n,nHV
(1058cm-3)t 3~+232 -88 T (keV)
net (lOI cmw3s) MT/HI
490+48s -29
0.239+;:,0;;
0 226fu.003
1.34fr9,
2 .67tu.07 -2.11
0.38:;:;;
-_
0.053
oot2.51
ootO.18 * -0.00
521+;:
117:;;
197:;;
0.223+;:;$
0.224:;:;;;
0.222:;:;;;
0 .58+0.08 -0.06
0.61:;:;;
[Si/H]
0.94$~~
0 .g7to.12 -0.11
1.10+“.27 -0.23
[S/HI
0.;92_0:,2
[Fe/H] NH (1O22 cmw2) y2/d.o.f.
0.33fi:ik 2.87+’ ” -0:12 341/254
5 .60+1.03 -0.83 0 .o(po.15 -0.00 2.82+’ O7 -0:04 11421559
6 .48t2.63 -1.83 o oo+o.25 * -0.00
0.48:;:;; 0 g7to.20 -0.17 12+1.79 4’. -1.60 0.00+0.35 0.00
2.66+” ” -0:07 613/559
3.08+* l7 -0:06 593/559
tabulated. We also tried with powerlaw. were not satisfying.
u 00
3.04f,:,, 4161258
bremsstrahlung,
and blackbody
components,
but the results
We find that the cool component plasma contributes to the emission mainly below about 2 keV, while the hot component contributes above 2 keV. The cool component emission is mainly composed of the lines of Mg, Si, Fe L, and continuum, while the hot emission gives rise to the strong S line and Fe K complex. It is noticeable that the ionization parameter of the cool component n,t 2 1 x 1012 cmm3 s is considerably high and indicates that the component is approaching an ionization equilibrium (e.g., Masai 1984). In view of the possibility of the state of equilibrium ionization (EI), we also applied the XSPEC code (using the VMEKAL model therein) (Mewe et al. 1995) to fit the spectrum in the energy range 0.5 8.8 keV. Besides the spectrum extracted from the circle mentioned above, we also extracted the spectra
0.5
1
a
2 Energy
(keV)
Fig. 1 The X-ray spectrum extracted from the SISO data, fitted with the two component the “separated case” using the SPEX code.
NE1 model in
565
ASCADataAnalysis of the SupernovaRemnant G41.1-0.3
7Olo.-
?08. -
Po6. -
,9”07-40-
19”07”30’
,9"07"40S 19"07"30' 19*07"20'
,9"07"20*
Fig. 2 Smoothed X-ray image of G41.1.0.3 restored from the ASCA SIS data, with an overlay of 20cm radio emission contours. Contours are from 0.1 to 0.9 of maximum with 8 linear intervals. from the two circular portions results are listed in the Table.
A and B (labeled
Fig. 3 Smoothed ROSAT HRI image superposed with the restored SIS 2-10 keV (hard) contours. Contours are from 0.1 to 0.9 of maximum with 8 linear intervals.
in Figure
2), each with a radius
Morphology Using the SIS data we made 0.5-10 keV and 2-10 keV exposureimages, respectively. To restore the images from the severe influence of PSF, by running the LUCY task in the IRAF code. The 0.5-10 keV image with emission contours is shown in Figure 2. The ROSAT HRI grayscale image is 2-10 keV (hard) contours in Figure 3.
of 1’.65. The fitting
and vignetting-corrected we did the deconvolution an overlay of 20cm radio superposed with the SIS
The GIS data have been used in the temporal analysis owing to their better time Pulsed Signal ? resolution. A standard FFT method is used to search the possible period. No pulsed signal was found evident in the range of 0.1 - 10 s.
3
DISCUSSION
Parameters of the Remnant The NE1 and EI fitting results give a hydrogen column density 2.9 x 1O22 cmW2. The extinction per unit distance in the direction of G41.1-0.3 can be estimated NH N from the contour diagrams given by Lucke (1978): (E,_,)/d N 0.60 magkpc-‘. Using the relation NH = 5.9 x 1021(EB_-V) cm- 2 (Spitzer 1978), a distance d N 8 kpc was obtained. This is in agreement with the limit d 2 1.5 kpc (Caswell et al.1975). Adopting an average angular radius of 2’, the radius of the remnant is r N 4.7~~. From the Table we see that the EM of the cool component is much higher than that of the hot component. This result cannot be explained by the ordinary two-component scenario in which the hot component is ascribed to the surrounding medium swept up by the blastwave, and the cool component to the ejecta heated by the reverse shock. Instead, a competing interpretation may be that the surrounding medium is inhomogeneous. The hot component may correspond to the shocked low density medium, and the cool component to the shocked cloudy matter. If the EM of the cool component is taken as fn,n H,CV z 340 x 1O58cmT3, where f is the filling factor of dense cloudlets, then the hydrogen number density in the cloudlets is IZ~,~ z 30(f/0.25)-‘/2cm-3, and the electron density n, is N 36(f/0.25)-1/2.F rom the EM of the hot component (1 - f)n,nH,rCMV z 1 x 1058cm-3, the intercloud hydrogen density is obtained nN,ICM z 0.9[(1- f)/0.75]-'/2cm-3. If the high temperature
N 2.6 keV is ascribed
to the blastwave,
the postshock
temperature
may be
Y Chenetal.
566
N 1.3 x 108cms-’ (where the T, N 2 keV. The blastwave velocity would be v, = (16kT,/3pmH)‘/’ mean atomic weight p = O.Sl), and the age could be estimated t = 2r/5v, N 1.4 x 103yr. On the other hands, the ionization parameter n,t w 1.3 x 10’2cm-3 s of the cool component implies an age t ;L 1.1 x 103(f/0.25)‘/2yr, and n,t N 6.3 x 1010cm-3 s of the hot component implies an age t 2 1.8 x 103[(1 f)/0.75]1/2yr. Th ese values of age are greater than the previous estimation of 600 yr. For t - 1.4 x 103yr, the estimation of the explosion energy is E = (1.4n H,ICMmH/[)(r5/t2) - 3 X 1050[(1 - f)/0.7rj]-‘/‘ergs where < = 2.026 (under assumption of the Sedov model).
In Figures. 2 & 3, the ASCA and ROSAT X-ray emissions again have two concenMorphology trations. Two bright arcs in the hard contour map are roughly coincident with the two concentrations (Figure 3). Therefore the hot component is mainly concentrated in the two bright portions. The two arcs seem to be two parts of a bubble in the western half. The eastern half of the HRI image (Figure 3) seems to be a broken bubble-like structure, with the eastern concentration of emission on its western boundary. The hard contour map, the HRI image, and the VLA map all show an outward protrusion on the east side. The elongation direction of the whole remnant is essentially perpendicular to the galactic plane. The high EM of the cool component suggests that the remnant is evolving in a cloud. The brighter radio and X-ray emissions on the west side close to the galactic plane imply a density gradient of the cloud toward the plane. Also the NH values for the portion A and B obtained from the VMEKAL code are in favor of such a gradient. We envisage a possible scenario that the remnant takes a shape like a bipolar bubble (eastern bubble + western bubble) with its axis tilted with regard to the line of sight. The two bright peaks result from the bipolar bubble interacting with the western denser medium. On the other side, the plasma leaks out of the the broken eastern bubble, so that the X-ray emission from the eastern half is softer than from the western half. This work is supported by grants from the NSF of China, Commission of China, and the Education Ministry of China.
4
the Ascent
Project
of the State
Scientific
REFERENCES
Becker, R. H., Markert, Caswell, J. L., Haynes,
T., & Donahue,
Caswell, J. L., & Lerche, I., MNRAS, Caswell, J. L., Roger, R. S., Murray, Kaastra,
M., ApJ, 296, 461 (1985)
R. F., Milne, D. K., & Wellington,
K. J., MNRAS,
200, 1143 (1982)
187, 201 (1979) J. D., Cole, D. J., & Cooke, D. J., A&A, 45, 239 (1975)
J. S., Mewe, R., Nieuwenhuijzen, H., in The 11th ~011. on UV and X-ray Spectr. Plasmas, Watanabe, T.(ed.), p.411 (1996)
of Astroph.
and Laboratory
Lucke, P. B., A&A, 64, 367 (1978) Masai, K., ApSS, 98, 367 (1984) Mewe, R., Kaastra,
J. S., & Liedahl,
Milne, D. K., Astralian Spitzer,
D. A., Legacy-The
JournaE of the HEASARC,
No.6, 16 (1995)
J. Phys., 32, 83 (1979)
L. Jr., Physical Processes
in the Interstellar
Medium
(Wiley, New York) (1978)
0
Pergamon www.elsevier.nl/locate/asr
ASCA NANT
DATA ANALYSIS G41.1-0.3
Yang Chen’,
Ming Sun’, Zhen-Ru
2000
OF THE
Wang’,
and Qi-Feng
YOUNG
SUPERNOVA
REM-
Yin’
1. Department of Astronomy, Nanjing University, Nanjing 210093, P.R.China 2. NRAO, 520 Edgemont Road, Charlottesville, LA 22903-2475, USA
ABSTRACT We fit the ASCA SIS spectral data of SNR G41.1-0.3 with both a two-component, NE1 model and an EI model. Hard ( 2 2 keV) X-ray emission arises essentially from the hot component, containing S and Fe K lines. The cool component, which was found to be arriving at an ionization equilibrium, generates mainly Mg, Si, Fe L lines, and continuum, contributing essentially to the soft emission. The high emission measure of the cool component suggests that the remnant evolves in a cloudy medium. We did not find powerlaw, bremsstrahlung, and blackbody components, as well as pulsed signals. The ASCA SIS images were restored and compared with the ROSAT HRI and 20cm VLA maps. The X-ray image might suggest a bipolar structure for the remnant that encounters a denser medium in the west. Relevant parameters of the remnant were derived. 0 2000 COSPAR. Published by Elsevier Science Ltd.
1
INTRODUCTION
SNR G41.1-0.3 has a clear shell structure with a small average diameter (D N 3’.6) in 20 cm radio observation (Caswell et al. 1982). The HI absorption measurements provide a distance estimate d 2 7.5 kpc (Caswell et al. 1975), and the C - D relationship infers a distance N 12-13 kpc (Caswell & Lerche 1979; Milne 1979). The significant deviations from circular symmetry would suggest a young age, and a preliminary estimation of the age is t N 600yr (Caswell et al. 1982; Becker, Markert, & Donahue 1985). The EINSTEIN observation (Becker et al., 1985) showed two concentrations of soft X-ray emission in the IPC image, and yielded roughly an estimate of the plasma temperature and the hydrogen column density. Here we present a spectral analysis of the ASCA SIS data and a comparison of the ASCA SIS, ROSAT HRI, and radio morphologies.
2
OBSERVATIONAL
DATA
ANALYSIS
We acquired the X-ray data of SNR G41.1-0.3 from the archival ASCA data. The ROSAT HRI data and VLA 20cm data are also used for comparing the remnant morphology. The SNR G41.1-0.3 was observed by the ASCA satellite on April 8, 1995, with the detectors SIS and GIS, in l-CCD clocking mode. The data were screened using the standard processing. The effective exposure times after screening were 34.1 kiloseconds for SISO and 33.7 kiloseconds for SISl, and the total events amount 33943 and 27116 for SISO and SIS 1, respectively. Spectrum
The spectrum
of the remnant
was extracted
from the SIS data within
a circular
region
of radius N 4’, centered at R.A. = 19.:07”33’, decl. = 07’08’00” (J2000). Background spectra were extracted from the archival data of a blank sky near G41.1-0.3. The SISO spectrum was fitted with two thermal component non-equilibrium ionization (NEI) model using the spectral code SPEX (Kaastra et al. 1996) in the energy range 0.5 - 8.4 keV. Two cases, in which the element abundances of the two components either vary separately (Figure 1) or are coupled, were investigated. The fitting results are 563
564
Y Chen ernl.
Table: Fitting simultaneously
results obtained by applying SPEX to the SISO spectral data, and applying XSPEC to the SISO and SISl data. The 90% confidence range (Ax2 = 2.706) are affiliated.
t Here d= Rkpc is used
separated
case
SPEX coupled
case
whole
XSPEC portion A
portion
8
0
B
hot component
n,n~V
(10" cm-3)t
T (keV) net (lOI
cme3s)
Pk/ HI WHI
1.29+;:;;
0.99:;:;: 3.84+Oco -0.04
2,63+0.43 -0.53 6.26:;';; x lo-2
5*01+,0:;; x 1o-2
1 89+'.36 ,',,+IE . -0.00
0.69+;:$,
0
0.67+0.05 -0.00
O 6O+o.32 * -0.29
1.98+;:0,;
P/HI
3 72tO.65 . -0.56
0 .g1to.00 -0.00
P/HI
194-8.24
*
-4.32
,',,+Es . -0.00
0.52f;:;;
1 .oo+;:;; 5 .157+3.u2 -1.37
3.73tl.94 -1.06
cool component n,nHV
(1058cm-3)t 3~+232 -88 T (keV)
net (lOI cmw3s) MT/HI
490+48s -29
0.239+;:,0;;
0 226fu.003
1.34fr9,
2 .67tu.07 -2.11
0.38:;:;;
-_
0.053
oot2.51
ootO.18 * -0.00
521+;:
117:;;
197:;;
0.223+;:;$
0.224:;:;;;
0.222:;:;;;
0 .58+0.08 -0.06
0.61:;:;;
[Si/H]
0.94$~~
0 .g7to.12 -0.11
1.10+“.27 -0.23
[S/HI
0.;92_0:,2
[Fe/H] NH (1O22 cmw2) y2/d.o.f.
0.33fi:ik 2.87+’ ” -0:12 341/254
5 .60+1.03 -0.83 0 .o(po.15 -0.00 2.82+’ O7 -0:04 11421559
6 .48t2.63 -1.83 o oo+o.25 * -0.00
0.48:;:;; 0 g7to.20 -0.17 12+1.79 4’. -1.60 0.00+0.35 0.00
2.66+” ” -0:07 613/559
3.08+* l7 -0:06 593/559
tabulated. We also tried with powerlaw. were not satisfying.
u 00
3.04f,:,, 4161258
bremsstrahlung,
and blackbody
components,
but the results
We find that the cool component plasma contributes to the emission mainly below about 2 keV, while the hot component contributes above 2 keV. The cool component emission is mainly composed of the lines of Mg, Si, Fe L, and continuum, while the hot emission gives rise to the strong S line and Fe K complex. It is noticeable that the ionization parameter of the cool component n,t 2 1 x 1012 cmm3 s is considerably high and indicates that the component is approaching an ionization equilibrium (e.g., Masai 1984). In view of the possibility of the state of equilibrium ionization (EI), we also applied the XSPEC code (using the VMEKAL model therein) (Mewe et al. 1995) to fit the spectrum in the energy range 0.5 8.8 keV. Besides the spectrum extracted from the circle mentioned above, we also extracted the spectra
0.5
1
a
2 Energy
(keV)
Fig. 1 The X-ray spectrum extracted from the SISO data, fitted with the two component the “separated case” using the SPEX code.
NE1 model in
565
ASCADataAnalysis of the SupernovaRemnant G41.1-0.3
7Olo.-
?08. -
Po6. -
,9”07-40-
19”07”30’
,9"07"40S 19"07"30' 19*07"20'
,9"07"20*
Fig. 2 Smoothed X-ray image of G41.1.0.3 restored from the ASCA SIS data, with an overlay of 20cm radio emission contours. Contours are from 0.1 to 0.9 of maximum with 8 linear intervals. from the two circular portions results are listed in the Table.
A and B (labeled
Fig. 3 Smoothed ROSAT HRI image superposed with the restored SIS 2-10 keV (hard) contours. Contours are from 0.1 to 0.9 of maximum with 8 linear intervals.
in Figure
2), each with a radius
Morphology Using the SIS data we made 0.5-10 keV and 2-10 keV exposureimages, respectively. To restore the images from the severe influence of PSF, by running the LUCY task in the IRAF code. The 0.5-10 keV image with emission contours is shown in Figure 2. The ROSAT HRI grayscale image is 2-10 keV (hard) contours in Figure 3.
of 1’.65. The fitting
and vignetting-corrected we did the deconvolution an overlay of 20cm radio superposed with the SIS
The GIS data have been used in the temporal analysis owing to their better time Pulsed Signal ? resolution. A standard FFT method is used to search the possible period. No pulsed signal was found evident in the range of 0.1 - 10 s.
3
DISCUSSION
Parameters of the Remnant The NE1 and EI fitting results give a hydrogen column density 2.9 x 1O22 cmW2. The extinction per unit distance in the direction of G41.1-0.3 can be estimated NH N from the contour diagrams given by Lucke (1978): (E,_,)/d N 0.60 magkpc-‘. Using the relation NH = 5.9 x 1021(EB_-V) cm- 2 (Spitzer 1978), a distance d N 8 kpc was obtained. This is in agreement with the limit d 2 1.5 kpc (Caswell et al.1975). Adopting an average angular radius of 2’, the radius of the remnant is r N 4.7~~. From the Table we see that the EM of the cool component is much higher than that of the hot component. This result cannot be explained by the ordinary two-component scenario in which the hot component is ascribed to the surrounding medium swept up by the blastwave, and the cool component to the ejecta heated by the reverse shock. Instead, a competing interpretation may be that the surrounding medium is inhomogeneous. The hot component may correspond to the shocked low density medium, and the cool component to the shocked cloudy matter. If the EM of the cool component is taken as fn,n H,CV z 340 x 1O58cmT3, where f is the filling factor of dense cloudlets, then the hydrogen number density in the cloudlets is IZ~,~ z 30(f/0.25)-‘/2cm-3, and the electron density n, is N 36(f/0.25)-1/2.F rom the EM of the hot component (1 - f)n,nH,rCMV z 1 x 1058cm-3, the intercloud hydrogen density is obtained nN,ICM z 0.9[(1- f)/0.75]-'/2cm-3. If the high temperature
N 2.6 keV is ascribed
to the blastwave,
the postshock
temperature
may be
Y Chenetal.
566
N 1.3 x 108cms-’ (where the T, N 2 keV. The blastwave velocity would be v, = (16kT,/3pmH)‘/’ mean atomic weight p = O.Sl), and the age could be estimated t = 2r/5v, N 1.4 x 103yr. On the other hands, the ionization parameter n,t w 1.3 x 10’2cm-3 s of the cool component implies an age t ;L 1.1 x 103(f/0.25)‘/2yr, and n,t N 6.3 x 1010cm-3 s of the hot component implies an age t 2 1.8 x 103[(1 f)/0.75]1/2yr. Th ese values of age are greater than the previous estimation of 600 yr. For t - 1.4 x 103yr, the estimation of the explosion energy is E = (1.4n H,ICMmH/[)(r5/t2) - 3 X 1050[(1 - f)/0.7rj]-‘/‘ergs where < = 2.026 (under assumption of the Sedov model).
In Figures. 2 & 3, the ASCA and ROSAT X-ray emissions again have two concenMorphology trations. Two bright arcs in the hard contour map are roughly coincident with the two concentrations (Figure 3). Therefore the hot component is mainly concentrated in the two bright portions. The two arcs seem to be two parts of a bubble in the western half. The eastern half of the HRI image (Figure 3) seems to be a broken bubble-like structure, with the eastern concentration of emission on its western boundary. The hard contour map, the HRI image, and the VLA map all show an outward protrusion on the east side. The elongation direction of the whole remnant is essentially perpendicular to the galactic plane. The high EM of the cool component suggests that the remnant is evolving in a cloud. The brighter radio and X-ray emissions on the west side close to the galactic plane imply a density gradient of the cloud toward the plane. Also the NH values for the portion A and B obtained from the VMEKAL code are in favor of such a gradient. We envisage a possible scenario that the remnant takes a shape like a bipolar bubble (eastern bubble + western bubble) with its axis tilted with regard to the line of sight. The two bright peaks result from the bipolar bubble interacting with the western denser medium. On the other side, the plasma leaks out of the the broken eastern bubble, so that the X-ray emission from the eastern half is softer than from the western half. This work is supported by grants from the NSF of China, Commission of China, and the Education Ministry of China.
4
the Ascent
Project
of the State
Scientific
REFERENCES
Becker, R. H., Markert, Caswell, J. L., Haynes,
T., & Donahue,
Caswell, J. L., & Lerche, I., MNRAS, Caswell, J. L., Roger, R. S., Murray, Kaastra,
M., ApJ, 296, 461 (1985)
R. F., Milne, D. K., & Wellington,
K. J., MNRAS,
200, 1143 (1982)
187, 201 (1979) J. D., Cole, D. J., & Cooke, D. J., A&A, 45, 239 (1975)
J. S., Mewe, R., Nieuwenhuijzen, H., in The 11th ~011. on UV and X-ray Spectr. Plasmas, Watanabe, T.(ed.), p.411 (1996)
of Astroph.
and Laboratory
Lucke, P. B., A&A, 64, 367 (1978) Masai, K., ApSS, 98, 367 (1984) Mewe, R., Kaastra,
J. S., & Liedahl,
Milne, D. K., Astralian Spitzer,
D. A., Legacy-The
JournaE of the HEASARC,
No.6, 16 (1995)
J. Phys., 32, 83 (1979)
L. Jr., Physical Processes
in the Interstellar
Medium
(Wiley, New York) (1978)