A number of emission lines are clearly seen in the spectra of phase 0.50 and eclipse. The observed spectra in the entire energy range are shown in Figure 4.5 and Figure 4.6. It is apparent that the data obtained with an extremely high energy resolution leads to the
Figure 4.4: The spectrum of Vela X-1 obtained with HEG. The green, the blue and the red show the spectra of the phase 0.25, the phase 0.50 and the eclipse, respectively. The black lines are spectral fits results listed in Table 4.2.
detection and the identification of K-shell Si fluorescent lines from a wide range of charge state, for the first time from Vela X-1. The emission lines from highly ionized S, Si, Mg and Ne can be seen, in addition to fluorescent lines from Fe, Ca, S, and Si ions in lower charge states. Additionally, in the both spectra of phase 0.50 and eclipse, emission lines from the same ions are detected.
Blown-up spectra of the Si K lines region are shown in Figure 4.7. Intense Lyα line from H-like ions and fully-resolved He-like triplet lines are clearly seen in phase 0.50 and eclipse. At the lower energy end below 1.74 keV, fluorescent lines from near-neutral Si is also detected in both phases. Additionally, between the He-like lines and nearly neutral line, Si VII–Si XI Kα lines can be resolved. The forest of Si K lines is a clear evidence that plasma in various ionization states exist in the Vela X-1 system.
The centroid of energies, the widths and the intensities of each line are determined by fitting the data with a single gaussian model. In the spectral fittings, we use the Poisson likelihood statistics, in stead of the χ2 statisitics, because numbers of photons in some of the bins in the spectra are very small. As an example of the fittings, the Lyα line profiles from H-like ions of Si are shown in Figure 4.8, together with the best-fit models.
The derived parameters for phase 0.50 and eclipse are listed in Table 4.3 and Table 4.4, respectively. The line intensity ratios of phase 0.50 to eclipse are listed on Table 4.5 for lines from H- and He-like ions which were detected with statistical significance of more than 5 σ. These ratios are 8–10 for the H-like lines and are 4–7 for the He-like lines.
One of the striking results seen from Figure 4.8 is the Doppler shift of lines. Thanks to the resolving power of the HEG, Doppler shifts can be measured with an accuracy of
∼ 100 km s−1. Figure 4.9 compares the line profiles of Si Lyα and Mg Lyα between the phase 0.50 and the eclipse data. The differences in the line center energies are clearly seen in each line.
In Figure 4.10, the velocity shifts are plotted for both of the phases for all emission lines from H- and He-like ions. Though some fluctuations are seen, there is a trend that blue shifts are detected in phase 0.50 and red shifts are observed in eclipse. The shifts between phase 0.50 and eclipse (∆v) range in∼300–600 km s−1(Table 4.5). Additionally, the emission lines from highly ionized ions have widths of σ .300 km s−1.
The radiative recombination continuum (RRC) is detected clearly from H-like Ne.
The blow-up spectra of phase 0.50 and that of eclipse are shown in Figure 4.11. We fitted the RRC spectra using the “redge” model in XSPEC. The electron temperatures are derived to be kTe = 7.4+1.6−1.3 eV and kTe = 6.6+2.5−1.8 eV during phase 0.50 and eclipse, respectively.
Iron Kαfluorescent lines are detected in all three orbital phases. The profiles of these lines are shown in Figure 4.12. The parameters derived from the spectral fittings with a single gaussian model are listed in Table 4.4. The equivalent width of the iron Kα line is measured to be 116 eV and 51 eV for phase 0.50 and for phase 0.25, respectively. At the eclipse phase, a high equivalent width of 844 eV is observed. As shown in Figure 4.12,
there is a sign of a Compton shoulder in the iron Kα spectrum of phase 0.50.
Figure 4.5: The spectrum of Vela X-1 in the orbital phase of 0.50. The red shows MEG data and the blue shows HEG data. The green lines mark the energies of the emission lines listed in Table 4.3.
Figure 4.6: The spectrum of Vela X-1 in the eclipse phase. The red shows MEG data and the blue shows HEG data. The green lines mark the emission lines listed in Table 4.4.
Si XIV Ly α Si XIII
f r i Si II-VI
Si XI Si X Si IX Si VIII Si VII
phase 0.25
phase 0.50
eclipse
due to the detector response (Si-Kedge)
Figure 4.7: The spectrum of the Si K lines regions in each orbital phase.
Figure 4.8: The Lyα lines from H-like Si in phase 0.50 (left) and in eclipse (right). The bold lines show the best-fit models. Fitting model is the single gaussian.
Table 4.3. Derived Parameters of emission lines in the 0.5 orbital phase spectrum Center Energy Sigma Intensitya Candidate Line Shift
(keV) (eV) (photon cm−2 s−1) (energy keV) (km s−1)
3.69053 0.35 8.82e−5 Ca Kα
(3.68963–3.69273) (0.00–3.84) (5.73–11.87)
2.62213 4.04 1.77e−4 S XVI Lyα
(2.62094–2.62327) (2.81–5.53) (1.44–2.09)
2.46197 0.64 6.84e−5 S XV r
(2.46110–2.46273) (0.00–1.71) (5.18–8.66)
2.31112 0.20 1.03e−4 S Kα
(2.31070–2.31184) (0.00–2.12) (8.32–12.5)
2.00634 1.16 2.38e−4 Si XIV Lyα +127
(2.00618–2.00650) (0.89–1.42) (2.23–2.52) (2.00549) (+103–+151)
1.86614 1.38 1.36e−4 Si XIII r +183
(1.86592–1.86636) (1.09–1.67) (1.25–1.48) (1.86500) (+148–+219)
1.85537 0.82 3.05e−5 Si XIII i +257
(1.85483–1.85592) (0.00–1.77) (2.39–3.77) (1.85378) (+170–+346)
1.84136 2.15 1.41e−4 Si XIII f +311
(1.84109–1.84162) (1.86–2.47) (1.29–1.53) (1.83945) (+267–+353)
1.74447 2.35 1.46e−4 Si Kα
(1.74421–1.74473) (2.07–2.67) (1.36–1.58)
1.72998 0.44 3.36e−5 Al XIII Lyα?
(1.72962–1.73033) (0.00–1.10) (2.78–3.99)
1.59900 0.66 1.31e−5 Al XII r ?
(1.59832–1.59967) (0.00–1.68) (0.86–1.87)
1.57976 2.01 3.90e−5 Mg XI
(1.57920–1.58032) (1.38–2.64) (3.18–4.64)
1.55231 0.58 1.78e−5 Fe XXIV ?
(1.55175–1.55284) (0.00–1.42) (1.27–2.35)
1.47282 1.18 2.35e−4 Mg XII Lyα +102
(1.47268–1.47296) (1.02–1.35) (2.17–2.53) (1.47232) (+73–+130)
1.35279 1.00 1.47e−4 Mg XI r +120
(1.35263–1.35296) (0.81–1.20) (1.32–1.62) (1.35225) (+84–+157)
1.34346 0.69 7.34e−5 Mg XI i +80
(1.34324–1.34367) (0.36–1.02) (6.25–8.53) (1.34310) (+31–+127)
Table 4.3—Continued
Center Energy Sigma Intensitya Candidate Line Shift (keV) (eV) (photon cm−2 s−1) (energy keV) (km s−1)
1.33213 0.95 9.13e−5 Mg XI f +230
(1.33190–1.33236) (0.65–1.25) (7.89–10.5) (1.33111) (+178–281)
1.30808 1.06 4.99e−5 Fe XXI ?
(1.30766–1.30843) (0.61–1.49) (3.96–6.12)
1.27783 1.10 8.44e−5 Ne X Lyγ
(1.27756–1.27811) (0.78–1.38) (7.18–9.84)
1.21160 0.94 1.33e−4 Ne X Lyβ
(1.21139–1.21181) (0.72–1.17) (1.15–1.52)
1.12732 1.54 7.41e−5 Ne IX
(1.12683–1.12780) (1.13–2.03) (5.71–9.27)
1.07432 0.12 5.76e−5 Ne IX
(1.07397–1.07450) (0.00–0.73) (4.07–7.74)
1.02242 0.64 4.42e−4 Ne X Lyα +182
(1.02230–1.02253) (0.52–0.76) (3.91–4.97) (1.02180) (+147–+214)
0.922458 0.80 2.89e−4 Ne IX r +149
(0.922186–0.922725) (0.52–1.11) (2.24–3.64) (0.922001) (+60–+235)
0.916254 1.44 4.91e−4 Ne IX i +475
(0.915937–0.916584) (1.21–1.73) (4.03–5.89) (0.914803) (+371–+583)
0.905561 1.78 3.71e−4 Ne IX f +165
(0.905013–0.906071) (1.38–2.27) (2.88–4.65) (0.905062) (−16–+334)
aInter stellar gas absorption is corrected. The hydrogen column density of 6 × 1021 cm−2 is assumed, corresponding to the density of 1 H cm−3 and the distance of 1.9 kpc.
Note. — Errors correspond to 90 % confidence level.
Table 4.4. Derived Parameters of emission lines in the 0.0 orbital phase spectrum Center Energy Sigma Intensitya Candidate Line Shift
(keV) (eV) (photon cm−2 s−1) (energy keV) (km s−1)
3.69431 8.59 9.27e−6 Ca Kα
(3.69006–3.69836) (5.35–12.9) (6.57–12.3)
2.95657 0.00 4.92e−6 Ar Kα
(2.95507–2.95849) (0.00–3.35) (2.97–7.27)
2.61857 0.57 1.38e−5 S XVI Lyα
(2.61781–2.61947) (0.04–2.62) (1.08–1.73)
2.31035 2.19 1.76e−5 S Kα
(2.30935–2.31138) (0.87–3.50) (1.35–2.23)
2.00339 1.70 2.32e−5 Si XIV Lyα −314
(2.00305–2.00373) (1.28–2.14) (2.07–2.59) (2.00549) (−365–−263)
1.86299 1.58 2.34e−5 Si XIII r −323
(1.86267–1.86330) (1.25–1.94) (2.08–2.62) (1.86500) (−375–−273)
1.85271 0.00 2.76e−6 Si XIII i −173
(1.85254–1.85357) (0.00–1.12) (1.67–4.05) (1.85378) (−201–−34)
1.83924 2.92 2.11e−5 Si XIII f −34
(1.83878–1.83969) (2.48–3.43) (1.88–2.36) (1.83945) (−109–+39)
1.74247 1.67 1.96e−5 Si Kα
(1.74217–1.74276) (1.39–1.99) (1.74–2.20)
1.65752 0.12 2.84e−6 Mg XI or
(1.65697–1.65855) (0.00–1.53) (1.86–4.00) Fe XXIII
1.57719 1.34 6.02e−6 Mg XI
(1.57659–1.57776) (0.68–2.05) (4.59–7.64)
1.47068 1.23 2.63e−5 Mg XII Lyα −334
(1.47044–1.47093) (0.99–1.50) (2.29–2.99) (1.47232) (−383–−283)
1.35057 1.54 2.58e−5 Mg XI r −373
(1.35026–1.35088) (1.21–1.90) (2.23–2.97) (1.35225) (−442–−304)
1.34238 1.18 6.33e−6 Mg XI i −161
(1.34170–1.34307) (0.39–2.04) (4.33–8.65) (1.34310) (−313–−7)
1.33122 2.27 2.03e−5 Mg XI f +25
(1.33072–1.33171) (1.87–2.77) (1.70–2.40) (1.33111) (−89–+135)
1.30640 1.26 6.61e−6 Fe XXI ?
(1.30574–1.30701) (0.71–1.97) (4.56–9.05)
Table 4.4—Continued
Center Energy Sigma Intensitya Candidate Line Shift (keV) (eV) (photon cm−2 s−1) (energy keV) (km s−1)
1.27592 0.89 8.70e−6 Ne X Lyγ
(1.27547–1.27636) (0.00–1.44) (5.62–11.4)
1.23618 1.57 5.94e−6 Fe XX ?
(1.23507–1.23727) (0.71–2.79) (3.13–8.95)
1.20980 1.34 1.43e−5 Ne X Lyβ
(1.20930–1.21033) (0.96–1.82) (1.07–1.84)
1.07241 0.78 1.91e−5 Ne IX
(1.07205–1.07276) (0.44–1.19) (1.38–2.54)
1.02075 0.99 8.54e−5 Ne X Lyα −308
(1.02056–1.02095) (0.81–1.18) (7.25–9.98) (1.02180) (−364–−249)
0.920773 0.92 5.48e−5 Ne IX r −400
(0.920390–0.921159) (0.60–1.30) (3.86–7.43) (0.922001) (−524–−274)
0.914926 1.05 4.68e−5 Ne IX i +40
(0.914407–0.915448) (0.75–1.53) (3.03–6.70) (0.914803) (−130–+211)
0.904058 0.75 8.98e−5 Ne IX f −333
(0.903784–0.904342) (0.48–1.10) (6.75–11.6) (0.905062) (−424–−239)
aInter stellar gas absorption is corrected. The hydrogen column density of 6 × 1021 cm−2 is assumed, corresponding to the density of 1 H cm−3 and the distance of 1.9 kpc.
Note. — Errors correspond to 90 % confidence level.