X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

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26 CHAPTER 4. INSTRUMENTATIONThe ACIS array (Figure 4.6) consists of ten CCDs (Charge Coupled Devices) with 1024 ×1024 pixels each. ACIS-I (chip I0–I3) is laid out in 2 × 2 and is used for imaging observations,while ACIS-S (chip S0–S6) is in 1 × 6, which is optimized for both imaging and grating-spectroscopicobservations. The pixel size is 24 µm × 24 µm, corresponding to the angular size of 0. ′′ 5 × 0. ′′ 5.Two CCDs in the ACIS-S array (chip S1 and S3) are back-illuminated (BI) and the others arefront-illuminated (FI). For each observation, we can use any six CCDs simultaneously.Energy resolutionFigure 4.7 (left) shows the energy resolution (∆E) of ACIS estimated by ground calibrations. TheFI chips have a good resolution near the theoretical limit (∼120 eV @ 5.9 keV), while that of theBI chips is slightly poor. After the launch, however, the ACIS-I chips were significantly damagedby low energy protons encountered during radiation belt passages. These protons made manycharge traps at buried channels, which cause the increase of charge transfer inefficiency (CTI).Consequently, ∆E becomes worse as a function of source positions from the readout nodes; ∆E ∼130 eV (@ 5.9 keV) near the readout nodes, while ∆E ∼ 330 eV (@ 5.9 keV) at the edge of thechip. Since the gate structure of the BI chips (S1 and S3) is at the opposite side to HRMA, ∆Eof the BI chips remains at their pre-launch value and has little dependence on the source position.Now ACIS is not operated and not left at the focal position during the radiation belt passages,hence no further degradation of CTI is expected. Furthermore, in order to reduce the effect of CTI,the CCD temperature is lowered from −90 ◦ C to −120 ◦ C.Fig. 4.7.— (left) The pre-launch energy resolution of ACIS. (right) The energy resolution of chipS3 and I3 as a function of row number, after the proton damage on orbit. The data were taken atthe CCD temperature of −120 ◦ C.
4.3. ACIS – ADVANCED CCD IMAGING SPECTROMETER 27Quantum efficiency and effective areaFigure 4.8 shows the quantum efficiency of ACIS (left) and the combined effective area of HRMAand ACIS (right). A large fraction of low energy X-rays is absorbed at the optical blocking filter(OBF) and the gate structure for FI chips, hence the efficiency becomes worse at the lower energy.Since the gate of BI chips is at the opposite side to HRMA, the efficiency of BI chips at the lowenergy band is about two times better than that of FI chips. Due to the effect of CTI, the quantumefficiency at the further side from the readout node is about 5–15 % worse than that near thenode. The energy ranges with the effective area of larger than 10 cm 2 are ∼0.5–10.0 keV and∼0.1–10.0 keV for FI and BI chips, respectively.Fig. 4.8.— (left) The quantum efficiency of ACIS with the transmission of OBF. (right) Thecombined effective area of HRMA and ACIS. The sudden drop-off seen around 1.8 keV is due tothe K-edge of silicon.Event gradeX-ray events are selected with the criterion that the bias-subtracted pixel value exceeds the eventthreshold (38 ADU ∼ = 140 eV and 20 ADU ∼ = 70 eV for FI and BI chips) and is the highestamong the neighboring 9×9 pixels. In order to distinguish real events from background ones and toestimate the total number of electrons including those leak into the neighboring pixels, the grademethod is used. Its algorithm is basically the same as the ASCA/SIS grade method (Burke et al.,1991), but ACIS gives the specified numbers for each pixel around the event pixel (Figure 4.9) anddetermines the grade by summing up those pixel numbers that have a pixel value larger than the
Page 3: Contents1 Introduction 12 Review of
Page 9 and 10: List of Figures2.1 The H-R diagram
Page 11 and 12: LIST OF FIGURESix6.17 Light curves
Page 13 and 14: List of Tables3.1 Multiwavelength s
Page 15 and 16: Chapter 1IntroductionStar formation
Page 17 and 18: Chapter 2Review of Low-mass Young S
Page 19 and 20: 2.1. EVOLUTION OF LOW-MASS STARS 5w
Page 21 and 22: 2.2. MOLECULAR CLOUDS 72.2 Molecula
Page 23 and 24: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 25 and 26: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 27: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 30 and 31: 16 CHAPTER 3. REVIEW OF THE ρ OPHI
Page 36 and 37: 22 CHAPTER 4. INSTRUMENTATIONrespec
Page 38 and 39: 24 CHAPTER 4. INSTRUMENTATIONangle
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Page 44 and 45: 30 CHAPTER 4. INSTRUMENTATIONTable
Page 46 and 47: 32 CHAPTER 5. CHANDRA OBSERVATIONS
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78 CHAPTER 6. INDIVIDUAL SOURCES6.4
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92 CHAPTER 6. INDIVIDUAL SOURCESFig
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94 CHAPTER 6. INDIVIDUAL SOURCESPra
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96 CHAPTER 7. OVERALL FEATURE OF X-
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98 CHAPTER 7. OVERALL FEATURE OF X-
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100 CHAPTER 7. OVERALL FEATURE OF X
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Chapter 8Systematic Study of YSO Fl
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8.2. CORRELATION BETWEEN THE FLARE
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8.3. MAGNETIC RECONNECTION MODEL 11
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8.5. EFFECT OF THE QUIESCENT X-RAYS
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8.6. EVOLUTION OF YSOS AND THEIR FL
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Chapter 9ConclusionWe summarize the
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Appendix AFlare Light CurvesFig. A.
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Fig.A.2 (Continued)121
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Fig. A.4.— Same as Figure A.1, bu
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126 APPENDIX B. PHYSICAL PARAMETERS
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128 APPENDIX B. PHYSICAL PARAMETERS
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Appendix CModeling of the FlareIn t
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C.2. PREDICTED CORRELATIONS BETWEEN
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BibliographyAgeorges, N., Eckart, A
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BIBLIOGRAPHY 137Feigelson, E. D., &
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BIBLIOGRAPHY 139Johnstone, D., Wils
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BIBLIOGRAPHY 141Rutledge, R. E., Ba
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BIBLIOGRAPHY 143Yokoyama, T. & Shib
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X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

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