The ground calibration of the back- side illuminated CCD camera of XIS onboard Astro-E2 (Suzaku) H. Yamaguchi, H. Nakajima, H. Matsumoto, T. G. Tsuru, K. Koyama (Kyoto Univ., Japan), D. Matsuura, T. Miyauchi, S.Katsuda, M. Namiki, K. Torii, K. Hayashida, H. Tsunemi (Osaka Univ., Japan), and XIS team

Contents  Introduction Characteristic of XIS-BI  Event Detection Grade method  Charge Trailing Charge trailing against the transfer. We developed “ Charge Trail Correction ”.  Onboard Calibration Comparison with Chandra/ACIS

1. Introduction Suzaku (Astro-E2) XIS sensor XIS : Front-illuminated (FI) CCD×3 + Back-illuminated (BI) CCD×1 BI-CCD ‥ High quantum efficiency for soft X-ray Quantum efficiency of XIS XIS-FI XIS-BI

X-ray electrode 1. Introduction Chemisorption charging process strengthen the electric field Collection efficiency of electrons were improved !! structure of XIS-BI back surface spread Charge cloud spreads widely ↓ Energy resolution become worse ex. Chandra/ACIS ΔE(BI) ≒ 2×ΔE(FI)

1. Introduction Energy resolution of the XIS-BI is almost comparable with FI! ΔE= 49eV (BI) 42eV ΔE= 129eV (BI) 128eV counts/keV Spectrum of O-K line XIS-BI XIS-FI

ComponentsLocationX-ray SourceQE reference Chip levelCSR/MIT Fluorescent X-rays (C,O,F,Al,Si,P,Ti,Mn,Cu) ACIS chips calibrated at BESSY Camera without OBF +Flight Model AE Osaka Univ. Grating Spectrometer keV Polypro-window Gas PC & XIS-EU Kyoto Univ. Fluorescent X-rays (Al,Cl,Ti,Mn,Fe,Zn,Se) Window-less SSD OBF Synchrotron Facility Synchrotron X-rays + monochrometer (Transmission measurement with PIN diode) Camera onboard the satellite ISAS/JAXA55Fe 1. Introduction Ground Calibration Task Share

2. Event Detection Grade02346 are used as X-ray event The center pixel A pixel whose PH is larger than split threshold and added to the PHA (= summed PH) A pixel whose PH is larger than split threshold but NOT added to the PHA Grade0 Grade1 Grade2 Grade3 Grade4 Grade5 Grade6 Grade7 split over 2x2 region We analyzed BI data similarly to FI. ↓ Several problems were found.

3. Charge Trailing Vertical Imaging Area of XIS Uniform illumination of fluorescent X-ray Read out node Distribution of Grade0,2,3,4,6 events V (Vertical) Counts Distribution of Grade7 events Counts V (Vertical) not uniform! Ground Calibration transfer Several X-ray events escape to Grade7?

3. Charge Trailing transfer Trailing charge Some charges are deposited during the transfer. Grade0Grade2 Vertical Contribute to the increasing Grade0Grade2 PH

3. Charge Trailing transfer Trailing charge Grade6Grade7 Distribution of Grade7 events Counts V (Vertical) PH

3. Charge Trailing Q ’ (ADU) extracted only Mn-K event CTR = 4.5×10 (Mn-K) “ Charge Trail Ratio ” (CTR) ≡ the probability of charge trailing par 1 pixel transfer N (Number of transfer) V Trailing Charge ≡ Q ’ [ADU] Q ’ = C×N Q ’ = C×N ; C = 6.8×10 -3 Mean PHA ≡ Q[ADU] spectrum of Mn-K CTR [1/transfer] = C/Q

3. Charge Trailing CTR Q (PHA) CTR depends on the PHA of event Relation of the CTR and the PHA is able to be expressed by the power-law function CTR = (1.72×10 -4 )×(PHA [ADU] ) -0.5 We have developed “ Charge Trail Correction ”. BeforeAfter VV

3. Charge Trailing After the Charge Trail Correction … Distribution of Grade0,2,3,4,6 events not uniform! Counts V (Vertical) Distribution of Grade0,2,3,4,6 events becomes uniformly. Grade7 events due to charge trail are successfully reduced. The detection efficiency improve about 10-20%. → The detection efficiency improve about 10-20%.

4. Onboard Calibration XIS FI XIS BI XIS keeps their performance even after the launch!! ACIS BI Spectra of E

Summary  Suzaku/XIS is composed of 3 FI-CCD and 1 BI-CCD.  Good energy resolution of BI was achieved by chemisorption charging process.  We developed new analysis method, “ Charge Trail Correction ”. → Detection efficiency improved.  More detailed onboard calibration is proceeding now.

4. Spilt Threshold Optimization Split threshold = 20ADU (for XIS-FI) XIS-FI (SpTh.= 20ADU) XIS-BI (SpTh.= 20ADU) 20ADU is not optimum value of the split threshold for BI?

O-K line Zn-K line optimum split threshold Split Threshold (ADU) ΔE (eV) Efficiency O-K (0.5keV) Zn-K (8.6keV) 4. Spilt Threshold Optimization 10ADU for 0.5keV 13ADU for 8.6keV

4. Spilt Threshold Optimization The function for setting split threshold optimum split threshold [ADU] = log 10 (E [keV] ) Optimize the split threshold for each energy events we make Grade classification using variable split threshold XIS-FI (SpTh.= 20ADU) XIS-BI (SpTh.= 20ADU) XIS-BI (variable SpTh.) Energy resolution of the XIS-BI is almost comparable with FI ΔE= 49eV (BI) 42eV ΔE= 129eV (BI) 128eV