射线成像硬X射线望远镜在美国和欧洲的发展_英文_

2003 云 南 天 文 台 台刊 2003年

No11 Publications of Yunnan Observatory 第1期

成像硬X射线望

摘要:硬X射线能带(20 KeV1 MeV)是我们了解天体物理中高能辐射过程

的一个好窗口 。成像硬X射线望远镜可以提供更好的分辨率和灵敏度来研究

硬X射线天体物理学。本文介绍了成像硬X射线望远镜在美国和欧洲的发展,

这包括: 1) EXIT E2 ,由美国哈佛———史密松林天体物理中心设计和运行的一个

光电开关成像硬X射线望远镜; 2) EXI S T,一个成像硬X射线全天巡天望远镜,

将于2010年由ULDB (EXI S T- L I NE)或国际空间站(EXI S T- I S S)承载; 3)

HERO,由NASA/MSFC建造的一个新的硬X射线光学仪器。

关键词:仪器;望远镜;X射线

中图分类号: P111 文献标识码:A文章编号: 1001 - 7526 (2003) 01 - 0106-07

Imaging Hard X-ray Telescope Developments in

United States and Europe

ZHOU Yi

(Department of Physics,National Tsing Hua University,No 101 , Kuang Fu Rd Sec 2, Hsinchu 30043, Taiwan

E-mail : yichou@phys nthu edu tw)

Abstract :The hard X- ray(20 keV up to 1 MeV) energy band is a good window for us tolearn the high energy radiation processes in astrophysics.The imaging hard X- ray telescopesprovide better resolution and sensitivity for studying hard X- ray astrophysics. In this paper , therecent years developments of imaging hard X- ray telescopes in United States and Europe areintroduced , including 1)EXITE2- a phoswich imaging hard X- ray telescope designed andoperated by Harvard- Smithsonian Center for Astrophysics (CFA) ; 2) EXIST - an imaging hardX- ray all - sky survey ,which will be carried by ULDB(EXIST - LITE) or mounted oninternational space station (EXIST- ISS) in the year of 2010 and 3) HERO - a newly developedhard X- ray optics instrument , built by NASA/MSFC.

Key wor ds : instrumentation; telescope;X- ray

Ξ收稿日期: 2002- 10- 08

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1 INTRODUCTION

e-annihilation. These processes provide important information for studying the nature of the cosmicX- ray sources.

In the past few decades ,many interesting astrophysics phenomena in the hard X- ray bandhave been observed by non- imaging X- ray telescopes. However, although the collimators of non- imaging telescopes are able to restrict the field of view less than one degree , source confusionproblems are still unavoidable, (e. g.3C273 vs. GRS1227+025, see Jourdain 1992 and Chou etal . 2002) [ 12] , especial for the sources in crowded regions. Besides , simultaneously observingmultiple sources is impossible with non- imaging telescopes.

Soft X- ray (< 10 keV) imaging telescope using grazing incidence optics have achievedremarkable success (e. g.Chandra, angular resolution < 1 arcmin) . Focusing techniques are hardto applied to hard X- ray telescopes due to small effective area until early this century.Traditionally, the coded -mask technique (Ables 1968 ; Dicke 1968 and Fenimore et al . ,

1978) [ 3 5]has been widely applied to the hard X- ray/γ- ray imaging telescopes , either balloon-borne ( e. g.Harvard EXITE1 ,Grindlay et al . 1986 ; Caltech GRIP, see Finger 1987 andSchindler etal . 1997)6 8 or space-borne(e. g. the Fr[e9n]ch SIGMA telescope (30 keV- 1. 3 MeV)on Russian GRANATsatellite, see Paul et al . 1991) .

In this paper, I introduces the imaging hard X-ray telescopes in United States and Europe inrecentyears , including 1)EXITE2-a phoswich imaging hard X- ray telescope designed andoperated by Harvard- Smithsonian Center for Astrophysics (CFA) (Section 2. 1 ) ; 2) EXIST-an imaging hard X-ray all- sky survey,which will be carried by ULDB(EXIST- LITE) ormounted on international space station(EXIST- ISS)in the year of 2010 (Section 2.2) ; 3)HERO - a newly developed hard X- ray optics instrument , built by NASA/MSFC(Section 2.

3) .

2 EXITE2,EXIST AND HERO

2. 1 EXITE2

The second generation Energetic X- ray Imaging Telescope (EXITE2,Grindlay et al . 1990 ,Covault 1991 ,Lum et al . 1994,Manandhar 1995, Lum et al . 1997 , Chou et al . 1998 andChou 2001) [ 1016] is an imaging telescope built to observe astrophysical X- ray sources in the hard

108 云 南 天 文 台 台刊 2003年

X-ray band(20-600 keV)with high spatial and spectral resolution.The telescope observescosmic X- ray on a high altitude (40 km) s cientific balloon. Thepayload is shown schematicallyas Figure 1. The detector uses phoswich technique to reduce the background. EXITE2 is the first

Fig. 1 EXITE2 Payloadphoswich imaging telescope although this technique has been applied to many non - imagingtelescope ( e. g.RXTE/HEXTE) .The main detector consists of a 36cm ×36cm NaI (T1 )scintillator surrounded on four side and rear by 2cm thick CsI crystals. Using the differentscintillation light decay time constants for NaI (T1)and CsI (Na) , 230ns and 680ns respectively,we are able to filter most of the background with pulse - shape discrinimator (PSD) .The NaI/CsIcrystals absorb the X- ray and generate scintillation light proportional to the X- ray photon energycollected by a 7 ×7 photo -multiplier tube (PMT) array. The light signals are subsequentlyamplified and converted to electronic pulses that then are further amplified by the following pre -amplifiers before digitization.Laboratory tests showthat the spatial and spectral resolutions of thedetector are 10.4 mm(FWHM) and 14%(FWHM) at 60 keV.

The uniformredundant array(URA)coded mask technique is spplied to image the cosmic X-ray sources.Themask,which consists of an extended 13×12 pixel basic mask pattern with 72open and 71 close, is 2. 5 m away fromthe main detector.The co°llimator located right in front ofthe main detector is used to limit the detector view field of 4.64(FWHM). The phoswich crystalassembly is surrounded on five sides by active and passive shields to reduce X- ray background andthe X- ray induced by the cosmic ray interacting with the payload.

The EXITE2 has flown four times in 1993 , 1997 , 2000 and 2001. During the scientificflights from 1997 to 2001 , several astrophysical sources were significantly detected, including Crabnebula, black hole candidate Cyg X- 1 , an unidentified source GRS 1227+025 and microquasar

第1期 周翊:成像硬X射线望远镜在美国和欧洲的发展 109

GRS 1915+105 shown as Figure 2. The details about the instrument developments ,data analysismethods and observation results are presented in (Chou, 2001) [ 16]and (Bloser et al . 2001) [ 17] .

2.2 EXIST

The hard X- ray band is probably the final astronomical spectrum still without a sensitiveimaging all - sky survey. The last all - sky survey in hard X- ray band was performed by A- 4instrument on HEAO 1 (non- imaging) over a 16 -month period about 20 years ago. It took 6months to scan the entire sky. Therefore , a highly sensitive imaging all - sky survey is required inastrophysical researches in this new century.

Fig. 2 EXITE2 images observed during its 1997 and 2001 flights : upper left :Crab Nebula ;upper right : black hole candidate Cyg X- 1 ; lower left : GRS 1227+025, the crossat center indicates the expected position of QSO 3C273 ; lower right :microquasar GRS

1915+105

The Energetic X-ray Imaging SurveyTelescope(EXIST) is currently in the concept studystages.Like EXITE2, the coded-mask is applied to image the astrophysical sources but it uses anewgeneration hard X-ray detector,pixilated cadmium- zinc - telluride(CZT) .There are twoconcepts ofEXIST.EXIST-LITE, EXIST for Long Integration Time Experiment,will be carriedby Ultra -Long Duration Balloon(ULDB) .During its 100 days flying above 40 km,EXIST-LITE could achieve a sensitivity of 1m Crab for 20- 300 keV over half of the sky. The second

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Fig.3 EXIST- ISS: Energetic X- ray Imaging Survey Telescope on

International Space Station.concept is EXIST- ISS ,EX°IST° on International Space Statio°n(s°ee Figure 3) .There will be eighttelescopes , with each 40×40field of view and totally 80×160, mounted on the InternationalSpace Station. The entire sky is imaged every 90 minutes with angular resolution of 5 arcmin. Theinstrument may have sensitivities of 0. 05 m Crab for 10- 100keV and 0. 5 m Crab for 100- 500keV during its two year mission.More details about the EXIST are present in (Bloser, 2000) .

2. 3 HERO

The X-ray optics , using grazing incident optics ,was considered to be improper for the hardX- ray/γ -ray imaging telescope due to too small grazing angle which make too small effectivearea. However, owing to the improvements of the technology, High Energy Replicated Optics

第1期 周翊:成像硬X射线望远镜在美国和欧洲的发展 111

(HERO,up to75 keV, see Ramsey et al . 2000,Austin, et al . 1999) [ 1819] , a ballon-borne hard X-ray imaging teles cope developed by Marshall Space Flight Center,NASA,us es

inside each module.The first -ever focused hard X- ray astrophysical images , including Crabnebula ,Cyg X- 1 and GRS 1915+015 were obtained during its 24 - hour flight. HERO isscheduled for flight in late 2002 with 6 m focal length optics currently fabricated.

3 SUMMARY

The discovery of cosmic X- rays in the early 1960s has opened a new era in astronomy. Tofurther improve the sensitivity and resolution, astronomers have sucessfully designed, tested andflown the imaging hard X- ray telescopes.Owing to the developments of the techniques , the nextgeneration of imaging hard X- ray telescopes are able to solve more cosmic mysteries and give usclearer picture of our universe in new millennium.

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