Solar X-ray Spectrometer (Soxs) Mission on Board GSAT2 Indian Spacecraft: The Low-Energy Payload

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The first space-borne solar astronomy experiment of India, namely “Solar X-ray Spectrometer (SOXS)”, was successfully launched on 08 May 2003 on board geostationary satellite GSAT-2 of India. The SOXS is composed of two independent payloads, viz.
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  Solar Physics (2005) 227: 89–122  C   Springer 2005 SOLAR X-RAY SPECTROMETER (SOXS) MISSION ON BOARDGSAT2 INDIAN SPACECRAFT: THE LOW-ENERGY PAYLOAD RAJMAL JAIN, HEMANT DAVE, A. B. SHAH, N. M. VADHER, VISHAL M. SHAH,G. P. UBALE, K. S. B. MANIAN, CHIRAG M. SOLANKI, K. J. SHAH,SUMIT KUMAR, S. L. KAYASTH, V. D. PATEL, JAYSHREE J. TRIVEDIand M. R. DESHPANDE Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India(e-mail: rajmal@prl.ernet.in) (Received 11 August 2004; accepted 24 November 2004) Abstract.  The first space-borne solar astronomy experiment of India, namely “Solar X-ray Spec-trometer (SOXS)”, was successfully launched on 08 May 2003 on board geostationary satelliteGSAT-2 of India. The SOXS is composed of two independent payloads, viz. SOXS Low-EnergyDetector (SLD) Payload and SOXS High-Energy Detector (SHD) Payload. The SOXS aims to studythe full-disk integrated X-ray emission in the energy range from 4 keV to 10 MeV. In this pa-per we present the first report on the SLD instrumentation and its in-orbit performance. The SLDpayload was designed and developed at the Physical Research Laboratory in collaboration with var-ious centers of Indian Space Research Organisation (ISRO). The basic scientific aim of the SLDpayload is to study solar flares in the energy range from 4 to 60 keV with high spectral and tem-poral resolution. To meet these requirements, the SLD payload employs state-of-the-art solid statedetectors, the first time for a solar astronomy experiment, viz. Si PIN (4–25 keV), and cadmium–zinc–telluride (4–60 keV). With their superb high-energy resolution characteristics, SLD can ob-serve iron and iron–nickel complex lines that are visible only during solar flares. In view of its3.4 ◦ FOV, the detector package is mounted on a Sun Aspect System, for the first time, to getuninterrupted observations in a geostationary orbit. The SLD payload configuration, its in-flightoperation, and the response of the detectors are presented. We also present the first observationsof solar flares made by the SLD payload and briefly describe their temporal and spectral moderesults. 1. Introduction X-ray emission from a large variety of objects is mostly in continuum. Extract-ing the significant parameters from the X-ray continuum spectroscopic methodsrequires the detectors of sufficiently high spectral and temporal resolution. It isa technical challenge to meet these requirements. In the case of X-ray emis-sion from solar flares it is more complex because the flux at low energies (be-low 20 keV) is extremely high while at higher energies (above 20 keV) it isstarved of photons. On the other hand extraordinary high flux from the X-rayemission lines coming from the high-temperature corona ( < 5 keV) and dur-ing the flare ( > 5 keV) make the spectroscopy more and more complex. Thisrestricts the application of one single detector in general and when one looks  90  RAJMAL JAIN ET AL . for high spectral resolution in particular. The Solar X-ray Spectrometer (SOXS)experiment aims to do this by using a judicious combination of different types of X-ray detectors. The SOXS is the first space-borne solar astronomy experimentof India.The “Solar X-ray Spectrometer (SOXS)” mission (Jain  et al. , 2000a) waslaunched onboard an Indian geostationary satellite namely GSAT-2 on 8 May 2003by GSLV-D2 rocket. The SOXS aims to study the high-energy and temporal res-olution X-ray spectra from solar flares. The SOXS consists of two independentpayloads, viz. SOXS Low-Energy Detector (SLD) and SOXS High-Energy Detec-tor (SHD) payloads. The SLD is comprised of two semiconductor devices, viz.silicon PIN detector for 4–25 keV (area 11.56 mm 2 ); and cadmium–zinc–telluride(CZT) detector for 4–60 keV energy range (area 25 mm 2 ). These state-of-the-artsolidstatedetectorsinSLDhavesuperbsub-keVenergyresolutionand100mstem-poral resolution characteristics, which make them most appropriate for solar flareresearch in the context of energy transport and acceleration time scales of particles.On the other hand, the SHD payload, composed of a phoswich NaI(Tl)–CsI(Na)scintillation detector (area 125 cm 2 ) covering the energy range from 25 keV to10MeV,enablesthestudyofrelativisticelectronsandionsthroughhardX-raysandgamma rays. However, the energy resolution revealed by such detectors is mediumto poor.SeveraloftheproblemsdoggingthefieldofhardX-raycontinuumspectroscopylike background measurement, limited energy bandwidth, limited degrees of free-dom for spectral fitting due to poor energy resolution can be effectively tackled byincorporating the new generation near-room-temperature solid state devices likeSi PIN and CZT detectors. These detectors have very good efficiency and superiorenergyresolution(10%at6keVandalmost3%at60keV)comparedtoscintillationand proportional counters.The overlap in operating energy ranges among the three different detectors isdesignedtoprovidegoodcross-calibrationwhileutilizingthehighenergyresolutionof the Si-PIN detector, excellent hard X-ray capability of CZT detector, and thebroadcoverageofhardX-rayandgamma-rayregimesbythephoswichdetector.Theinstrumentation for the onboard processing of SOXS low energy and high-energydata as well as their flare trigger systems are independent. The SLD payload wasdesigned and developed at the Physical Research Laboratory in collaboration withISRO Satellite Centre (ISAC), Bangalore, and Space Application Centre (SAC),Ahmedabad, while the SHD payload was designed and developed by Tata Instituteof Fundamental Research (TIFR), Mumbai.In this paper we restrict our presentation to the SLD payload. Section 2describes scientific objectives and design considerations of the SLD payload.In Section 3, we present instrument details, while in Section 4 the data for-mat is briefly described. We report the in-flight performance of the SLD pay-load and first observations of solar flares in Section 5. We conclude the paperin Section 6.  SOXS  –  LOW - ENERGY PAYLOAD  91 2. Scientific Objectives and Design Considerations Shown in Figure 1 is a simulation of full-disk integrated photon spectrum from1 to 100 keV energy range considering the following plasma parameters for quiet(preflare) and various components of a M5 class flare. Preflare background,  T   = 4 MK, EM = 10 49 cm − 3 ; thermal,  T   = 13 MK, EM = 10 49 . 5 cm − 3 ; superhot, T   = 40 MK, EM = 10 47 cm − 3 , and nonthermal-spectral index (gamma) =− 3.5(range =− 2.5 to − 4.5), flux at 20 keV = 10 photons cm − 2 s − 1 keV − 1 . Figure 1.  Simulation of full-disk integrated X-ray photon emission spectrum from the Sun in theenergyrange1–100keVforquietandM5solarflareconditions.The dottedline ispreflarebackground, thin solid line  indicates the thermal component of the flare. The superhot hard X-ray component isshown by a medium dashed line while the  long dashed line  represents the nonthermal hard X-rayspectrum. The  solid thick line  is total flux from all these assumptions (see text). It may be noted thatiron and iron–nickel complex lines appear only when flare occurs, and there is a break in energy fromone to the other spectrum, which SLD aims to measure.  92  RAJMAL JAIN ET AL . Figure1showsunambiguouslythatironcomplexlines(Fe XXV , XXVI )at6.7keVand Fe Ni complex lines at 8 keV appear only during solar flare activity. On theother hand, it may also be noted that below 5 keV the corona is so hot that a largenumberofphotonsisemitted,mostlyintheformoflines,evenintheabsenceoftheflare. This restricts the design of the spectroscopy experiment from 1 to 20 keV inview of saturation of the detector, because of its limited count handling capability,to low energy photons only. However realizing that the synoptic observations atenergies below 10 keV may improve our current understanding of the temperatureenhancement during flares of different magnitude, requires optimization in designsuch that the detectors do not get saturated. The flare spectrum (cf., Figure 1) alsoreveals break energy points between 10 and 100 keV, the first between thermaland superhot components, and the second between superhot and nonthermal com-ponents. Precise measurements of these break energy points will improve currentknowledge of the acceleration of electrons and energy release in solar flares.2.1. T HERMAL AND NONTHERMAL CONTRIBUTIONS TO A SOLAR FLARE X- RAY FLUX The single-temperature approximations have been widely used in the past in inves-tigations of X-ray emissions from solar flares. However, the fact that the plasmais heated at different temperatures (multi-thermal plasma), and the emission mea-sure varies as a function of temperature emphasizes the crucial need to study theX-ray spectra with high energy and temporal resolution. Similarly, the continuumspectra above 10 keV with better energy and temporal resolution are essential toimprove our current understanding on the nonthermal acceleration of electronsduring impulsive phase of solar flares (cf., Figure 1). The relationship betweenthe thermal and nonthermal components of the total flare energy budget is notwell understood. Indeed, the total energy in neither component has generally beendetermined to better than an order of magnitude. However, there are strong in-dications that, in many flares, the nonthermal component contains a substantialfraction of the total flare energy (Dennis  et al. , 2003), while significant energyrelease in the soft X-ray domain ( < 10 keV) is also predicted. Thus, it is criticallyimportant to improve our present knowledge of the flare energy release process todetermine the energy in these two components and their relationship as a functionof time more precisely. The SLD payload, therefore, has the objective to answerreasonably the fundamental question of how much of the energy released in a flareis involved in direct plasma heating and how much in accelerating electrons thatsubsequently lose their energy to produce secondary plasma heating. SLD is thefirst solar astronomy instrument in geostationary orbit providing dedicated high-energy resolution X-ray spectra of the Sun in 4–60 keV. High-spectral resolutionmeasurements of the flare can be directly inverted to obtain the detailed spectrumof the parent X-ray producing electrons (Johns and Lin, 1992). SLD is designed  SOXS  –  LOW - ENERGY PAYLOAD  93to resolve the steep thermal spectra and determine the low energy limit of thenonthermal spectrum with  < 1 keV spectral resolution. However, the low energyrange should extend enough so as to determine the thermal–nonthermal transition(cf., Figure 1). This has been judiciously done in the SLD payload with the ap-plication of Si and CZT detectors. We plan to exploit the high-spectral resolutioncapabilityoftheSLDpayloadtodeterminethemagnitudeofthesetwocomponentsof the total flare X-ray emission. The classic ways of differentiating between thethermal and nonthermal components include separating exponential from power-law continuum spectra, impulsive from gradual varying flux, compact from ex-tended sources, and footpoint from loop-top sources. All of these techniques maybe combined together for individual flares using the SLD data and images in otherwavebands taken simultaneously by other space-borne missions and ground-basedtelescopes.2.2. S OFT  X- RAY LINE EMISSION The simulation of X-ray emission from a solar flare (cf., Figure 1) unambiguouslyindicates the possibility to determine the thermal spectrum from the measurementsof the soft X-ray line emission. However, this requires high sensitivity of the in-strument as well as  ∼ 1 keV (FWHM) energy resolution. It may be noted fromFigure 1 that the iron-line and Fe/Ni line complexes are visible only during theflare. Thus measurements of soft X-ray flux before and during the flare providea wonderful opportunity to study the soft X-ray characteristics of the active re-gion corona. The high sensitivity of the SLD and sub-keV energy resolution of the Si PIN detector allow the intensity and mean energy of the Fe-line complexat approximately 6.7 keV to be measured as a function of time in all classes of flares. This line complex is due mostly to the 1s–2p transitions in He-like andH-like iron, Fe XXV  and Fe XXVI  respectively, with associated satellite lines. An-other weaker line complex at  ∼ 8 keV made up of emission from He-like nickeland more highly excited Fe XXV  ions is also evident in the more intense flares(Phillips, 2004; Phillips  et al. , 2004). Detailed calculations of emission line inten-sities as a function of temperature, with provision for different element abundancesets (e.g., photospheric or coronal), are given by the MEKAL/SPEX atomic codes(Mewe, Gronenschild, and van den Oord, 1985; Mewe  et al. , 1985, Phillips  et al. ,2004) and the CHIANTI code (Dere  et al. , 1997). These codes also include ther-mal continuum intensities. These codes are used to interpret the SLD spectralobservations in terms of the plasma temperature and emission measure. The cen-troid energy and width of the iron-line complex at ∼ 6.7 keV, the intensity of theFe/Ni line complex at  ∼ 8 keV, and the line-to-continuum ratio are the functionsof the plasma temperature and can be used to limit the range of possible plasmaparameters.
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