PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2004-06-02, Lauri Alha, UH, 1.0" RECORD_TYPE = STREAM RELEASE_ID = 0001 REVISION_ID = 0000 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "S1" INSTRUMENT_ID = "XSM" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "X-RAY SOLAR MONITOR" INSTRUMENT_TYPE = "SPECTROMETER" INSTRUMENT_DESC = " Science and technology objectives ========================= Calibration of D-CIXS data The strength and spectral distribution of the fluoresence spectrum measured by D-CIXS is strongly dependent on the solar X-ray irradiance at the surface of the Moon, and the primary task for the XSM is to provide solar X-ray spectra for the calibration of the fluorescence lines. For the derivation of the expected fluorescence, the incident solar spectrum shape and intensity at energies higher than the absorption edges of relevant elements are necessary. XSM will observe the solar input at 2-20 keV, which includes all important absorption edges, and a broad enough energy range above them. Studies of the solar corona In addition to being a part of the D-CIXS measuring system, the XSM will provide significant independent information about the solar corona. The XSM spectral range is very sensitive to solar flare activity. During a flare the measured total spectrum will be largely dominated by the flux from the event, especially at higher energies above 2-3 keV. For example, 97% of the total count rate in the whole XSM energy range (2-20 keV) is covered by an X1 flare. Following the total evolution of a large number of various flare events during the long observing period will yield a very useful database for general studies of flare physics and flare evolution. On the other hand, XSM is sensitive enough to see the 2-10 keV tail of the quiescent Sun spectrum at solar minimum with 10 ksec integration times. Monitoring of solar corona in short time scales With the time resolution of the spectra obtained with XSM the variabilities of the mean coronal temperature and non-thermal tails can be followed. Earlier studies of solar corona suggest 1/f - type power spectral density which may be characteristic to magnetic systems (like Tokamaks). This flickering behaviour is also characteristic to the black hole candidate Cyg X-1. This indicates that magnetic flares on the disc may cause the variability and serve also as sites for the Comptonization process and soft-hard time-lags. In this way, the data provided by XSM may have a much broader significance. XSM can trace the evolution of the X-ray spectrum of solar flares during the declining phase, and for the strongest flares also along the rise of the flare. From the spectrum evolution it is possible to track the temperature evolution, and also much of the whole physical process during the flare. Combining with simultaneous observations of solar corona by other satellites Adding independent knowledge about the spatial extent and morphology of the flares from e.g. observations by SOHO will provide a possibility for significant improvements of flare modelling studies. It is important to also note that XSM will provide a significant extension of spectral range to higher energies, and will thus nicely complement e.g. the SOHO data. Long term monitoring of the solar corona and study of the solar-stellar connection * Long term monitoring of the X-ray spectral variability of the Sun has also significance, especially in comparison with similar studies of other stars. The coronal emission is known to have a strong connection with the magnetic activity of stars, and following the behaviour of the solar coronal emission together with other types of solar monitoring programmes (magnetograms, radio emission monitoring, XUV observations) will help in building a more complete picture about the connection between different aspects of the magnetic activity. Testing of stellar X-ray emission models Since the XSM will observe the Sun as a star, the interpretation of the resulting data will be similar to that obtained from distant point objects by large X-ray observatories. In addition, the spectral range of XSM overlaps partly with many of the modern astronomical X-ray satellites (e.g., BeppoSAX, ASCA, XMM-Newton, Chandra), and also the spectral resolution is closely similar. Therefore it will be possible to make direct comparisons of the X-ray emission models of the Sun, which are based on the XSM observations, with the observations of other stars and astronomical objects. Instrument overview ============== Detector The flux of the Sun in the energy range 0.1-20 keV, is very high (several hundred million photons/cm^2/s) and variable. The spectrum has a very steeply declining slope with increasing energy, which means that most of the photons are concentrated in the lower energies below 1 keV. The variability, on the other hand, is concentrated in the higher energies because it is mainly caused by the changes in the high temperature components of the solar spectrum (originated in active regions and flares). This means that tuning the low energy limit for the XSM is very important to achieve reasonable count rates. Nevertheless, even by cutting out the mostly invariable low energy part of the flux below 1 keV, the detector will receive high photon fluxes requiring a very small active detector active area. The lower limit for the energy range is determined by the external X-ray windows and surface contact layers of the detector, with a minor effect by the very thin dead layer of Si on the detector surface. The optimal energy passband is acquired with a 25 micron Beryllium window. The standard design includes an Aluminium contact of 500 nm thickness, and the estimated Si dead layer is 200 nm. The coadded effects of these lead to a bandpass with 30% efficiency at about 2 keV. The upper limit of the energy range for the HPSi Pin-detector depends on the thickness of the detector, which dominates the upper energy limit via the QE. The thickness, 0.5 mm, of the standard Si diodes manufactured by Metorex suits excellently for the purpose. The range extends to about 20 keV with well sufficient QE for our purpose. The inefficient charge collection in the edge area of the detector is handled by manufacturing a larger detector bulk with the edge area covered by an aperture stop of gold. Simulations with various flare intensities show that the optimal area size not covered by the golden ring is about 1.5 mm in diameter, and the optimal thickness is 125 to 500 microns. Electronics The electronics of the XSM consists of 1) pre-amplifier stages and shaping amplifier, which are in the sensor unit box, and 2) an electronics board in the main D-CIXS instrument box, which includes further stages of the signal processing electronics. The electrical and data interfaces connect the XSM with the D-CIXS. There will be no direct electrical or data interfaces from the XSM to the spacecraft. Performance and field of view The detector features will, according to simulations, yield about 1 cps in the solar activity minimum (i.e., sunspot cycle minimum), about 200 cps in the solar maximum (i.e., average sunspot cycle maximum), and about 7000 cps during an X1 flare (a strong flare). Class X10 flares have been detected, and photon count rates above 10000 cps are therefore not ruled out, albeit very rare. From these we can set the requirement for the dynamic range of the detector. The optimisation of that range should, however, be done simultaneously with that of the energy resolution, which is equally important for our science goals. Technically, there is a clear tradeoff between these goals, since high dynamic range, and capability to handle very high count rates will inevitably lead to a loss of energy resolution, and to an untolerable event pile-up. Therefore, we set the requirement for the energy resolution to be about 230 eV at 6 keV. This is quite sufficient for a good spectral analysis, with the capability of resolving the major spectral lines expected in the lower energy part of the spectrum (thin thermal plasma spectrum). The resolution will also be comparable to those of the instruments on the other X-ray missions (XMM- Newton, Chandra, SRG). The suitable number of equally spaced energy channels in the range 0-20 keV is 512, leading to at least 3-4 channels per resolution element. The dynamic range will be optimized by using a hardware technique to compensate for the detector dead time losses. This will also lead to significantly decreased amount of signal pile-up. The estimated fraction of piled up events will be about 1% at the signal level of 20000 cps. The X-ray flux from the Sun will overwhelmingly dominate the signal in the energy range of the instrument over the sky background or any other possible source simultaneously in the FOV. In fact, the open aperture of the detector should be maximized in order to have the Sun in the field of view for a sufficiently large range of attitudes. Thus, the instrument requires no collimation or a focusing system (telescope), and the full aperture of the detector will be used. Energy resolution The XSM detector is a PIN-diode made of High Purity Silicon. Its initial energy resolution will be about 250 eV at 5.9 keV at the beginning of the SMART-1 mission. Bombardment of solar protons and cosmic particles will deteriorate the operation of the PIN-diode by degrading the energy resolution. These radiation damages will cause higher leakage current across the PIN- diode, which will be the dominant noise source in the system. Performance summary =================== Parameter Unit Comments / precision ------------ ------ -------------------------- Energy resolution eV 250 @ 6 keV Time resolution s 16 Energy range keV 2.0 ... 20.0 keV Number of channels number 512 Operational temp. deg (C) 0 ...-30 FOV deg (angle) 105, off axis circular shape Read out dead time s 0.000005 Pile-up % 2 @ 20000 ct Calibration =========== See XSM_CALI.TXT in CALIB directory! Operational modes ================= XSM has two operational modes. 1. Calibration (=shutter closed and acquisition on) 2. Observation (=shutter open and acquisition on) The acquisition time per spectrum is 16 seconds, which is also the time of each particular data file. Miscellaneous notes of instrument performance (updated 02 June 2004) ========================================================================= The time cap between the HV on and the start of the acquisition is too short causing bad energy resolution in the couple of first spectra due to the too high leakage current. The observation mode must be delayed to exted the time of calibration mode to obtain good calibration spectra. In practise this means, that the shutter open TM comand must be delyed e.g. for 64 seconds. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "N/A" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END