The Low Energy X-ray Telescope on HXMT

  The Low Energy X-ray telescope (LE) is one of the scientific payloads on the Hard X-ray Modulation Telescope (HXMT) satellite. The scientific objectives of LE focus on the sky survey and pointing observation in soft X-ray range (0.7-15 keV).

  Unlike Chandra X-ray Observatory and XMM-Newton utilizing grazing incidence telescope to focus photons, LE used collimators to shield the photons outside the field of view (FOV).

  The Swept Charge Devices (SCDs) of LE detector are able to work in a continuous readout mode, recording the energy and the arrival time of the incident photons, so as to achieve a higher time resolution, compared to traditional Coupled Charged Devices (CCDs). In addition, with good energy and time resolution, LE will play an important role in the soft X-ray astronomy.

Table 1.Summary of the LE instrument characteristics

 

  LE contains three identical detector boxes (DB) and one electric control (EC) box. The three detector boxes are mounted upon the +Z side of the main payload's plate, with an angle of 120° to each other. Therefore, in the survey observation of HXMT, the signals from the X-ray sources will be modulated by those three detector boxes, one can get the image information by a restoration technique, e.g. Direct Demodulation method. Moreover, the EC box is located in the payload cabinet of HXMT satellite.

  The principle of the SCD is as follows. The SCD sensors can convert the energy of X-ray photons to analog voltage signals proportionally. The LE Detector Box (DB) electronics processes the CCD output signal and converts it into digital representation to generate the event data containing both information of energy and arrival time of the photon. The event data are sent to the EC box through the Low-Voltage Differential Signaling (LVDS) interface.

  The LE detector box contains two parts, i.e. the upper part and the lower part, as shown in Figure 1, which are mounted upon and bellow the main plate, respectively. Flexible cables are employed for electrical connection between these two parts. The DB consists of eight SCD modules, two types of collimators, optical blocking filters and remainder-proof films, heat pipes as well as several thermal and mechanical supporters. The lower part of the detector box contains mainly the electronics.

Figure 1 The structure of LE detector box

  Pointing observation and survey collimators (POS collimators) are employed in LE system with combined wide (4°×6°) and narrow (1.6°×6°) FOVs. Besides the two FOVs, the closed FOVs are added in LE system which enable source and two kinds of background distinguished in both pointing and sky survey modes. Moreover, the all sky monitoring (ASM) collimators with FOVs of 50~60° × 2~6° are used to survey the large sky area in scanning mode.

Figure 2 The layout of collimator FOVs in the detector box

  LE is a soft X-ray telescope based on SCD detectors. The advantages of SCDs are good energy resolution and high time resolution comparing to traditional CCD detectors. The whole calibration of LE has been finished.

  The energy of X-ray and energy channel from electronic box should be match well. The energy-channel relationship can be evaluated by integral nonlinearity, which can be described as below:


Ei are the central positions of arriving X-ray peaks through Gauss fitting, Êi are the positions obtained from Energy-Channel curves, | E- Ê|max is the maximal deviation among the above two kinds of positions. The linearity of the temperature and signal amplitude (channel) is excellent; the integral nonlinearity of this relation is about 0.3%.

Figure 3 A spectrum in the thermal test of LE flight module

  With a copper target bombard by an X-ray tube, the temperature-dependent Cu-Kα(8.048keV) signal mean curves of 32 channels are obtained and are shown as below. As shown in Figure 4, the differences of the amplitudes of each channel is within ±1%.

Figure 4 The typical INL in the thermal test of LE flight model

  For LE-A detector box, the merged spectrum of 32 channels are similar to spectra of each channel. As shown in Figure 5, the FWHM of merged spectrum could also achieve a good performance ( 该Email地址已收到反垃圾邮件插件保护。要显示它您需要在浏览器中启用JavaScript。 ).

Figure 5 The merged spectrum of 32 channels, obtained from flight model of the LE-A

  The energy responses of LE have been measured with double crystals monochromator. From these tests, the RMF (Redistribution Matrix File) of LE can be obtained by an interpolating technique (Figure 6).

Figure 6 LE RMF

Figure 7 The energy spectrums of CCD236(-100℃) and HPGe.

  The depleted layer of LE is about 50 um, the theoretical efficiency line is obtained from the "henke.lbl.gov/optical_constants/". According to the simulated and normalized result, the relative efficiency at 6.7keV is 0.698, the relative efficiencies other energy points could achieved through the same methods. Through kinds of comparisons, the relative efficiencies of the six energy points are reliable.

  In addition, the relative efficiency at 6.4keV is about 0.8, which is obtained from the comparison between CCD 236 and standard detector. This experimental value can meet theoretical value well, as shown in Figure 8.

Figure 8 Experimental relative efficiency and theoretical relative efficiency of CCD236 (Liu et al. 2016)

  Since the charge transfer in CCD236 and the readout of charge are continuous, the proportion of pileup events is less than 1% for the brightest source Sco X-1 with a maximum count rate about 18000 cts/s, which is much more lower than that of the focusing telescope, e.g. XMM-Newton.

Figure 9 The pileup of LE compared to the XMM-Newton MOS and PN in small window mode.