8. Hard X-Ray Detector (HXD)

Figure 8.1: The Hard X-ray Detector before installation.

The Hard X-ray Detector (HXD, see Figure 8.1) is a non-imaging, collimated hard X-ray scintillating instrument sensitive in the $\sim 10$keV to $\sim 600$keV band. It has been developed jointly by the University of Tokyo, Aoyama Gakuin University, Hiroshima University, ISAS/JAXA, Kanazawa University, Osaka University, Saitama University, SLAC, and RIKEN. Its main purpose is to extend the bandpass of the Suzaku observatory to the highest feasible energies, thus allowing broad-band studies of celestial objects.

This AO-7 document is based on the calibration of the PIN and the GSO detectors as of October 2010 (HEASOFT 6.9). The recommended procedure for feasibility simulations is basically unchanged from previuos AOs, the same response and background files as provided for AO-6 can be used for simulations. Note that the HXD aim point is not supported anymore (see chapter 2 and section 5.5.2).

Figure 8.2: Schematic picture of the HXD instrument, which consists of two types of detectors: the PIN diodes located in the front of the GSO scintillator, and the scintillator itself.

The HXD sensor (HXD-S) is a compound-eye detector instrument, consisting of 16 main detectors (arranged as a 4 $\times$ 4 array) and the surrounding 20 crystal scintillators for active shielding. Each unit actually consists of two types of detectors: a GSO/BGO phoswich counter, and 2mm-thick PIN silicon diodes located inside the well, but in front of the GSO scintillator. The PIN diodes are mainly sensitive below $\sim 60$keV, while the GSO/BGO phoswich counter (scintillator) is sensitive above $\sim 40$keV. The scintillator signals are read out by photomultiplier tubes (PMTs). A schematic drawing of the HXD is given in Fig. 8.2. The HXD features an effective area of $\sim 160$cm$^{2}$ at 20keV, and $\sim 260$cm$^{2}$ at 100keV (Fig. 3.5). The energy resolution $\sim$ 4.5keV (FWHM) for the PIN diodes, and $7.6 / \sqrt{E}$ % (FWHM) for the scintillators, where $E$ is energy in MeV. The HXD time resolution is 61$\mu$s.

8.1 GSO/BGO Counter Units

Each main detector unit is of a well-type design with active anti-coincidence shields. The shields and the coarse collimator itself are made of Bismuth Germanate (BGO, Bi$_{4}$Ge$_{3}$O$_{12}$) crystals, while the X-ray sensing material ``inside the well'' consists of Gadolinium Silicate (GSO, Gd$_{2}$SiO$_{5}$(Ce)) crystals. The aspect ratio of the coarse collimators yields an acceptance angle for the GSO of 4.5$^{\rm o}$ (FWHM). Each unit forms a 2 $\times$ 2 matrix, containing four 24mm $\times$ 24mm, 5mm thick GSO crystals, each placed behind a PIN diode. BGO crystals are also placed underneath of the GSO sensors, and thus each well is a five-sided anti-coincidence system. The effective thickness of the BGO active shield is about 6cm for any direction from the PIN and GSO, except for the pointing direction.

The reason for the choice of the two different crystals for the sensor and the shield is dictated by the large stopping ability of both, yet the very different rise/decay times, of $\sim 700$ns for BGO, and $\sim 120$ns for GSO, at a working temperature of $-20^{\rm o}$C. This allows for an easy discrimination of the shield vs. X-ray sensor signals, where a single PMT can discriminate between the two types of scintillators in which an event may have occurred. Any particle events or Compton events that are registered by both the BGO and GSO can be rejected by this phoswich technique, utilizing custom-made pulse-shaping LSI circuits.

In early 2010, a new GSO gain calibration with associated response files has been released. With this update, GSO data become usable down to 50keV. See the Suzaku web pages for more details ^8.1. Note that proposers do not need to consider the details of the new responses, since the differences to the old ones are minor with respect to performing feasibility simulations (although not negligible in a real data analysis).

8.2 PIN-Si Diodes

The low energy response of the HXD is provided by 2mm thick PIN silicon diodes, placed in front of each GSO crystal. The geometrical area of the diodes is 21.5 $\times$ 21.5mm$^2$, while the effective area is limited to $\sim$16.5 $\times$ 16.5mm$^2$ by the guard ring structure. The temperature of the PIN diodes is controlled to be $-15 \pm3$ $^{\circ}$C to suppress electrical noise caused by the leakage current, and they are almost fully depleted by applying a bias voltage of 400$\sim$500V ^8.2. The PIN diodes absorb X-rays with energies below $\sim 70$keV, but gradually become transparent at harder X-rays, which reach and are registered by the GSO detectors. The X-rays are photoelectrically absorbed in the PIN diodes, and the signal is amplified, converted to digital form, and read out by the associated electronics. The PIN diodes are of course also actively shielded from particle events by the BGO shields, as they are placed inside the deep BGO wells. In addition, in order to reduce contamination by the cosmic X-ray background, passive shields called ``fine collimators'' are inserted in the well-type BGO collimator above the PIN diodes. The fine collimator is made of 50$\mu$m thick phosphor bronze sheets, arranged to form 8 $\times$ 8 square meshes, 3mm wide and 300mm long, each.

The lower threshold of the PIN diodes has gradually become higher due to the increase of leakage current by cosmic-ray damage. Updated response files are regularly provided by the HXD team, for well defined ``epochs'' in time. As of August 2011 calibration epoch 11 is the newest/current one.

8.3 HXD Field of View

The field of view of the HXD changes with incoming energy. Below $\sim 100$keV the passive fine collimators define a $34'\times34'$ FWHM square opening as shown in Figure 8.3. The narrow field of view compared to the Beppo-SAX-PDS and RXTE-HEXTE experiments is one of the key advantages of HXD observations. Above $\sim 100$keV the fine collimators become transparent and the BGO active collimator defines a 4.5 $^{\rm o}\times$ 4.5 $^{\rm o}$ FWHM square opening. In summary, the full PIN energy range and the lower quarter of the GSO range have a field of view of $34'$, while the GSO events above $\sim 100$keV have a wider field of view, up to 4.5$^{\rm o}$.

Figure 8.3: Angular response of a single fine-collimator along the satellite X-axis, obtained from offset observations of the Crab nebula.

8.4 In-Orbit HXD Background

Although the HXD is a non-imaging instrument, its instantaneous background can be reproduced through modeling, without requiring separate off-source observations. The HXD has been designed to achieve an extremely low in-orbit background ($\sim 10^{-4}$cps cm$^{-2}$ keV$^{-1}$), based on a combination of novel techniques: (1) the five-sided tight BGO shielding as mentioned above, (2) the use of the 20 shielding counters made of thick BGO crystals which surround the 16 main GSO/BGO counters, (3) sophisticated on-board signal processing and on-board event selection, employing both high-speed parallel hardware circuits in the analog electronics, and CPU-based signal handling in the digital electronics, and (4) the careful choice of materials that do not become strongly activated under in-orbit particle bombardment. Finally, (5) the narrow field of view below $\sim 100$keV defined by the fine collimator effectively reduces both the CXB contribution and the source confusion.

Figure 8.4: [Left] Comparison of average non X-ray background spectra of the PIN, obtained in various epochs. The Crab spectrum, scaled down by two orders of magnitude, is shown as well. [Right] Evolution of average GSO-NXB spectra.

The non X-ray background (NXB) of the PIN diodes, measured in orbit, is plotted in the left panel of Fig. 8.4. The average background count rate summed over the 64 PIN diodes is $\sim$0.6counts s$^{-1}$, which is roughly equal to an intensity of 10mCrab. In addition, almost no long-term growth has been observed in the PIN-NXB during the first three years of Suzaku, thanks to the small activation effect of silicon. In contrast, as shown in the right panel of Fig. 8.4, a significant long-term increase caused by in-orbit activation has been observed for the GSO-NXB, especially during the early phase of the mission. The background spectrum of the GSO contains several activation peaks, with intensities exponentially increasing with their half-lives. Since the longest half-life is about one year, the GSO-NXB level will have almost saturated.

Figure 8.5: Comparison of the in-orbit detector background of the PIN/GSO, normalized by the individual effective areas, with that of the RXTE-PCA, RXTE-HEXTE, and BeppoSAX-PDS. Dotted lines indicate 1Crab, 100mCrab, and 10mCrab intensities.

Figure 8.5 illustrates the comparison between detector backgrounds of several hard X-ray missions. The lowest background level per effective area is achieved by the HXD in an energy range of 12-70 and 150-500keV. The in-orbit sensitivity of the experiment can be roughly estimated by comparing the background level with celestial source intensities indicated by dotted lines. Below 30keV, the level is smaller than 10mCrab, which means a sensitivity better than 0.3mCrab can be obtained, if an accuracy of 3% is achieved in the background modeling.

Figure 8.6: [Left] Light curve of the non X-ray background of the PIN, folded with the elapsed time after the SAA passage (top), and the average cut-off rigidity at the corresponding position (bottom). [Right] The same folded light curves for the GSO background, in the 40-90, 260-440, and 440-700keV energy bands.

Since the long-term variation of both PIN-NXB and GSO-NXB can be expected to be stable, the main uncertainties of the background come from temporal and spectral short-term variations. As shown in Fig. 8.6, the PIN-NXB displays significant short-term variability, with a peak-to-peak amplitude of a factor of 3, anti-correlated with the Cut-Off Rigidity (COR) over the orbit. Since the COR affects the flux of incoming primary cosmic-ray particles, most of the PIN-NXB is considered to originate in the secondary emission produced by interactions between cosmic-ray particles and materials surrounding the detector. When a selection criterion of COR$>$6, a standard value used in the pipeline processing, is applied for the event extraction, the amplitude decreases to a factor of $\sim$2. During this temporal variation of the PIN-NXB, its spectral shape also changes slightly (larger deviations from the average are observed at a higher energy range, Kokubun et al. 2007). In case of the GSO-NXB the temporal variation differs for different energy bands, as shown in the right panel of Fig 8.6. In the lowest energy range a rapid decline after the SAA passage is clearly observed, in addition to a similar anti-correlation with the COR. All these temporal and spectral behaviors have to be properly handled in the background modeling.

8.5 Background Modeling

As is the case for every non-imaging instrument (and in particular, for those sensitive in the hard X-ray range), the limiting factor for the sensitivity of the HXD is the reproducibility of the background estimation. Since this is the first space flight of an HXD-type detector, and the reproduction of the in-orbit background is not at all an easy task, the modeling accuracy evolves with the experience with in-orbit data. The latest status of the estimation procedures and their uncertainties will be regularly posted on the Suzaku web-sites listed in Appendix B. For proposal preparation, methods, limitations, and reproducibilities (as a function of time-scale and energy range) of the current background modeling are briefly described below. Note that this document is based on ``Suzaku-memo-2008-03'' (Mizuno et al. 2008) and ``Suzaku-memo-2008-01'' (Fukazawa et. al 2008), which can be found at http://www.astro.isas.jaxa.jp/suzaku/doc/suzakumemo, and on Fukazawa et. al (2010, PASJ 61, S17). We recommend that proposers properly take into account the expected uncertainties for their observation, based on the following information. Note that all uncertainties are reported at the 90% confidence level in this section.

8.5.1 PIN Background Model

8.5.1.1 Modeling Methods

Since there is a strong anti-correlation between the PIN-NXB and the COR, the background modeling of the PIN is primarily based on the count rate of high-energy charged particles, directly measured by the PIN diodes. Due to large energy deposits in the silicon, penetrations of cosmic-ray particles cause large signals in the corresponding PIN diodes. Hence they activate the Upper Discriminator (UD) in the analog electronics and are then recorded as PIN-UD monitor count in the HK data. The PIN-UD rate is considered to directly indicate the flux of primary cosmic-ray particles. The background count rate at any time can be generally estimated based on the corresponding PIN-UD rate.

In the actual modeling procedure of the so called ``tuned-bgd'', the PIN-NXB rate is described by adding the raw PIN-UD rate and the integrated PIN-UD rate with a fixed decay time constant, to take into account the small effect of activation during SAA passages. In addition, several parameters such as GSO count rate, Earth elevation angle and cut-off rigidity, are included as input parameters.

The spectral shape of the PIN-NXB is assumed to depend on the COR and the elapsed time after the SAA passage. For each estimated rate it is extracted from a database of PIN-NXB spectra, which has been compiled from Earth occultation data.

8.5.1.2 Comparison With Sky Data

Figure 8.7: Comparison between the data and the NXB model count rate of sky observations with 10ks integration time in the 15-40keV band. Observations with no apparent hard X-ray objects in the XIS FOV were selected (see text for details of the data selection).

Figure 8.8: The same as Figure 8.7, but for observations of E0102-72 (black) and the Cygnus LOOP (red).

We first selected observations with no strong X-ray emission above 7keV (less than 20% above the XIS-FI NXB in the entire XIS field-of-view) and compared the HXD-PIN data and the NXB model count rate (10ks exposure) in the 15-40keV band, as shown in Figure 8.7. The 90% confidence region (including a statistical uncertainty of $\sim 3.3$%) of the residual is 5.8% of the mean NXB count rate, which is larger than that obtained from the Earth occultation data ($\sim 3.8$% including 3.1% statistical uncertainty, see § 3.2 of ``Suzaku-memo-2008-03'').

Figure 8.8 shows the same comparison of sky data and the NXB model for E0102$-$72 observations (the same sky region is observed regularly for XIS calibration purposes). Some observations do not satisfy the selection criteria using XIS due to sources or diffuse emission in the XIS field-of-view, but we used all E0102$-$72 observations in order to compare sky data and the NXB model for as big a data set as possible. We also plot the data and the NXB model for Cygnus LOOP multi-pointing observations (regions within a radius of 1.5degrees were observed). We see a clear difference of the residual between the two sets of observations. Especially, the width of the residual for the E0102$-$72 data is much narrower than that seen in Fig. 8.7.

The 90% confidence region of the residual obtained from the E0102$-$72 observations is $\pm \sim 0.015$counts s$^{-1}$, or $\pm \sim 5\%$ of the mean NXB rate, including the statistical uncertainty of $\sim 3.3$%. This is somewhat larger, but comparable to, the residual distribution of 3.8% (including the statistical uncertainty of 3.1%) obtained from Earth occultation data. After subtracting the statistical uncertainty, the residual systematic uncertainty of the E0102$-$72 observations is estimated to be 3.8%, for a typical 10ks exposure. As described above, the E0102$-$72 data might suffer from contamination from sources within the field-of-view, and thus this confidence region should be regarded as a conservative estimate. For longer exposures the systematic uncertainty is expected to become a bit smaller, though the lack of long-exposure sample observations makes it difficult to verify this effect at the moment.

In order to check the NXB reproducibility for a sample of observations, we also investigated the background subtraction for eight objects for which the source signal is expected to be negligible in the HXD-PIN. The spectra are summarized in Figure 8.9. The background-subtracted spectra and the CXB model of Boldt (1987) ^8.3 are displayed as blue and green histograms, respectively. No systematic difference between them is seen up to 60keV.

From these arguments, it is clear that the current NXB model reproducibility at the 90% confidence level (excluding the statistical error) is better than 5%, and will be as good as 3% in most observations with exposures longer than 10ks. When analyzing HXD data, the user should carefully estimate the reproducibility depending on the given observational conditions. For simplicity, we suggest to employ 3% as nominal value of the 15-40keV PIN NXB reproducibility at the 90% confidence level for the preparation of proposals (see, e.g., section 5.5.2).

Figure 8.9: Comparison between the measured PIN spectra (black) and the PIN NXB model spectra (red) for observations of objects with no strong hard X-ray contribution. Their fractional residuals are given by purple crosses in the bottom panel of each figure. The blue and green histograms in the top panel indicate the background-subtracted spectrum and the typical CXB spectrum (Boldt 1987), respectively.

8.5.2 GSO Background Model

Figure 8.10: Comparison of the GSO spectra between the data (black) and BGD model (red) for observations of objects with no known strong hard X-rays. Their fractional residuals are given by blue crosses.

8.5.2.1 Modeling Methods

The GSO background is higher than that of the PIN, and hence the background modeling accuracy is very important. The background model generation methods are similar to those applied for the PIN background.

8.5.2.2 Comparison With Dark Objects

We compared the NXB model with the on-source data of dark objects, for which the source signal is expected to be negligible in the HXD-GSO. Examples for the comparison of spectra for eight dark objects are summarized in Figure 8.10. Unlike for the PIN, the CXB is negligible in the GSO band. The overall spectral shape is similar between the data and the background model spectra, where the latter is solely based on a template derived from Earth occultation data. In most cases, the residuals amount to 1-1.5% of the data. For simplicity, we therefore suggest to employ 1.5% as nominal value of the GSO NXB reproducibility at the 90% confidence level for the preparation of proposals (see, e.g., section 5.5.2).

8.5.3 Theoretical Sensitivity

Figure 8.11: Calculated detection limits of the HXD, for continuum measurements. The solid lines denote the statistical 3$\sigma$ limit for a 100ks exposure, while the dashed lines show the assumed systematic uncertainties of 3% and 1.5% for the PIN- and GSO-NXB modeling, respectively.

As a reference, Fig. 8.11 presents the theoretical sensitivity calculation results, that is, expected sensitivities defined by a certain systematic uncertainty of the background modeling, and those solely determined by the statistical uncertainty for a given exposure. In the plot, background reproducibility uncertainties of 3% and 1.5% are assumed as an example, for the PIN and the GSO, respectively. Since the actual statistical and systematic uncertainties that are to be expected for a proposed observation differ from case to case, they should be carefully verified using the data-NXB residual distribution plots.

8.6 Data Analysis Procedure

HXD data are accumulated on event by event basis. After on-board data selection, event data are further screened by the ground pipeline analysis process. By referring to the trigger and flag information (including the inter-unit anti-coincidence hit patterns), the pipeline assigns specific grades to the HXD events such as pure PIN events and pure GSO events. Detector responses and background files that match the particular (i.e., default) grade of events are provided by the HXD team. There are no user-specified parameters for the HXD.

8.7 Wide-Band All-Sky Monitor (WAM)

Tight active shielding of HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of $\sim 4$cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. With limited angular ( $\sim 5^{\rm o}$) and energy ($\sim 30$% at 662keV) resolution, they work as a Wide-band All-sky Monitor (WAM).

Analog signals from normally four counters on each side of an HXD sensor are summed up and a pulse height histogram is recorded every second. If a transient event such as a Gamma-Ray Burst (GRB) is detected, light curves with finer (31.25ms) time resolution are also recorded in four energy bands. The energy coverage of the WAM extends from $\sim 50$keV to $\sim 5$MeV, and its effective area is $\sim 800$cm$^2$ at 100keV and 400cm$^2$ at 1MeV. These data are shared among the PI and the HXD team, i.e., the PI can use the full WAM data set. Since such transient events, especially GRBs, require immediate distribution to the community, the HXD team will make the analysis products, such as light curves and spectra, public as soon as possible at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB.