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Ref:D D Issue: 2 Date: 27/06/03 page: 1/23

A GAMMA-RAY BURST DETECTOR FOR

LOBSTER – ISS

Preliminary Definition Document

for Phase A study

Draft 3.0 August 7, 2003 Prepared by F. Frontera (1,2), L. Amati (1), N. Auricchio (1), A. Bogliolo (2,3), E. Caroli (1), G. Di Domenico (2), C. Guidorzi (2), E. Montanari (2), J.B. Stephen (1), G. Ventura (1)

(1) IASF, CNR, Bologna (2) University of Ferrara, Dept. of Physics (3) University of Urbino

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Preface

The LOBSTER experiment, submitted by George Fraser, University of Leicester, in response to

the Call for Mission proposals for two flexi-missions (F2 and F3) issued in October 1999) is now approved by ESA for a Phase A study for a future flight (2009) aboard the Columbus Exposed Payload Facility (CEPF) of the International Space Station (ISS). Previously an accommodation study of LOBSTER aboard CEPF was performed by ESA (see ESA/MSM-GU/2000.461/AP/RDA Report of Dec. 2000). In this study the instrument was designed to be launched within an EXPRESS Pallet Adapter (ExPA) envelope, with the following features(see http://stationpayloads.jsc.nasa.gov/E-basicaccomodations/E3.html):

• Max Mass 227 Kg (?); • Max dimensions : 864 mm wide, 1168mm long and 1245 mm high (now revised in the

following 820mm, 1050mm, 1250mm, respectively); • 2500 W at 120 Vdc (160 W for Lobster) • Passive thermal • Low rate experiment data, control, and telemetry on the 1553 bus at a rate of <100 kbps • High-rate data by extension of the Columbus video/data link at 32 Mbps; • 10 Mbps Ethernet connection; • On-orbit stay time: 3 yrs. • For other resources see the above mentioned ESA report.

In the mentioned report, a small ancillary Gamma Ray Burst Monitor (GRBM), is assumed to be part of the final instrument configuration in order to maximize the LOBSTER science return. In a LOBSTER Consortium meeting held in ESTEC on March 21—22, 2002, in which we presented our ideas, we were charged to propose a GRBM configuration, capable of satisfying the Lobster requirements and, compatibly with the financial resources, to improve the LOBSTER—ISS scientific goals. Our ideas were positively evaluated. In the first version (draft 2.0) we presented two possible options of the GRBM configuration for the preliminary LOBSTER—ISS Definition Document. In a meeting held in Milan on 19 May 2003 with G. Fraser and N. Bannister (Leicester University), D. Lumb (ESA), F. Tominetti (CGS), our proposal was discussed and the option 2 (based on CZT detector units) was positively evaluated in relation to the scientific case discussed in the document. However, two critical points were found: the large power budget (70 W), and, less critical, the mass budget (21 kg) foreseen for option 2. In the present document we take into account the meeting results , limiting our study to the option 2 and proposing a new configuration of this option, which is lighter and meets the power budget.

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1. Why a GRBM for Lobster – ISS In the above mentioned accommodation study of LOBSTER aboard CEPF (see ESA/MSM-

GU/2000.461/AP/RDA Report of Dec. 2000) the main goal of a GRBM which was considered was the following: a. To distinguish between classical GRBs and other fast transients with similar duration, like X-

ray spikes due to charged particles, type I X-ray bursts from LMXRBs (mainly atoll sources), type II X-ray bursts from peculiar LMXBs (so far the Rapid Burster and the bursting pulsar GRO J1744-28), X—ray shots from BHCs, flares from stars, X-ray flashes of terrestrial origin. A time flag is provided to Lobster when a true GRB is detected.

In the mentioned report it was implicitely supposed that the GRBM was a single, completely

open, detection unit. However, three main reasons make this configuration unsuitable to provide the needed function (see a.): 1. Interactions of charged particles with the detector material make very difficult to distinguish

between spikes to due to charged particles and GRBs or other transient events. 2. the very wide field of view of the Lobster telescope makes not negligible the chance that

more than one transient event is detected by Lobster simultaneously with the event detected by the single GRBM. Which is the true X-ray counterpart of the GRB cannot be established;

3. the GRBM sensitivity becomes unacceptably low at high angular offsets with respect to the GRBM single detector axis.

Thus other constraints for the GRBM should be included: b. an almost uniform flux sensitivity within the FOV of Lobster; c. capability of identifying the new discovered (with BeppoSAX) X-ray rich GRBs and X-ray

flashes (XRFs); d. a rough GRB localization capability, with the GRB derived coordinates to be automatically

provided to Lobster. Finally the GRBM should: e. maximize the scientific objectives of Lobster for the study of classical GRBs, XRFs, Soft

Gamma-Ray Repeaters (SGR) and transient X—ray sources.

2. GRB detector Requirements

In order to achieve the goals listed in 1.1 a., b. and c. and d., the detector should satisfy the following requirements: • Energy passband from 3 keV up to 300 keV. The lower threshold has been chosen to have a

small superposition of the GRBM energy band with that of Lobster (0.1—3.5 keV), The wide band is of key importance for the identification of both XRFs (most of the energy in the 3-10 keV band) and GRBs (most of the energy emitted in 40 - 200 keV band).

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• Enough sensitivity to derive the hardness ratio (HR) of the transient events. On the basis of our

experience acquired with the GRBM aboard BeppoSAX (e. g., Guidorzi et al. 2001), HR is of key importance in order identify the true nature of the events.

• Good energy resolution for the study of the emission/absorption features from GRBs (see science case).

• Field of View (FOV) which almost uniformly cover the instantaneous Lobster FOV (162o x 22.5o).

• Capability of providing a rough GRB position information (≤10o error radius for the weakest events). This capability is needed in order to unambiguously and promptly identify the X-ray counterpart of the GRB (or XRF) event in the Lobster data. Position information is also requested for the cases in which Lobster does not detect the burst because of a strong GRB absorption or a very hard GRB spectrum as that expected in the case of a short (<1 s) GRBs). In these cases, the position is needed to deconvolve the GRB spectra.

• Prompt transmission of the GRB position to the Lobster Data Analysis System for an accurate GRB localization by Lobster and prompt alert distribution.

3. GRB monitor configuration

The best detector configuration is a compromise between the requirement of covering the entire Field of View (FOV) of Lobster and that of maximizing the GRB positioning capability and instrument sensitivity. We propose here an instrument made of 4 detection units, with a rectangular FOV (55o x 35° FWHM, with 35° along the ISS direction of motion) and their axes misaligned with each other by 45° in the direction perpendicular to the ISS motion and 10° along the ISS direction of motion (see Fig. 3.1). The FOV of each unit is obtained through the use of mechanical collimators, which also limit the background level. With this configuration, a source

Figur

e 3.1 Sketch of the Lobster payload. The location of the GRBM modules is indicated

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is seen by at least 2 detection units for most (95%) of the directions within the FOV of Lobster. As a consequence, the area exposed by the GRBM to these directions ranges from 0.70 to 1.30 times the useful area of a single detection unit.

3.1 Detector material

The best choice for the detector material appears to be CdZnTe (CZT), cooled along with its front-end electronics, at a temperature of about 250 K. This temperature is not difficult to achieve, e.g., using Peltier effect or passive cooling systems. The CZT detectors are now well known for lab uses and soon will be tested in the space environment with the SWIFT mission, while CdTe detectors are flying aboard the INTEGRAL mission. They: 1) exhibit a very good energy resolution (<5 keV FWHM @60 keV); 2) thanks to the density (5.85-6.06 g/cm3) and atomic numbers (48-52) of their component materials, show a high detection efficiency even at high energies (absorption coefficient µ = 10.03 cm-1 @100 keV); 3) at the temperature of 250 K, the electronic noise corresponds to a photon signal of about 1 keV, thus 3 keV photons can be easily detected. The strip readout technique (e.g., Budtz-Jorgensen et al. 2002) should be investigated for rejecting charged particles which interact with the detector. This technique has been developed mainly to compensate the signal loss caused by trapping of positive carriers. The technique makes use of a planar electrode on one side (where X-rays are incoming in the detector) and a set of drift strip electrodes plus one anode readout strip on the opposite side. Planar electrode and drift strip electrodes are biased in such a way that holes move to the planar electrode and electrons to the anode strip. The information on the depth of the interaction is measured on the ratio between the collected charges Qplanar and Qstrip.

3.2 Detection unit Each detection unit (see Fig. 3.2) is made of an array of CZT elementary crystals (pixels). Each

pixel has a cross section of 8 x 8 mm2. The detector thickness is still TBD, with 3 mm as baseline, and 5 mm in the case this thickness is crucial to exploit the GRBM as polarimeter as well. This study is in progress. The size of each elementary crystal has to satisfy the requirements on the low energy threshold and energy resolution. In the negative, this size should be decreased.

Figure 3.2 Sketch of each detection unit for both options

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Figure 3.3. A possible assembly of a CZT-Unit and of a CZT Sub-Unit (16 CZT elementary detectors). The mother board connects CZT-Sub-Units and also houses the analog post processing electronics.

Elementary crystals should be packed together in sub-units of 4x4 pixels, while each detection unit is made of 3x6 sub-units. Thus the X-ray sensitive array is made of 288 pixels. The active area of each unit is 184 cm2 while its geometric area, which takes into account of a 0.5 mm pitch between each two pixels, is 208 cm2. The crystals are assembled on thin (1 mm) ceramic plates. Below each module it is located the front-end electronics with multiplexers and ADCs. A possible grouping of the CZT detector elementary cells is shown in Fig. 4.4, which also simplifies the replacement (on the ground) of a faulty group without changing the entire assembly. The structure of the detection units can also help to design the front end electronics, to debug the hardware, and to simplify the burst trigger logic by partitioning the signals to be processed. The anodes are directly connected to the inputs of the first-stage Charge Sensitive Preamplifiers (CSPs’) (see sketch in Fig. 3.3).

3.3 Collimator Each detection unit should be surmounted by a collimator in order to limit its FOV. The

collimator is assumed to be made of four slabs of Tungsten 1.5 mm thick. The use of a graded shield (e.g., Sn, Cu and Al), to decrease the X—ray fluorescence produced in the collimator, will be evaluated during the phase B. To get a rectangular FOV of 55° x 35o (FWHM), the height of each slab is required to be 14.2 cm.

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3.4 In-flight calibration system and gain control

Each detection unit should be periodically calibrated. The design of the calibration system is

TBD. Also a gain control system of each pixel will be evaluated in order to equalize the gain of each detection unit.

4. Particle Monitor

In order to automatically switch off the GRBM in the case of very high particle fluxes (e.g., during the passage of ISS through the South Atlantic Geomagnetic Anomaly, SAGA) a particle monitor like that mounted aboard the BeppoSAX satellite, should be foreseen.

5. Front end Electronics

A possible block diagram of Front End Electronics (FEE), which was designed for a 4 x 4 (instead of 3 x 6) subunit CZT detector for a balloon experiment, is shown in Fig. 5.1. It makes use of ASIC (16 input Channels, Chs) integrated electronics. The design of the front-end ASIC strongly depends on the analog functions required. Taking into account the present technological progress in ASIC, it would be also possible to envisage the use of 32 Chs ASICs (TBD). This should permit both reliable spectroscopic and counting mode performances up to 4x105 cts/sec.

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In order to get a reliable measurement of the energy deposited in a CZT detector by an X-ray event, we intend to use the following method. The analog signal from each of the preamplifiers is processed by two separate shaping active stages, one giving a filter action selective to the electron collection time (“fast” signal component), the other to the electrons+holes (i.e. energy) collection time. This process should provide two energy components, EF and E, which are evaluated for all events in the energy window. EF and E will be converted in digital form (ADC) with 10 bit

Fig. 5.1 Possible Functional Block diagram of a Sub-Unit Front-End Electronics (16 pixels). Also schematically shown is the housekeeping data handling system.

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resolution. The ratio of the fast energy EF to the energy E will be used to correct possible energy shifts introduced by the detector. Thus the double chain allows the analysis of the signal rise time in order to correct for the charge losses due to hole trapping in the detectors. We will also evaluate possibility to use the double chain analysis to eliminate the charged particle contribution due to the impact of charged particles on CZT crystals (e.g. Budtz-Jorgensen et al. 2001).

6. Data Handling Electronics

The GRBM Data Handling Electronics (G-DHE) performs two main tasks: 1) Management of the GRBM scientific and housekeeping data and their transmission to the

Lobster OBDH; 2) Prompt recognition and localization of GRBs.

6.1 Management of the GRBM scientific and housekeeping data and their transmission to the Lobster On Board Data Handling (OBDH).

6.1.1 Scientific data collection modes

The data collection mode in the case of GRBs and for instrument calibration or diagnostic

purposes will be event by event, with possible compression of the number of bits/event. Instead, in the absence of a burst, the basic collection mode will be continuous transmission. In the case of event by event transmission, the maximum number of bits are the following: 10 bits (energy); 10 bits (fast energy or equivalent), 8 bits (crystal pixel identifier), 3 bits (detection unit identifier), 12 bits (time of occurrence). The time bits can be likely halved transmitting the time distance between two contiguous events Total bit number/event become: 37 bits. We foresee the following transmission modes from the GDHE to the Lobster OBDH.

6.1.1.1 Burst transmission mode This is the transmission mode when the GRBM is triggered by a burst. In this case the telemetry stored in the Pre-Burst Telemetry Memory (PBTM, see below) is first transmitted. With no interruption, from the burst trigger time, the direct transmission (event by event) is continued. The data rate expected from the entire instrument is the sum of background count rate (8000 cts/s) + GRB count rate. For GRBs with the highest intensity and incoming from the most favourable directions (i.e. those for which the detector area exposed to the source is maximum, see Sect. 3), the total counts expected are ~64000. Assuming a time duration of the event of 100 s and 37 bits/event, the amount of bits to transmit (inclusive of the content of the PBTM) is approximately given by (8000x100 + 64000) x 37 = 3.19x107. This bit volume has to be stored on board the G-DHE and safely transmitted at a low rate telemetry. Given that we expect to detect at most 1 GRB/day (TBC), assuming a duty cycle of 90%, we need to transmit these bits in about 8000 s. This constraint requires a telemetry devoted to the GRB data of 4kb/s. In addition a telemetry bit rate of 1kb/s has to be reserved to the continuous transmission mode (see below). Thus the total telemetry required by GRBM is 5k/s.

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6.1.1.2 Continuous transmission mode

This mode is required to be continuously operative, with an automatic parameter change

during the burst transmission mode (see below). Taking into account that a Crab-like source gives in each detection unit a count rate of about 500 cts/s in the 3-300 keV energy band vs. a background level of about 2000 cts/s/unit in the same energy band, the major telemetry load is the transmission of the background level. In the worst case, in which 10 Crab-like sources are in the GRBM FOV, a counting rate of 10x500 + 8000 = 13000cts/s has to be continuously transmitted . Assuming a telemetry budget of 1kb/s , a spectra collection mode is mandatory. We foresee to transmit r count spectra for each detection unit, each corresponding to one of r subsections of the detector, with s channels/spectrum and b bits/channels. The integration time of the spectra is t (sec). The channels are assumed to have equal logarithmic width. In the absence of GRB data transmission, the entire telemetry assigned to GRBM (12 kbits/s have been assumed) will be available. For example, if, e.g., r = 18, s =16 and t =1 (thresholds of the energy channels approximately given by 3.0, 4.0, 5.3, 7.1, 9.5, 12.6, 22.3, 29.7, 39.5, 52.7, 70.3, 93.7, 124.7, 165.9, 221.2, 300 keV), the needed number of bits/channel is conservatively b = 7 (maximum count rate/subsection expected is about 30 cts/s assuming a power-law with index Γ= -1.4 and 50 cts/s for Γ= -2). In this case the telemetry needed is 18x16x4x7 = 8064 bits/s, well below the telemetry budget available. The integration time t, number of spectra/detection unit r, number of energy channels/spectrum s, should selectable from the ground. During the GRB data transmission mode the telemetry budget shrinks to 1 kb/s, thus it is required to adjust the parameters r, s, and t. Possible permitted values are r = 1, s =16 and t =1. In this case the needed number of bits/channel is b = 11 (maximum count rate/channel expected is about 850 cts/s). The telemetry needed becomes 1x16x4x11= ~700 bits/s. The counters are required to restart from 0 if the count rate exceeds the maximum counter value. During the continuous transmission mode, the spectral correction ( to take into account losses in charge collection) is needed to be performed on-board. The correction consists of the addition of an energy term ∆E to the measured energy E of the event. It is required that this correction can be inhibited by means of a command from the ground. The term is calculated using the measured energy of the event and a set of either 2 (for linear polynomial) or 4 (for cubic polynomial) 16-bit parameters characterising each pixel. The steps are:

• Calculate the ration R = EF / E • Read the 2 (or 4) parameters for the specific pixel • Calculate the new energy as E + ∆E = E + (a +b R+[c R2+d R3]).

6.1.1.3 Diagnostic mode

The diagnostic mode is activated for functional tests, in flight-calibration tests, diagnostic tests. During the diagnostic mode the transmission is done event by event, with 43 bits/event as in the burst mode.

6.1.2 Housekeeping data In parallel with the burst or continuous transmission mode, housekeeping data are transmitted to ground. The telemetry required for this data is negligible and will be extracted from the 5kb/s budget. The housekeeping list is TBD.

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6.2 Prompt recognition and localization of GRBs

This task requires three steps: i. Continuous check whether the burst trigger criteria are satisfied; ii. Further checks (if any) on candidate bursts; iii. Sky direction localization of the burst source.

6.2.1 Burst trigger

Eight counters, each comprising 1024 (TBV) bins, will be available in a buffer (e.g. a shift register): 4 counters (one per each detection unit) of data in the [E1, E2] energy band and 4 in the [E3, E4] energy band. The accumulation time/bin is 7.8125 (=1000/128) ms. With this accumulation time, the shift register stores data for 7.8125x1024 = 8000 ms = 8 s. The shift register is continuously updated. The number of bits/counter bin is 10 (TBV). In addition to the counters, a memory (pre-burst telemetry memory, PBTM) should contain in the same time interval (8 s) the direct telemetry with the same information associated to each event as in the burst transmission mode. The tasks to be performed on these available data are the following: a. The mean count rates C[E1, E2](i) and C[E3, E4](i) over a long time window of size W (8, 16, 32 s)

in the two energy bands [E1, E2] and [E3, E4], for each detection unit i, are computed and continuously updated.

b. In parallel the mean count rates c[E1, E2](i,j) and c[E3, E4] (i,j) (j=1,2,3) over three short time windows of size w1, w2, w3 (wj<<W) are computed in the two energy bands [E1, E2] and [E3, E4], for each detection unit i.

c. For each unit i and for each time window wj, a comparison between both C[E1, E2](i) and C[E3,E4](i) and the mean count rates c[E1, E2](i,j) and c[E3, E4](i,j) at the current time t is performed.

d. The trigger is satisfied if one of the following conditions is simultaneously matched for at least two detection units and for at least one time window wj: 1. c[E1, E2](i,j) - C[E1, E2](i) > step[E1, E2](j) * [C[E1, E2](i)/ wj]1/2 2. c[E3, E4](i,j) - C[E3, E4](i) > step[E3, E4](j) * [C[E3, E4](i)/ wj]1/2

where step[E1, E2](j) and step[E3, E4](j) are the minimum significance levels, that depend on the energy band and on the short time window wj: e.g., with values step[E1, E2](j) = 4, 5, …, 16, step[E3, E4](j) = 4, 5, …, 16 (TBV).

Spurious bursts will be filtered on the basis of the hardness of their energy spectra, using the experience acquired with the BeppoSAX satellite.

6.2.2 Further checks on candidate bursts

These are TBD. Problem to be faced: how the trigger should work in order to detect X-ray flashes (XRF).

6.2.3 GRB localization

The source location will be estimated by comparing the GRB counts in each of the four detection unit, taking into account the angular response of the detectors, whose analytical shape is stored on-board in the DE memory. The stored angular response R(θ,φ) is given in a local reference frame with z axis perpendicular to the instrument main frame, and (θ,φ) coordinates with φ being

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the azimuthal angle (along the plane perpendicular to the ISS direction of motion) and theta is the elevation angle. Ri(φj,θk) is the angular response of the detection unit i in the direction φj θk, where (φj θk) is the generic element of a grid of values which cover the entire field of view of the GRBM. If Ci are the measured total GRB counts in the detection unit i, the source direction is determined by minimizing over the grid the following χ2 -like statistics: χ2(φj,θk) = ∑i ((Ci - (C/R(φj,θk)) Ri(φj,θk))/σ)2 where C = ∑i Ci and R(φj,θk) = ∑i Ri(φj,θk). Given that the GRBM units are made of matrices of elementary crystals, a more accurate estimate of the GRB direction can be later (off-line analysis) obtained by determining the barycentre of the illuminated pixels. The position of the detected GRB will be provided to the Lobster Data Analysis System.

6.3 Time tag of the events The On-Board Time (OBT) and its synchronization with the absolute time (UTC) is expected to be provided by the Lobster OBDH. A counter between two marks of the OBT is thus needed in order to mark with the due accuracy the arrival time of the GRBM detected events.

6.4 Memory requirements Memory requirements concern:

a. the input buffer needed to compensate the delay of GRB detection, b. the look-up tables used to represent the pre-characterized models of the detection crystals, c. the non-volatile memory used to store either the configuration stream of programmable

logic elements or the software running on a microprocessor, d. the output buffers used to dynamically adapt the output data stream to the available

bandwidth.

6.5 Reliability requirements (preliminary) In order to achieve a high reliability of the GDHE, the following precautions should be adopted:

a) the choice of radiation-tolerant components; b) triple (TBV) modular redundancy (fault tolerance); c) built-in self test; d) self checking; e) possibility of reconfiguration / reprogramming. f) test campaign for characterization.

6.6 Design trade-off

The following design trade-offs will be investigated:

a) FPGA vs. Microprocessor; b) parallel vs. sequential circuits; c) data compression policy.

6.7 GDHE development activity

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The following activity has been planned: a) Working prototype of DHE; b) Simulation/emulation of the GRBM data production.

7. Mass, Power and Telemetry Budgets

7.1 Mass Budget

The mass budget detail is reported in Table 7.1.

Table 7.1 Mass budget for 4 GRBM units

Subsystem Kg Collimator 11.2 Gain control plus calibrator 1.2 Detection plane 2.0 Electronics + Harness 2.5 Total (no contingency) 16.9

7.2 Power budget

The power budget is reported in Table 7.2. The power values reported are based on the following considerations.

7.2.1 FEE-AE We can consider to use ASIC that are currently in an advanced stage of realisation such as

the chip we have developed in collaboration with the Politecnico of Milan for solid state large array detectors. The Milano group has already developed several prototype chips with 8 channel that are under functional evaluation and that we foresee to use with a pixellated CdTe spectrometer by the end of this year. The current version of this ASIC contains the CSP, the shaping amplifier and the peak detector stages, and during the functional test a power consumption less that 1 mW was measured (0.8 mW). The ASIC is already designed for a large energy band operation (dynamic range: 100) and can be tuned to optimize the coupling depending on the dark current level of the used CZT/CdTe device.

Thus, adding the ADC consumption, we can assume 1 mW for each channel. For 4 detection units with 6x3x16 = 288 detection pixels per unit and two channel for each pixel, the total expected power consumption of the FEE-AE is 2.3 W.

7.2.2 High Voltage Supply units The pwer consumption of the HV Supply Units (HVSUs) is proportional to the number of

channels, the count rate and the event energy. Assuming as HVSUs the EMCO CAxxP/N devices (1 mA output at 1000 V), for 4 GRBM detection units (1152 pixels), the total expected power consumption of the HVSUs is 1.6 W.

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7.2.3 Digital Electronics (DE)

Concerning the DE unit, using as components the last generation of mobile processors and motherboards, we foresee a consumption of 2-3 Watt for this subsystem. Given that part of which is due to the management of the telemetry, this figure can be allocated to the Lobster OBDH. In spite of that, for time being, we assume a power consumption of 2.5 W.

7.2.4 Peltier cooling system Both the CZT detection elements and ASICs will be supported by the same ceramic layer of

Al2O3 (the CdTe element on one side and the ASIC on the other side), which has the advantage of a high thermal conductivity. Thus the low temperature (250 K) of both CZT and ASIC FET could be achieved by cooling only the ceramic layer with a Peltier cooling system.

Assuming a direct proportionality between the power to be dissipated and the power consumption, the total expected power required by the Peltier cells is a few Watts.

We will also evaluate using passive cooling systems or a combination of passive and active cooling methods. For time being, an estimate of the cooling system consumption is 2-4 Watt or less.

Table 7.2 Power budget (4 GRBM units) Subsystem Watt Front End plus Analog Electronics (FEE-AE) 2.3 Digital Electronics (DE) 2.5 High Voltage Supply 1.6 Peltier cooling system (see 7.2.4) Total (no contingency) 6.4 + cooling system

Prospects

We cannot exclude the possibility of operating CdTe/CZT units at a temperature higher than 250 K with no noticeable degradation of the energy resolution and with the same energy threshold of 3 keV. In fact, in the case of p-n (Schottky) CdTe detectors (ACRORAD, Japan), good spectral capabilities and an energy threshold of about 1 keV (with a hybrid preamplifier and Indium/Pt electrodes) have been obtained even at 5o C. Unfortunately this type of very low dark current detector is still limited in thickness (1 mm). This limitation is due to the technology used to produce the Shottky barrier using Indium/Pt electrodes. This kind of detector is now used by AMPTEK (instead of the previously commercialized CZT) for their off-the-shelf high performance spectrometer probes. A novel technique to produce p-n CdTe detectors has now been proposed by a large Italian collaboration within the 6th European Framework program. This technique should not suffer this thickness limitation and would be able to operate at room temperature. The development of this kind of detectors is compatible with the LOBSTER time scale.

7.3 Telemetry budget The telemetry budget, based on the requirements discussed in the Section 6, are summarized in Table 7.3.

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Table 7.3 Telemetry budget

Burst mode

(kbit/s) Continuous mode

(kbit/s) Event by event transmission (37 bits/event) 4 Continuous transmission of low resolution spectra (see text)

1

Transmission of energy spectra plus housekeeping data [1] 1

Total 5 1 [1] In the continuous mode, the number of channels, subunits and the accumulation time should be selectable from the ground (see section 6).

8. Expected functional Performance

8.1 Background level

The mean background level which is expected at the ISS orbit is reported in Table 8.1. A FOV

of 55° x 35° (FWHM) is assumed. The intrinsic background spectrum is assumed to have a power law shape with count index of -1.4 and normalization derived from the assumption that the count intensity in the 30-200 keV range is 3x10-3 cts/(cm2 s keV).

Table 8.1 Expected mean background level/detection unit

Energy band

(keV) Diffuse cosmic BKG

(cts/s) Intrinsic BKG

(cts/s)

3 – 10 1077 167 10 – 30 294 96 30 – 200 96 92 50 – 300 42 72

Given the wide field of view of the GRBM, the contribution of bright galactic surces to the total background level is not negligible. In particular, when the Galactic Bulge is inside the FOV of a detection unit, the background level is expected to increase of about 30%. This has been taken into account in the estimates and simulations reported in the next sections.

8.2 Flux Sensitivity

The 5 σ sensitivity to GRB events for both on-line and off-line data analysis is reported in Table 8.2 for the same configurations assumed in Table 8.1. Given that a GRB in the FOV of the Lobster telescope is viewed by at least 2 GRBM detection units (see sect. 3), we assumed the background level corresponding to an area of 2 x 184 = 368 cm2 as input for the on-board source localization and detection algorithms (on-line analysis). For the off-line analysis we expect to be able to identify the pixels (or groups of pixels) illuminated by the source. In this case the detection area contributing to the background level will be reduced to that exposed to the source; here we assumed a minimum exposed area/unit of 129 cm2 corresponding to 70% of the area of each detection unit

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(see sect. 3). As source spectrum we assumed a typical Band function with α = -1, β = -2 and E0 = 200 keV .

Table 8.2 Flux sensitivity in 1 s integration time (5 σ)

Energy band (keV)

Total background level (cts/cm2/s)

Flux sensitivity (1 s) (ph/cm2 s)

Flux sensitivity (1 s) (10-8 erg/ cm2 s)

On-line Off-line On-line Off-line 3 – 10 6.7 2.0 1.24 1.8 1.1 10 – 30 2.1 1.2 0.74 9.5 2.1 30 – 200 1.0 0.85 0.55 10 6.8 50 – 300 0.6 0.68 0.45 13 8.6

This sensitivity matches the 0.1 – 3.5 keV Lobster sensitivity for classical GRBs. The 5σ sensitivity of the GRBM to a Crab-like source as a function of exposure, with on-line data analysis and always assuming a total illuminated area of 129 cm2 , is shown in Figure 8.1 for the cases of a source located in the galactic center region and of a source with high galactic latitude.

Figure 8.1 The 5σ sensitivity of the GRBM to a Crab-like source as a function of exposure time for the on-line data analysis and assuming a total illuminated area of 129 cm2 (worst case). Two cases have been considered: a source located in the galactic center region and source at high galactic latitude.

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8.3 Source location accuracy The simulated source location accuracy (90% confidence level, c.l.) as a function of the burst

fluence in the 50-300 keV energy band is shown in Figs. 8.2 and 8.3 for the cases of a ‘short’ (1 s duration) and a ‘long’ (20 s duration) GRB, respectively. In each figure, the upper line corresponds to the on-line analysis, whereas the lower line corresponds to the off-line analysis technique. The dashed vertical line indicates the fluence corresponding to the sensitivity limit of the Lobster telescope. In these simulations we assumed the values of the total detector area illuminated by the source, background level and source spectrum assumed for the flux sensitivity evaluation. We used as source location reconstruction algorithm that expected to be used during the flight ( see Sect. 6. 2. 3).

Fig

ure 8.1 Localization accuracy as a function of fluence for a ‘short’ (1s) burst.

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8.4 Hardness ratio sensitivity

As mentioned in Section 2, the minimum requirement for the GRBM is its capability of

recognizing true GRBs. To this end, from the BeppoSAX experience, we need to impose a proper condition to the hardness ratio (HR) between the count rates in two energy bands. HR should have a sufficient statistical significance to allow the discrimination between GRBs and other transient events. In Table 8.3 we report, as an example, the significance of the count rates in the 30-70 keV and 70-200 keV energy bands and their ratio, for different GRB fluences and durations, starting from the value corresponding to the minimum intensity detectable by the Lobster telescope in 1 s. We assumed the same total illuminated area, background levels and source spectrum as in the previous sub-sections (worst case). Table 8.3 Hardness ratio sensitivity vs. GRB fluence and duration. The fluence of 2.5x10-7 erg cm-2 , for 1 s duration, corresponds, in the 50 – 300 keV energy band, to the limit 1 s sensitivity of Lobster in soft X-rays. Fluence (erg cm-2)

∆T (s)

30 – 70 keV (σ)

70 - 200 keV (σ)

HR (70-200)/(30-70) (σ)

On-line Off-line On-line Off-line On-line Off-line 2.5x10-7 1 5.5 7.7 6.9 7.7 4.3 5.7 2.5x10-6 15 15 22 19 25 12 16 2.5x10-5 40 74 92 84 94 56 66

Figure 8.2 Localization accuracy as a function of fluence for a ‘long’ (20s) burst.

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8.5 Sensitvity to absorption features in the GRB spectra

One of the main goals of the present GRBM configuration is the possibility of detecting transient absorption features during the rise time of the GRB events. Such features, possibly associated with a variable column density, are predicted by several models and have already been observed in two GRBs (GRB990705, Amati et al. 2000; GRB011211, Frontera et al. 2003). In Figure 8.4 we show the spectrum expected to be detected by the GRBM for Lobster-ISS using as template that measured, with the BeppoSAX WFC plus GRBM, from GRB990705 during the first 13 s. The presence of the feature is visible at much higher statistical significance than that observed with BeppoSAX from GRB990705. In Figure 8.5 we show the 3σ sensitivity of the GRBM to an absorption feature at ~3.8-4.8 keV as a function of exposure time (or feature duration, if transient). The joint fitting of the GRBM and Lobster expected spectra will extend the analysis down to 0.1 keV, allowing not only the study of the column density behaviour with time, but also the increase of of the edge significance.

Figure 8.4 The spectrum of GRB990705 in the earliest 13 s as expected to be measured with the GRBM in the Option 2 configuration assuming an illuminated area of 129 cm2 (worst case). The fit with a simple power-law is unacceptable ( χ2/ d.o.f. = 1066/654). The absorption edge at ~3.8 keV is apparent at a >12 σ confidence level .

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Figure 8.5 Sensitivity (3σ) to a narrow absorption edge at 3.8 keV or a narrow negative Gaussian at 6.8 keV, as a function of the exposure time (or feature duration, if transient) ) for 3 different GRB intensities. Such features have been observed from GRB990705 and GRB011211, respectively (Amati et al. 2000 and Frontera et al. 2003). We assumed an illuminated area of 129 cm2 (worst case) and the off-line data analysis technique.

8.6 Sensitvity to emission features in the GRB spectra

In addition to absorption features, also transient emission components are expected to contribute to the early GRB emission, in particular black-body emission from the photosphere of the fireball (Meszaros & Rees 1998). Evidence of such a transient broad emission component has actually been found in the prompt emission of GRB990712, and could be modelled with a blackbody with temperature kT~1.3 keV (Frontera et al. 2001). In Figure 8.6 we show a GRBM spectrum simulated by assuming as template the spectrum of GRB990712 in the time interval in which the transient emission feature was observed. The excess with respect to the power-law continuum is apparent and the feature is detected at ~8σ significance. As above, a total exposed area of 129 cm2 (worst case) and the off-line analysis technique were assumed.

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Figure 8.6 The spectrum of GRB990712, as expected to be observed with the Lobster GRBM, in the time interval in which a transient blackbody component with kT ~1.3 keV was detected with BeppoSAX WFC plus GRBM (Frontera et al. 2001). The fit with a simple power-law, shown in the figure, is clearly unsatisfactory and the black-body component is detected at 8σ significance. The minimum exposed area of 129 cm2 and the off-line analysis technique were assumed.

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References Amati, L., Frontera, F., Vietri, M., et al., 2000, Science, 290, 398 Budtz-Jorgensen, C., et al., 2001, NIM A, 458, 132 Frontera, F., et al., 2001, ApJ, 550, L47 Frontera, F. et al., 2003, to be submitted to ApJ Guidorzi, C., et al., 2001 , in “ Gamma Ray Bursts in the Afterglow Era” , E. Costa, F. Frontera, J. Hjorth (Eds.), ESO Astrophysical Symposia (Springer, Berlin), p. 43. Meszaros, P. & Rees, M.J., 1998, ApJ, 502, L105

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