Proposed (to) EXIST: Hard X-ray Imaging All Sky Survey/Monitor

6 Pages·0.15 MB·English

by  J. E. Grindlay

Proposed (to) EXIST: Hard X-ray Imaging All Sky Survey/Monitor
J.E. Grindlay,1 T.A. Prince,2 F. Harrison, 2 N. Gehrels, 3 C.J. Hailey, 4
B. Ramsey,5 M.C. Weisskopf,5 G.K. Skinner,6 P. Ubertini,7
1
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138
2California Institute of Technology, Pasadena,CA 91125
8 3Goddard Space Flight Center, Greenbelt, MD 20771
9 4Columbia University, New York, NY 10027
9 5Marshall Space Flight Center, Huntsville, AL 35812
1
6University of Birmingham, Birmingham B15 2TT, UK
n 7IAS, I-00044 Frascati, IT
a
J E-mail: [email protected]

1
Abstract

1
Thehardx-ray(10-600keV)skyisinherentlytimevariableandyethasneitherbeensurveyednorbeen
v
9 monitored with a sensitive imaging telescope. The Energetic X-ray Imaging Survey Telescope (EXIST)
5 is a mission concept, proposed for MIDEX, which would conduct the first imaging all-sky hard x-ray
3 survey as well as provide a sensitive all sky monitor (ASM). With ∼60% sky coverage each orbit, and
2 full sky coverage each ∼50 days, hard x-ray studies of gamma-ray bursts, AGN, galactic transients, x-ray
1
binaries and accretion-poweredpulsars can be conducted over a wide range of timescales. We summarize
7
9 the scientific objectivesof EXIST for both the survey andmonitoring objectives. We describe the mission
/ concept and the instrumentation approach, which incorporates a large area array of Cd-Zn-Te (CZT)
h
p detectors, as well as some of our ongoing development of CZT array detectors.
- Key words: surveys: X-rays — X-ray sources: binaries, black holes, AGN — GRBs — instruments:
o
all sky monitors
r
t
s
a
v: 1. Introduction Zn-Te detectors proposed, and community input for the
i mission design has been sought. A detailed summary
X TheEnergeticX-rayImagingSurveyTelescope(EXIST)
and description of the Mission Concept Study will be
r was proposed (in December 1994) and accepted (April
given by Grindlay et al (1997).
a
1995) as one of the 27 New Mission Concept (NMC)
studies for a satellite-borne astrophysics mission. It
would conduct the first imaging survey of the sky at 2. Overview of EXIST
hardx-rayenergies(10-600keV)withasensitivity some A survey telescope at hard x-ray energies can be con-
100× greater than the only previous all-sky survey car- structed as a coded aperture telescope with a field of
ried out by HEAO-A4 experiment in 1978-80(Levine et view of up to ∼45◦ without significant projection ef-
al1984). Theneed,andpriority,forsuchanallskyimag- fects or collimation by the mask aperture, assumed pla-
inghardx-raysurveymissionhasbeenpointedoutinthe nar. Such a telescope would execute a continuous scan
recentreportofthe NASA Gamma-RayProgramWork- rather than fixed target pointing to cover a maximum
ing Group. An overall description of the initial EXIST sky fraction in minimum time and would be more sensi-
conceptisgivenbyGrindlayetal(1995)(andontheWeb tive (above 30 keV) than a scanning grazing incidence
site http://hea-www.harvard.edu/EXIST/EXIST.html). (multi-layer) telescope of comparable (or even larger)
EXIST has been developed extensively in the course of physical size though smaller effective area and field of
preparationandsubmissionofa successful“Step 1”and view. For example, a multi-layer telescope with (cur-
invited (1995) “Step 2” proposal for the MIDEX pro- rentlyambitious)parametersofFOV∼10′andeffective
gram. The mission is being further developed for the area A ∼ 500cm2 in a scanning (ROSAT-like) satel-
eff
next MIDEX solicitation, including additional labora- lite mission would be a factor of ∼10 less sensitive than
tory development and balloon flight testing of the Cd- a wide-field (FOV ∼ 20-40◦ ) scanning coded aperture
1
Fig.1. Schematic viewofEXISTinorbitshowinglayoutofcodedaperture telescopes andspacecraft.
telescopewithA ∼5000cm2 inthe30-100keVband. by the broad objectives of surveys and monitoring stud-
eff
In addition, the wide-field coded aperture imager allows ies. Inthis paper weemphasize the monitoringandgen-
the survey to extend up to the poorly explored 100-600 eral time-variability studies that EXIST can carry out.
keV range, totally unaccessible to focussing optics (ex- However,sincemanyoftheserequirethehardx-raysam-
cept for Bragg concentrators, which can work only in a ples and populations of (many) classes of object to be
very narrow energy band). first studied and established beyond the currently lim-
EXIST would incorporate two coded aperture tele- ited samples, we begin with the survey objectives.
scopes, each with FOV = 40◦ × 40◦ (above ∼40 keV;
3.1. Surveys
at 10-30 keV a low energy collimator would restrict the
◦ Hardx-rayspectraandluminosityfunctionofAGNs: Ac-
FOV to 3.5 in one dimension), for a combined FOV =
40◦ × 80◦ ,. A Cd-Zn-Te (CZT) detector array of total tive galactic nuclei (AGN) are now measured by OSSE
area of 2500 cm2 is at the focal plane of each telescope. to have hard spectra with breaks typically in the 50-100
keV range for Seyferts (cf. Zdjiarski et al 1995) and
Theschematiclayoutofthetwotelescopesandapossible
withmulti-componentornon-thermalspectraextending
spacecraft implementation is shown in Figure 1.
tohigherenergieslikelyfortheblazars. EXISTwillhave
BecauseofitsverylargeFOVandlargeareadetectors
an all-sky sensitivity some 10× better than that needed
withhighintrinsicresolution(bothspatialandspectral),
to detect the “typical” Seyferts seen with OSSE. More
EXIST could approach the unprecedented all-sky sensi-
than 1000AGN should be detected in the all sky survey
tivity shown in Figure 2. The sensitivity plots are for
andEXISThastherequiredsensitivityinthepoorlyex-
EXISTforitsproposed9-monthall-skysurvey(followed
plored 30-200keV band to measure accurate spectra for
by a pointed mission phase), which would allow total
integration times of ∼106 sec for any source. allknownAGNdetectedwithGingaorwiththeEinstein
slew survey.
Although the mission is designed primarily as a sur-
A major objective for AGN studies and surveys is
vey and monitor mission, it could be operated as a
the detection and inventory of heavily obscured or self-
pointed (observatory) mission for selected high priority
absorbed AGN. Such objects, primarily Seyfert 2’s but
targets and very long exposure times (e.g. M31 GRB
also including (some) star-formation galaxies, are now
and BHC surveys; galactic bulge survey) in which case
being discovered in pointed observations with SAX (cf.
even greater total exposure times and thus sensitivities
Piro,theseproceedings)andwillalsolikelybediscovered
can be achieved.
with the focussing ABRIXAS all sky survey up to ∼10
keV(cf. Staubert,theseproceedings). However,themost
3. Scientific Objectives: All Sky Survey and Monitoring heavily obscured objects, with absorbing column densi-
The scientific objectives for the EXIST mission may be ties >∼ 1024−25cm−2 yielding low energy cutoffs in the 5-
summarized either by classes of object to be studied or 10keVrange,willbemorereadilydetectedwithEXIST.
Fig.2. SensitivityofEXIST(MIDEX)forcontinuum (left)andnarrowline(right) spectra andcompared withothermissions.
This,togetherwiththeEXISTmeasureofthehardspec- sources. Similarly, 511 keV emission from black hole bi-
traandthustotalluminosityofthe stilllargersampleof naries (or AGN) can be searched for (e.g. in transient
obscuredAGNwithlowercutoffsdiscoveredwithcompa- outbursts),andthediffusegalactic511keVemissionim-
rablesensitivity(∼0.1mCrab)withABRIXAS,willyield agedwithsensitivitycomparabletoOSSE(cf. Figure2).
the first measure of the luminosity function of this im- Study of the diffuse hard x-ray background: The spec-
portant yet poorly studied class of AGN. tra of a significant sample of AGN will test the AGN
origin of the diffuse background for the poorly explored
Survey for black hole and neutron star compact bina-
hard x-ray band. Because the background measured by
ries. Studies of compact objects over a wide range of
the EXIST detectors below 100 keV is dominated by
timescales and luminosity are possible throughout the
the cosmicdiffuse spectrum, its isotropyandfluctuation
Galaxy. A deep galactic survey for x-ray binaries con-
spectrum can be studied with much higher sensitivity
taining black holes vs. neutron starsand pulsars will al-
than before.
lowthe relativepopulationsofblackholesinthe Galaxy
to be constrained. All previous galactic hard x-ray sur-
3.2. Monitoring
veys have been constrained to the brightest decade in EXIST surveys 60% of the sky each day, yielding ∼10
sourceflux(andluminosity); EXISTwillextendthis1-2 orbits/day (allowing for SAA, etc.) × >∼ 10 min ex-
decadesdeeper. WhereastheINTEGRALgalacticplane
posure/orbit or >∼ 6000 sec/day for each source ob-
survey(s) will also make great strides, the EXIST (all
served. This yields a daily flux sensitivity (30-100 keV)
sky) survey will be more sensitive and not be limited of ∼1-2 mCrab, sufficient for the brightest AGN and
to the central ∼ ±7◦ of galactic latitude covered by the
essentially all known accretion-powered binaries in the
smallerFOVofINTEGRAL.TheEXISTsurveywillalso
Galaxy. Pulsar timing allows even fainter flux limits,
samplethegalacticplaneonawiderrangeoftimescales.
as demonstrated with the extensive BATSE monitoring
Emission line surveys: hidden supernovae via 44Ti project (cf. Bildsten et al 1997). Over one sky survey
emission and 511 keV sources: The arrayof CZT imag- epoch (∼50d), each source is observed for >∼ 25d, and
ing detectorsproposedforEXIST achieveshighspectral variability flux limits for ∼month timescales are thus
resolution(e.g. ∼5%at60keV).Thusemissionline sur- ∼0.3mCrab.
veys can be conducted. The decay of 44Ti (lines at 68 Faint hard x-ray transients: black hole population
and 78 keV) with long (68 y) halflife allows a search for in Galaxy: The sensitivity to ∼1-10d transients is
thelong-soughtpopulationofobscuredsupernovaeinthe >∼ 30 × better than BATSE, so that the low resolution
galacticplaneatsensitivitiessignificantlybetterthanthe occulatation-imaging survey for faint transients being
possibledetectionofCas-A(cf. Figure2). Theseobjects conducted with BATSE (cf. Grindlay et al 1996) can
wouldlikelyappearasdiscrete(unresolved)emissionline beextendedtocorrespondinglyloweroutburstluminosi-
tiesorgreatersourcedistances. Witha1-10dsensitivity studies are currently underway by us (at CfA) to verify
of only ∼1 mCrab, BH transients can be detected with this.
their characteristic peak luminosities of ∼ 1037−38erg/s The individual 4 × 4 pixel detector elements would
(10-100 keV) out to 100 kpc, so that the LMC/SMC be grouped into a 2 × 2 sub-array for a 64-channel
can be surveyed in hard x-rays for the first time. Since basic detector element (BDE) read out by a preamp-
the transients containing BHs are characterizedby hard shaper-multiplexer ASIC readout circuit with very low
x-ray spectral components (extending usually to >∼ 100 power dissipation) <∼ 1mW/channel). The ASIC is self-
keV)whicharebothmoreluminous(atpeak)andlonger triggeredandmultiplexesits64-channeloutputtoapro-
lived than than the hardtails found for NS systems, the cessing chain which could encode the 1, 4, or 16 peak
hardx-rayall-skysurveyprovidesperhapsthebeststrat- channels(configuredoncommand)sothatmulti-sitede-
egy for discovering x-ray binaries containing BHs. tection (e.g. Compton events and internal background
Monitoring and Study of X-ray Pulsars: The mea- rejection) could be accomplished. We are currently in-
surement and monitoring of spin periods, pulse shapes vestigatingpackagingschemestoassembletheseunitde-
and luminosity/spectra of a large sample of accretion- tectorelements(BDE)intoatiledarray. Itislikelythat
powered pulsars would extend the BATSE sample of tiling would be done by combining a sub-array of 4 × 4
Bildsten et al (1997) to the entire sample (>∼ 30) of BDEs contiguously into a single basic detector module
knownaccretion-poweredpulsars. Thehighspectralres- (BDM), with area ∼100 cm2 (depending on final choice
olution of the CZT detectors on EXIST will allow high of unit detector and thus also pixel size).
◦
sensitivitystudiesofcyclotronfeaturesinpulsarspectra, The 40 collimator could be either passive (e.g. Ta
greatlyextending currentRXTE/HEXTEstudiesofrel- slats) or, more effective, active (BGO). We are studying
atively few objects to a much larger sample. the tradeoffs but an active collimator would allow each
Studies of Gamma-ray Bursts: EXIST would have a BDM to be separately shielded with a rear (2cm)/side
GRB sensitivity approximately 20× that of BATSE so (0.5-1cm) BGO shield for a well-type geometry yielding
that a 2 month pointed exposure could both detect and both collimationandlowerbackground(due to forward-
map a halo in the Andromeda galaxy (M31) should one hemisphere rejection of internal CZT activation back-
exist as a component of the GRB population. Given ground as well as lack of production in the passive col-
◦
theobservedGRBlogN-logSrelation,EXISTshouldde- limator). For the 40 (FWHM) field of view desired for
tectGRBsoverallatabout1/2therate,or∼0.5/day,as eachEXIST telescope, the BGO shield/collimatorblade
BATSE with its much larger FOV but reduced sensitiv- height is 12cm so that the BDM modules are nearly cu-
ity. GRBs will be located to <∼ 1-5′ positions, thereby bic. Each of the two complete EXIST detectors would
providing definitive tests of repeaters. Bright burst po- thusconsistofanarrayof5×5or25suchBDMs. These
sitions and spectra could be brought down in real time modules would be close-packed and mounted in a com-
for automated followup searches. mon frame. Although the collimator shields will require
anetgapof∼1.5cmbetweeneachofthe5BDMsacross
thedetectorarray,thisdoesnotaffectimagereconstruc-
4. Detector, Telescope and Overall Mission Concepts
tionbutonlymakesthedetector∼8cmlargeronaside.
4.1. Detector Concept
The sensitivity and resolution (both angular and spec- 4.2. Telescope Concept
tral)desiredforEXISTcanbeachievedwithalargearea The coded aperture telescopes can be relatively com-
array of pixellated CZT detectors used as the position- pact design with coded mask at focal length 1.4m and
sensitive readout for a wide-field coded aperture tele- mask pixel size 5mm. This yields an imaging resolution
′
scope. The wide-field needed for the monitoring as well of 12 , which is appropriate to resolve even the most
assurveyobjectives,combinedwiththedesiredsensitiv- crowded galactic bulge fields at the high sensitivity ex-
ity up to ∼600 keV, lead to a relatively thick (3-5mm, pected. In order to cover the full 40◦ field of view, each
minimum)detectorwithmoderatelylargepixelsize. For telescope would have a URA mask of approximate di-
the 40◦ FOV,anda 5mmthick detector,the pixels need mensions 1.2m × 1.2m and format 257 × 255 to fully
be >∼ 5mm × tan(20◦ ) ∼ 1.9mm across for minimal image the 40◦ FWHM field of view. However, since the
charge spreading due to projection effects. Since CZT BGO collimator on each BDM segment of the detector
crystals are much more readily available (currently, at array would produce partial coding for sources off-axis,
least) in <∼ 10-12mm crystal sizes, this leads naturally the mask must be either smaller format and repeated
to a unit detector element of 12 mm (square) × 5mm (e.g. 4 contiguous 129 × 127 masks, leading to ambigu-
(thick) on which a 4 × 4 array of 2.5mm (square) pixels ous sourcepositions)orrandom. Arandommask would
isinscribedwith0.5mmgapsbetweenpixels. Suchade- be, in any case, as effective as a URA of such large for-
tectorpixelsize/thicknessratioshouldyieldareasonable mat. Since the coded mask shouldnot collimate the im-
small pixel effect advantage (cf. Barret et al 1995), and age significantly, the mask thickness is restricted to be
<∼ 5mm,which(forTamaskelements)restrictsitsupper In collaborationwith Caltech and JPL, we assembled
energy limit to be <∼ 600 keV for partial shadowing. aflightunittotestthepromptanti-coincidenceshielding
efficiencyofaplanarBGOshieldimmediatelybehindthe
4.3. Mission Concept CZTdetectorplane,asproposedoriginallyforEXIST(a
EXIST is proposed as a MIDEX with nominal lifetime future experiment will test the possible well-type geom-
of 2 years (although with no consumables, an extended etry). The BGO (75mm diameter × 75mm thick, and
mission is desirable). The baseline plan is for the all supplied by JPL) was centered below a single element
sky survey to be conducted in the first 10 months (in- CZT detector (10mm × 10mm × 2mm, and supplied
cluding a 1 month verification phase) which allows 6 by eV Products to Caltech). The detector-shield and
passes through the sky. The entire sky is then effec- preamp was mounted in a pressure vessel and shielded
tively surveyedto a totalexposure of∼1.5×106 sec for with a 1.8mm thick Pb + 0.8mm thick Sn and 1.2mm
◦
any source. The two-telescope combined FOV of 80 is Cugradedshieldtosimulatetheapproximategrammage
pointed North-South, and the pointing direction “nods” ofthe passivehighenergycollimatorinthe baselineEX-
towardthe poles during the high latitude portion of the IST detector. The raw CZT and BGO (shield) detector
orbit so that all-sky coverageis uniform over the survey preampoutputs areinterfacedtoshaping ampsanddig-
to within <∼ 10%. A pointed mission phase of 2 months ital (discriminator and 12 bit ADC) electronics built at
withinthe firstyearwouldallowthe deepsurveyofM31 CfA to interface to the flight computer and data system
(for GRBs and BHCs) and the galactic center and/or fortheEXITE2balloon-bornetelescope(Lumetal1994,
the LMC/SMC to be conducted (during M31 occulta- 1997).
tions). These deep pointed surveys would more than Aballoonflightwas(finally)obtainedonMay8,1997.
doublethe totalexposureonthese targetsthathasbeen The background spectrum in the CZT is reduced by a
acquired during the all sky survey phase, as well as al- factor (energy-dependent) of ∼4-8 with the BGO veto
low for broader timescales of coverage. The second year and is at flux level at 100 keV of ∼ 1.0 × 10−3 cts/
of the nominal mission would be a pointed phase, with cm2-sec,orwithinafactorof2ofthe EXITE2phoswich
a series of deep pointings (∼1 month each) designed to backgroundlevelmeasuredsimultaneously. Withthead-
both allow deep surveys and TOOs as well as to effec- ditional shielding efficiency possible with a well-type ac-
tivelyextend the allsky surveyandASMfunction given tive BGO collimator, the background may be reduced
the large FOV covered. Data from the entire mission is another factor of ∼2. Full details are given in Bloser et
open (by proposal) to the entire community, and point- al (1997).
ing directions/durations (and surveys) are proposed by
Balloon Flight Measure of Neutron Backgrounds: In
a GI program.
order to fully calibrate the CZT backgroundand shield-
ing experiment so that balloon results may be extrapo-
5. Ongoing CZT and Array Studies latedtothefullspaceenvironment,asimultaneousmea-
sureoftheneutronfluxexperiencedbythedetectorisde-
As part of the effort to both conduct the EXIST Mis-
sirable. The atmospheric neutron fluxes as tabulated by
sion Concept study and optimize the design for a future
Armstrong et al (1973) are sufficiently uncertain (prob-
MIDEX proposal as well as a prototype balloon-borne
ablyby afactorof>∼ 2)thatwehaveattemptedtomea-
implementation, we are conducting a variety of studies
sure the flux by a simple passive experiment: an array
of CZT detectors and array technologies at our respec-
(7×6)ofgoldfoils(each∼6cm2)mountedontopofthe
tive institutions. Here we outline briefly the projects
gondola in which the n-γ reaction
underway at CfA; space does not permit description of
thesignificantdevelopmenteffortsunderwayattheother Au-197(n,γ) → Au-198(,e)Hg-198
institutions in the EXIST collaboration. was (attempted to be) measured after the flight by ob-
Balloon Flight Tests of Backgrounds and Shielding Ef- serving the resulting 412 keV decay γ-ray (2.7d halflife)
ficiency: Thelargeneutroncrosssection(s)forCd,which with a low background Ge spectrometer at JPL (by L.
result in prompt gamma-ray decays, may yield high in- Varnell). This experiment, conducted in collaboration
ternal backgrounds for CZT detectors in space. Balloon with G. Skinner and L. Varnell, is currently being ana-
flighttestsofsingleisolatedCZTdetectorsbytheGSFC lyzed.
and Caltech groups in May and September-October, Spatial Uniformity of CZT: Pixellated CZT detector
1995, suggested disturbingly large in-flight backgrounds arrays, as proposed for EXIST, will require relatively
compared to those expected for similar scintillation de- uniform response across both the projected surface area
tectors (e.g. Parsons et al 1996). However the GSFC and depth of the detector elements. Non-uniformities of
measurementof a markedreduction in backgroundwith detector responsecanbe calibratedout(by flatfielding)
an external anti-coincidence shield (NaI) suggested this butwillbesimplifiedtotheextentthedetectorsareuni-
could be effectively reduced by suitable active shielding. form (and may be less of a problem with the relatively
large pixel detectors for EXIST than with small pixel verypromisingandwillbereportedinShahetal(1997).
CZT imagers for focussing optics). We have conducted At CfA, we are now testing this array for its small-pixel
aprogramofmappingthespectralresponseofsinglede- effect properties and we have developed an interface to
tectors and comparing the observed variations with IR a 16-channel ASIC preamp/shaper supplied by the IDE
micrographs (obtained at eV Products) of the detector Corp(Oslo). The CZT arraydetectors (10mm;16chan-
to correlatespectralresponsewith grainboundariesand nels)aremountedonchipcarriersforeasyplug-ininter-
inclusions in the detector. Spectra (Am-241) obtained comparison of the M-S-M vs. P-I-N detectors through
ina 3 ×3 rasterscanofa 0.5mmbeam acrossa4mm × the same ASIC readout system, with results presented
4mm×3mmCZTdetectorshowvariationinspectralre- inaforthcomingpaperbyBloseretal(1997). Aballoon
sponsewhichcorrelateswiththegrainboundariesaswell flighttestofbotharraydetectors,withawell-typeBGO
asinclusionsandprecipitates. Resultswerepresentedat shield, may be conducted in 1998.
the NASA-SEUS Workshop in December 1996.
This work was supported in part by NASA grant
Development of PIN Readouts for CZT Detectors:
NAG8-1212.
CZT detectors are conventionally fabricated with metal
(gold) contacts deposited directly on the CZT crystal.
Thesemetal-semiconductor-metal(M-S-M)detectorsare References
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tigated alternative readouts incorporating P-I-N junc- ration.
tions. The Spire Corp. (Bedford, MA) has developed a Grindlay,J., Prince,T.,Gehrels, N.., Tueller,J., Hailey,
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byusingCdS(p-type)andZnTe(n-type)layersdeposited Grindlay, J., Prince, T., Gehrels, N.., et al. 1997, in
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Development of Thick CZT Detector Array Readouts:
Thick detectors (5mm or greater) as desired for EXIST
pose special challenges for the optimum design of the
detector and readout. In particular, the electric field
configuration needed for the small pixel effect (Barrett
et al 1995) must be carefully considered, and the effects
of charge diffusion and spreading become more impor-
tant. We are exploring these effects in collaboration
with both Spire Corp. and the RMD Corporation (Wa-
tertown, MA), who have just completed fabrication of
a prototype 4 × 4 array M-S-M detector on a 10mm
× 10mm × 5mm CZT substrate. Initial results appear