The Reuven Ramaty High Energy Solar Spectroscopic Imager

(RHESSI) was designed to measure hard X-rays and γ-rays from solar flares. With its big detection area and thin side shielding it also proved to be well suited for studying

Gamma Ray Bursts (GRB). Polarization analysis is feasible as well, due to a big modulation factor (MF), though serious constraints on the minimum detectable polarization

(MDP) come from detection efficiency of double scattered photons. More constraints are given by background of accidental and real coincidences

Hajdas, W.

Wigger, C.

Zehnder, A.

English

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DOI 10.1393/ncc/i2005-10161-xIL NUOVO CIMENTO Vol. 28 C, N. 4-5 Luglio-Ottobre 2005RHESSI as Gamma Ray Burst Polarimeter(∗)W. Hajdas, C. Wigger and A. ZehnderPaul Scherrer Institut, CH-5232 PSI Villigen, Switzerland(ricevuto il 23 Maggio 2005; pubblicato online il 20 Ottobre 2005)Summary. — The Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) was designed to measure hard X-rays and γ-rays from solar flares. Withits big detection area and thin side shielding it also proved to be well suited for study-ing Gamma Ray Bursts (GRB). Polarization analysis is feasible as well, due to a bigmodulation factor (MF), though serious constraints on the minimum detectable po-larization (MDP) come from detection efficiency of double scattered photons. Moreconstraints are given by background of accidental and real coincidences.PACS 95.55.Ka – X- and γ-ray telescopes and instrumentation.PACS 95.75.Hi – Astronomical polarimetry.PACS 95.85.Pw – Gamma-ray astronomical observations.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.1. – IntroductionData analysis of astronomical objects typically uses spectral characteristics and timevariability in order to derive physical properties of the source. It often results in am-biguous explanations by several “equally good”, but different models. The measurementof photon polarization may give us an additional tool to pick the best one. It also en-ables us to study various astrophysical phenomena and permits us to distinguish betweenphotons coming from synchrotron radiation, Compton scattering or bremsstrahlung. Todate, the only positive and conclusive results from various γ-ray polarization measure-ments in space are for the Crab nebula. It is largely due to technical difficulties withγ-ray polarimetric measurements. Nonetheless, in the past few years several novel astro-nomical γ-ray polarimeters were proposed, aiming to provide first successful polarizationdata. Here we report results of our studies of the polarization detection potential withthe already flying RHESSI satellite [1] and its germanium spectrometer. The mission isequipped with a passive beryllium scatterer for measurements of the solar photon po-larization [2]. In this paper we concentrate on polarization studies for non-solar sources(like GRBs) using a double scattering, coincident mode in which one detector acts as anactive scatterer and the other one as an analyzer.(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 817818 W. HAJDAS, C. WIGGER and A. ZEHNDERFig. 1. – Modulation factor as a function of the initial photon energy (left) and the photonincoming angle (right).2. – RHESSI and its spectrometerRHESSI was launched into a Low Earth Orbit on 5th February 2002. It is a NASASMEX mission devoted to observation of solar explosive phenomena and providing γ-raydynamic images of solar events. The satellite permanently points toward the sun androtates with a period of about 4 s. Its mass is equal to 238 kg including 120 kg of scientificpayload. There is only one instrument on board consisting of the imager- telescope systemand Ge-spectrometer [3] detecting photons with energies from 3 keV up to 20MeV. Thespectrometer has nine super-pure Ge-detectors (φ = 7.1 cm, h = 8.5 cm) working inthe event-by-event mode. Photon arrival time, deposited energy and a number for thedetector segment hit define each detected event. The energy and time resolution (∆E ≈3 keV, ∆t ≈ 1µs) allow for time-resolved spectroscopy. Polarization measurements arealso possible using the photon event list and a coincidence mode between two nearbydetectors. Due to a thin side shielding γ-rays from all directions are detectable. Theeffective solid angle for observations of the GRBs is half the sky and the energy rangefor their detection is between ca. 100 keV and a few MeV. Usually, more than 5 GRBsper month are observed.3. – Response and analyzing power modellingResponse and analyzing power calculations were performed using the complete satel-lite mass model based on the GEANT3 code from CERN. The Ge-spectrometer and itsnearest surroundings were described with a high level of accuracy while for more distantparts some approximations were used [4]. Polarization sensitive processes (Compton)and corresponding tracing routines were implemented using the GEANT4 approach. Itallowed us to generate detector responses for photons with predefined energies, incomingangles and polarization vector directions. Information about the incoming photon polar-ization could be extracted using coincident events between two detectors. It was shownthat for typical spectral distributions of the GRB, the relative number of such events(multiplicity = 2) is only about 1–2%. This is a direct consequence of two factors: lowmean free path values for γ-rays of a few hundred keV energy in Germanium and smallvalues of the solid angle for photon scattering from one Ge-detector to another. Thus,RHESSI AS GAMMA RAY BURST POLARIMETER 819Fig. 2. – Minimum detectable polarization (2σ) as a function of the number of detected photons(left) and burst duration (right). (f(E) = c · E−2, E ∈ (25− 2000) keV, background included).despite the dominating role of the Compton cross section for energies above 150 keV, avery strong event is still required for a statistically significant result. In the next step weextracted the RHESSI MF by utilizing the satellite rotation and comparing the relativenumber of coincidences between perpendicular detector pairs.The low energy threshold imposed on the single detector for the polarization analysiswith RHESSI is usually around 25–30 keV. It corresponds to an energy threshold ofthe incoming photon of around 150 keV. These criteria were applied to compute theRHESSI MF and its energy dependence. As one can see in fig. 1, the MF is highest atlow energies. Thus, the best burst candidates for RHESSI polarization analysis shouldhave most counts at lower energies and be very strong. Typical MF values for GRBscoming close to the satellite rotation axis are around 0.2.Calculations point out that the modulation curve is well preserved for polarized burstscoming from within about 25◦ of the RHESSI rotation axis (front and rear). In this regionthe MF changes less than 30%. At other angles the curve flattens and the analyzingpower rapidly decreases. The MF dependence on the photon incoming angle is shown infig. 1. One expects to detect about 10 GRBs/year with directions close to the satelliterotation axis and thus suited for the polarization analysis [5].4. – Background sourcesThree different sources of background come into consideration while extracting theRHESSI GRB modulation curve. The real coincidences background is exhibited as truecoincident events between detectors, and its sources are diffuse background photons andγ-rays generated by cosmic-ray–induced nuclear reactions. After applying cuts on thephoton energies and setting the coincidence time width between detectors (usually 1µs),its rate is relatively constant and amounts to about 120 events/s. It can be dominant forvery long but less intense bursts. The second background source comes from accidentalcoincidences. It is directly related to the GRB intensity, being a product of two singledetector rates multiplied by their coincidence time width. Such a quadratic function ofthe rate makes it a dominant background source for very strong GRB events. Althoughthe accidental background can be measured accurately, it effectively increases the MDPvalue. The last source of background depends on the event geometry with respect to the820 W. HAJDAS, C. WIGGER and A. ZEHNDEREarth and the spacecraft itself. It is caused by photons initially re-scattered by the Earthatmosphere. As such photons may change their polarization direction, this backgroundsource decreases the observed value of the burst polarization. At the same time it alsodecreases the instrument response matrix and its MF value. It can be partially takeninto account by proper modelling though the uncertainties are rather hard to control.5. – Minimum detectable polarizationThe MDP value is defined in terms of the source and background rates (R), theRHESSI MF, and the whole event duration T—see equation below.MDP =nσMF ·Rcoi(Rsng) ·√2(Rcoi(Rsng) +Raccid(Rsng) +Rrealbckg)T.(1)The multiplicative factor nσ indicates the significance level. The background is a sumof real (Rrealbckg) (e.g., from photons generated by Cosmic Rays) and accidental (Raccid)coincidences. Both the coincidence signal rate Rcoi and the accidental background rateRaccid are related with the single detector rate Rsng. Combining them allows us togenerate the MDP curves as a function of the number of detected photons or the burstduration—see fig. 2. The curves were obtained for a coincidence time width of 1.5µs and aratio between coincident and single events equal to 0.015 . The results are in accordancewith the GRB polarization analysis from [6] and indicate rather limited polarimetricpotential of the instrument even for the strongest bursts (Nsng ≈ 105 events).6. – SummaryThe RHESSI spectrometer can be characterised by a large detection area and highenergy and time resolution. Nonetheless, the effective area for polarimetric detection ofdouble coincidences is much smaller. The average value of its MF is about 0.2 for eventscoming close to the axis. One can expect about 10 such events per year. However, aftertaking into account both real and accidental background, even for the strongest GRBs,the MDP values are high (∼ 50%). This seriously limits the RHESSI usefulness as a GRBpolarimeter. A novel device should be characterised by many smaller detectors but withlarger total effective area for both single and double event detection. The coincidencewidth should be much lower than 1µs and the lower energy threshold set at few tens ofkeV favouring higher values of the instrument MF.REFERENCES[1] Lin R.P. et al., Solar Physics, 210 (2002) 3.[2] McConnell M. et al., Solar Physics, 210 (2002) 202.[3] Smith D. et al., Solar Physics , 201 (2002) 33.[4] Hajdas W. et al., PSI Annual Report 2001.[5] See grb.web.psi.ch .[6] Wigger C. et al., ApJ, 613 (2004) 1088 , see also Wigger C. et al., these proceedings(2005).

RHESSI as Gamma Ray Burst Polarimeter

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RHESSI as Gamma Ray Burst Polarimeter

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