Laboratorio in banda X V. Fioretti - INAF/IASF Bologna The Simbol-X mission and the background rejection Corso di Strumentazione per l'Astrofisica 21/05/08

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Outline The sensitivity of a focusing X-ray telescope The background problem: The photonic component The hadronic component Active and passive shielding The case of the Simbol-X mission: Background evaluation from balloon experiments The passive shielding design (material, thickness, mass) Soft proton flares detected by XMM/Newton The magnetic proton diverter

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The sensitivity – 1 The sensitivity is the minimum detectable flux A fundamental parameter in the case of far and faint sources (as in the majority of extragalattic X-ray sources) In X-ray Astronomy, the standard flux unity is the Crab: - Power law F(E) = k×E -Γ (k=9.7 ± 1, Γ=2.1 ± 0.03) ph. cm -2 s -1 keV -1 - Integrating in the energy range…. Energy range [keV] 1 μCrab [erg cm -2 s -1 ] 2 – × – × – ×

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The sensitivity – 2 The sensitivity is different for collimated (A) and direct (B) imaging X- ray telescopes Collimated system Focusing system A1A1 A2A2 B1B1 B2B2

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Telescopes with collimated or indirect imaging system BeppoSAX/PDS Field of view = 1.3 ° Sensitivity = 1 mCrab INTEGRAL/IBIS Angular resolution = 12 Field of view = 9° x 9° Sensitivity = 0.5 mCrab

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Telescopes with direct imaging system Chandra Angular resolution = 0.5 Field of view = 30 x 30 Sensitivity = 0.1 µCrab keV) XMM-Newton Angular resolution = 15 Field of view = 30 x 30 Sensitivity = 1 µCrab keV)

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The sensitivity of a focusing X-ray telescope The effective area: The critical reflecting angle: The detection area:

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The background Main components Photonic (induced by CXB photons) Hadronic (caused by hadronic interactions) Prompt emission Delayed emission Solar flares Particles trapped in the Earth magnetosphere Aperture component Shield leakage Scattering Compton Stray-light

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The shielding system Passive shielding: - An high Z material (more efficiency to photoelectric absorption) that, with a specified geometry, shields the focal plane from photons coming from outside the telescope field of view - Materials (high Z, low cost) = Lead, Tungsten, Tantalum - Often a grading is applied (a set of materials with decreasing atomic number until the fluorescence of the last layer falls below the telescope energy range) Active shielding - It is a detector itself, posed around the focal plane - When an hadronic particle interacts with the active shield, i.e. deposits an energy above the operative lower energy limit (500 keV, 1 MeV), it causes a detected event. - All the coincidence events with the telescope detectors are rejected - Material = BGO, NaI, Plastic

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The photonic background Aperture component: The unfocused Cosmic X-ray photons arriving at the focal plane cause background events Important at E < 100 keV (power law of index ~1.4) The CXB is isotropic and it increases for greater aperture angle Solution = passive shield

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The photonic background The shield leakage: Photons that dont interact with the passive shield or deposit an energy below the active shield threshold Scattering Compton: High energy photons (E > 100 keV) can be deflected and arrive at the focal plane Production of couples: Photons with E > 1 MeV interact with the material producing electrons and positrons

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The hadronic component It is mostly caused by incident cosmic rays (~ 90 – 95 % protons) It depends on the type of orbit and mission duration Prompt emission: Secondary particles and gamma photons produced by cosmic rays interactions that arrive to the focal plane. Solution = Active shielding Delayed emission: Cosmic rays can also produce radioactive nuclei, that decay with times longer than active shielding coincidence time. This so calles spallation effects increases with mission duration.

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 The hadronic component The Soft Proton flares: Protons trapped by the Earth magnetic field (E < 500 keV) can be funnelled by the grazing optics of an X-ray telescope and arrive to the focal plane, causing CCD damage and increasing the background level Damage occurred in ACIS instrument of Chandra Rapidly increasing background flares are detected by XMM- Newton Solution = magnetic diverter

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Background detected by balloon experiments Objective: evaluate how the background flux depends on aperture angle Energy range = [ ] keV

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 A case study: Simbol-X Energy range = from ~0.5 keV to ~100 keV Angular resolution: ~ (HPD) - IBIS ->12 Field of view: 10 Sensitivity F min 1 µCrab (~ 20 keV) - A factor of ~ 500 respect to SAX/PDS and INTEGRAL/IBIS

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 A case study: Simbol-X E > 15 keV F min < 1 µCrab Formation flight tecnique

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the background as a critical point The fraction of background increases The formation flight tecnique doesnt allow the use of an optical tube ΔE = keV implies the use of active shielding Total maximum background flux: B = count cm -2 s -1 keV -1

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the passive shielding D opt D det

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the passive shielding D opt D MSC D det

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the shielding composition MaterialZ Density [g/cm 3 ] K α [keV] Lead Tungsten Tantalum MaterialZ Density [g/cm 3 ] K α [keV] Tin Copper Aluminum Carbon

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the mass Collimator mass Skirt mass

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 …..end?

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Acronyms: FL = focal length HPD = half power diameter (angular resolution) Refl = reflectivity B = background CXB = cosmic X-ray background BGO = Cristallo di Germanato di Bismuto NaI = Ioduro di Sodio

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna Corso di Strumentazione per l'Astrofisica 21/05/08 Appendix: FL = focal length HPD = half power diameter (angular resolution) Refl = reflectivity B = background CXB = cosmic X-ray background BGO = Cristallo di Germanato di Bismuto NaI = Ioduro di Sodio