Calibration of the IXPE Instrument

Transcription

1 Calibration of the IXPE Instrument Fabio Muleri (INAF-IAPS) On behalf of the IXPE Italian Team 13th IACHEC Meeting 2018 Avigliano Umbro (Italy), 9-12 April 2018

2 IXPE MISSION IXPE will (re-)open the polarimetric window in X-rays Only one source (Crab Nebula) has a known polarization Sco X-1 has a tight upper limit, but extremely variable Calibration of IXPE will rely primarily on calibration with laboratory sources, both on ground and in flight Astrophysical sources will be observed to verify calibrations from laboratory sources 2

3 IXPE CALIBRATION Instrument calibration will be performed at INAF-IAPS The Gas Pixel Detectors on-board IXPE will provide all photon information Polarization (on statistical basis) Time of arrival Energy Arrival direction All of these quantities will be calibrated We opted for in-house calibration with home-made sources for maximum versatility and control 3

4 Sources of polarized X-rays 4

5 BRAGG DIFFRACTION AT 45 DEGREES 5

6 BRAGG DIFFRACTION AT 45 DEG Polarization depends on the diffraction angle: Diffraction angle and energy are related by Bragg s law E = n hc 2d sin θ = n d kev d: lattice spacing n: order of diffraction Spectrum is a series of equally-spaced monochromatic lines Source can be used to characterized spectral behaviour and spatial resolution data from Henke et al., Atomic Data and Nuclear Data Tables 54,

7 LINES VS CONTINUUM DIFFRACTION Bragg diffraction occurs on a very narrow energy band pass for fixed incident angle (tens of ev) Very ineffective for continuum spectra Source much more intense if input photons have a welldefined energy, tuned with diffraction at nearly 45 degrees Different X-ray tubes and crystals to match fluorescence energy and crystal spacing for ϑ 45 deg Flux increase of a factor 50 Polarization degree precisely known Beam direction and polarization direction completely defined by crystal orientation 7

8 PRACTICAL IMPLEMENTATION TEST SOURCE Crystals On a tip-tilt adjustment stage Commercial X-ray tube (50 W) Capillary plate collimator Constrains incident and/or diffracted photon direction Muleri et al. 2007, 2008 X-rays 8

9 THOMSON/COMPTON SOURCE AT 90 DEG Polarization in Thomson regime: P= 1 cos2 φ 1+cos 2 φ Source is partially polarized, exact value to be calculated by means of simulations Continuum spectrum 9

10 TEST FACILITY FOR GPD DEVELOPMENT Developed over the years for GPD characterization Movements to Align the beam and the GPD Rotate the polarization degree Incline the beam for diagnostic and simulate focusing Diaphragms from 2 mm to 25 μm

11 IXPE INSTRUMENT CALIBRATION EQUIPMENT Same concept as test facility, with a few improvements: More versatile alignment and measurement of X-ray incident direction (arcmin level) Set of test detectors (Si-PIN, Si-SDD, CCD) Helium flowing for the large part of the photon path for reducing air absorption Hosted in a clean room better than class 100,000

12 AVAILABLE ENERGIES WITH KNOWN POLARIZATION High polarization degree, typically >95%, calculated with Henke et al. table Crystal surface aligned with crystal lattice better than 0.1 deg (3 arcmin measured) 12

13 UNPOLARIZED SOURCES Radioactive sources 55 Fe, residual polarization <<0.5% Head-on X-ray tube 3.7 kev, 8.0 kev Residual polarization < 0.5% 13

14 DU ON-GROUND CALIBRATION PLAN Automatic sequence of measurements, space out with source replacement, test and alignment Map at several energies of (as a minimum) modulation factor energy resolution spatial resolution quantum efficiency gain disuniformities Duration: 45 days for each DU (x3 Flight Models + x1 Flight Spare) Many monochromatic sources will be used to track the energy-dependent response of the GPD Continuum radiation used to check data analysis

15 TIME MANAGEMENT AND CALIBRATION Two GPS antennas will be present on-board IXPE spacecraft to distribute a Pulse-Per-Second signal to the Instrument A ultra-stable Temperature-Compensated Oscillator with 4 ppm stability in the DSU will generate the PPS when (occasionally) the GPS is unlocked Local time counters will be present in the DUs and synchronized every second with the master clock in the DSU Differences will be stored for correction on-ground Time of arrival of photons will be tagged in the DU when a trigger is detected (within a few μs from absorption) Instrument timing with respect to the PPS will be checked during Instrument IAPS The Instrument Ground Support Equipment will be able to generate a time-tagged signal with an adjustable delay with respect to the GSE-PPS to check how events are time-tagged 15

16 Instrument on-orbit calibration 16

17 FILTER & CALIBRATION WHEEL Each DU includes a Filter & Calibration Wheel (FCW) Filters, for specific observations Open position (normal observation mode) Closed position (background measurement) Gray filter (very bright sources, >2 Crab) x4 calibration sources based on radioactive nuclides ( 55 Fe) 17

18 CAL A POLARIZED Powered by a single 55 Fe source emitting at 5.9 and 6.5 kev A thin silver foil is used to produce Ag fluorescence at 2.9 kev 2.9 and 5.9 kev incident at 38 deg on graphite crystal are diffracted at first and second order Polarization is 67% Counting rate ( 55 Fe 100 mci) kev kev Pre-collimator Graphite crystal GPD Silver foil 55 Fe 5.9 kev 2.9 kev 18

19 CAL B - UNPOLARIZED Unpolarized 55 Fe source, collimated to a spot Check the absence of spurious signal to a very low level Spot representative of the image of a point-like source over an IXPE observation Spot size of a few mm Counting rate >60 c/s with 20 mci on a spot of 3 mm diameter measured

20 CAL C GAIN MAP Unpolarized 55 Fe source Full illumination of the GPD to map the gain Counting rate >100 cts/s with 0.4 mci

21 CAL D GAIN MAP Uncollimated, unpolarized fluorescence source Full illumination of the GPD to map gain Target baseline is CaF 2 and illuminating source is 55 Fe cover at best DU energy range maximize counting rate Counting rate about 40 cts/s with 10 mci source

22 55 Fe nuclides decay with half-time 2.7 years Flight nuclides will be mounted as late as possible in the integration flow Design allows to replace sources after DU integration through a dedicated cover Current baseline is to replace them during Instrument integration on the spacecraft Fainter sources will be used to functionally test the units during on-ground testing RADIOACTIVE SOURCES REPLACEMENT

23 CONCLUSIONS IXPE Instrument on-ground calibration will be performed at INAF-IAPS Dry run with GPD EM from September 2018 First FM arrives in December 2018 Set-up is based on the heritage of GPD characterization over more than 10 years All information provided by the GPD (photon arrival time, energy, direction and polarization) will be calibrated In-flight calibration will be possible thanks to 4 calibration sources 2 energies for measuring the response to polarization Check for spurious signals Map of the gain at two energies 23