Pileup and the ACIS/HRMA Point Spread Function

Pileup and the ACIS/HRMA Point Spread FunctionT. J. Gaetz, D. Jerius, D. PeaseChandra X-ray Center (SAO)2005 October 31PSF PileupACIS Operation (TE mode)• ACIS is shutterless CCD detector.◦ frame exposed for τframe(typically ∼3 s);X-ray photons ⇒ e−charge clouds, #e−∝ photon energy◦ frame xfr: transfer collected charge rapidly to frame store;parallel transfer rate ≡ 40µs per parallel transfer.◦ each pixel of column sees short exposure of all emissionalong the column ⇒ transfer streak or smear.◦ transfer  dither; “instantaneous smeared snapshot”2005 Chandra Calibration Workshop2005 Oct 31 1/20PSF PileupACIS and Pileup• Overlapping charge clouds in same 3 × 3 pixel detect island ⇒ pileup◦ spectral distortion: piled event energy is too large◦ grade distortion:◦ altered charge distribution in 3 × 3 island⇒ “grade morphing”◦ count rate depression:◦ good grades ⇒ worse grades ⇒ bad grades(e.g., g0 ⇒ g6; g6 ⇒ g7 or worse)◦ some bad grades rejected on board.◦ spatial shift of detected event:◦ pixel with maximum charge shifted;pix with largest charge ⇒ center of detect island◦ worst for center of near on-axis point source PSFnarrow PSF core ⇒ high count rate for single pixel2005 Chandra Calibration Workshop2005 Oct 31 2/20PSF PileupGround Data• Pileup effect on PSF: depression of central part of the PSF.• PSF “peakier” on orbit ⇒ larger effect2005 Chandra Calibration Workshop2005 Oct 31 3/20PSF PileupPileup ObservationsExperiment design• source: isolated pulsar PSR J0437-4715;count rate ∼0.2 ct s−1• use different frame times to obtain different pileup rates.◦ Initial choices:τframe= 0.4s, 1.5s, 3s, 6s, 10s (∼0.08–2 ct/frame)◦ Observed so far:τframe= 0.4s, 1.5s, 3s2005 Chandra Calibration Workshop2005 Oct 31 4/20PSF PileupPileup Observations: DitherAim: make data as similar to real data with different count rate.• dither: max velocity is 0.3 pix/sec◦ Different frame times ⇒ max travel/frame varies⇒ shorter τframe, too “slow”; longer τframe, “too fast”.◦ 6s, 10s frame time:dither travel comparable to pixel size⇒ aspect reconstruction degradation• solution: scale dither periods inversely with τframe/3 sec.◦ limitation: operational limitation on dither period; shortestobservation not quite scaled to τframe• shorter frame times ⇒ more frame transfers◦ larger deadtime: ∝ 1/τframe; ∼10% for τframe= 0.4s.◦ more transfer streak events; ∼10% for τframe= 0.4s2005 Chandra Calibration Workshop2005 Oct 31 5/20PSF PileupThe ObservationsObsID τframeOntime Exposure # Frames 1− DTC(s) ( ks) ( ks)6154 0.4 25.188 22.438 57110 0.093056157†0.4 9.450 8.570 21426 0.093056156 1.5 21.179 20.615 13743 0.026636155 3.0 24.628 24.295 8098 0.01350†∼40off-axis2005 Chandra Calibration Workshop2005 Oct 31 6/20PSF PileupPileup: PSF Profile2005 Chandra Calibration Workshop2005 Oct 31 7/20PSF PileupPileup: PSF Profile2005 Chandra Calibration Workshop2005 Oct 31 8/20PSF PileupDiagnosing PileupIs τframe= 0.4 s data effectively unpiled?Tools:• grade fractions: look for grade morphing effects, but...what is zero-pileup limit?◦ ground: subassembly data◦ on-orbit: faint sky sources; external calibration source• spectral fitting: look for spectral distortion◦ simultaneous fit to all three obsids using pileup model:pileup(phabs(bbodyrad + bbodyrad + powerlaw))2005 Chandra Calibration Workshop2005 Oct 31 9/20PSF PileupGrade Fractions [gn/g02346], Subassembly Data2005 Chandra Calibration Workshop2005 Oct 31 10/20PSF PileupGrade Fractions [gn/g02346], Ext Cal Source2005 Chandra Calibration Workshop2005 Oct 31 11/20PSF PileupGrade Fractions [gn/g02346] vs. Frame Time2005 Chandra Calibration Workshop2005 Oct 31 12/20PSF Pileup[gn/g02346] vs. Frame Time; 0.4s obs off-axis2005 Chandra Calibration Workshop2005 Oct 31 13/20PSF PileupGrade Fractions [gn/g02346] vs. Grade2005 Chandra Calibration Workshop2005 Oct 31 14/20PSF PileupPSF Pileup Fitting• simultaneous fit to all three obsids using pileup model:pileup(phabs(bbodyrad + bbodyrad + powerlaw))• circular extraction radii: 200, 500, 10002005 Chandra Calibration Workshop2005 Oct 31 15/20PSF PileupPileup Model Spectral Fits: χ2/dof = 0.982005 Chandra Calibration Workshop2005 Oct 31 16/20PSF PileupObserved Count Rates2005 Chandra Calibration Workshop2005 Oct 31 17/20PSF PileupPileup-Corrected Rates2005 Chandra Calibration Workshop2005 Oct 31 18/20PSF PileupPileup Fraction2005 Chandra Calibration Workshop2005 Oct 31 19/20PSF PileupSummary• analyzed initial set of pileup observations◦ frame times 0.4s, 1.5s, 3.0s• how unpiled is the shortest frame time data?◦ grade ratios ⇒ still significant; off-axis data closer tosubassembly values◦ spectral fitting with pileup model ⇒ ∼5% pileup• depression of PSF core examined for pileup ∼5%–35%2005 Chandra Calibration Workshop2005 Oct 31