IXO Instrument Working Group Meeting January 27, 2009 — Cambridge, MA Count-Rate Performance for X-Ray Microcalorimeter Spectrometer Richard Kelley NASA/GSFC

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA2 Counting Rate Issues Most of the IXO science will be from what we think of as faint sources. But note that a 5 mCrab source corresponds to about 625 cps on the array! Probably higher given latest effective area calculations. Further, we are still promoting IXO, so we need to be as inclusive as possible to the user community. X-ray binary sources as very strong (typically Crab), but sometime several times higher, including x-ray busters. 1 Crab ~ 125,000 cps. So, we need to consider counting rates much higher than what we normally like to deal with for x-ray calorimeters

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA3 Dealing with Pulse Pile-up: Event Grades (developed for XRS) Grade events by relative arrival times: Apply different pulse height algorithms according to grade. Δt > T2 - “High-Res” Full template T1 < Δt < T2 – “Mid-Res” Truncated template Δt < T1, - “Low-Res” Simple PHA (e.g., low- pass filter and single sample)

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA4 Energy Resolution vs. Analysis Record Length We generally ignore the DC, or “zero”, frequency bin for the optimal filter., where f 0 = 1/T and T = record length Device model: α = 80 τ = C/G = sec ΔE 0 = 2.5 eV τ crit = 1.5 x sec Hi-resMid-res

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA5 Single-Pixel Hi-res Fraction vs. Rate

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA6 Point Spread Function on array – spreading out the counts RingFraction per pixel a32.38% b46.60%5.82% c17.34%1.08% d3.33%0.14% Gaussian PSF with HPD of 5 arcsec 20 m f/l  10.3 arcsec/mm 0.3 mm pixels  3 arcsec pixels a b c d 5” HPD

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA7 Output rate vs. Input rate

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA8 Event Grade Fractions vs. Input Rate

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA9 Bright Source Diffuser – Slumped Micro Channel Plate (MCP) R. Willingale 2008 Inner array Outer array

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA10 Performance of Diffuser ~ 1180 pixels in “donut”

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA11 Output Rates vs. Input Rate with Diffuser 38% transmission of diffuser 40,000 cps

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA12 Comparison of Hi-Res Event Rate with and without Diffuser with BSDO without alone

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA13 Summary & Conclusions Use digitized data for optimal filtering based on pulse inter-arrival times (“event grades”). Minimizes dead-time overall – all events are counted (up to rates higher than the Crab) Need to develop algorithm for low-res events (e.g., low-pass and sample peak, or weighted average near peak, etc.) For bright, Galactic sources, we should have a filter position with a MCP diffuser. Can observe sources with flux comparable to Crab with 10% of the events in Hi-Res + Mid-Res. Cross-over point is about 2000 cps input. Invest some effort to optimize the performance of the diffuser.