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, MA 2

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 0.1-1 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, MA 3

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, MA 4

Energy Resolution vs. Analysis Record Length

We generally ignore the DC, or “zero”, frequency bin for the optimal filter.

€

1

ΔE 2=

4df

NEP 2( f )f0

∞

∫ , where f0 = 1/T and T = record length

Device model:

α = 80

τ = C/G = 0.006 sec

ΔE0 = 2.5 eV

τcrit = 1.5 x 10-4 sec

Hi-resMid-res

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 5

Single-Pixel Hi-res Fraction vs. Rate

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 6

Point Spread Function on array – spreading out the counts

Ring FractionFractionper pixel

a 32.38% 32.38%

b 46.60% 5.82%

c 17.34% 1.08%

d 3.33% 0.14%

Gaussian PSF with HPD of 5 arcsec

20 m f/l 10.3 arcsec/mm

0.3 mm pixels 3 arcsec pixels

ab

cd

5” HPD

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 7

Output rate vs. Input rate

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 8

Event Grade Fractions vs. Input Rate

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 9

Bright Source Diffuser – Slumped Micro Channel Plate (MCP)

R. Willingale 2008

Inner array

Outer array

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 10

Performance of Diffuser

~ 1180 pixels in “donut”

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 11

Output Rates vs. Input Rate with Diffuser

38% transmissionof diffuser

40,000 cps

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 12

Comparison of Hi-Res Event Rate with and without Diffuser

with BSDO

without alone

R. Kelley - IXO IWG Meeting, January 27, 2009, Cambridge, MA 13

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.