Swift Calibration status On ground & in-flight Experience

Olivier Godet, Andy Beardmore, Giancarlo Cusumanoon behalf the XRT calibration team

Outlines

• Introduction - The Swift mission - The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

• Introduction - The Swift mission

The Swift mission(Gehrels et al., 2004, ApJ, 611, 1005)

Localisation of short GRBs - Study of the early GRB afterglow

BAT

XRT

Spacecraft

UVOT

BAT

UVOT

XRT

(0.2-10 keV)

(14-150 keV)BAT trigger (3 arc-minutes, 90%)

Swift automatically slews

NFIs observed after a few 100s (XRT <3 arc-seconds, 90%)

(UVOT < 0.5 arc-seconds, 90%)

Outlines

• Introduction- The Swift mission - The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

The X-ray Telescope (XRT)

JET-X mirrors

Focal plane

(Burrows et al., 2005, Space Sci. Rev., 120, 165)

EEV CCD22: same as for the MOS cameras600 x 600 pixels – 2.36’’ per pixel

Effective area 135 cm2 1.5 keV20 cm2 8.1 keV

Spec. Res. (at launch): 140 eV at 5.9 keV

FOV: 23.6 arcmin x 23.6 arcmin

XRT band: 0.2-10 keV

The X-ray Telescope (XRT)4 observation modes (3 spectral modes)

(Hill et al., 2004, SPIE, 5165, 217)

Increasing count rate

Photon Counting(2-D info)

Windowed Timing(1-D info)

Low-rate Photo-Diode(no spatial info)

Readout time2.5 s

Readout time1.8 ms

Readout time0.14 ms

10 ro

ws

200 central pixels

Good events: grades 0-2 Good events: grades 0-5Pattern recognition same as the MOS cameras

Good events: grades 0-12

Outlines

• Introduction- The Swift mission - The X-ray Telescope (XRT)- Problems in orbit

- Loss of the thermo-electronic cooler- Micro-meteorite event- Bright Earth

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

- Loss of the thermo-electronic cooler- Micro-meteoroid event- Bright Earth

Loss of the Thermo-electronic Cooler

Swift LEO CCD not cooled by passive way like XMM-Newton

TEC to regulate actively the CCD temperature (at -100ºC)XRT

ThermalTests

UVOTActivation

Beginning XRTThermal Control

TEC Power

HeatingCCD

Without control, XRT regularly heats above -50ºC

Passive control of the temperature

The CCD temperature correlated with the Earth elevation angle. Take the dependency of the roll angle allows to optimize the predictions + Avoid to the Sun.

Condition: The XRT CCD temperature needs to stay below -50ºC to ensure good data.

(J. Kennea)

Predictions at +/-2ºC (still large variations depending on the weather ?)

How good is our prediction model?

Outlines

• Introduction- The Swift mission - The X-ray Telescope (XRT)- Problems in orbit

- Loss of the thermo-electronic cooler- Micro-meteoroid event- Bright Earth

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

Micro-meteorite event on 27th May 2005• Apparition of several bad columns in the central part of the CCD

Raw image

Micro-meteorite event on 27th May 2005

Loss of the LrPD mode

Mask out on-board of the main bad columns (PC, WT)Mask out on-ground of the adjacent bad columns ….

Outlines

• Introduction- The Swift mission - The X-ray Telescope (XRT)- Problems in orbit

- Loss of the thermo-electronic cooler- Micro-meteoroid event- Bright Earth

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

Bright Earth

Symptoms:• High background rates near Earth limb• Typically affects beginning and end of snapshots• Typically very soft

Start of snapshot Scattered Light approaching Earth Limb End of snapshot

EARTH ANGLE=160 EARTH ANGLE=125 EARTH ANGLE=104

T

Central blob

(C. Pagani)

Bright Earth

Mirrors

TAM

Inner thermal baffle

Outer thermal baffle

Telescope tube

Star tracker

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

• The spectral response computation

Spectral response

Spectral response = ARF * RMF (QE)Spectral response = ARF * RMF (QE)

ARF generates using a Mirror Area ray-tracing code ARF = Mirror response + Filter responseCorrection for the vignetting applied to the ARF

C. Cusumano, ISAF INAF

ARF is tweaked on several calibratorsfor each XRT mode

Spectral response(O. Godet, A. Beardmore, UL)

RMF generates for each mode using a Monte-Carlo code(RMF computes for the central part of the CCD i.e. 200x200 pixels)

Input parameters

Core

OutputsNum

ber o

f eve

nts

to g

ener

ate

- Geometry (depletion layer, …)- Serial and parallel CTE- CCD temperature, EN- Photon energy + number of events- Readout mode (PC, WT and PD)

- Spectrum at a given energy - Response Matrix, QE

Photon interactions in depletion, field-free and substrate regionsSiKα,β fluorescenceCharge loss between Si02 and SiCharge spreadingCharge mappingReadout noiseEvent recognition

1 pixel of theCCD-22

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors

• On-ground & In-flight calibration strategy

On-ground calibration

The XRT mirrors were calibrated at Panter (Germany) in 1996 for the JET-X calibration plan and again in 2000 during the XRT end-to-end calibration.

Quantum efficiency measurements of the EEV CCD22 detectors have been made at Leicester University and at the Orsay Synchrotron(from C(0.27 keV) to Cu(8.04 keV)).

On-ground calibration

On-ground calibration

Measured spectrum of the Si Kα and β lines + Bremsstrahlung continuum + OKα background fit by the pre-flight RMF

(K. Murkajee, A. Beardmore, UL)

Quantum Efficiency

27 µm depletion region35 µm depletion region

E (keV)

QE

PC Mode grade 0

Lab measurements of the QE in the PC mode at -80ºC(A. Abbey et al., UL)

(O. Godet, UL)

In-flight calibration sourcesCalibration phase: from Dec. 2004 to April 2005

WT

H1426+428 PC Effective area

Door source Fe55 line shoulder

Inter-calibration with other X-ray instruments (RXTE, XMM)

*

*

*

*

*

*

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status - Gain- PC & WT Low-energy response- Line shoulder- High energy redistribution- Si, O, Al edges- Status of the ARF

• Current status

Gain

The XRT gain as a function of temperature measured in PC mode using emission lines of Si, S, Ar, Ca and Fe from the supernova remnant Cas A.

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status - Gain- Line shoulder- PC & WT low-energy response- High energy redistribution- Si, O, Al edges- Status of the ARF

• Current status

Line shoulder(O. Godet, A. Beardmore)

Fits using the pre-flight RMF (v006)

PC Mode grades 0-12

Spectrum of the door sources

WT Mode grade 0

Spectrum of the door sources

Line shoulderA possible solution: increase artificially the threshold

PC Mode grades 0-12WT Mode grade 0

Spectrum of the door sources

WT and PC RMFs released (v007)

Issues in the RMFs v007Steps in the Quantum Efficiency plot (variations < 5%)

No noticeable consequences to fit the data (GRBs or transients)

Charge cloud RMF

Change of the 2-D Gaussian shape of the charge cloud in the field-free region of the CCD.

Grade 4

threshold

2-D Gaussian

threshold

New 2-D shapewith extended wings

Grade 22

Increasing of the sub-threshold losses

Charge cloud RMF

RMF v007

RMF v009 in development

PC RMF new charge cloud shape

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status - Gain- Line shoulder- PC & WT low-energy response- High energy redistribution- Si, O, Al edges- Status of the ARF

• Current status

Low energy response in the PC mode

RMF v007

Response at low energy

Introduction of a loss function given in Popp et al. (2000) at low energy (E < 2 keV) (already used by EPIC-pn)

=)(zFc

lzBS

+

0

τ/)(1 lzAe −−−

Dz ≥0<z

lz ≤≤0

Dzl <≤

or

where L, S, B, c and τ are free parametersD=280 µm

Response at low energy

RXJ 1856 in PC mode grade 0

Spectral parameters

NH = 4(+6/-3) 1019 cm-2

kT = 62 (+4/-3) eVerg cm-2 s-1

Chandra measurements

NH = 9.5±0.3 1019 cm-2

kT = 63.5±0.2 eVF = 0.99 10-11erg cm-2 s-1

1126.019.0 1081.0 −+

− ×=F

PC RMFs with improvement of the low-energy response released(v008)

Model: wabs*bb

Flux given in 0.2-10 keV band

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status - Gain- Line shoulder- PC & WT low-energy response- High energy redistribution- Si, O, Al edges- Status of the ARF

• Current status

Stratford - XRT Team Meeting – May 16-17th 2006

RMF v008

nH = 5.6 x 1021 cm-2

High energy shelf (WT)

High energy shelf (WT)

Rescale the shelf Need to use several absorbed sources

High energy shelf (WT)

WT g0-2

Model: wabs * (pow+bb)

WT g0

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status - Gain- Line shoulder- PC & WT low-energy response- High energy redistribution- Si, O, Al edges- Status of the ARF

• Current status

Model: vphabs*pow+ Al edge (1.5596 keV) Abund loddersAbund 0, Ne left freeAb(Fe) = 0.456 (fixed RGSvalue) + gain in XSPEC

WT grade 0 RMF v007

Crab

Dip residuals seen in the fits around Si edge. Depend on brightnessof the source.

Si edge

O edge?

WT grade 0-2 RMF v008

Possible energy scale offset?Contamination?RMF problem?

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status - Gain- Line shoulder- PC & WT low-energy response- High energy redistribution- Si, O, Al edges- Status of the ARF

• Current status

Status of the ARF

NH = 0.35 ± 0.04

Γ = 2.10 ± 0.01

Pre-flight ARF v006RMF v007

Status of the ARF

V008: New ARF with a new spider design

v008

v007

Empirical patch for the Si edge

v008

v007

(G. Cusumano, A. Beardmore, O. Godet)

Performance of the WT ARF

Performance of the PC ARF

Systematic uncertainty (0.3-10 keV) ~ 5%Systematic in the absolute flux ~ 10%

Outlines

• Introduction - The mission Swift- The X-ray Telescope (XRT)- Problems in orbit

• The spectral response computation

• On-ground & In-flight calibration strategy

• Current status

• Limiting factors• Limiting factors- Bright Earth- Bias subtraction problems

Energy offsets in PC and WT

PC energy scale offset

PC energy scale offset(A. Beardmore, UL)

Pre-dip dip

Bright Earth or Glint

PC energy scale offset

PC energy scale offset

12 3 45 67 8 9

Corner pixel mean (Abbey, Tyler, Osborne, UL)grade 0 event

Mainly due to Bright Earth and thermally induced charge

Correlation with Bright Earth angle (and temperature)

Grade 0

PC energy scale offset

Dip Pre-dip

Dip

Pre-dip

offsets: -10eV-20eV

Summary •Even if not nominally operates, the instrument is globally well understood and calibrated (astrometry, centroid, psf, boresight, …).

•Still some issues about the spectral response (energy scale offset, difference of the effective area between PC and WT mode, edges)

•Start the planning of long term calibration (mainly the evolution of the low energy response. So far, no change noted)

•Now regular monitoring of the CTE and regular update of the gain file, RMFs

How the CCD will evolve with time and proton damage?

WT energy scale offset

•WT bias row taken during slew. Subtracted on-board.• Last 20 pixels telemetred with each WT pseudo-frame

Example bias row + 10 last 20 pixels

WT energy scale offset

Possible contamination by Bright Earth

WT energy scale offset

30-40eV shift

With bias correction• Computes last 20 pix median value • Apply a column-by-column correction

Comparison of the PC/WT energy scale

Comparison of the PC/WT energy scale

PC/WT ARF differences (A. Beardmore, C. Cusumano, O. Godet)

WT arf is 15-20 % higher than PC at ~ 1 keV, 5-10 % higher at ~5 keVsimilar area at higher energies

G. Cusumano

PC/WT ARF differences

20%

10%

Bad columns & hot pixels

RXJ 0720

Effect of the bad columns

The Burst Alert Telescope (BAT)(Barthelmy et al., 2005, Space Sci. Rev., 120, 143)

5 times more sensitive than BATSE

The UV/Optical Telescope (UVOT)(Roming et al., 2005, Space Sci. Rev., 120, 95)

UVOT “copy” of the Optical Monitor onboard XMM-Newton

UVOT covers the range 170-650 nm

FOV = 17’ x 17’

Photons register on a microchannelplate intensified CCD (MIC)

M101

Micro-meteorite event on 25th May 2005

Conclusion: The rapid evolution in the brightness of the main hot column seen late 2005 has stopped.

Difference in Raw frame, 2005 days 277 and day 355

Difference in Raw frame, 2005 day 358 and 2006 day 124

XDS XDS