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John Nousek & David Burrows (Penn State University)Alberto Moretti (Osservatorio Astronomico di Brera)

X-Ray Telescope (XRT):

Performance after Five Years

International Workshop on Astronomical X-ray Optics - Prague, Czech Rep. – 6-9 Dec. 2009

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Swift launch:

20 Nov 2004 !!

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Observatory Science Up-time: 97.5%

Ground Station Status: Nominal Malindi 18454 passes since Launch, 98.8% successful USN 1532 passes since Launch, 94.3% successful TDRSS currently providing 99.5% success rate

Observatory Status: Nominal ACS: executed 152811 slews, >99% within 3’ accuracy

Observatory Lifetime: Above prediction

Flight Operations Team Response: Excellent After hours response once every four days

Science Operations Team Response: Excellent Respond to 1.6 ToO requests per day, conduct ~4-8 ToO obs. per day

Swift Observatory Status

Swift continues to run smoothly after nearly five years!

Statistics from Rob Laverghetta, FOT – as of 31 Oct 2009

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BAT

XRT

Spacecraft

UVOT

BAT

UVOT

XRT

The XRT instrument

The Swift X-ray Telescope :

• 0.2-10.0 keV• FOV: 23.6’ x 23.6’

• Energy resolution 140 eV @ 5.9 keV• HPD: 17” @ 1.5 keV

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BAT Burst Image

T~30 sec

BAT Error Circle

XRT Image

T~70 sec

• The typical XRT observation starts ~100s from the burst and lasts 15 days , with ~ 10 ks effective exposure each day

•1 orbit is 5800 s ; ~ 4 targets per orbit for each source: ~1500s of observation and ~ 4000s of occultation

XRT Operation

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From calibration observations (2005, MKN786, RXJ0720.4) we

found thatthe PSF is well-fit by a King

function: ))(1()( 2

cr

rrPSF

Testing the PSF (I)

2 free parameters that linearlydepend on energy and off-axis angle

EdEcbaE

EdEcbaErc

2222

1111

),(

),(

The PSF is entirely described by 8 numbersTypical values of the parameters are

rc:5.5” ; β: ~1.5, with very weak dependence on energy and off- axis angle.

[Feb 2005]

Moretti et al. 2005, SPIE 5898, 325

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Testing the PSF (II)

• used 34 high - fluence serendipitous sources to test the PSF calibration

• fitted the 34 sources with a King function • compared the best fit values with the expected ones

Energy Off-axis angle

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Testing the PSF (III)

2 different sources, with different spectra and off-axis angle , but same PSF:

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Boresight calibration (I)

We used our serendipitous catalog to calibrate the

boresight.

Cross-correlation with optical catalog shows the boresight is

time-dependent.

1010Moretti et al. 2006 A&A 448 L9

Boresight calibration (I)

There is a misalignment betweenTelescope and star – tracker and

it varies with time !

We used our serendipitous catalog to calibrate the

boresight.

Cross-correlation with optical catalog shows the boresight is

time-dependent.

1111

Boresight calibration (II)

Δ dec = A sin (PA + δ)Δ ra = A cos (PA + δ)

We quantified (and corrected) theboresight drift, by studying the

residuals’ dependence with roll angle.

It is a simple trigonometric function: the amplitude is the size of the misalignment,

and the phase is the direction

1212

Boresight calibration (III)

Studying the dependence of A and δ on time allowed us to correct

boresight drift.

Δ dec = A(t) sin (PA + δ(t))Δ ra = A(t) cos (PA + δ(t))

The original XRT positional accuracy

was determined by the star tracker

accuracy (3”) and the XRT statistical error

which is < 2” for a source with 100 counts.

Use of UVOT as ‘super-star tracker’ has eliminated star tracker error.

Typical XRT positional error is ~2.2”

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Flux limits

cfacft

dbn

t

cF

cell

cellsig

)(expexp

minlim

nsig = 3 ; dcell =8 ; cmin =7

For each field we defined the flux limit as the flux of the source

Detected with the lowest significance(>3 σ) within the 5 arcmin

radius central part of the FOV

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Sky coverage and Log N – Log S

The sky coverage calculation contains:

•exposure maps•flux limits and completeness function

•Eddington bias correction (Kenter & Murray 2003, ApJ 584,1016)

Soft band : 0.5-2.0 keV

Extra-galactic fields

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Extended sources

Output of wavelet detection algorithm:84 sources are extended with 4 σ confidence

Sky coverage for extended sources

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Conclusions

Swift - XRT has been performing beautiful photometric and

spectroscopic observations of GRB afterglows from their early phases

Serendipitous sources are useful to test PSF calibration

We used serendipitous sources to map the telescope boresight variations

The serendipitous survey for extended sources is potentially very interesting