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John Nousek & David Burrows (Penn State University)

X-Ray Telescope (XRT): Performance after Five Years. John Nousek & David Burrows (Penn State University) Alberto Moretti (Osservatorio Astronomico di Brera). International Workshop on Astronomical X-ray Optics - Prague, Czech Rep. – 6-9 Dec. 2009. Swift launch: 20 Nov 2004 !!.

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John Nousek & David Burrows (Penn State University)

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  1. X-Ray Telescope (XRT): Performance after Five Years John Nousek & David Burrows (Penn State University) Alberto Moretti (Osservatorio Astronomico di Brera) International Workshop on Astronomical X-ray Optics - Prague, Czech Rep. – 6-9 Dec. 2009

  2. Swift launch: 20 Nov 2004 !!

  3. Swift Observatory Status Swift continues to run smoothly after nearly five years! • 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 Statistics from Rob Laverghetta, FOT – as of 31 Oct 2009

  4. UVOT BAT BAT XRT UVOT XRT Spacecraft 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

  5. BAT Burst Image XRT Image T~30 sec T~70 sec BAT Error Circle XRT Operation • 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

  6. Testing the PSF (I) From calibration observations (2005, MKN786, RXJ0720.4) we found that the PSF is well-fit by a King function: 2 free parameters that linearly depend on energy and off-axis angle [Feb 2005] The PSF is entirely described by 8 numbers Typical values of the parameters are rc:5.5” ; β: ~1.5, with very weak dependence on energy and off- axis angle. Moretti et al. 2005, SPIE 5898, 325

  7. 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

  8. Testing the PSF (III) 2 different sources, with different spectra and off-axis angle , but same PSF:

  9. Boresight calibration (I) We used our serendipitous catalog to calibrate the boresight. Cross-correlation with optical catalog shows the boresight is time-dependent.

  10. Boresight calibration (I) We used our serendipitous catalog to calibrate the boresight. Cross-correlation with optical catalog shows the boresight is time-dependent. There is a misalignment between Telescope and star – tracker and it varies with time ! Moretti et al. 2006 A&A 448 L9

  11. Boresight calibration (II) Δ dec = A sin (PA + δ) Δ ra = A cos (PA + δ) We quantified (and corrected) the boresight 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

  12. 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”

  13. Flux limits 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 nsig= 3 ; dcell =8 ; cmin =7

  14. 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

  15. Extended sources Output of wavelet detection algorithm: 84 sources are extended with 4 σ confidence Sky coverage for extended sources

  16. 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

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