XRT Data Analysis I E. Troja (NASA/GSFC/ORAU)

Transcription

1 XRT Data Analysis I E. Troja (NASA/GSFC/ORAU) with thanks to Kim Page (U. Leicester) 1

2 Introduction to XRT Basic steps: Outline - XRT data processing (from Level 1 to Level 3) - create images, light curves with XSELECT PILE-UP CORRECTION (mainly PC) - introduction to QDP Create a spectrum (mainly PC) - Fit it with XSPEC more advanced analysis will be covered in the next two lectures

3 The X-Ray Telescope UVOT XRT BAT * Bandpass: kev FoV: 23.6 x 23.6 Precision: a few arcsec Effective area: 110 cm 1.5 kev Sensitivity: erg/cm 2 /s

4 XRT read-out modes Imaging mode Exposure of 0.1 or 2.5 s No spectroscopy data Photo-Diode mode Disabled since May 2005 Windowed Timing mode 1-dimensional imaging 1.7 ms time resolution NEW: WT settling Photon Counting mode 2-dimensional imaging 2.5 s time resolution

5 Run the xrtpipeline The pipeline requires the auxil and xrt directories to run. punlearn xrtpipeline xrtpipeline cleanup=no createexpomap=yes punlearn: reset the default parameters cleanup=no keeps all the temporary files the pipeline produces; this is mainly useful because it keeps the extraction region used, which is centred on the position given. createexpomap=yes will save us time, but if you forget it there is xrtexpomap TO EXPLORE THE COMMAND PARAMETERS USE FHELP namecommand

6 xrtpipeline example panormus: ~/swiftdb >xrtpipeline cleanup=no createexpomap=yes ============================================================ Running SWIFT XRT pipeline Task: xrtpipeline Version: Release Date: ============================================================ Source RA position (degrees or hh mm ss.s) or POINT or OBJECT[ ] : Source DEC position (degrees or dd mm ss.s) or POINT or OBJECT[ ] : Target Archive Directory Path [ ] : Stem for FITS input files [i.e. sw ] [ ] :sw Directory for outputs [ ] : GRB xrt min later... xrtpipeline_0.12.4: Exit with no errors - Tue Nov 9 13:20:31 EST 2010 ==============================================================

7 GRB position: some useful links GCNs: Swift XRT enhanced position (GCN 9339) optical afterglow position (GCN 9332) GCN Reports: Heasarc Coordinate Converter:

8 Products - PC mode sw xpcw3po_uf.evt sw xpcw3po_cl.evt Level 1 Level 3 - Calibrated ds9 sw xpcw3po_cl.evt -regions sw xpcw3po.reg Create source (30 pix) and background (annulus) regions for lightcurves and spectra

9 Products - PC mode sw xpcw3po_uf.evt sw xpcw3po_cl.evt Level 1 Level 3 - Calibrated ds9 sw xpcw3po_cl.evt -regions sw xpcw3po.reg Create source (30 pix) and background (annulus) regions for lightcurves and spectra

10 Products - WT mode ds9 sw xwtw2po_cl.evt RED: source region (40 x 20 pixels wide) GREEN: background region

11 XSELECT - Extract an image XSELECT is an easy way to extract X-ray images, spectra and light-curves: > xselect Enter session name >[xsel] > read event Enter the Event file dir >[ ]./ Enter Event file list > [ ] sw xpcw3po_cl.evt Got new mission: SWIFT Reset the mission? >[ ] yes xsel:swift-xrt-photon > filter pha_cutoff ! kev energy band xsel:swift-xrt-photon > extract image xsel:swift-xrt-photon > plot image

12 XSELECT - Extract a spectrum xsel > read ev sw xwtw2po_cl.evt > Enter the Event file dir >[./] xsel:swift-xrt-windowed > filter region srcwt.reg xsel:swift-xrt-windowed > extract spectrum xsel:swift-xrt-windowed > save spectrum > Give output file name >[] grb090510_wt.pha Wrote spectrum to grb090510_wt.pha xsel:swift-xrt-windowed > clear region xsel:swift-xrt-windowed > filter region bkgwt.reg xsel:swift-xrt-windowed > extract spectrum xsel:swift-xrt-windowed > save spectrum > Give output file name >[] grb090510_wt_bkg.pha Wrote spectrum to grb090510_wt_bkg.pha

13 XSELECT - Extract a light curve xsel:swift-xrt-windowed > clear region xsel:swift-xrt-windowed > filter region srcwt.reg xsel:swift-xrt-windowed > filter pha_cutoff xsel:swift-xrt-windowed > set binsize 10 xsel:swift-xrt-windowed > extract curve > plot curve > save curve wtsrc.lc > clear region > filter region srcbkg.reg > set binsize 10 > extract curve > save curve wtbkg.lc

14 Pile-up in PC data Although Swift has different modes designed for different count-rates, there can be problems with pile-up (generally in PC mode, but really bright objects can be piled-up in WT), where more than one photon hits a single (or adjacent) pixel before the charge has been read out. Where this is the case, an annular region should be used to extract the spectrum, rather than a circle, in order to exclude the core of the PSF. If the count-rate for the source is more than ~ count s -1, the PC data are likely to suffer from pile-up. A piled-up spectrum will be flatter/ harder than one which has the pile-up correctly accounted for. > xselect > read ev sw xpcw3po_cl.evt > Enter the Event file dir >[./] > filter region src.reg > filter pha_cutoff > extra curve

15 Pile-up - 2 Reducing the source region decreases the statistics. Pile-up usually affects only the first orbit of PC data.

16 Pile-up - 3 > filter time cursor OPENS A QDP SESSION > rescale x e3 To start selection enter quit at the PLT prompt then - to select a time range click the left mouse button on the start time then again on the stop time. To cancel the selection click the second time outside the plot. - press x on the keyboard to finish cursor selection and return to the xselect prompt > clear region > extract event copyall=yes! copyall=yes makes a copy of the BADPIX extension > save event grb090510_pc_orb1.evt

17 Pile-up - 4 A quick way to estimate the radius which should be excluded is to run the PSF command in XIMAGE: > ximage > read grb090510_pc_orb1.evt > cpd /xtk > disp > psf/cur click in centre and as far out as you want to determine the PSF This then takes you into QDP, with the PSF profile, which can be fitted with the King function ( model king ) out in the wings away from pile-up. r c ~ 5.8; β ~ 1.55 where PSF(r) = [1 + (r/r c ) 2 ] -β

18 Pile-up - 5 PLT> model king 1 RC: VAL( ), ,-1 2 IN: VAL( ), ,-1 3 S0: VAL( ), The cyan-coloured data points in the lower panel are the only ones we re interested in. Either use the command col off fit or fit on 5

19 Pile-up arcsec ~ 3 pixels The yellow line shows the King profile fitted to the wings of the PSF where pile-up is negligible. Extrapolating down to smaller radii shows where the model and data start to diverge; this is the radius which should be excluded when extracting spectra/light-curves.

20 Apply the pile-up correction Edit your src.reg file in order to exclude the central pixels: circle( , ,30) -circle( , ,3) save it as srcnopu.reg EXERCISE: Create PC source and background spectra during the first orbit. Tips: - use the event file grb090510_pc_orb1.evt or use the original PC file and select the time interval with filter time cursor - use the corrected source region file (srcnopu.reg) to extract the spectrum

21 Build an exposure map Exposure maps are used to correct artificial features, like vignetting effects or for the loss of flux caused by some of the CCD pixels not being used to collect data (bad pixels, hot columns). The Swift software allows the user to correct for the loss of flux which occurs when the source is positioned over a hot column. When running xrtpipeline, the command createexpomap=yes will produce an exposure map for each full eventlist. They can also be produced independently: Copy sw sat.fits.gz and sw xhd.hk.gz in your working directory. They are in /auxil and /xrt/hk. > punlearn xrtexpomap > xrtexpomap infile=grb090510_pc_orb1.evt stemout=grb090510_pc_orb1 attfile= sw sat.fits.gz hdfile= sw xhd.hk.gz outdir=./ If everything goes well, the result is a file called grb090510_pc_orb1_ex.img

22 PC exposure map ds9 grb090510_pc_orb1_ex.img -regions src.reg

23 Ancillary response file Ancillary Response Files (ARFs) can be built using the tool xrtmkarf. > punlearn xrtmkarf > xrtmkarf expofile=grb090510_pc_orb1_ex.img Name of the input PHA FITS file [ ] : grb090510_pc_orb1_nopu.pha PSF correction active?(yes/no) [ ] : yes Name of the output ARF FITS file [ ] : grb090510_pc_orb1_nopu.arf Source X coordinate (SKY for PC and WT modes, DET for PD mode): [ ] : -1 Source Y coordinate (SKY for PC and WT modes, DET for PD mode): [ ] : -1 When an exposure map is included, the PSF correction must be active (i.e. yes as the input to the second xrtmkarf prompt); this ensures the ARF is corrected for hot columns, bad pixels and any loss of counts caused by using an annular extraction region (if the source is piled-up). If -1 is given at the prompts for the source coordinates, the position of the source is extracted from the header of the spectrum (and will, thus, correctly be the centre of the extraction region used).

24 Almost there... Source spectrum - corrected for pile-up Background spectrum Ancillary Response File - Response matrix: the relevant RMF to use is given in the output of xrtmkarf xrtmkarf_0.5.6: Info: Processing '/software/heasoft-6.10/swift/caldb// data/swift/xrt/cpf/rmf/swxpc0to12s6_ v011.rmf' CALDB file. > grppha grb090510_pc_orb1_nopu.pha grb090510_fin.pha > bad > group min 20! for chi-squared* > chkey backfile grb090510_pc_orb1_bkg.pha > chkey respfile swxpc0to12s6_ v011.rmf > chkey ancrfile grb090510_pc_orb1_nopu.arf * Alternatively you can use the Cash statistics. In this case: 1) group min 1 2) the source region must be smaller (<10 pix) so that the bkg is negligible

25 XSPEC - Start > xspec > data grb090510_fin.pha 1 spectrum in use Spectral Data File: grb090510_fin.pha Spectrum 1 Net count rate (cts/s) for Spectrum: e-01 +/ e-02 (98.7 % total)... > ignore bad > setplot energy > plot ldata

26 PC Spectrum

27 > model wabs*zwabs*pow XSPEC - Afterglow model wabs: absorption component, modeling the Galactic absorption zwabs: redshift dependant absoprtion component, to model the GRB intrinsic absorption pow: simple power law, suitable for afterglow emission FTOOL NH >nh Equinox (d/f 2000)[2000] RA in hh mm ss.s or degrees[ ] DEC in dd mm ss.s or degrees[ ] LAB >> Average nh (cm**-2) 1.67E+20 LAB >> Weighted average nh (cm**-2) 1.66E+20

28 XSPEC - Fit Input parameter value, delta, min, bot, top, and max values for e+06 1:wabs:nH>0.017,-1! XSPEC model uses units of cm e+06 2:zwabs:nH> :zwabs:Redshift> :powerlaw:PhoIndex> e+24 1e+24 5:powerlaw:norm> > renorm > fit 999! statistic cstat to use the Cash statistics; default is chi-squared

29 XSPEC - RESULTS ================================================================= Model wabs<1>*zwabs<2>*powerlaw<3> Source No.: 1 Active/On Model Model Component Parameter Unit Value par comp 1 1 wabs nh 10^ E-02 frozen 2 2 zwabs nh 10^ E-02 +/ E zwabs Redshift frozen 4 3 powerlaw PhoIndex / E powerlaw norm E-03 +/ E-04 Chi-Squared = using 55 PHA bins. Reduced chi-squared = for 52 degrees of freedom Null hypothesis probability = e-03 XSPEC12>error Parameter Confidence Range (2.706) 90% confidence level ( , ) XSPEC12>error 1. 4 Parameter Confidence Range (1) 68% confidence level ( , ) XSPEC12> plot ldata del

30 XSPEC - RESULTS

31 As a comparison Spectral results without PILE-UP correction ================================================================= Model wabs<1>*zwabs<2>*powerlaw<3> Source No.: 1 Active/On Model Model Component Parameter Unit Value par comp 1 1 wabs nh 10^ E-02 frozen 2 2 zwabs nh 10^ E-02 +/ E zwabs Redshift frozen 4 3 powerlaw PhoIndex / E powerlaw norm E-03 +/ E-04

32 HOMEWORK Create the light curve of GRB and find its best fit model: - you can use xrtgrblc as showed in the next lecture - download the custom QDP models from the WORKSHOP wikipage and fit the light curve - play with QDP and make a nice plot