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Leicester XRT digest

Things to know, or to look out for, when analysing XRT data. See also the XRT threads page for step-by-step guides to analysing the data.

XRT Science Analysis Status

See also the XRT Calibration digest page

Index

Optical Loading

Bright optical sources can deposit significant charge in the CCD pixels, affecting the detection of X-rays. This is described on the optical loading page, where we provide graphs showing when optical loading becomes an issue for different stellar types. We also provide an optical loading calculator to predict the level of optical loading for a given star.

OPEN ISSUES

Bad columns

At the end of May 2005, the XRT CCD was hit by a micrometeorite. This has led to a small number of hot columns in PC and WT modes being vetoed in order to prevent saturation of the telemetry. Because of the way the CCD is read out, this method cannot be used for Photo-Diode mode, so this mode is currently disabled.

Unfortunately, because some of the bad columns run down the centre of the CCD, sources occasionally lie on top of, or very close to, them. In this case, the loss of counts have to be corrected for, both when extracting spectra and light-curves. Examples of such a chance alignment are shown below.

The bad columns closest to the centre of the field of view (and, therefore, the most likely to affect any observations) are located at DETX positions of 291-294 for PC (291-295 for WT) and 319-321 (both PC and WT). DETX=290 is an additional partial bad column for PC mode (between DETY=199 and 290). Note that the columns will only be obvious when plotting the image in detector coordinates, or for a single orbit at a time, since using sky coordinates will blur different pointings together, concealing the fact that some orbits may be over the bad columns while others are not.

PC bad columns
WT bad columns
Figure 1. These panels show the supernova remnant, Cas A, positioned over the bad columns (top panel: PC; bottom panel. Clearly, the number of counts and the flux will be underestimated if this occurs. The software (version 2.3 and after) compensates for these columns during spectral analysis through the production of an exposure map (though see below for a caveat). The extraction of light-curves, however, is still an open issue.

To correct for the bad columns, exposure maps need to be created, and incorporated into the ARFs generated. This procedure is covered by the XRT exposure map thread.

For sources that lie close to the bad columns, the correction factor which compensates for the exposure lost to those columns is very sensitive to the exact location of the object on the CCD. When using the standard attitude information (from the sw*sat.fits.gz file), the XRT astrometry is accurate to 3.5′′ 90% of the time. Thus a catalogued position of the object in question may not correspond to the correct position in the XRT astrometric frame, and this can result in the correction factor being wrong by up to a factor of ~2 in extreme cases. It is therefore strongly advised that the object position used to calculate the correction factors should be found from XRT Photon Counting mode data taken as close as possible in time to the WT mode data. Where no such data are available, we advise analysing the data (from xrtpipeline onwards) using the most accurate celestial co-ordinates (e.g. optical or radio) available. However even in these cases, if the object centre is within 2-3 pixels of the bad columns (this can be determined by plotting the image in ds9) users should be aware that the correction factor may be uncertain, and flares/drop-outs seen in the WT mode could be spurious. Also, spacecraft movement may affect this correction, see the Pointing Stability section, below.

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Pointing stability

During the first ~150 sec of a snapshot the spacecraft may still be slewing very slightly. If the source is situated near to the bad columns it is necessary to calculate a time-dependent correction factor to compensate for this movement. This is the default behaviour of the xrtlccorr code, although in some cases residual features of 5-10% of the mean flux level may still exist. If these are seen early in the snapshot, we advise users to examine the sw*s.mkf.gz file in the auxil directory of the observation. By plotting RA or Dec against time in this file, one can determine whether the spacecraft is moving at the time of a flare or dip. If it is, and the source is near the bad columns (as can be determined by viewing the WT mode image in, e.g., ds9) the feature should be viewed with scepticism.

As of 2014 January 14, the user objects tools correctly calculate time-dependent correction factors. The xrtgrblc tool still does not currently calculate time-dependent corrections, however.

The bad column correction factor is also strongly sensitive to the assumed position of the X-ray source. This was discussed above (under Bad columns).

Occasionally, when Swift is settled on a target (rather than slewing), an instability can cause the source to drift around on the detector. If the source is close to one of the bad columns, a small movement can cause the core of the PSF to become hidden by the bad columns, leading to a drop in the detected count rate. An example of such an event is shown below.

jumping attitude
Figure 2. The plot above shows an observation of RS Oph, where an instability in the pointing direction caused the source to wander towards and over the bad columns, then jump away again. The ordinate shows the position of the source in terms of DETX, while the abscissa shows increasing time (from left to right).

The xrtlccorr tool allows for time-dependent corrections to be made, which correct for this effect. This option is enabled by default within xrtlccorr. See the Exposure Correction thread for details.

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Background estimation in ximage

When analysing PC-mode data, it is common practice to determine the background level using the back command in ximage. However, because the Swift-XRT has a very low background level, this tool often gives biased or incorrect results for XRT images, and we advise against its use. The problems, are twofold, as described below. For estimating the background we instead suggest examining the image and exposure map by eye, and identifying a region on the detector which is source free and uniformly illuminated. Then you can place a region on this and identify the number of counts per pixel in this, using the counts command in ximage or the funtools plugin in ds9, for example. This can then be used in ximage, for example the detect command takes an option flat=x to specify the background level across the image, in counts per pixel.

The first issue with the back command is that at the present time it does not make use of the exposure map. Therefore the effects of vignetting, bad columns, and field of view are not included in the background calculation. Additionally, any cell containing zero counts (the back command splits the image into a series of cells, and measures the number of counts in each) is excluded from the estimation of the mean background. This has the advantage of ignoring parts of the image outside the field of view, however it also means that any fully exposed portions of the image which contain no events will be ignored, biasing the measured background.

The second issue is with the sigma clipping method employed by ximage. Background cells containing a number of counts more then 3-σ from the mean value of all cells are discarded and the mean level is recalculated. However, whether a cell is more than 3-σ from the mean is determined using Gaussian statistics, which is not appropraite for the low-background Swift-XRT data, where Poisson statistics should be used. The practical upshot of this is that the 3-σ lower limit, below which cells are ignored, is frequently negative, e.g. cells containing fewer than -2 counts are excluded; this of course is impossible in Poissonian datasets such as XRT images, and therefore in reality the sigma clipping only rejects cells lying above the mean, again biassing the background.

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Attitude files

There are three types of Swift attitude file, with between one and three of these being available for any given observation:

The sat file contains the attitude determined from the spacecraft star trackers. The pat file is almost identical to the sat file, but the task attjumpcorr has been applied to it. From the point of view of XRT analysis, these files are indistinguishable. The attitude in the uat file has been determined using the UVOT as a star tracker.

There are a number of tasks which use an attitude file (e.g., xrtlccorr, xrtexpomap, pointxform). It is essential that the attitude file used by such tasks is the same as that used to create the corresponding event list. In principle, the file used by xrtpipeline can be determined by examining the ATTFLAG keyword in the EVENTS extension of the event list. The flags are defined as:

NB This must be read from the EVENTS extension, since this is the only one to be altered when xrtpipeline is run.

However, a significant number of datasets taken before August 2007 have incorrectly formatted ATTFLAG values -- these have been corrected in the UKSSDC archive. When running xrtpipeline on data from this time frame which have been downloaded from the GSFC SDC, the patched version of HEASOFT 6.15.1 should be used.

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Pile-up

Pile-up in CCD cameras occurs when there is a significant probability that two or more photons registering within a given CCD frame will have overlapping charge distributions. This can lead to a spectral distortion if the resulting distribution is recognised as a single event whose energy is the sum of the overlapping events (i.e., two or more soft X-ray photons can be registered as a single higher-energy photon), or a flux loss if the charge distribution has a pattern, or grade, outside that clasified as a true X-ray event (0-12 for Swift Photon Counting mode; 0-2 for Windowed Timing).

For basic details on how to estimate the level of pile-up, and how to correct for it, see the pile-up analysis thread.

Useful papers containing details about Swift pile-up analysis:

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Event screening

There are a number of issues of which users should be aware when it comes to data screening.

Temperature

Because of the failure of the Thermo-Electric Cooler power supply, the XRT CCD routinely operates between about -70 and -50C. By default, xrtpipeline excludes any data which were obtained at temperatures higher than -47C (from v1.2 of the software onwards). This can be changed either by including a GTI expression directly when running the pipeline (this method is needed to include data taken at higher temperatures which would usually be thrown away, e.g. xrtpipeline gtiexpr="CCDTemp>=-102&&CCDTemp=<-45"), or by later filtering within XSELECT (e.g. select mkf mkf_dir=./ mkf_name=sw[obsid]s.mkf "CCDTemp<-55").

While less stringent filtering on the temperature will sometimes lead to more exposure time, there is the possibility of more hot pixels appearing within the data.

Bright Earth and Elevation angles

When the angle between the XRT and the limb of the Earth is small, the average background level is higher. An extreme example is shown below.

Bright earth in GRB 050223 GRB 050223 after Bright Earth filtering
Figures 3 and 4. The left-hand image shows the field of GRB 050223 before filtering. The right-hand image has been cleaned within XSELECT (filter pha_cut 30 1000; i.e., photons below 300 eV have been removed). The green circle shows the position of the burst in both cases.

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Velocity Aiding

Velocity aiding was switched off onboard the spacecraft at 21:57 UT on 31st January 2005. Data obtained before this date should include the command aberration = yes when running xrtpipeline (the default for all recent versions of the pipeline is aberration = no).

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Mode-switching

When the CCD temperature is around -52C, certain hot pixels become active. Although many are masked out, this can lead to "mode-switching"; this occurs when the count-rate within the centre of the field of view is close to the PC/WT switch point. This means that, even if you expect your object to be in PC mode, a substantial portion can end up in WT event files. Exposure time is lost with each mode change. This is annoying, but there is nothing that the user can do about it. The science planners at PSU do a very good job at keeping the temperature down, but sometimes it's not possible (or a new burst turns out to be in a part of the sky which causes the XRT to become particularly hot).

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Timing

This file explains how to check for a periodicity in XRT data.

Randomisation

The times listed in an event file occur at intervals of the fundamental time resolution (TIMEDEL keyword) for the mode being analysed. When binned light curves are created with arbitrary time bins close to small number multiples of TIMEDEL, artefacts can sometimes be seen in subsequent powerspectral analysis of the data.

A common method to deal with this is to randomise the event times over the timebin interval (TIMEDEL) before the data are binned. The Swift-XRT software does not currently perform this randomisation but it can be achieved using the fcalc ftool and its built in random() function using the expression 'TIME-TIMEPIXR*TIMEDEL+RANDOM()*TIMEDEL'. For example,

ftcalc infile=input.evt+1 outfile=output.evt column=TIME expression='TIME-TIMEPIXR*TIMEDEL+RANDOM()*TIMEDEL' history=yes

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The xrtgrblc tool

Up to the present HEASOFT version (v6.15), there are known issues affecting the xrtgrblc ftool: it does not correct for pile-up below 300 ct/sec in WT mode (correction down to ~150 ct/sec is needed), and the exposure corrections do not have the necessary time resolution (see Pointing stability for details). We do not advise use of this tool at the present time. For information on how to create XRT light curves, see the XRT analysis thread.


CLOSED ISSUES

Hotspots

There are 3 "hot spots" or "burn marks" in the centre of the XRT CCD. These are areas of enhanced dark current due to focussing of X-rays during the ground calibration before launch. These would not be visible below about -90C, but, because the CCD is not as cool as expected, the spots were sometime visible in earlier data and could be mistaken for a GRB afterglow if the user is not vigilant! The positions are known in detector coordinates. These are labelled in the image below. Filtering out the lowest energies tends to make these spots disappear. Do this by using the pha_cut command in XSELECT (e.g., filter pha_cut 30 1000 filters between 0.3 and 10 keV).

These areas are now masked out as bad pixels, so should seldom be a problem.

CCD hotspots
Figure 5. The three "hot spots" are located at (321, 298), (325, 263) and (340, 285) in detector coordinates (set xyname detx dety before extracting an image in XSELECT).

Incorrect RA/Dec in the header

For some of the early (start of 2005) Swift data, the RA and Dec in the headers of the raw files are set to zero or 90, rather than the position of the target. This causes the pipeline to crash in normal circumstances. To side-step this problem (very rare now), use the dummy attitude file sw00000000000sat.fits.gz as follows: xrtpipeline attfile=sw00000000000sat.fits.gz

The problem is believed to be fixed for all recently-processed versions of the data.

New GRB during a Manual State observation

This event can only occur if the XRT is in Manual State, rather than its usual Auto State; some of the earlier (before April 2005) datasets were interrupted in this manner. If this happens, the event-list for the original target may also include data for the slew to the burst and subsequent snapshots. This complicates matters, since the RA and Dec have changed part way through the observation. The header files will include the coordinates of the original target, rather than those for the burst, thus processing the data in the normal way will show a field of view not containing the GRB.

To fix this problem, RA_PNT and DEC_PNT in the xhd.hk file must be changed to the required position:

Within the sw<obsid>/xrt/hk directory there will be a file called sw<obsid>xhd.hk, within which can be found keywords called RA_PNT and DEC_PNT (in 2 separate extensions). These should be changed from the values of the original target to those of the GRB of interest using the fparkey command

For example, if the required RA and Dec are 278.103333 and 42.365000 respectively, use the commands (obviously including the correct <obsid>):

fparkey 278.103333 sw<obsid>xhd.hk+0 RA_PNT
fparkey 278.103333 sw<obsid>xhd.hk+1 RA_PNT
fparkey 42.365000 sw<obsid>xhd.hk+0 DEC_PNT
fparkey 42.365000 sw<obsid>xhd.hk+1 DEC_PNT

Then xrtpipeline can be run as normal. Following this, the cleaned event-list should be read into XSELECT as normal. Then the housekeeping file also needs to be read in, using the command read hk; XSELECT then prompts for the location of the housekeeping directory (sw<obsid>/xrt/hk) and the name of the HK file (sw<obsid>xhd.hk). The select command can then be used to choose the relevant RA and Dec. For the example here, this would be:

select hk "RA>260&&Dec<50"

Failure of xrthotpix

Occasionally xrthotpix failed with older versions of the Swift software, giving the message:

ERROR: Operation not permitted
Task xrthotpix 0.1.5 terminating with status 1

If this happens, the cleaned event-list will not be generated for the pointing observation.

When running xrtpipeline, include impfac=2000 on the command line. This parameter is used to compute the background level. The problem has been fixed for v1.2 (and later versions) of the software.