6. Resolve Data Analysis

6.1 Introduction

The unprecedented spectral resolution in the hard ( $>$ 2 keV) band is the hallmark of Resolve data. It can be realized with the High and Mid primary events (Hp and Mp) among five X-ray grade events (Table 4.1. See also POG for details). Users who want to take full advantage of the Resolve instrument should use these events for spectral analysis. The lower-grade events (Ms, Lp, Ls) also have very good energy resolutions, especially compared with X-ray CCD data (cf. Table 6.1)^6.1. They may also help obtain the highest statistical significance. Section 6.3 describes how to choose specific grades for analysis.

An essential characteristic of the Resolve data is that the grade branching ratios significantly change at high count rates, above $\sim$ 1 cts s $^{-1}$ array $^{-1}$ for on-axis point sources (see Figure 5.9 in POG). We categorize these sources as Resolve Bright sources and briefly describe the treatments, including other high count rate features, in Section 6.7. This threshold is not so high among typical Resolve point sources, so users should exercise extra caution in analyzing such data, particularly strongly variable sources, as observed spectral variations can be artificial. Note that this threshold is based on count rate per pixel, so it is much higher for extended sources, and nearly all extended sources are unaffected by this issue.

We strongly encourage users to familiarize themselves with the Resolve detector properties described in Chapter 5 in POG. There are ongoing efforts on the detector response calibration. Currently (November 2024), we recommend using Hp events only for spectral analysis because the Hp and Mp events have significant gain inconsistencies: merging these two event grades degrades spectral resolution significantly. Software released in the future will address this issue. Please check Section 2 and the XRISM web page regularly for the latest information.

6.2 Cleaned Event File

The pipeline creates 2 cleaned event files in the resolve/event_cl directory.

xa000126000 rsl_p0px1000_cl.evt

xa000126000 rsl_p0px5000_cl.evt

The first file corresponds to the science data. The digit after px can be 1 $-$ 4, depending on the X-ray filter chosen for the science observation (see Section 4.3). The file is screened with the standard screening condition in Table 5.1 unless users change the screening condition. The second file (px5000) contains the data obtained during the gain calibration. The directory may have one more event file, “*_p0px0000_cl.evt", which includes data not optimal for the primary target.

6.3 Additional Screenings and Processings

We recommend that users consider the following additional processes for the cleaned events. The pipeline does not perform these processes primarily because the applications depend on the science goal, observation condition, or the nature of the target. We recommend running some processes on a Unix terminal while the others are after loading data into xselect (Section 6.4.2), partly for historical reasons.

Screening Out Pixel-Pixel Coincident Events

Command line/Highly recommended

When energy is absorbed into the silicon frame around the Resolve array, it pulses the temperature of the heat sink of all of the pixels, resulting in pulses in the temperatures of the pixels themselves. For very large depositions of energy (MeV scale), the resulting pulses on the pixels can produce signals that trigger in the Pulse Shape Processor (PSP). We refer to the resulting clusters of events as frame events. Most of these events have significantly different pulse rise times from regular events, so they can be removed efficiently with a rise time cut. Because Ls events have a very large spread in rise times, Ls events are excluded from the cut.

Frame events can also be identified by pixel-pixel coincidence and removed by screening on STATUS[4]. The rise-time and STATUS[4] cuts largely remove the same events, but coincidence screening is more effective at very low energies for which the determination of rise time is noisy. STATUS[4] also removes more rare background events that can be identified by coincidence, such as secondary events associated with the same primary cosmic ray. For very weak sources for which the probability of two source photons hitting the array within the coincidence window, currently set at $\pm$ 0.72 ms, is negligible, the STATUS[4] cut is a useful multi-purpose cleaner. However, it is important to consider the false-coincidence probability before applying it to your data. If the $\pm$ 0.72 ms window will result in excessive data loss due to false coincidence, the STATUS[4] screening may be omitted.

Currently, electrical-crosstalk screening with STATUS[6] is not recommended. By requiring PI $>=$ 600, both for analysis and for flagging pixel-pixel coincidence, the only cross-talk events in the data will be associated with very energetic parent events ( $>$ 12 keV) most of which are background events, and a STATUS[4] cut will remove both events. However, cross talk that is much too small to be confused with an X-ray can still be large enough to contaminate a real pulse if it overlaps with it. Thus, the coincidence of normal events on electrically adjacent channels is flagged (STATUS[13]) so that these events can be removed from spectral analysis. The window of overlap over which such contamination can significantly degrade the determination of the photon energy is large, $\pm$ 25 ms. For bright sources, consider screening on STATUS[13] and comparing the result to the case of retaining such events.

The Resolve instrument team and the SDC are currently working to refine the various pixel-pixel coincidence definitions (general (STATUS[4]), electrical cross talk (STATUS[6]), and coincident real events that contaminate each other via untriggered crosstalk (STATUS[13])) through criteria based on PI threshold, PI ratio, and group size, to minimize the overlap in the definitions and allow more precise cleaning at the lowest energies.

To apply screening on PI, rise time, and STATUS[4], execute the following HEASoft command on the Resolve cleaned event file.

term> ftcopy infile="xa000126000rsl_p0px1000_cl.evt[EVENTS][(PI>=600)
&&(((((RISE_TIME+0.00075*DERIV_MAX)>46)&&
((RISE_TIME+0.00075*DERIV_MAX)<58))&&ITYPE<4)||(ITYPE==4))&&
STATUS[4]==b0]" outfile=xa000126000rsl_p0px1000_cl2.evt 
copyall=yes clobber=yes history=yes

Users use the output cleaned event file (*_cl2.evt) for further analysis.

**Figure 6.1:** *Left*: Ls/Hp ratio over the highest count rate pixel (*black dot*: on-axis point source, *red cross*: extended source, *blue circle*: off-axis or other source). Below 0.2 cts/s/pixel where the first Ls component dominates, the Ls fraction increases as the source count rate decreases. Above 0.2 cts/s/pixel where the second Ls component dominates, the Ls fraction increases strongly as the source count rate increases. *Right*: Hp fraction over the total event counts including (*red*)/excluding (*blue*) Ls events from 48 point source data. The fractions of these two cases do not change until 2-3 cts/s, but then start to deviate.
$\begin{figure}\centering \includegraphics[width=\textwidth]{Figure/LsAnomaly.pdf} \end{figure}$

Removing the Anomalous Ls Events

Command line/Highly recommended for Relatively Weak Sources/Temporary Measure

The onboard calibration found more Ls events than expected from the ground study. These so-called anomalous Ls events do not originate from direct X-ray signals from celestial objects. However, the current analysis scheme assumes that: the rslmkrmf response generator divides the X-ray collecting area by the number ratio of the selected grade events over the total Grade 0 $-$ 4 events, which include the Ls events. The produced response systematically has a lower collective area than it should be, which overestimates the total flux.

The anomalous Ls events have two components (See Figure 6.1 left). The first component prominent in the low count rate range ( $\lesssim$ 0.4 cts/s/pixel) does not correlate with the source count rate, probably originating from cosmic-ray particles or instrumental X-rays induced by cosmic rays. The second component is more robust with brighter sources, significantly above the 0-30 keV pixel count rate range greater than 1 cts/s/pixel, so it can be related to secondary signals produced by initial (probably energetic) X-rays.

Currently, we have a temporary measure for weak sources whose X-rays do not significantly produce Ls events (maximum pixel count rate below $\sim$ 0.4 cts/s/pixel or count rate of $\sim$ 1 cts/s for a point source). Since the detected Ls events originate from particle background, users can safely remove them from the event files as non-source origin. The following command produces an event file without Ls events.

term> ftcopy infile="xa000126000 rsl_p0px1000_cl.evt[EVENTS][ITYPE $<$ 4]"
outfile=xa000126000 rsl_p0px1000_wols_cl.evt copyall=yes clobber=yes
history=yes

Users may combine this command with the above command for "Screening Out Pixel-Pixel Coincident Events" by removing the " $\vert\vert$ (ITYPE==4)" part. Users must use the cleaned event file produced for further data reduction or analysis.

Moderately bright sources, up to $\sim$ 10 cts/s for a point source, produce the second Ls component events, but not so significantly between 3 $-$ 10 keV. For such sources, uses may remove Ls events with the above command and analyze only the data between 3 $-$ 10 keV. The derived source flux should be considered a lower limit, as the source's direct X-rays also produce some Ls events.

The Ls events of brighter sources have complicated behavior, so we do not yet have an effective solution. With currently available response files, their absolute flux and global spectral shape are highly uncertain, so users should limit analyses to narrow energy bands. The instrument team is conducting a comprehensive study to determine the most effective strategy for mitigating this problem.

Selecting Event Grades

Xselect/Highly recommended

The cleaned event files processed with the standard screening include five X-ray grade events [0:4], each having a different spectral resolution.(The High and Mid-primary events (Hp and Mp) achieve the highest energy resolution (see Section 5.3.3 in POG), while the other events have up to $\sim$ 6 times worse spectral resolutions. To select only Hp and Mp events for spectral analysis, type on the xselect command line,

xsel> filter GRADE "0:1"

We do not recommend grade selection for image and light curve analysis, particularly of Bright sources, as the grade branching ratios change with the count rate (see Section 6.7 for details). Users should create response matrices matching the selected grades (Section 6.5.1).

Excluding Pixel 27 Data

Xselect/Highly recommended

Pixel 27 at an edge of the array (Figure 5.1 in POG) has significantly different gain variation characteristics from the other pixels. Including its data can degrade the spectral energy resolution. Add the following filtering condition to exclude Pixel 27 events from your data.

xsel> filter column "PIXEL=0:11,13:26,28:35"

Users must also exclude this pixel from the pixel list or the detector region when generating rmf and arf response files (see Sections 6.5.1, 6.5.2). Since this pixel is on the edge of the pixel array, an on-axis point source loses only a fraction of events. The above formula also excludes the calibration pixel 12, but the description is redundant as the science cleaned event data do not contain pixel 12 events.

Excluding High Particle Background Periods

Xselect/Depending on source flux or science goal

Particle background in low Earth orbits is roughly inversely correlated to the geomagnetic cut-off rigidity or COR. Excluding low COR intervals may improve the signal-to-noise ratio of relatively faint sources, with a sacrifice of exposure time. If the background contribution is not negligible, users may study the dependence of COR on the signal quality.

The standard screening does not exclude low COR intervals. The HK file, xa000126000 .ehk, collects four different COR values during the observation (COR, COR2, COR3, CORTIME). CORTIME is the latest available table, and should best reflect the COR condition on XRISM orbit. The following xselect command selects only the CORTIME $>$ 8 interval.

xsel> select mkf "CORTIME.gt.8" mkf_name=xa000126000 .ehk
mkf_dir=path/to/ehk/directory

A caveat is that users cannot use the command's prompt mode for this selection; they must type all the command option on an xselect command line. Typing only xselect> select mkf automatically launches the "FIND MKF" task, which searches for an mkf file in the specified filter file directory. However, the XRISM mkf files (xaOBSID .mkf) do not contain the COR information, so xselect returns an error not finding the COR column.

Applying Barycentric Correction

Command line/Pulse search

With a timing resolution of $\sim$ 80 $\mu$ sec, Resolve is also good for pulse searches. With the following command, users can convert the event time to those in the solar system barycenter time system.

term> barycen xa000126000rsl_p0px1000_cl2.evt
xa000126000rsl_p0px1000_bc_cl2.evt xa000126000.orb 81.2596 -69.6441
orbext=ORBIT

Users must specify the orbext hidden option at ORBIT because the default value, PAR_ORBIT, does not work for XRISM data. Users must set the precise target position in the RA and DEC options to get the best timing information for the target.

Excluding Off-Pointing Intervals

Xselect/Spatially-resolved spectral analysis

Some users may divide Resolve FOV into multiple regions for spatially-resolved spectral analysis or exclusion of nearby bright sources. The resolve arf generator only supports data in the DET coordinates, which does not correct the observatory's attitude fluctuation. XRISM's attitude control switches between the star tracker control and the inertial reference unit (IRU) propagation in every $\sim$ 96-minute orbital cycle. During the IRU propagation periods, the attitude typically drifts by around 10 $-$ 15 arcseconds, about half of the Resolve pixel size. This drift affects approximately 20% of typical Resolve GTIs, so users must be cautious in analyzing data in the DET coordinates. One practical mitigation of spatial contamination is a stricter constraint on the pointing allowance. The following command limits the pointings within 0.1 arcmin from the nominal aim point.

xsel> select mkf "ANG_DIST.lt.0.1" mkf_name=xa000126000 .ehk
mkf_dir=path/to/ehk/directory

This screening cannot entirely exclude pointing offsets that could remain during the attitude control transition phases, each of which may take up to 4 minutes to converge. We plan to introduce a more straightforward and effective method in a future guide. We note that this drift does not affect the Xtend data analyses using the SKY coordinates.

Relaxing Screening Criteria

Command line (xapipeline)/Expert use only

If users understand Resolve data well and want to relax the screening criteria in Table 5.1 or extract data under a different condition, they may rescreen data with xapipeline with appropriate options. In this case, a run starting from stage 2 (entry_stage=2) saves processing time (see Section 5.5).

6.4 Extracting Products with Xselect

Xselect is the primary tool for extracting Resolve data products. It can filter events with areas, times, energies, or event flags and use the filtered events to create images, light curves, and spectra.

6.4.1 Loading Event Data

Go to the analysis/ directory, start a new xselect session, and read a Resolve cleaned event file with science data:

xsel> read events xa000126000rsl_p0px1000_cl2.evt .

It may ask if the new mission name is XRISM. If so, return for responding yes.

6.4.2 Making Additional Screenings

Type xselect commands in Section 6.3 after loading event data for additional screenings.

6.4.3 Extracting Images

The following commands create a 2 $-$ 10 keV SKY image.

xsel> set image sky
xsel> filter pha_cut 4000 19999
xsel> extract image
xsel> saoimage

The SKY images use a much smaller pixel size than the Resolve 's physical pixels (see section 4.5.1). They show multiple fine pixels inside a box corresponding to Resolve 's physical pixel, but these structures are artificial and should not be considered to reflect the source's spatial distribution.

Users can make images in any energy band by changing the filter pha_cut command options. The Energy $-$ PI relation is:

PI = 2000 $\times$ Energy_in_keV

Users can also look up the relation in the EBOUND extension of a Resolve rmf file. To make a DET image, change the first command to

xsel> set image det

To save the image, type:

xsel> save image N132D_rsl_sky_020100.img

Finally, remove the PI filter for further reductions if necessary.

xsel> clear pha_cutoff

6.4.4 Region (Pixel) Selection for Light Curve or Spectral Analysis

Resolve 's FOV is comparable to the mirror PSF size, so events from a source are spread over all pixels. If users analyze point sources without significant contamination from nearby sources or diffuse sources whose spatial structures are unimportant, they can choose all pixels for light curve or spectral extractions. No spatial selections are necessary.

However, some users may study spatial structures of extended sources within the FOV or need to exclude areas with significant contamination from nearby sources. Then, they need to extract events from a sub-array region. The Resolve spectrum response generator only supports analysis in the DET coordinates while Resolve has only 35 imaging pixels. So, we recommend that users define Resolve event extraction regions with pixel numbers. The following example extracts events from all pixels on the positive DETX coordinates except for Pixel 27 (see Figure 5.1, Resolve pixel map in POG).

xsel> filter column "PIXEL=0:11,13:26,28:35"

Users should investigate if attitude fluctuation during the observations is insignificant concerning the sizes of the source extraction regions. Please check "Excluding Off-Pointing Intervals" in Section 6.3.

6.4.5 Extracting Light Curves

The following example extracts a 128 sec bin light curve in the 2 $-$ 10 keV band.

xsel> filter pha_cutoff 4000 19999
xsel> set binsize 128.0
xsel> extr curve exposure=0.0
xsel> save curve N132D_rsl_b128.lc

If users do not want narrow light curve bins, increase the exposure option value between 0.0 $-$ 1.0 (see the xselect extract manual).

The grade branching ratios significantly change with the source count rates above $\sim$ 1 cts s $^{-1}$ array $^{-1}$ for on-axis point sources (see Chapter 6.7, Figure 5.9 in POG). Above this threshold, the Hp (+ Mp) event rate is nonlinear with the source flux. Users should use all X-ray event grades (Hp, Mp, Ms, Lp)^6.2 to track the flux variation.

Again, remove the PI filter for further reductions if necessary.

xsel> clear pha_cutoff

6.4.6 Extracting Spectra

Whole Spectrum

The following commands extract and save a Resolve spectrum of the whole observation. We extract only high-res primary (Hp) events using the grade filter.

xsel> filter GRADE "0:0"
xsel> extr spectrum
xsel> save spectrum N132D_rsl_Hp_src.pi

Remove the grade filter for further reductions if necessary.

xsel> clear grade

Time-resolved spectrum

Making a time-resolved spectrum requires a few additional steps. First, users set up a time filter.

xsel> filter time file N132D_int0.gti

The filter command has a few methods to apply a time filter. Please check the filter time section in the xselect user manual.

The rmf response generator needs an event file of this time interval. Users make one with the "extr events" command.

xsel> extr events
xsel> save events N132D_rsl_int0.evt
> Use filtered events as input data file ? >[yes] no

Type "no" to the last question, or users cannot use event data outside of this time interval afterward. Do not apply a grade filter as the event file needs all X-ray Grades (0 $-$ 3) events.

After this operation, users set a grade filter and extract a spectrum.

xsel> filter GRADE "0:0"
xsel> extr spectrum
xsel> save spectrum N132D_rsl_Hp_int0_src.pi

Users may repeat this procedure to create spectra at other intervals. Before that, make sure to clear the time and grade filters.

xsel> clear time
xsel> clear grade

6.5 Generating Response Files

An extracted spectrum needs the corresponding instrument response for spectral analysis. The Resolve spectral response comprises two parts. The rmf describes how the detector redistributes X-rays onto the detector PI channels. It is a 2-dimensional matrix that maps photon energy bins to a PI space. The arf describes the mirror's energy-dependent effective collecting area, assuming an X-ray source's spatial distribution in the sky. It is a one-dimensional matrix with the rmf's photon energy bins. Resolve and Xtend have nearly identical mirrors and so share the arf generation software.

6.5.1 Making an RMF file

The HEASoft XRISM tool, rslmkrmf, generates Resolve rmf files. Below is a command example.

term> punlearn rslmkrmf
term> rslmkrmfinfile=xa000126000rsl_p0px1000_cl2.evt
   outfileroot=N132D_rsl_Hp_L_src regmode=DET whichrmf=L
   resolist=0 regionfile=ALLPIX

The first command, punlearn, initializes any optional parameters of rslmkrmf that remain from the previous run. The tool, rslmkrmf, obtains the grade branching ratios from the event file fed with the infile option and calculates the efficiency for the selected grades specified at the resolist option and the region specified at the regionfile option. The input event file must contain all events of all X-ray grades 0 $-$ 3. For a time-sliced spectrum, use the event file produced at the "Time-resolved spectrum" bullet in Section 6.4.6 to match the time interval. If users do not make any region selection for extracting a spectrum, the command option can be regmode=DET and regionfile=ALLPIX^6.3. If the spectrum is extracted from a subset of the pixels, users must specify the pixel list using the pixlist option with regionfile=“NONE”. For example, the command for a spectrum without Pixel 27 is:

term> rslmkrmf infile=xa000126000rsl_p0px1000_cl2.evt
   outfileroot=N132D_rsl_Hp_L_src regmode=DET whichrmf=L
   resolist=0 regionfile=None pixlist=0-11,13-26,28-35

Make sure to connect the pixel number with "-", not ":" as for xselect.

The resolist option specifies the grade types used for the spectrum. The above example assumes a spectrum with only Hp events. If users make a spectrum from multiple grade types, list the grade numbers separated by commas (e.g., resolist=0,1 for Hp+Mp). Please refer to Table 4.1 for the number for each grade type.

A Resolve spectrum has 6 $\times$ 10 $^{4}$ PI channels, and a physical spectral model needs at least the same 6 $\times$ 10 $^{4}$ bins to reproduce the data accurately. With 3.6 $\times$ 10 $^{9}$ elements, a response matrix can be as big as $\sim$ 7 GB, which enormously slows spectral fitting processes. So, the tool can generate four types of rmfs with different file sizes.

Size	Option	Model Components
small	S	Gaussian core
medium	M	small + exponential tail & Si K $\alpha$ instrumental line
large	L	medium + escape peak
x-large	X	large + electron loss continuum (ELC)

Users can choose the rmf type using the whichrmf option. The larger rmfs reproduce the response more accurately. For example, if the response matrix does not include ELC, the source may appear to have a soft excess. However, they give progressively heavier loads for spectral fittings, so users might want to consider hybrid approaches in which an “S" matrix is used for exploratory spectral fitting, followed by finer fitting, possibly in restricted energy bands. Since the extra-large (whichrmf=X) Resolve rmfs are extremely large, users can split them into two matrices, with a coarse grid for ELC and a fine grid for the other components with splitrmf=yes and splitcomb=yes, and store them in two separate extensions of the output rmfs file. Xspec can handle such response files as regular response files. See Section Line spread function in POG on the different components of the response and the rslmkrmf help file for more details.

6.5.2 Making an arf file

There are two steps to make an arf file. The first step is to create an exposure map with xaexpmap, which calculates exposure time for each portion of the sky during the observation using the observatory's attitude and instrument FOV information.

term> punlearn xaexpmap
term> xaexpmap ehkfile=xa000126000.ehk gtifile=xa000126000rsl_p0px1000_cl2.evt
   instrume=RESOLVE badimgfile=NONE pixgtifile=xa000126000rsl_px1000_exp.gti
   outfile=N132D_rsl.expo outmaptype=EXPOSURE delta=20.0 numphi=1

The second step is to calculate the effective X-ray collecting area of the target with xaarfgen. The result depends strongly on the X-ray source's spatial distribution and position on the detector plane. Xaarfgen can, in principle, handle any X-ray sources in the sky, but that, in turn, means that users need to input multiple parameters to the tool. Below, we introduce examples of a point source and an extended source.

Xaarfgen runs a raytracing simulation by calling two tasks: i) xrtraytrace calculates the reflection and transmission of raytracing simulation photons from the assumed X-ray source, and ii) xaxmaarfgen counts the photons detected in the selected pixels. The simulation is more accurate with more raytracing photons until it hits the calibration or simulation limitation; the numphoton option controls the number of simulation photons. For a complete list of parameters and their description, please refer to the command line (term> fhelp xaarfgen) or online help.

If multiple sources with various shapes or at different locations significantly contribute to the spectrum, users should make an arf file for each source. Some contributions can come from outside the FOV. Users should assess the contributions using the simultaneously obtained Xtend data and/or other observatory data. We note that sources more than a few arcminutes away from the FOV do not significantly contribute to the X-ray event data unless they are extremely bright. Xaarfgen simulations of these sources require numerous simulation photons, many of which do not fall onto the detector, and take a long time.

Point Source

The following command shows an example for a point source (sourcetype=POINT).

term> punlearn xaarfgen
term> xaarfgen xrtevtfile=raytrace_N132D_rsl_pt.fits source_ra=81.2596
   source_dec=-69.6441 telescop=XRISM instrume=RESOLVE
   emapfile=N132D_rsl.expo regmode=DET regionfile=N132D_DET.reg
   sourcetype=POINT rmffile=N132D_rsl_Hp_L_src.rmf 
   erange="1.5 18.0 0 0" outfile=N132D_rsl_Hp_L_ptsrc.arf 
   numphoton=300000 qefile=CALDB contamifile=CALDB gatevalvefile=CALDB 
   onaxisffile=CALDB onaxiscfile=CALDB mirrorfile=CALDB 
   obstructfile=CALDB frontreffile=CALDB backreffile=CALDB 
   pcolreffile=CALDB scatterfile=CALDB imgfile=NONE

The source_ra/source_dec options input the source's J2000/FK5/ICRS coordinates.
The regmode and regionfile options define the source regions used for the spectrum extraction. Since xaarfgen supports only the DET coordinates for Resolve, regmode=DET. If users do not have a region file of the selected pixels in the DET coordinates, copy and edit the pixel region template in Section 6.8.
The rmffile option specifies the rmf file produced with rslmkrmf. Xaarfgen makes the output arf energy grid to match the energy grid of the input rmf file. The arf works only with an rmf that has a matched energy grid.
The erange option consists of four energy values. The first two values are the range in which arf values are calculated. The wider the range, the longer it will take for arf to be generated. The latter two values are only used in the IMAGE mode and are filled with zeros.
The emapfile option specifies the exposure map filename created by xaexpmap.
The xrtevtfile option specifies the simulation event file name outputted by xrtraytrace. If a named file is in the directory, xaarfgen skips the xrtraytrace run and uses the existing file for the second step.
The numphoton option specifies the number of raytracing photons allocated to each attitude histogram bin per energy grid point in the exposure map file. If the number of photons is too low, xaarfgen complains and fails to provide an arf. More raytracing photons improve the quality of arfs, but the quality is limited by the calibration accuracy or the simulator's architectural restriction. It does not improve with Poisson statistics near the higher end, while the simulation takes proportionally to the number of raytracing photons.

Extended Source: IMAGE Mode

The point source mode arf described above assumes a point-like source placed at RA = 81.2596 $\deg$ and DEC = -69.6441 $\deg$ in the Resolve array. The case of extended sources is more complicated because the PSF also has a spatially-dependent shape and extent. In addition, the typical width of the PSF is comparable to the size of the Resolve array, meaning that in a large number of cases a significant fraction of photons initially emitted from the source will leak outside the detector region. Conversely, the extracted spectrum inside a given region will also contain photons coming originally from outside the selected region. Whereas these effects are limited in the case of point-like sources, this spatial-spectral mixing (SSM) has a much more important impact in the case of extended sources. This effect, and how to deal with it, is described in detail in POG. Making and using arfs appropriately is thus critical for many science cases involving extended sources.

If the user has a high-quality image of the extended source taken with another telescope (ideally Chandra/ACIS, otherwise XMM-Newton/EPIC), they can use the arf generator in IMAGE mode (as opposed to POINT source mode):

term> punlearn xaarfgen
term> xaarfgen xrtevtfile=raytrace_N132D_rsl_img.fits source_ra=81.2596
    source_dec=-69.6441 telescop=XRISM instrume=RESOLVE
    emapfile=N132D_rsl.expo regmode=DET regionfile=N132D_DET.reg 
    sourcetype=IMAGE imgfile="/path/to/my_chandra_image.img" 
    rmffile=N132D_rsl_Hp_L_src.rmf erange="0.3 18.0 2.0 8.0" 
    outfile=N132D_rsl_Hp_L_img.arf numphoton=600000 
    qefile=CALDB contamifile=CALDB gatevalvefile=CALDB 
    onaxisffile=CALDB onaxiscfile=CALDB mirrorfile=CALDB obstructfile=CALDB
    frontreffile=CALDB backreffile=CALDB pcolreffile=CALDB scatterfile=CALDB

The two last numbers of the erange parameter now represent the energy range in which the provided image has been made. The source_ra and source_dec coordinates should now be set to the coordinates of the target source within the input image. If there are no obvious source, these coordinates should be set to the center of the image.

Important warning: A parameter that is important in the IMAGE mode is the number of input simulated photons, numphoton (see also above). If the user creates an arf for a source that is far away from the array, only a small percentage of the input photons simulated from this source will be scattered into the detector. In such a case, the raytracing sub-task xrtraytrace will require a large number of photons to create a reliable arf. Similarly, if the user uses a too wide input image, a large percentage of the input photons will be used to simulate features that are out of interest (e.g. point sources or residual background too far away from the detector region). These photons will thus be “lost” and the arf may be created with poor statistics and/or even biased low. It is strongly recommended to use an input image that is cropped to a moderate size ( $\sim$ 10 $-$ 15 arcmin wide at most). The appropriate image size depends on the brightness and distribution of surrounding sources. For example, if the Resolve FOV is on a dim part of an extended source with a very bright core, the bright core could contribute more to the outer pixels than the local dim emission. One way to check would be to run xaarfgen twice, the second time with a slightly larger image, to see what difference it makes.

Extended Source: Alternative Methods

In some cases, users may not have a proper Chandra/XMM-Newton image of their extended source. Providing that the surface brightness distribution of that source can be approximated with a 2D beta-model or a flat circle with known parameters, users can also make an arf using either the BETAMODEL or the FLATCIRCLE mode, respectively:

term> punlearn xaarfgen
term> xaarfgen xrtevtfile=raytrace_N132D_rsl_beta.fits source_ra=81.2596 
    source_dec=-69.6441 telescop=XRISM instrume=RESOLVE
    emapfile=N132D_rsl.expo regmode=DET regionfile=N132D_DET.reg
    sourcetype=BETAMODEL betapars="0.50 0.60 5.0" 
    rmffile=N132D_rsl_Hp_L_src.rmf erange="0.3 18.0 0 0"
    outfile=N132D_rsl_Hp_L_beta.arf numphoton=600000
    qefile=CALDB contamifile=CALDB gatevalvefile=CALDB
    onaxisffile=CALDB onaxiscfile=CALDB mirrorfile=CALDB obstructfile=CALDB 
    frontreffile=CALDB backreffile=CALDB pcolreffile=CALDB scatterfile=CALDB
    imgfile=NONE

This beta-model can be parametrized as $N(r) = C [1 + (r/r_c)^2]^{1.5-3\beta}$ and its parameters (betapars) are, in order: $r_c$

, $\beta$ , and the cutoff limit of the profile (0.5 $'$

, 0.60, and 5 $'$

in this example).

term> punlearn xaarfgen
term> xaarfgen xrtevtfile=raytrace_N132D_rsl_flatcircle.fits source_ra=81.2596 
    source_dec=-69.6441 telescop=XRISM instrume=RESOLVE
    emapfile=N132D_rsl.expo regmode=DET regionfile=N132D_DETx.reg
    sourcetype=FLATCIRCLE flatradius=10.0 
    rmffile=N132D_rsl_Hp_L_src.rmf erange="0.3 18.0 0 0"
    outfile=N132D_rsl_Hp_L_flatcircle.arf numphoton=600000
    qefile=CALDB contamifile=CALDB gatevalvefile=CALDB
    onaxisffile=CALDB onaxiscfile=CALDB mirrorfile=CALDB obstructfile=CALDB 
    frontreffile=CALDB backreffile=CALDB pcolreffile=CALDB scatterfile=CALDB 
    imgfile=NONE

The only parameter relevant to the flat circle model is its radius (10 $'$

in this example).

Also, note that the last two parameters of erange are set to zero. By definition, these numbers are relevant only for the IMAGE mode as they are set to the lower and upper energy bound, respectively, of the image data.

Although the BETAMODEL and FLATCIRCLE modes can be useful in a few specific cases, using the arf generator in IMAGE mode is the recommended procedure for the analysis of most extended sources.

6.6 Generating Non X-ray Background Spectra

Because of the small field of view, users cannot create Resolve background spectra from the same observation data, but must generate them using the XRISM tool rslnxbgen, which requires sufficient time to have passed since the XRISM launch to have a database that it can use. A provisional version of this data base was used during the PV phase and is being made available to GOs. Links to pages describing how to access the data base and use rslnxbgen and how to model the background can be found at

https://heasarc.gsfc.nasa.gov/docs/xrism/analysis/index.html

Updates will be reflected in those links as they are made available. If users produce a background spectrum, they should ensure that the keywords BACKSCAL in the source and background spectrum have identical values (both spectra should be drawn from the same Resolve pixels). Note also that the background and the data must be subjected to the same screening.

6.7 Bright Source Analysis

Resolve 's detector response changes at high count rates. The following list shows three count rate thresholds for an on-axis point source, above which the performance changes significantly. The thresholds for extended sources are higher as X-ray events spread over multiple pixels more evenly. There are few XRISM extended sources that fall in this category. Detailed instructions on such data are beyond the scope of this document. Please, see Chapter 8 in POG for the details.

Cause		Threshold
		(cts s $^{-1}$ )
	Significant change in the Hp grade branching ratio $^\dagger$	$\sim$ 1
	Anomalous Ls events complicate accounting for the source's Ls events $^\dagger$
	Pulse Shape Processor processing limit	$\sim$ 200
	X-ray pulse contamination by untriggered electrical cross-talk	$\gg$ 1

$^\dagger$ See Figure 5.9 in POG.

6.8 DET Pixel Region

Below are the pixel positions in the DET coordinates. Copy those selected pixels into a region file and feed it into xaarfgen through the regionfile option (see Section 6.5.2).

box(4,3,1,1,0)  # pixel 0
box(6,3,1,1,0)  # pixel 1
box(5,3,1,1,0)  # pixel 2
box(6,2,1,1,0)  # pixel 3
box(5,2,1,1,0)  # pixel 4
box(6,1,1,1,0)  # pixel 5
box(5,1,1,1,0)  # pixel 6
box(4,2,1,1,0)  # pixel 7
box(4,1,1,1,0)  # pixel 8
box(1,3,1,1,0)  # pixel 9
box(2,3,1,1,0)  # pixel 10
box(1,2,1,1,0)  # pixel 11
box(2,2,1,1,0)  # pixel 13
box(2,1,1,1,0)  # pixel 14
box(3,2,1,1,0)  # pixel 15
box(3,1,1,1,0)  # pixel 16
box(3,3,1,1,0)  # pixel 17
box(3,4,1,1,0)  # pixel 18
box(1,4,1,1,0)  # pixel 19
box(2,4,1,1,0)  # pixel 20
box(1,5,1,1,0)  # pixel 21
box(2,5,1,1,0)  # pixel 22
box(1,6,1,1,0)  # pixel 23
box(2,6,1,1,0)  # pixel 24
box(3,5,1,1,0)  # pixel 25
box(3,6,1,1,0)  # pixel 26
box(6,4,1,1,0)  # pixel 27
box(5,4,1,1,0)  # pixel 28
box(6,5,1,1,0)  # pixel 29
box(6,6,1,1,0)  # pixel 30
box(5,5,1,1,0)  # pixel 31
box(5,6,1,1,0)  # pixel 32
box(4,5,1,1,0)  # pixel 33
box(4,6,1,1,0)  # pixel 34
box(4,4,1,1,0)  # pixel 35