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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
A New PROS Task for Calculating HRI Source Intensities
or Upper Limits
J. C. Chen, M. A. Conroy, J. DePonte, and F. A. Primini
Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA
02138
Abstract. The srcinten task provides PROS users with a tool to comí
pute count rates for point sources in the ROSAT High Resolution Imager
(HRI). Count rates are corrected for point response function, vignetting,
and quantum efficiency effects. Corrected upper limits are cited at locaí
tions where the count rate falls below the source detection threshold.
1. Introduction
Approximate count rates for point sources observed with the ROSAT HRI can
easily be calculated by using the IRAF/PROS imcnts task to accumulate Xíray
events in a (usually) circular aperture about the source, with background derived
from either a model background map or a region adjacent to the source. Howí
ever, more accurate determinations require corrections for various instrumení
tal effects, such as reduction in effective area for offíaxis sources (vignetting),
fraction of total source counts in the source aperture (encircled energy), and
variations in detector quantum efficiency with event position on the detector.
Although levelí1 processing properly corrects for such effects, PROS users who
wish to apply nonístandard data screens, or who wish to study sources not
detected in standard processing, are required to compute and apply such correcí
tions by hand. The task is complicated by the fact that the correction factors
include contributions from a range of offíaxis angles or detector positions, due
to the induced ¦3 0
satellite wobble present in most observations.
Srcinten is a new PROS task that computes and applies the above correcí
tion factors to the net counts in a useríspecified aperture. If significance of the
net counts is less than a useríspecified threshold, an upper limit is calculated.
As a pedagogical exercise, the task was designed to minimize additional coding
and maximize the use of other existing tools in PROS and IRAF. It will be fully
integrated into IRAF and available in the PROS xray.xspatial package in the
next PROS release.
2. Calculation of Correction Factors
Both the vignetting and encircled energy corrections are primarily functions of
source offíaxis angle. The existing PROS task QPSPEC is used both to compute
net counts in the source aperture and to compile an offíaxis angle histogram,
1

2
whose entries comprise the fraction, f i , of counts in the aperture corresponding
to offíaxis angle ` i .
For each angle ` i , the vignetting correction V (` i ) at 1 keV is evaluated using
the function in Figure 1 (David et al. 1993). The average vignetting correction
is then
! V ? =
X
i
f i V (` i ):
The encircled energy correction is simply the integral of the point response
function (PRF) within the source aperture. Although in general the ROSAT
HRI PRF exhibits significant azimuthal asymmetries for offíaxis sources, the
azimuthallyíaveraged function P (r; ` i ) may be used to estimate encircled ení
ergy (David et al. 1993). For each angle ` i in the offíaxis angle histogram, the
encircled energy within aperture radius R is given by
E(R; ` i ) =
Z R
0
P (r; ` i )2‹rdr:
E(R) for various offíaxis angles ` is shown in Figure 1. The average encircled
energy is given by
! E(R) ? =
X
i
f i E(R; ` i ):
Relative quantum efficiency variations of approximately \Sigma10% across the
detector are present in the ROSAT HRI, and their correction requires knowledge
of the event positions in detector coordinates (i.e., a coordinate system fixed to
the detector). These coordinates are present in the event structure of QPOE
files generated with PROS version 2.3, and may be added to earlier QPOE files
using the PROS task upqpoerdf.
The PROS task qpcopy is used to generate an image of the events in the
source aperture, blocked at a resolution equal to that of the calibration quantum
efficiency map (QE). The average quantum efficiency correction is then
! QE ? =
X
i;j
f i;j QE i;j ;
where f i;j is the fraction of total counts in binned detector pixel (i, j).
3. Structure Chart
Srcinten uses the IRAF CL script language and, as described above, takes
advantage of a number of existing IRAF/PROS tools. The structure chart in
Figure 2 shows all the tools used in srcinten, and a brief overview is given
below.
qpspec extracts the total counts from the source and background regions
and computes net counts and error. It also produces an offíaxis angle histogram
file in table format.
qpcopy copies one QPOE file to another through region filtering, to gení
erate a source image in detector coordinates.
imcalc multiplies the source and QE images.

3
Figure 1. ROSAT XRT vignetting function at the energy 1 keV
(left). ROSAT HRI encircled energy at various offíaxis angles (right).
& PRF corrections
QE map
Output ccr.tab
Correct Count Rate or Upper Limit
. QPOE file
. source position & region
. background position & region
Entries From User
. compute vignetting,
TABPAR, KEYPAR
. get net count,
ra,dec,x & y
QPSPEC
. get source aspect histogram
QPCOPY, IMCALC, & IMCNTS
. compute QE correction
TCALC
Figure 2. Structure Chart for srcinten.
imcnts computes the total counts in the source image.
tabpar, keypar, tinfo get keywords and values from tables, needed in
computing correction factors.
tcalc performs arithmetic operations on columns to compute corrections.

4
4. Using srcinten in PROS
Before using srcinten, users are required to load the xspectral, xspatial,
ximages, images, and TABLES packages. An example srcinten run is shown
below, and the output table is displayed in Table 1. Both the count rate and
upper limit columns are included in the output, but for sources above the userí
specified significance threshold, only the count rate column is filled.
xs? srcinten
source qpoe file: xdata$rh110267n00.qp
source region descriptor: circle 2427. 2806. 82.
background qpoe file: xdata$rh110267n00.qp
bkgd region descriptor: annulus 2427. 2806. 82. 100.
output root name (×ccr.tab): src
íí Creating aspect histogram table from qpoe íí
íí Computing vignetting correction factor íí
íí Computing prf correction factor íí
íí Computing qe correction factor íí
íí SRCINTEN created Output table: src×ccr.tab íí
ra dec x y radius cntrate cntraterr
deg deg pix pix asec c=ks c=ks
331.8 45.7 2427. 2806. 41. 83.1 10.7
uplrat totcnt bkgcnt exptim prf qe vign
c=ks c c sec none none none
INDEF 188. 43. 2124.4 0.96 0.93 0.92
Table 1. Output table src ccr.tab
5. Conclusions
Although srcinten is a simple tool, it reproduces much of the functionality of its
HRI levelí1 processing counterpart and demonstrates the utility of assembling
existing PROS tasks into CL scripts to address real analysis problems. Possible
future enhancements include computing an energyíaveraged vignetting function
weighted by the source spectrum, allowing apertures of arbitrary shape, and
correcting for spatiallyívarying exposure through an HRI exposure map.
Acknowledgments. This work is partially supported by NASA contract
NAS5--30934.

5
References
David, L. P., Harden, F. R., Kearns, K. E., & Zombeck, M. V. 1993, The ROSAT
High Resolution Imager