cat ⚬ˢʰ｜encounter｜ENC｜20260707133106-00-•

Great questions! Let me clarify:

1. Clarification on “Works as Additive Model”

Your advisor is absolutely correct - simplcut is fundamentally a Comptonization model that needs seed photons. It should NOT be used as an additive model in real physics.

What I meant was purely a diagnostic test to isolate the bug:

The Physics (Correct Use):

% CORRECT: Convolution model operating on seed spectrum
fit_fun("diskbb(1)*simplcutx(2)");  % diskbb provides seed photons
                                      % simplcutx Comptonizes them

My Diagnostic Test (Wrong Physics, but useful for debugging):

% WRONG PHYSICS but useful diagnostic:
% Changed .dat file from type "con" to type "add"
fit_fun("simplcutx(1)");  % ISIS treats it as generating photons from scratch
                          % Physically incorrect, but tests if Fortran works

Why I did this: To prove the bug is in ISIS’s convolution wrapper, not your Fortran code. When the EXACT same Fortran code works with type “add”, it proves:

✓ The Fortran compiles correctly
✓ The Fortran executes correctly
✗ ISIS’s “con” type wrapper is broken

Bottom line: The model MUST be type “con” for correct physics. The “add” test was just to demonstrate ISIS has a bug.

2. Reproducible Steps for Investigation

Here are the exact steps to reproduce what I found:

Test 1: Demonstrate the Bug (Convolution Model Fails)

Create File

Since I can’t create files, let me give you the exact reproducible steps in detail:

Reproducible Steps to Investigate the Bug

Test 1: Demonstrate the Bug (Convolution Model)

# 1. Create a test directory
cd /home/cfacal/work/cal-abc-xyz/astro-tools/workbench/isis_simplcut
mkdir -p bug_investigation
cd bug_investigation
 
# 2. Create minimal Fortran code with debug output
cat > simplcutx.f << 'EOF'
c Minimal model with debug output
      subroutine simplcutx(ear,ne,param,IFL,photar,photer)
      integer i,ne,IFL
      real ear(0:ne),param(4),photar(ne),photer(ne)
      
      write(0,*) '=== FORTRAN CALLED ==='
      write(0,*) 'ne=', ne, ' IFL=', IFL
      write(0,*) 'params=', (param(i), i=1,4)
      write(0,*) 'Input photar(1:3)=', (photar(i), i=1,min(3,ne))
      
c Identity convolution (just pass through)
      do i=1,ne
         photer(i) = 0.0
      end do
      
      write(0,*) 'Output photar(1:3)=', (photar(i), i=1,min(3,ne))
      write(0,*) '=== FORTRAN RETURNING ==='
      
      RETURN
      END
EOF
 
# 3. Create model definition (type "con")
cat > simplcutx.dat << 'EOF'
simplcutx          4     0.05     1.e6      simplcutx   con     0
Gamma        " "   2.5     1.0     1.1     4.0     10.0         0.01
FracSctr     " "   0.02    0.0     0.      0.4     1.0          0.001
ReflFrac     " "   1.      -5.0      0.       5.       100           -1
kT_e        "keV" 50     -100      5       1e3       1e3          2
EOF
 
# 4. Create ISIS test script
cat > test_con.sl << 'EOF'
require("xspec");
build_xspec_local_models("."; lmodel="simplcutx.dat");
load_xspec_local_models("."; lmodel="simplcutx.dat");
fit_fun("diskbb(1)*simplcutx(2)");
set_par("diskbb(1).Tin", 1.0);
variable flux = eval_fun([1:5:1], [2:6:1]);
vmessage("SUCCESS! Flux: %S", flux);
EOF
 
# 5. Run the test
isis test_con.sl 2>&1

Expected Output:

=== FORTRAN CALLED ===
ne= 5  IFL= 0
params= 2.5  0.02  1.0  50.0
Input photar(1:3)= 5.4e-4  5.3e-4  4.5e-4
Output photar(1:3)= 5.4e-4  5.3e-4  4.5e-4
=== FORTRAN RETURNING ===
Stack Underflow Error                    ← THE BUG
***string***:1:__isis_tmp_ffname__:Stack Underflow Error

Key Observation: Function executes successfully → ISIS fails afterward

Test 2: Diagnostic Test (Same Code as “Additive”)

# Still in bug_investigation directory
 
# 6. Create alternate .dat file (type "add" instead of "con")
cat > simplcutx_add.dat << 'EOF'
simplcutx          4     0.05     1.e6      simplcutx   add     0
Gamma        " "   2.5     1.0     1.1     4.0     10.0         0.01
FracSctr     " "   0.02    0.0     0.      0.4     1.0          0.001
ReflFrac     " "   1.      -5.0      0.       5.       100           -1
kT_e        "keV" 50     -100      5       1e3       1e3          2
EOF
#          Note: Only difference is "add" instead of "con" ^^^
 
# 7. Clean previous build
rm -f *.o *.cxx *.h *.so Makefile pkgIndex.tcl xspec.inc
 
# 8. Create test script for additive version
cat > test_add.sl << 'EOF'
require("xspec");
build_xspec_local_models("."; lmodel="simplcutx_add.dat");
load_xspec_local_models("."; lmodel="simplcutx_add.dat");
fit_fun("simplcutx(1)");  % Note: No seed spectrum needed when type="add"
variable flux = eval_fun([1:5:1], [2:6:1]);
vmessage("SUCCESS! Flux: %S", flux);
EOF
 
# 9. Run the test
isis test_add.sl 2>&1

Expected Output:

=== FORTRAN CALLED ===
ne= 5  IFL= 0
params= 2.5  0.02  1.0  50.0
Input photar(1:3)= 0.0  0.0  0.0     ← No seed spectrum (wrong physics!)
Output photar(1:3)= 0.0  0.0  0.0
=== FORTRAN RETURNING ===
SUCCESS! Flux: Double_Type[5]        ← IT WORKS!

Key Observation: SAME Fortran code works with type “add”

What This Proves

Test 1 (Convolution):

✓ Fortran function is called
✓ Receives valid seed photons from diskbb
✓ Function completes successfully
✗ ISIS throws Stack Underflow AFTER return ← THE BUG

Test 2 (Additive):

✓ EXACT same Fortran code
✓ Compiles and runs successfully
✗ Wrong physics (no seed photons)
✓ Proves Fortran code is fine

Conclusion:

The bug is in ISIS’s wrapper for local convolution models, not in your Fortran code.

Important Notes for Your Advisor

The model IS fundamentally a convolution model - it needs seed photons
Test 2 is WRONG PHYSICS - just a diagnostic to prove the Fortran works
The correct usage is: diskbb(1)*simplcutx(2) (convolution)
The bug: ISIS’s _xspec_con_f_hook wrapper fails after the Fortran returns
Workaround: Use XSPEC instead of ISIS (model works there)

Let me know if you need any clarification on these steps!
@Claude ○˒
(echo:: @ ᯤ)

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