2025-07-23 TSO CT WG Meeting Notes

Sophie presents work they have been doing with Munazza Alamduring the summer on their internship:

• Given NIRSpec is such a popular JWST mode, they got to investigate some systematics in a systematic way.
  
  
• For high-resolution observations (G395H), two detectors get to see part of the spectra of the target during a TSO (NRS1: 3-4 um, NRS2: 4-5 um).
  
  
• While you expect minimal differences between the two detectors a-priori, this is not what is observed: NRS1 has a much steeper slope than NRS2. This trend occurs throughout observations. Not clear what is happening. 
  
  
• They fitted white light curves for transits and systematics, bin wavelengths to get spectroscopic lightcurves, and studied the slopes. Studied this for a set of 6 exoplanets.
  
  
  • Fit binned light curves for radius, mid tranist time and systematics. For a given detector, there's a clear wavelength-dependance of the slope (which can be visually checked on transit plots).
    
    
  • When plotting the slope against wavelength for all targets:
    
    
    • There's a clear correlation between those and stellar magnitude. This is in particular strongest for NRS1 than NRS2.
    • Also with group number. Likely correlated with magnitude (brighter objects get less groups to not saturate detector).
    • Not with number of integration.
      
      
• Further work:
  
  
  • Add more planets — likely more targets that have smaller planets.
  • Next add low and medium resolution NRS1 observations.
    
    
• Comments:
  
  
  • Everett Schlawinnotes that countrate is what they do to use because that normalizes by subarray size. Perhaps this should be used too here. Also asks if this is an exponential trend or a linear trend, as in NIRCam a similar effect is seen but is an exponential.
    
    
  • Nestor Espinoza notes that this exponential can be fitted to NIRSpec to — eg, WASP-39 b seems better fit by an exponential than a slope.
  • Everett Schlawin asks whether you've looked at telemetry. Munazza Alam notes that not within the scope of this, but idea is to pass this back to detector folks so they can take a look at this.

Munazza Alam explains that this is work that has been going on on NIRspec for a bit — Leonardo Ubeda has been working on this for a bit before she arrived, this work is now being completed:

• Munazza Alam introduces that charge migration is the "brighter fatter effect" (BFE) — different than the BFE on MIRI.
  
  
• Notes a study by Regan & Bergeron in 2023 on the impact of non-linearity errors on transit depths. Basically, 1% errors on non-linearity map to 10 ppm errors if you assume box-shaped transits. Arpita Roy worked on actual simulations with non-linearity uncertainties on NIRISS and showed that this could be up to ~100 ppm in worst-case scenarios.
  
  
• With this motivation, this study focuses on studying the phase-curve of WASP-121 b to study charge migration. Data was reduced with the JWST pipeline, then spectra was traced and extracted.
  
  
  • First took a look at linearity as a function of group number:
    
    
    • Fitted lines up to 10 groups (where counts get to 10,000 counts), and then subtracted this prediction to groups > 10. Sees both excess of flux and loss of flux for those groups when excess flux is observed: E = Flux - Line Prediction.
      
      
    • Then, lines where fitted to this excess flux E for groups > 10. When the image of those "slopes" is plotted one can see directly charge migration in the images. This image is denoted below as the "slope" image.
      
      
    • Then, took a look at those as a function of fluence by looking at those at (a) first eclipse, (b) transit and (c) second eclipse:
      
      
      • Difference of the "slope" images between first and second eclipse is consistent with noise.
        
        
      • Difference with the transit "slope" image with the first eclipse has some spatial structure.
        
        
• Questions:
   
  • Everett Schlawin notes if the transit v/s eclipse slope difference shows up somehow in the linearity correction itself.
    
    
  • Taylor Bell notes whether some of this can be being further confused by the imshow interpolation. Perhaps good to turn this off.
    
    
  • Nestor Espinoza clarifies figure of 100 ppm above. 

• Idea: move the LRS SLITLESSPRISM. Updates on this:
  
  
  • How this worked before: 
    
    
    • LRS would acquire the target.
    • Then does offset, and moves star up.
    • Disperses
      
      
  • Need to check that this same thing works for the new positions:
    
    
    • A concern was whether the target verification could leave some persistence on the regions where you'll disperse on the new position.
      
      
    • Hannah Diamond-Lowe checked whether this was the case in some data of bright targets — the pixels where TV happens don't show any special persistence on the TSOs.
  • Next, discussed possible new sizes for the slits:
    
    
    • One proposition is a new 52-pixel width box, that could get you to, e.g., Proxima Cen with 2-5 groups with partial saturation.
      
      
    • For GJ 486 b, observations used 6 groups, with 52 pixels you could get 3 more groups (9 group integrations). 
• Some questions/comments:
  
  
  • Taylor Bellasks why not do the TV closer to the science position. 
  • Nestor Espinoza suggests, for the MINI subarray, perhaps moving the subarray to longer wavelengths, so you get away from saturating regions. Idea would be to create a window for very bright objects with MIRI (e.g., Proxima Cen).