CFHT SITELLE Sets Deepest Optical Upper Limit on Crab Nebula Forward Shock at 3.79e-17 erg cm-2 s-1 arcsec-2

The observations targeted coronal iron emission expected from the long-sought supernova remnant shell. After full-field processing, no statistically significant signal appeared in any annulus. This non-detection rules out bright, highly ionized gas at the radii where models predict the forward shock should currently reside, tightening constraints on ambient density and ionization state beyond the visible nebula. The result aligns with scenarios of expansion into a very low-density medium or delayed shock heating.

Context from prior X-ray and radio studies shows the Crab's pulsar wind nebula expanding at ~1500 km s^{-1}, yet the forward shock has remained elusive at optical wavelengths. The new limit is an order of magnitude deeper than earlier narrow-band searches and demonstrates that integral-field spectroscopy can efficiently map faint, extended structures across degree-scale fields. It also highlights the need for multi-wavelength stacking to distinguish between under-ionized gas and truly absent shell material.

Future deeper observations with XRISM or Athena could detect the shock in soft X-rays if the preshock density exceeds 0.05 cm^{-3}. Optical limits like these already exclude the brightest predicted shells and will guide target selection for those missions.

The study underscores how non-detections in well-designed wide-field surveys can still deliver quantitative astrophysical constraints when paired with realistic shock models.