We present late-time optical R -band imaging data from the Palomar Transient Factory ( PTF ) for the nearby type Ia supernova SN 2011fe .
The stacked PTF light curve provides densely sampled coverage down to R \simeq 22 mag over 200 to 620 days past explosion .
Combining with literature data , we estimate the pseudo-bolometric light curve for this event from 200 to 1600 days after explosion , and constrain the likely near-infrared contribution .
This light curve shows a smooth decline consistent with radioactive decay , except over \sim 450 to \sim 600 days where the light curve appears to decrease faster than expected based on the radioactive isotopes presumed to be present , before flattening at around 600 days .
We model the 200–1600 d pseudo-bolometric light curve with the luminosity generated by the radioactive decay chains of ^ { 56 } Ni , ^ { 57 } Ni and ^ { 55 } Co , and find it is not consistent with models that have full positron trapping and no infrared catastrophe ( IRC ) ; some additional energy escape other than optical/near-IR photons is required .
However , the light curve is consistent with models that allow for positron escape ( reaching 75 % by day 500 ) and/or an IRC ( with 85 % of the flux emerging in non-optical wavelengths by day 600 ) .
The presence of the ^ { 57 } Ni decay chain is robustly detected , but the ^ { 55 } Co decay chain is not formally required , with an upper mass limit estimated at 0.014 M _ { \sun } .
The measurement of the ^ { 57 } Ni/ ^ { 56 } Ni mass ratio is subject to significant systematic uncertainties , but all of our fits require a high ratio > 0.031 ( > 1.3 in solar abundances ) .