On the basis of the recently developed universal decline law of classical novae , we propose prediction formulae for supersoft X-ray on and off times , i.e. , t _ { X - on } = ( 10 \pm 1.8 ) t _ { 3 } days and t _ { X - off } = ( 5.3 \pm 1.4 ) ( t _ { 3 } ) ^ { 1.5 } days for 8 \lesssim t _ { 3 } \lesssim 80 days .
Here t _ { 3 } is the newly proposed “ intrinsic ” decay time during which the brightness drops by 3 magnitudes from optical maximum along our universal decline law fitted with observation .
We have determined the absolute magnitude of our free-free emission model light curves and derived maximum magnitude vs. rate of decline ( MMRD ) relations .
Our theoretical MMRD relations are governed by two parameters , one is the white dwarf ( WD ) mass and the other is the initial envelope mass at a nova outburst ; this second parameter explains the scatter of MMRD points of individual novae .
Our theoretical MMRD relations are also in good agreement with the well-known empirical formulae .
We also show another empirical relation of M _ { V } ( 15 ) \sim - 5.7 \pm 0.3 based on the absolute magnitude of our model light curves , i.e. , the absolute magnitude at 15 days after optical maximum is almost common among various novae .
We analyzed ten nova light curves , in which a supersoft X-ray phase was detected , and estimated their WD masses .
The models best reproducing simultaneously the optical and supersoft X-ray observations are ONeMg WDs with 1.28 \pm 0.04 ~ { } M _ { \sun } ( V598 Pup ) , 1.23 \pm 0.05 ~ { } M _ { \sun } ( V382 Vel ) , 1.15 \pm 0.06 ~ { } M _ { \sun } ( V4743 Sgr ) , 1.13 \pm 0.06 ~ { } M _ { \sun } ( V1281 Sco ) , 1.2 \pm 0.05 ~ { } M _ { \sun } ( V597 Pup ) , 1.06 \pm 0.07 ~ { } M _ { \sun } ( V1494 Aql ) , 1.04 \pm 0.07 ~ { } M _ { \sun } ( V2467 Cyg ) , 1.07 \pm 0.07 ~ { } M _ { \sun } ( V5116 Sgr ) , 1.05 \pm 0.05 ~ { } M _ { \sun } ( V574 Pup ) , and a CO WD with 0.93 \pm 0.08 ~ { } M _ { \sun } ( V458 Vul ) .
The newly proposed relationships are consistent with the emergence or decay epoch of the supersoft X-ray phase of these ten novae .
Finally , we discuss the mechanism of shock-origin hard X-ray component in relation to the emergence of companion star from the WD envelope .