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 ± 1.8)t 3days and t X-off = (5.3 ± 1.4)(t 3)1.5days for 8 ≲ t 3 ≲ 80days. Here t 3 is the newly proposed "intrinsic" decay time during which the brightness drops by 3mag 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 versus 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 MV (15)∼ -5.7 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 10 nova light curves, in which a supersoft X-ray phase was detected, and estimated their WD masses. The models best simultaneously reproducing the optical and supersoft X-ray observations are ONeMg WDs with 1.28 ± 0.04 M ⊙ (V598 Pup), 1.23 ± 0.05 M ⊙ (V382 Vel), 1.15 ± 0.06 M ⊙ (V4743 Sgr), 1.13 ± 0.06 M ⊙ (V1281 Sco), 1.2 ± 0.05 M ⊙ (V597 Pup), 1.06 ± 0.07 M ⊙ (V1494 Aql), 1.04 ± 0.07 M ⊙ (V2467Cyg), 1.07 ± 0.07 M ⊙ (V5116 Sgr), 1.05 ± 0.05 M ⊙ (V574 Pup), and a CO WD with 0.93 ± 0.08 M ⊙ (V458 Vul). The newly proposed relationships are consistent with the emergence or decay epoch of the supersoft X-ray phase of these 10 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.
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