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Classical novae are expected to emit gamma rays during their explosions. Themost important contribution to the early gamma-ray emission comes from theannihilation with electrons of the positrons generated by the decay of 13N and18F. The photons are expected to be down-scattered to a few tens of keV, andthe emission is predicted to occur some days before the visual discovery and tolast ~2 days. Despite a number of attempts, no positive detections of suchemission have been made, due to lack of sensitivity and of sky coverage.Because of its huge field of view, good sensitivity, and well-adapted energyband, Swift/BAT offers a new opportunity for such searches. BAT data can beretrospectively used to search for prompt gamma-ray emission from the directionof novae after their optical discovery. We have estimated the expected successrate for the detection with BAT of gamma rays from classical novae using aMonte Carlo approach. Searches were performed for emission from novae occurringsince the launch of Swift. Using the actual observing program during the first2.3 years of BAT operations as an example, and sensitivity achieved, weestimate the expected rate of detection of classical novae with BAT as~0.2-0.5/yr, implying that several should be seen within a 10 yr mission. Thesearch for emission in the directions of the 24 classical novae discoveredsince the Swift launch yielded no positive results, but none of these was knownto be close enough for this to be a surprise. Detections of a recurrent nova(RS Oph) and a nearby dwarf nova (V455 And) demonstrate the efficacy of thetechnique. The absence of detections is consistent with the expectations fromthe Monte Carlo simulations, but the long-term prospects are encouraging givenan anticipated Swift operating lifetime of ~10 years.

Detectability of gamma-ray emission from classical novae with Swift/BAT