Classical Novae Masquerading as Dwarf Novae? Outburst Properties of Cataclysmic Variables with ASAS-SN

TL;DR

This study compares the outburst properties of dwarf novae and classical novae using ASAS-SN data, revealing key differences in brightness and decline rates, and assessing the potential for misclassification between these two types of cataclysmic variables.

Contribution

It provides the first detailed comparison of outburst amplitudes and decline times between dwarf and classical novae, demonstrating minimal confusion in their classification.

Findings

Classical novae brighten by ~11 magnitudes, dwarf novae by ~5 magnitudes.

Outburst amplitude and decline time are positively correlated in dwarf novae.

Misclassification of distant dwarf novae as classical novae is minimal.

Abstract

The unprecedented sky coverage and observing cadence of the All-Sky Automated Survey for SuperNovae (ASAS-SN) has resulted in the discovery and continued monitoring of a large sample of Galactic transients. The vast majority of these are accretion-powered dwarf nova outbursts in cataclysmic variable systems, but a small subset are thermonuclear-powered classical novae. Despite improved monitoring of the Galaxy for novae from ASAS-SN and other surveys, the observed Galactic nova rate is still lower than predictions. One way classical novae could be missed is if they are confused with the much larger population of dwarf novae. Here, we examine the properties of 1617 dwarf nova outbursts detected by ASAS-SN and compare them to classical novae. We find that the mean classical nova brightens by ~11 magnitudes during outburst, while the mean dwarf nova brightens by only ~5 magnitudes, with the outburst amplitude distributions overlapping by roughly 15%. For the first time, we show that the amplitude of an outburst and the time it takes to decline by two magnitudes from maximum are positively correlated for dwarf nova outbursts. For classical novae, we find that these quantities are negatively correlated, but only weakly, compared to the strong anti-correlation of these quantities found in some previous work. We show that, even if located at large distances, only a small number of putative dwarf novae could be mis-classified classical novae suggesting that there is minimal confusion between these populations. Future spectroscopic follow-up of these candidates can show whether any are indeed classical novae.