A Systematic Search for Optical Outbursts in IPs Using ASAS-SN

TL;DR

This study uses ASAS-SN data to identify short optical outbursts in intermediate polars, revealing these events are more common than previously thought and may include micronovae.

Contribution

First systematic detection of short outbursts in IPs, suggesting a higher prevalence of micronovae and providing a list of candidates for further study.

Findings

At least half of the bursts may be micronovae.

Detection rate of bursts is higher than expected if all IPs exhibit micronovae.

Limited cadence affects classification confidence.

Abstract

Cataclysmic variables can show rapid increases in optical flux. Intermediate polars (IPs), a subset with strong magnetic fields that disrupt the inner accretion disc, have been thought to possess truncated discs that rarely undergo the disc-instability outbursts seen in dwarf novae. However, the discovery of micronovae and magnetic-gating bursts suggests that such events may occur even without a fully developed disc. Using data from the All-Sky Automated Survey for Supernovae (ASAS-SN), we identify a previously unrecognised population of short-timescale optical outbursts in IPs. Initial energy estimates indicate that at least half of these bursts may be consistent with micronovae, though limited cadence reduces our ability to classify each event with high confidence. These detections should therefore be regarded as evidence of short outbursts in IPs rather than definitive micronova identifications. Our results show that such bursts are more common in IPs than assumed and may include a substantial fraction of micronovae. Simulations reveal that if micronovae occur once per year at regular intervals, up to 30\% of the shortest bursts could be missed over a 10-year observing baseline. Under the same assumptions, and assuming all IPs display micronovae, we would expect 50--70\% of IPs to show these bursts, yet only 14\% of known IPs in our sample do so. This discrepancy suggests that not all IPs undergo micronovae. Overall, this work establishes the first comprehensive set of short burst detections in IPs, providing a foundation for future investigations and a list of candidate micronova systems for follow-up analysis.