# Multiwavelength monitoring of a very active dwarf nova AX J1549.8$-$5416 with an unusually high duty cycle

## TL;DR

This study presents a comprehensive multiwavelength analysis of the highly variable dwarf nova AX J1549.8$-$5416, revealing detailed timing, spectral, and emission characteristics across optical, UV, and X-ray bands, and estimating its high duty cycle.

## Contribution

It provides one of the most complete multiwavelength datasets for a dwarf nova, refining understanding of emission relationships and accretion processes in such systems.

## Key findings

- UV delay of 1.0-5.4 days during outburst rise
- X-ray suppression during optical peak and recovery at outburst end
- High duty cycle (~50%) indicating significant quiescent X-ray luminosity

## Abstract

We present the results of our analysis of new optical, ultraviolet (UV) and X-ray observations of a highly variable source $-$ AX J1549.8$-$5416. Both the detection of several fast rise, exponential decay outbursts in the optical light curve and the lack of He II emission lines in the optical spectra suggest AX J1549.8$-$5416 is a cataclysmic variable of the dwarf nova (DN) type. The multiwavelength analysis of three mini-outbursts and one normal outburst represent one of the most complete multiwavelength studies of a DN and help to refine the relationship between the X-ray, UV and optical emission in this system. We find that the UV emission is delayed with respect to the optical by $1.0-5.4$ days during the rising phase of the outburst. The X-ray emission is suppressed during the peak of the optical outburst and recovers during the end of the outburst. From our analysis of archival Swift, Chandra and XMM-Newton observations of AX J1549.8$-$5416, we estimate this DN has a high duty cycle ($\sim50\%$), suggesting a quiescent X-ray luminosity larger than $10^{32}$ erg/s. We also find the X-ray and UV flux are roughly anti-correlated. Furthermore, we find that, at low X-ray fluxes, the X-ray spectrum is well described by a single temperature thermal plasma model, while at high X-ray fluxes, an isobaric cooling flow model also works. We find that the maximum temperature of the plasma in quiescence is significantly higher than that in outburst.