  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.
