Atacama Compact Array observations of the pulsar-wind nebula of SNR 0540-69.3

We present observations of the pulsar-wind nebula (PWN) region of SNR 0540-69.3. The observations were made with the Atacama Compact Array (ACA) in Bands 4 and 6. We also add radio observations from the Australia Compact Array at 3 cm. For 1.449-233.50 GHz, we obtain a synchrotron spectrum $F_{\nu } \propto \nu ^{-\alpha _{\nu }}$ , with the spectral index α_ν = 0.17 ± 0.02. To conclude how this joins the synchrotron spectrum at higher frequencies, we include hitherto unpublished AKARI mid-infrared data, and evaluate published data in the ultraviolet (UV), optical, and infrared (IR). In particular, some broad-band filter data in the optical must be discarded from our analysis due to contamination by spectral line emission. For the UV/IR part of the synchrotron spectrum, we arrive at $\alpha _{\nu } = 0.87^{+0.08}_{-0.10}$ . There is room for 2.5 × 10^-3 M_⊙ of dust with a temperature of ∼55 K if there are dual breaks in the synchrotron spectrum, one around ∼9 × 10¹⁰ Hz and another at ∼2 × 10¹³ Hz. The spectral index then changes at ∼9 × 10¹⁰ Hz from α_ν = 0.14 ± 0.07 in the radio to $\alpha _{\nu } = 0.35^{-0.07}_{+0.05}$ in the millimetre-to-far-IR range. The ACA Band 6 data marginally resolve the PWN. In particular, the strong emission $\text{$\sim$} 1\hbox{$.\!\!^{\prime \prime }$}5$ south-west of the pulsar, seen at other wavelengths, and resolved in the 3 cm data with its 0.^″8 spatial resolution, is also strong in the millimetre range. The ACA data clearly reveal the supernova remnant shell ∼20-35 arcsec west of the pulsar, and for the shell we derive α_ν = 0.64 ± 0.05 for the range 8.6-145 GHz.