High-Resolution Radio Observations of the Remnant of Sn 1987a at High Frequencies

Abstract

We present new imaging observations of the remnant of Supernova (SN) 1987A at 44 GHz, performed in 2011 with the Australia Telescope Compact Array (ATCA). The 0. ′′ 35 × 0. ′′ 23 resolution of the diffraction-limited image is the highest achieved to date in high-dynamic range. We also present a new ATCA image at 18 GHz derived from 2011 observations, which is super-resolved to 0. ′′ 25. The flux density is 40±2 mJy at 44 GHz and 81±6 mJy at 18 GHz. At both frequencies, the remnant exhibits a ring-like emission with two prominent lobes, and an east-west brightness asymmetry that peaks on the eastern lobe. A central feature of fainter emission appears at 44 GHz. A comparison with previous ATCA observations at 18 and 36 GHz highlights higher expansion velocities of the remnant eastern side. The 18-44 GHz spectral index is α = −0.80 (S ν ∝ ν α ). The spectral index map suggests slightly steeper values at the brightest sites on the eastern lobe, whereas flatter values are associated with the inner regions. The remnant morphology at 44 GHz generally matches the structure seen with contemporaneous X-ray and Hα observations. Unlike the Hα emission, both the radio and X-ray emission peaks on the eastern lobe. The regions of flatter spectral index align and partially overlap with the optically-visible ejecta. Simple free-free absorption models suggest that emission from a pulsar wind nebula or a compact source inside the remnant may now be detectable at high frequencies, or at low frequencies if there are holes in the ionised component of the ejecta.

Radio Emission from Supernova Remnants

“…Observations at higher frequencies (18, 36, 44 and 94 GHz) undertaken with ATCA (Manchester et al 2005;Potter et al 2009;Lakićević et al 2012;Zanardo et al 2013), followed by observations at 110, 215, and 345 GHz with the Atacama Millimeter/Submillimeter Array (ALMA), show the same asymmetric double lobe morphology Zanardo et al 2014). Also ALMA observations revealed the presence of one of the largest masses of cold dust (greater than 0.2 M ) ever measured in a SNR.…”

The Radio Remnant of Supernova 1987A − A Broader View

“…On a local scale, high-resolution spectral maps have consistently shown steeper synchrotron spectral indices and, thus, lower compression ratios, mainly localised on the eastern lobe of the remnant (see Figure 5; Zanardo et al 2013Zanardo et al , 2014. A softer particle spectrum on the eastern regions and the higher Mach numbers measured for the eastbound shocks , are likely to create the conditions for magnetic field amplification (Bell & Lucek 2001) with efficient cosmic ray production.…”

“…2, while the VLBI image has a restoring beam of 61 × 95 mas. The VLBI observations have been instrumental to align images prior to the derivation of spectral index maps (see Zanardo et al 2013Zanardo et al , 2014. Credit: Zanardo (2014).…”

“…To match the average epoch of ALMA observations, ATCA data are scaled to day 9280, via exponential fitting parameters derived for ATCA flux densities from day 8000, as measured at 1.4, 8.6 and 9 GHz (Zanardo 2014). The hollow red diamond indicates the central emission measured at 44 GHz as reported by Zanardo et al (2013) scaled to day 9280. The difference between possible spectral fits is highlighted in light orange.…”

Ultraviolet spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analogue

“…After about 1500 d, however, the radio emission brightened and the expansion speed slowed to only 3500 km s −1 (Gaensler et al 1997(Gaensler et al , 2000Zanardo et al 2013). In order to reach the angular scale of the resolved radio emission when it turned on, the blast wave must have been expanding at about 35 000 km s −1 for those first 1500 d (Staveley-Smith et al 1993).…”