Evolution of the Radio Remnant of Supernova 1987A: Morphological Changes from Day 7000

We present radio imaging observations of supernova remnant 1987A at 9 GHz, taken with the Australia Telescope Compact Array over 21 years from 1992 to 2013. By employing a Fourier modeling technique to fit the visibility data, we show that the remnant structure has evolved significantly since day 7000 (mid-2006): the emission latitude has gradually decreased, such that the overall geometry has become more similar to a ring structure. Around the same time, we find a decreasing trend in the east-west asymmetry of the surface emissivity. These results could reflect the increasing interaction of the forward shock with material around the circumstellar ring, and the relative weakening of the interaction with the lower-density material at higher latitudes. The morphological evolution caused an apparent break in the remnant expansion measured with a torus model, from a velocity of 4600+150-200 km/s between day 4000 and 7000 to 2400+100-200 km/s after day 7000. However, we emphasize that there is no conclusive evidence for a physical slowing of the shock at any given latitude in the expanding remnant, and that a change of radio morphology alone appears to dominate the evolution. This is supported by our ring-only fits which show a constant expansion of 3890+/-50 km/s without deceleration between days 4000 and 9000. We suggest that once the emission latitude no longer decreases, the expansion velocity obtained from the torus model should return to the same value as that measured with the ring model.Comment: 11 pages, 5 figures, accepted for publication in ApJ, Figure 1 has been scaled dow

Search for a Radio Pulsar in the Remnant of Supernova 1987A

We have observed the remnant of supernova SN~1987A (SNR~1987A), located in the Large Magellanic Cloud (LMC), to search for periodic and/or transient radio emission with the Parkes 64\,m-diameter radio telescope. We found no evidence of a radio pulsar in our periodicity search and derived 8$\sigma$ upper bounds on the flux density of any such source of $31\,\mu$Jy at 1.4~GHz and $21\,\mu$Jy at 3~GHz. Four candidate transient events were detected with greater than $7\sigma$ significance, with dispersion measures (DMs) in the range 150 to 840\,cm$^{-3}\,$pc. For two of them, we found a second pulse at slightly lower significance. However, we cannot at present conclude that any of these are associated with a pulsar in SNR~1987A. As a check on the system, we also observed PSR~B0540$-$69, a young pulsar which also lies in the LMC. We found eight giant pulses at the DM of this pulsar. We discuss the implications of these results for models of the supernova remnant, neutron star formation and pulsar evolution.Comment: 7 pages, 3 figures, 2 tables. Accepted for publication in MNRA

High-resolution radio observations of SNR 1987A at high frequencies

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\farcs35\times0\farcs23 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\farcs25. The flux density is 40$\pm$2 mJy at 44 GHz and 81$\pm$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 $\alpha=-0.80$ ($S_{\nu}\propto\nu^{\alpha}$). 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 H$\alpha$ observations. Unlike the H$\alpha$ 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.Comment: References updated. High resolution version may be found at http://ict.icrar.org/store/staff/gio/Papers/Zanardo_2013.pd

Spectral and morphological analysis of the remnant of Supernova 1987A with ALMA & ATCA

We present a comprehensive spectral and morphological analysis of the remnant of Supernova (SN) 1987A with the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/submillimeter Array (ALMA). The non-thermal and thermal components of the radio emission are investigated in images from 94 to 672 GHz ($\lambda$ 3.2 mm to 450 $\mu$m), with the assistance of a high-resolution 44 GHz synchrotron template from the ATCA, and a dust template from ALMA observations at 672 GHz. An analysis of the emission distribution over the equatorial ring in images from 44 to 345 GHz highlights a gradual decrease of the east-to-west asymmetry ratio with frequency. We attribute this to the shorter synchrotron lifetime at high frequencies. Across the transition from radio to far infrared, both the synchrotron/dust-subtracted images and the spectral energy distribution (SED) suggest additional emission beside the main synchrotron component ($S_{\nu}\propto\nu^{-0.73}$) and the thermal component originating from dust grains at $T\sim22$ K. This excess could be due to free-free flux or emission from grains of colder dust. However, a second flat-spectrum synchrotron component appears to better fit the SED, implying that the emission could be attributed to a pulsar wind nebula (PWN). The residual emission is mainly localised west of the SN site, as the spectral analysis yields $-0.4\lesssim\alpha\lesssim-0.1$ across the western regions, with $\alpha\sim0$ around the central region. If there is a PWN in the remnant interior, these data suggest that the pulsar may be offset westward from the SN position.Comment: ApJ accepted. 21 pages, emulateapj. References update

Dust Production and Particle Acceleration in Supernova 1987A Revealed with ALMA

Supernova (SN) explosions are crucial engines driving the evolution of galaxies by shock heating gas, increasing the metallicity, creating dust, and accelerating energetic particles. In 2012 we used the Atacama Large Millimeter/Submillimeter Array to observe SN 1987A, one of the best-observed supernovae since the invention of the telescope. We present spatially resolved images at 450um, 870um, 1.4mm, and 2.8mm, an important transition wavelength range. Longer wavelength emission is dominated by synchrotron radiation from shock-accelerated particles, shorter wavelengths by emission from the largest mass of dust measured in a supernova remnant (>0.2Msun). For the first time we show unambiguously that this dust has formed in the inner ejecta (the cold remnants of the exploded star's core). The dust emission is concentrated to the center of the remnant, so the dust has not yet been affected by the shocks. If a significant fraction survives, and if SN 1987A is typical, supernovae are important cosmological dust producers.Comment: ApJL accepte

The First VLBI Image of the Young, Oxygen-Rich Supernova Remnant in NGC 4449

We report on sensitive 1.4-GHz VLBI radio observations of the unusually luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the first well-resolved image of this object. The remnant's radio morphology consists of two approximately parallel bright ridges, suggesting similarities to the barrel shape seen for many older Galactic supernova remnants or possibly to SN 1987A. The angular extent of the remnant is 65 x 40 mas, corresponding to (3.7 x 2.3) x 10^{18} (D/3.8 Mpc) cm. We also present a new, high signal-to-noise optical spectrum. By comparing the remnant's linear size to the maximum velocities measured from optical lines, as well as using constraints from historical images, we conclude that the supernova explosion occurred between ~1905 and 1961, likely around 1940. The age of the remnant is therefore likely ~70 yr. We find that SNR 4449-1's shock wave is likely still interacting with the circumstellar rather than interstellar medium.Comment: 7 pages, Accepted for publication in MNRA