The Salamander: A case study of the magnetic field and peculiar morphology of G309.8-2.6 through radio polarimetry

Abstract

Studying the interaction between core-collapse supernova remnants (SNRs) and their surrounding environments is essential to understanding the mechanism for energy transfer to the interstellar medium (ISM) and the intrinsic physical properties of these remnants. In this paper, we focus on G309.8-2.6. Our new observations reveal that this object includes an SNR shell with a relic pulsar wind nebula (PWN) that extends well beyond the emission that has been previously observed in X-rays. We present new radio continuum and polarization images of G309.8-2.6 from the Evolutionary Map of the Universe (EMU) and Polarization Sky Survey of the Universe's Magnetism (POSSUM) surveys with the Australian Square Kilometre Array Pathfinder (ASKAP). The images reveal the complex and peculiar morphology of G309.8-2.6. The linear polarization displays an atypical S-shaped morphology and a highly ordered magnetic field. The rotation measure (RM) map shows a large-scale gradient or possible sign reversal, depending on the foreground RM. We reprocessed archival X-ray observations from Chandra and eROSITA, and retrieved archival H$α$ and infrared observations. We performed a joint analysis of the multiwavelength data and proposed scenarios to explain the unusual shape. Our results place new constraints on the magnetic field of G309.8-2.6, including its environment, and demonstrate the power of polarization observations in probing the properties of SNRs.

Figures (15)

• Figure 1: Illustration of the main features of G309.8$-$2.6 mentioned in Section \ref{['sec:introduction']}. The contours are from the ASKAP total intensity. Note that the SNR here includes the relic PWN and the eastern shell outlined by the orange polygon.
• Figure 2: Left panel: The combined total intensity image from EMU and STAPS over a $4\degr \times 3\degr$ region. Right panel: The total intensity image from EMU for the $1.5\degr \times 1.5\degr$ region outlined with the yellow square in the left panel. Both images are at 943 MHz with an angular resolution of $18\arcsec$, and displayed in cubehelix colour scheme Green2011.
• Figure 3: Histogram of added flux densities after combining EMU and STAPS images.
• Figure 4: The EMU and STAPS total intensity combined image of the G309.8$-$2.6 at 943 MHz, where the background and point sources have been subtracted. The resolution of this image is 18$\arcsec$. The location of PSR J1357$-$6429 is illustrated by a yellow diamond. Cyan and white polygons mark the primary emission structures with the texts showing the nomenclature, and red arrows indicate the three limbs discussed in Section \ref{['sec:overall-radio']}.
• Figure 5: Upper panel: Peak linearly polarized intensity map at 943 MHz in square root color scale, with white contours for Stokes $I$ at levels of 5$\sigma_I$, 10$\sigma_I$, and 15$\sigma_I$. Middle panel: RM map, where pixels with $P < 8\sigma_P$ are masked. The contours outline the boundary of $P$. Lower panel: Same as the middle panel, but overlaid with bars showing the orientation of the transverse magnetic field, corrected for Faraday rotation. The bar length is proportional to the square root of the magnitude of $P$. Dashed lines mark the regions defined in Figure \ref{['fig:radio-overall']}. In all panels, a cross marks the position of PSR J1357$-$6429.