A New Distance to the Supernova Remnant DA 530 Based on HI Absorption of Polarized Emission

TL;DR

The study develops and applies a polarized HI absorption technique to measure distances to SNR DA 530, mitigating HI self-absorption and background-subtraction issues that impede traditional methods. By combining NRAO-VLA and DRAO-ST data and deriving optical-depth spectra from Stokes Q and U, the authors detect absorption at velocities of $-$28 and $-$67 km s$^{-1}$, corresponding to minimum distances of $4.4^{+0.4}_{-0.2}$ kpc and $8.3$ kpc, respectively. The results imply a diameter of $34^{+4}_{-1}$ pc and an elevation of $537^{+40}_{-32}$ pc at the preferred distance, with the larger distance being tentative due to non-circular motions. Comparison with Sedov-Taylor and radiative-phase models, along with CHANDRA X-ray inferences, favors a Sedov-phase, $\sim$SNR at $d\approx 4.4$ kpc, providing a physically plausible set of parameters and demonstrating the polarization-based absorption method as a powerful tool for distance measurements to many faint SNRs.

Abstract

Supernova remnants (SNRs) are significant contributors of matter and energy to the interstellar medium. Understanding the impact and the mechanism of this contribution requires knowledge of the physical size, energy, and expansion rate of individual SNRs, which can only come if reliable distances can be obtained. We aim to determine the distance to the SNR DA 530 (G93.3+6.9), an object of low surface brightness. To achieve this, we used the Dominion Radio Astrophysical Observatory Synthesis Telescope and the National Radio Astronomy Observatory Very Large Array to observe the absorption by intervening HI of the polarized emission from DA 530. Significant absorption was detected at velocities $-28$ and -67 km/s (relative to the local standard of rest), corresponding to distances of 4.4 and 8.3 kpc, respectively. Based on the radio and X-ray characteristics of DA 530, we conclude that the minimum distance is 4.4$^{+0.4}_{-0.2}$ kpc. At this minimum distance, the diameter of the SNR is 34$^{+4}_{-1}$ pc, and the elevation above the Galactic plane is 537$^{+40}_{-32}$ pc. The $-67$ km/s absorption likely occurs in gas whose velocity is not determined by Galactic rotation. We present a new data processing method for combining Stokes $Q$ and $U$ observations of the emission from an SNR into a single HI absorption spectrum, which avoids the difficulties of the noise-bias subtraction required for the calculation of polarized intensity. The polarized absorption technique can be applied to determine distances to many more SNRs.

Figures (9)

• Figure 1: An illustration of the HI absorption method for distance determination to an SNR. Top: Absorption by the farthest HI cloud along the line-of-sight indicates the minimum distance to the SNR. Bottom: The emission "off" the SNR is subtracted from that "on" the SNR in order to remove excess HI emission and self-absorption structures from the spectrum.
• Figure 2: Continuum images of DA 530 observed by the NRAO-VLA. These continuum images of DA 530 show (clockwise from top left) Stokes $I$, polarized intensity (PI), $U$, and $Q$ and were created by averaging across the end channels of the VLA-2004 data cubes. The Stokes $I$ image includes HI emission in addition to the continuum emission from DA 530, as all frequency channels included in the observations contained some emitting HI.
• Figure 3: DRAO-ST continuum images of DA 530. Images of (clockwise from top left) Stokes $I$, polarized intensity (PI), $U$, and $Q$ made by combining the four continuum frequency channels of the C21-2020 images.
• Figure 4: The spectra for DA 530 plotted in the optical depth variable ($e^{-\tau} -1$) as a function of velocity (black) and HI emission (blue). The optical depth spectra were produced by combining the Stokes $Q$ and $U$ spectra, which were determined using Equation 15. 1. Top: the DRAO-ST spectra, with ($e^{-\tau} -1$) spectra from the S21-2020 data and HI emission spectra from the S21-2012 data. 2. Bottom: the NRAO-VLA spectra from the VLA-2004 data. The spectra are shown for four regions on DA 530: (a) the whole SNR, (b) the south-west shell, (c) the south-east shell and center, and (d) the northern shell. Shading indicates one [red], two [blue], and three [yellow] standard deviations from the unabsorbed emission line ($e^\tau -1 = 0$).
• Figure 5: Foreground HI column density for DA 530. The HI column density profile (red curve) was calculated using the DRAO-ST absorption profile over the whole remnant (top right plot of Figure \ref{['fig:Spectra']}). The HI emission profile (black curve) was obtained from the S21-2012 Stokes $I$ data cube averaged in each velocity channel over the area covering the whole remnant.