A New Distance to the Supernova Remnant DA 530 Based on H IAbsorption of Polarized Emission Rebecca A. Booth 1Roland Kothes

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

Rebecca A. Booth ,1Roland Kothes ,2Tom Landecker ,2Jo-Anne Brown ,1Andrew Gray,2

Tyler Foster ,3and Eric Greisen4

1Department of Physics and Astronomy, University of Calgary, 2500 University Dr NW, Calgary, AB T2N 1N4

2Dominion Radio Astrophysical Observatory, Herzberg Astronomy and Astrophysics Research Centre, National Research Council Canada,

PO Box 248, Penticton, BC V2A 6J9, Canada

3Department of Physics and Astronomy, Brandon University, 270-18th Street, Brandon, MB, R7A 6A9

4National Radio Astronomy Observatory, P. O. Box O Socorro NM 87801-0387 USA

(Accepted October 20, 2022)

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 HIof the polarized emission from DA 530. Signiﬁcant absorption was

detected at velocities −28 and −67 km s−1(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

−0.2kpc. At this minimum distance, the diameter of the

SNR is 34+4

−1pc, and the elevation above the Galactic plane is 537+40

−32 pc. The −67 km s−1absorption

likely occurs in gas whose velocity is not determined by Galactic rotation. We present a new data

processing method for combining Stokes Qand Uobservations of the emission from an SNR into a

single HIabsorption 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.

Keywords: Interstellar line absorption (843), Supernova remnants (1667), Interstellar medium (847),

H I line emission (690), Distance indicators (394)

1. INTRODUCTION

A supernova remnant (SNR) is an expanding structure bounded by a blast wave that began with a supernova

(SN). Understanding SNRs is vital to understanding many fundamental processes in the interstellar medium (ISM).

For example, for some 105years after the initial explosion, an SNR disperses the products of stellar fusion and

nucleosynthesis into the ISM, enriching its surroundings. At the same time, the expanding shock front compresses and

heats the ISM, initiating chemical reactions that would not be possible otherwise (Dubner & Giacani 2015). Given

that a star explodes as a supernova roughly every 40±10 years in our Galaxy (Tammann et al. 1994), and the age of

our Galaxy is estimated to be around 1010 years (Sharma et al. 2019), the sheer number of Galactic SNRs that have

existed means that a signiﬁcant fraction of the ISM has been processed through an SNR at some point (Padmanabhan

2001).

A reliable distance measurement to an SNR is required to model its physical attributes, such as size, age, expansion

rate, energy, mass, and evolutionary stage. Unfortunately, due to the diverse characteristics of SNRs, no simple

relationship between distance and their measurable attributes (e.g., surface brightness and angular size) can be derived

(Green 1984). As a result, distance determination has proven to be challenging for many SNRs. Of the 294 Galactic

arXiv:2210.12207v1 [astro-ph.GA] 21 Oct 2022

2Booth et al.

SNRs in Green’s catalog (Green 2019), only 112 (38%) have recorded distances, and many of those are broad estimates

with signiﬁcant uncertainties1.

One of the most widely applied methods to date for determining distances to SNRs uses the absorption of their

radio emissions by intervening neutral hydrogen gas (HI). Of the 112 SNR distances recorded in Green’s catalog, 51

have been determined by this method. HIbetween an SNR and an observer absorbs the SNR’s emission via the 21

cm (1420 MHz) electron spin-ﬂip transition. This absorption can be used to estimate the distance to the SNR by

placing lower limits on the SNR location. If there is HIabsorption detected in the spectrum of an SNR, we can be

certain that the SNR is some distance beyond the absorbing cloud. Since the frequency of the detected absorption

line represents the motion of the cool HI, the distance to the absorbing HIcloud can be determined through Galactic

kinematics. There may be several absorption features in the spectrum of an SNR, as its emission may pass through

many cool HIclouds; the furthest absorption provides a minimum distance to the SNR (see the top panel of Figure

1; e.g., Schwarz et al. 1980;Foster et al. 2004;Kothes 2013;Tian et al. 2019).

Despite its success, the HIabsorption technique is limited to measuring distances for SNRs that are bright emission

sources (Kothes et al. 2004). While cool HIprimarily absorbs radio waves at 21 cm, warm HIprimarily emits at 21

cm. As a result, the raw spectrum observed towards an SNR usually shows a mixture of continuum emission from the

SNR and line emission from Galactic HI. In addition, cool HIabsorbs the emission from warm HIat the same radial

velocity (HIself-absorption). Unless the SNR has a brightness temperature signiﬁcantly greater than the background

warm HIemissions, it can be diﬃcult to untangle these diﬀerent eﬀects.

In order to detect only the absorption of emission from the SNR, removal of the signal contributions from HI

emission and self-absorption may be attempted. To do this, spectra are measured from a position on the SNR and

from a nearby background position oﬀ the SNR (see the bottom panel of Figure 1). Assuming that the HIemission

and absorption features in the oﬀ-spectrum are identical to those in the on-spectrum, subtracting the oﬀ-spectrum

from the on-spectrum yields only the absorption spectrum for the SNR (e.g., Tian et al. 2007;Ranasinghe & Leahy

2018).

There are two problems with applying this background subtraction technique for a low-intensity SNR. First, the

subtraction of two noisy measurements further reduces the signal-to-noise. To improve this situation, the average

emission from a much larger background region can be calculated to reduce the contribution of noise from the oﬀ-

spectrum. However, this leads to the second problem: with a larger background region, it is less likely that the

oﬀ-source HIbrightness temperature matches what is actually on the source. If there is excess HIemission in the

oﬀ-spectrum, then background subtraction will fabricate artiﬁcial absorption features. If there is HIself-absorption in

the on-spectrum that is unmatched in the oﬀ-spectrum, the additional HIself-absorption features will remain in the

spectrum after subtraction. Therefore, in order to correctly identify HIabsorption of the continuum emission from

the SNR, the SNR must be signiﬁcantly brighter than the emitting HIalong the line-of-sight (LOS).

We use the novel technique of measuring the absorption of polarized SNR emission in order to circumvent the

ambiguity of HIself-absorption or the emission from small warm HIclouds at the same velocity. All SNRs are

linearly polarized radio sources (via synchrotron emission), while HIemission is not polarized. As a result, if the

linear polarization parameters, Stokes Qand U(hereafter Qand U), are measured from an SNR rather than total

intensity, Stokes I, the excess emission from background HIis eliminated from the spectrum. Since there is no HI

emission in the polarized on-spectrum, there is no need for background subtraction. Consequently, the problems due

to background subtraction are no longer an issue.

Dickey (1997) pioneered the polarized HIabsorption method when he demonstrated HIabsorption of polarized

Galactic extended emission (diﬀuse synchrotron radiation from relativistic electrons spiraling around interstellar mag-

netic ﬁeld lines). Kothes et al. (2004) were the ﬁrst to explore the use of polarized absorption for SNRs. From

observations of the spectra of three SNRs, Tycho’s SNR, DA 495, and G106.3 +2.7, they showed that polarized HI

absorption features tend to be deeper than their counterpart in total intensity and avoid the systematic noise contri-

bution from small warm clouds. They concluded that polarized HIabsorption could be used for any SNR and showed

particular potential for detecting HIabsorption towards weaker sources.

With the distances to a considerable number of Galactic SNRs still undetermined, the development of new techniques

is essential for advancing our understanding of SNRs. Polarized HIabsorption opens up the possibility of reliable

1See https://www.mrao.cam.ac.uk/surveys/snrs/ for the current web version of Green’s catalog

New Distance to SNR DA530 3

Distance

!!!"

Intensity

Absorption at !!but not !"

SNR is between !!and !"

Cool!

SNR d = ?

d1d2

Cool!

Warm!

SNR d = ?

OFF

Line-of-sight

Intensity

OFF

Distance

Intensity

ON -OFFSNR or HI self-absorption?

Impossible to know

Distance

!!!"

Intensity

Absorption at !!but not !"

SNR is between !!and !"

Cool!

SNR d = ?

d1d2

Cool!

Warm!

SNR d = ?

OFF

Line-of-sight

Intensity

OFF

Distance

Intensity

ON -OFFSNR or HI self-absorption?

Impossible to know

Observer

Distance

!!!"

Intensity

Absorption at !!but not !"

SNR is between !!and !"

Cool!

SNR d = ?

d1d2

Cool!

Warm!

SNR d = ?

OFF

Line-of-sight

Intensity

OFF

Distance

Intensity

ON -OFFSNR or HI self-absorption?

Impossible to know

line-of-sight

Observer

SNR

Warm HI

Distance

!!!"

Intensity

Absorption at !!but not !"

SNR is between !!and !"

Cool!

SNR d = ?

d1d2

Cool!

Warm!

SNR d = ?

OFF

Line-of-sight

Intensity

OFF

Distance

Intensity

ON -OFFSNR or HI self-absorption?

Impossible to know

Cool HI

Distance

!!!"

Intensity

Absorption at !!but not !"

SNR is between !!and !"

Cool!

SNR d = ?

d1d2

Cool!

Warm!

SNR d = ?

OFF

Line-of-sight

Intensity

OFF

Distance

Intensity

ON -OFFSNR or HI self-absorption?

Impossible to know

off-spectrum

on-spectrum

SNR

velocity

SNR absorption spectrum

Minimum distance

Intensity

Off-spectrum

On-spectrum

SNR absorption spectrum (on -off)

velocity

Intensity Intensity Intensity

velocity

SNR absorption spectrum

Minimum distance

Intensity

Off-spectrum

On-spectrum

SNR absorption spectrum (on -off)

velocity

Intensity Intensity Intensity

velocity

SNR absorption spectrum

Minimum distance

Intensity

Off-spectrum

On-spectrum

SNR absorption spectrum (on -off)

velocity

Intensity Intensity Intensity

Figure 1. An illustration of the HIabsorption method for distance determination to an SNR. Top: Absorption by the farthest

HIcloud 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 HIemission and self-absorption structures from the spectrum.

detection of HIabsorption in the spectra of very faint SNRs in order to establish their distance. This paper continues

the investigation initiated by Kothes et al. (2004) by measuring the polarized absorption spectrum for the SNR DA 530.

DA 530 is an example of an SNR with low continuum emission to which it has been a challenge to establish a distance

using traditional HIabsorption. Landecker et al. (1999; hereafter L99) observed DA 530 with the Dominion Radio

Astrophysical Observatory Synthesis Telescope (DRAO-ST). However, DA 530 is too faint to be able to detect HI

absorption of its emission in total intensity, and the DRAO-ST did not have polarized spectrometry at the time. As

a result, in 1999 a distance estimate by HIabsorption was not possible with the DRAO-ST. Instead, L99 identiﬁed

HIemission structures at v=−12 km s−1that they interpreted to be a stellar wind bubble associated with the

progenitor star of DA 530. They concluded the systemic velocity of DA 530 to be the same as that of the proposed

bubble, v=−12 km s−1.

Assuming a systemic velocity of −12 km s−1for DA 530 led to distance estimates ranging from 2.2 kpc (Foster &

Routledge 2003) to 3.5 kpc (L99), as well as conclusions about associated parameters such as energy, age, and swept-up

mass, which have informed the literature about DA 530 for over 20 years. Still, questions have persisted about the

consistency of this distance estimate with accepted SNR models. For example, Jiang et al. (2007; hereafter Jiang07)

found that this distance implies kinetic energy of the order of 1049 erg for DA 530, which is at the low end of the

expected kinetic energy. A greater distance would be required to obtain a more typical energy estimate. In addition,

when West et al. (2016) used computer modelling to describe the expansion of SNRs into the interstellar magnetic

ﬁeld, they determined that the tangential magnetic ﬁeld of DA 530 could only be reproduced if the distance to the

SNR were 4+2

−2kpc.

In this paper, we use the polarized HIabsorption method to investigate a new distance to DA 530, which improves

the agreement with the models discussed above. DA 530 is one of the most strongly polarised SNRs in the Galaxy,

with a percentage polarisation at 1420 MHz higher than 40% over a signiﬁcant fraction of the remnant (L99). The

high degree of polarization of the emission from DA 530 makes it an excellent candidate for the application of the

polarized absorption technique.

2. OBSERVATIONS OF DA 530

To obtain a polarized HIabsorption spectrum for DA 530, we ﬁrst used the National Radio Astronomy Observatory

Karl G. Jansky Very Large Array (NRAO-VLA) in 2004 (data set referred to as VLA-2004). Subsequently, we

used the DRAO-ST HIspectrometer and continuum correlator (data sets referred to as S21-2020 and C21-2020).

4Booth et al.

Parameter NRAO-VLA DRAO-ST DRAO-ST DRAO-ST

(VLA-2004) (S21-2012) (S21-2020) (C21-2020)

Date of observation July 22, 2004 2012 2020 June-July 2020 June-July

Duration of observations 6 h ×1 ﬁeld 144 h ×4 ﬁelds 288 h ×3 ﬁelds 288 h ×3 ﬁelds

Conﬁguration D east-west east-west east-west

Field center: 20h52m+ 55.4◦20h50m13.0s+55◦50049” 20h52m+55.3◦20h52m+ 55.3◦

RA DEC (J2000) 20h51m10.0s+56◦53030” 20h52m+55.3◦20h52m+ 55.3◦

20h54m46.8s+54◦06017” 20h52m+55.3◦20h52m+ 55.3◦

21h03m10.7s+55◦31004”

Field of view at 1.4 GHz 320to half power 2.65◦diameter 2.65◦diameter 2.65◦diameter

to 20% to 20% to 20%

Synthesized beam at 1.4 GHz 4600 10×1.2010×1.2010×1.20

Sensitivity 0.4 K 2.9 K 1.3 K 0.4 K

Frequency center - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - νA= 1406.9 MHz

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - νB= 1413.8 MHz

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - νC= 1427.4 MHz

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - νD= 1434.3 MHz

Velocity coverage v=−109.4 km s−1v=−144.65 km s−1v=−144.65 km s−1- - - - - - - - - - - -

to 19.4 km s−1to 65.51 km s−1to 65.51 km s−1- - - - - - - - - - - -

Bandwidth ∆v=1.29 km s−1∆v=0.82 km s−1∆v=0.82 km s−1∆ν=7.5 MHz

Velocity resolution ∆v=2.4 km s−1∆v=1.3 km s−1∆v=1.3 km s−1

Polarization I,Q,U I Q,U I,Q,U

Calibrator 3C 147 3C 147 3C 147 3C 147

3C 286 3C 295 3C 295 3C 295

J2052+365 3C 286 3C 286

Table 1. The observing parameters for the four sets of DA 530 observations taken over three observation windows.

To supplement our 2020 polarization data, we also have a Stokes IHIdata cube observed along the LOS towards

DA 530 in 2012 by the DRAO-ST (referred to as S21-2012), with short spacings provided by the HI4PI survey (HI4PI

Collaboration et al. 2016). The observing parameters for the four observations are given in Table 1. All four data

sets are used in our analysis. The NRAO-VLA can achieve higher signal-to-noise with a shorter observation time. Set

against this advantage, the ﬁeld of view of the NRAO-VLA is smaller than that of the DRAO-ST, and only the central

part of the NRAO-VLA ﬁeld is usable due to instrumental polarization problems. In spite of these diﬀerences, the

ability to compare polarized absorption spectra from two entirely diﬀerent instruments, observed sixteen years apart,

provides a unique opportunity to validate our results.

2.1. Observations by the NRAO-VLA

The VLA-2004 observations were made using the D conﬁguration of the NRAO-VLA (baselines 35 m to 1 km).

The primary beam of the 25 m NRAO-VLA antennas has a half-power beamwidth of 320at 1420 MHz, which is

very close to the angular diameter of DA 530, and the ﬁeld of view did not include all of the SNR. In addition, the

NRAO-VLA has strong oﬀ-axis instrumental polarization (Uson & Cotton 2008) and yields reliable data only very

close to the ﬁeld center, prompting us to center our observations on the southwest shell of DA 530, where the polarized

intensity from the remnant is highest. Continuum images of DA 530 in Stokes I,Q,U, and linear polarized intensity

(P I =pQ2+U2), created by averaging across the end channels of the VLA-2004 data cubes, are shown in Figure 2.

2.2. Observations by the DRAO-ST

As the VLA-2004 observations were not able to provide data for the entire remnant, we supplemented our study

of DA 530 with observations from the DRAO-ST (Landecker et al. 2000) and obtained the S21-2020 and C21-2020

data. The DRAO-ST is an east-west interferometer consisting of seven dishes, 9 m in diameter, with a baseline range

of 12.86 m to 617.2 m. The larger ﬁeld-of-view, 2.65◦diameter to 20% power, is more than suﬃcient to observe the

New Distance to SNR DA530 5

Galactic latitude (deg)

mJy/beam

Galactic longitude (deg)

Galactic latitude (deg)

mJy/beam

Galactic longitude (deg)

mJy/beam

Galactic longitude (deg)

mJy/beam

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 Qand were created by averaging across the end channels of the VLA-2004

data cubes. The Stokes Iimage includes HIemission in addition to the continuum emission from DA 530, as all frequency

channels included in the observations contained some emitting HI.

full angular extent of DA 530. The S21-2020 observations of DA 530 were made using the HIspectrometer (the S21

spectrometer) of the DRAO-ST, which was designed to image Stokes Ionly (Hovey 1998), so we had to adapt it to

measure Qand U(see Section 2.3). The duration of the 2020 observations, about six times longer than the normal

observing mode for this telescope, was chosen to ensure adequate sensitivity (see Table 1).

As a result of the polarimetry modiﬁcations to the S21 spectrometer, the S21-2020 observations of DA 530 only

include Qand U; however, we were able to supplement our polarized data with the S21-2012 Stokes Iobservations

towards DA 530 (available in the DRAO data archive).

The continuum (C21) and S21 correlators of the DRAO-ST operate simultaneously during an observation. The

C21 correlator has four frequency channels (centered at frequencies νA= 1406.9 MHz, νB= 1413.8 MHz, νC= 1427.4

MHz, and νD= 1434.3 MHz), which are outside the expected Galactic HIfrequency range and therefore detects

continuum emission in these bands. The resulting data products are Stokes I,Q, and Uimages for all four frequency

bands (Landecker et al. 2000). The C21-2020 images of DA 530, made by combining the four continuum frequency

channels, are shown in Figure 3.

2.3. Adapting the S21 spectrometer to be able to observe polarization

In order to observe Qand Uspectra for our S21-2020 data, the S21 spectrometer needed to be adapted to make

polarimetry possible. Radio observations of polarization are achieved by correlating the signals from orthogonally

polarized antenna feeds (Cohen 1958). In the case of the DRAO-ST, each antenna is equipped with circular feeds

measuring left (L) and right (R) circular polarization. When an interferometer correlates circularly polarized signals,

each permutation of the Rand Lcorrelation products is comprised of two of the four Stokes visibilities, Iv,Qv,Uv,

and Vv. Stokes visibilities are Fourier transformed during the imaging process to obtain maps of their corresponding

Stokes parameters. The four possible correlation products are (Conway & Kronberg 1969):

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ANewDistancetotheSupernovaRemnantDA530BasedonHIAbsorptionofPolarizedEmissionRebeccaA.Booth,1RolandKothes,2TomLandecker,2Jo-AnneBrown,1AndrewGray,2TylerFoster,3andEricGreisen41DepartmentofPhysicsandAstronomy,UniversityofCalgary,2500UniversityDrNW,Calgary,ABT2N1N42DominionRadioAstrophysicalObservatory...

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A New Distance to the Supernova Remnant DA 530 Based on H IAbsorption of Polarized Emission Rebecca A. Booth 1Roland Kothes

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