Distances and Evolutionary States of Supernova Remnant G18.9-1.1 and Candidate G28.6+0.0

HI spectra of the supernova remnant G18.9-1.1 and the supernova remnant candidate G28.6+0.0 are analyzed. We compared the spectra to 13 CO emission spectra and to spectra of HII regions in the area to determine kinematic distances. G18.9-1.1 is at 2.1 ± 0.4 kpc and G28.6+0.0 is at 9.0 ± 0.3 kpc from the Sun. Using the published X-ray spectra of G18.9-1.1, we apply supernova remnant models for shocked-ISM temperature and emission measure. We find that G18.9-1.1 has low, but not atypical, explosion energy of ≈3 × 10 50 erg and is in a low-density region of the ISM, ~0.1 cm −3 . It has age ~3700 yr if the ejecta mass is 1.4 M ʘ , typical of Type Ia SNe, or ~4700 yr if the ejecta mass is 5 M ʘ typical of core-collapse SN. The candidate G28.6+0.0 does not have reported X-ray emission, so we apply a basic Sedov model. The Sedov age is ~600 yr if the ISM density is 1 cm −3 but could be as old as ~6000 yr if the ISM density is as high as 100 cm −3 .


Introduction
Supernova remnants (SNRs) remain an important area of study due to their role in interstellar medium (ISM) and Galactic evolution. To determine the basic parameters (e.g. age and size) of a SNR, a distance estimation is necessary. There are ~294 Galactic SNRs [1], and ~62 SNRs in the area covered by the VLA Galactic Plane Survey (VGPS). Distances to 28 of the radio-bright SNRs 1 were determined by previous work (see [2] [3] [4] and [5]). Two more distances to SNRs were estimated by [6], using data from the HI, OH, Recombination line survey 1 Two are no longer classified as SNRs ([1] [12]). Journal of High Energy Physics, Gravitation and Cosmology of the Milky Way (THOR). Furthermore, the evolutionary state of 15 SNRs was determined by [7].
For this work, we chose radio-faint SNRs that were omitted from the previous selection, by carefully investigating their HI channel maps. Due to its diffuse nature, determining the distance to the SNR G18.9-1.1 using HI absorption spectra has been a difficult task. First detected by [8], the morphology, spectrum and presence of the center bar directed it to be classified as a composite SNR ( [9] [10]). Reference [11] determined a radio spectral index of −0.39 ± 0.03 ( S α ν ν ∝ ) for the SNR.
The SNR candidate G28.6+0.0 (G28.56+0.00 in [12]) is located in the G28.6-0.1 complex which consists of HII regions and radio sources. Initially [13] suggested a shell-like object to be an SNR due to its non-thermal radio spectrum. However, [14] presented an X-ray analysis of the SNR and found that it emits predominantly synchrotron X-rays from the shell and retained the original SNR ID. Reference [13] argued that the integrated flux density at 20 cm for G28.6+0.0 was clearly higher than the 6 cm and 3 cm flux densities to suggest the non-thermal nature of the SNR candidate. Using THOR data and the 1400 MHz radio continuum data from the VGPS data along with the Spitzer GLIMPSE 8.0 μm and Spitzer MIPSGAL 24 μm data, [12] classified the object as a SNR candidate.
In this paper, we present the HI and 13 CO data and analysis in Section 2. We derive distances for the SNR and SNR candidate in Section 3. Evolutionary models for the two objects are presented in Section 4.

Radio and 13 CO Emission Data
The 1420 MHz and HI data were retrieved from the [16] and the 13 CO spectral line from the Galactic Ring Survey of the Five College Radio Astronomical Observatory (FCRAO) 14 m telescope [17]. We followed the method of HI absorption spectra construction, spectral and error analysis as outlined by [18] and [2].
We used MEANLEV, a software program in the DRAO EXPORT package to construct HI absorption spectra. A main advantage of MEANLEV is that, the "on" (source) and "off" (background) spectra could be extracted by user-specified threshold continuum brightness (T B ) levels. Once a spatial region, normally a box is selected, the source and background spectra are extracted using pixels above (for source) and below (for background) the given T B level. This maximizes the contrast difference in (T B ), which maximizes the signal-to-noise in the HI spectrum. and HI data angular resolution of 40" and spectral resolution of 1.6 km•s −1 [19].
We constructed HI emission spectra using the THOR data for the same source (on) and background (off) regions as chosen for the VGPS spectra. This was done in order to assess whether we could obtain an improvement of the spectra. However, we found there to be almost no difference between the THOR and the VGPS spectra: the prominent emission and absorption features of the source regions were present in the spectra from both surveys. Even though the THOR data has a higher angular resolution, the spectral resolution is comparable to the VGPS data. Thus, for this analysis, we found that the VGPS data was sufficient.
To estimate the kinematic distances to SNRs, a reliable rotation curve is necessary. For this work, we adopt the universal rotation curve (URC) of [20] with the [21] parameters, where the Galactocentric radius of R 0 = 8.34 ± 0.16 kpc and orbital velocity of the sun V 0 = 241 ± 8 km•s −1 .

SNR G18.9-1.1
The diffuse nature of the SNR G18.9-1.1 has made extracting absorption spectra quite difficult, and care needs to be taken to avoid being dominated by noise. An inspection of the HI channel maps by [10] found a depression at 18 km•s −1 giving a distance of 2 kpc to the SNR. However, [10] doesn't present HI absorption spectra. Using red clump stars [22] estimated the distance to be 1.8 ± 0.2 kpc, an estimation consistent with [10].
The SNR consists of two brighter arcs (A & B in Figure 1). We extracted multiple spectra for both regions and found that arc B's brightest region produces the best absorption spectrum. Figure

SNR Candidate G28.6+0.0
We present the 1420 MHz radio continuum image of SNR candidate G28.6+0.0 including the HII regions and radio sources of the G28.6-0.1 complex in Figure 4.
The labelling convention is the same as that in [13]. The HII region labelled as D by [13] is in fact two separate HII regions G028.610+0.020 and G028.600-0.011 with the radio recombination line (RRL) velocities of 96.6 and 41.9/92.8 km•s −1 , respectively [23]. For this work, we re-label the two HII regions as D and D' (see     Figure 4). We extracted HI absorption spectra for the SNR candidate and HII region (D), which are shown in Figure 5. The scaled 13 CO emission spectra of the background (black curve) and source (purple curve) regions are shown in the bottom panels of Figure 5.
The HI absorption feature seen in the negative velocity range (near −40 km•s −1 ) is false, because HI emission is found coincidentally in the chosen background region ( Figure 6). Furthermore, this feature is seen in both the SNR candidate and HII region spectra. For both sources, HI absorption is seen up to the tangent point, which yields a lower limit distance of 7.3 kpc (the tangent point distance). 13 CO emission features are seen at the velocities ~40, ~75 and Figure 5. HI absorption spectra for the SNR candidate G28.6+0.0 (top) and the HII region G028.610+0.020 (bottom). For both panels, HI emission spectra for the source are in black, HI background spectra are red and the difference spectra are blue. The HI absorption spectra are green, their 2σ noise levels are the dashed lines. The scaled 13 CO emission spectra of the background are in black and of the source are in purple. Journal of High Energy Physics, Gravitation and Cosmology ~100 km•s −1 , and are consistent with the HI absorption features for both objects.
Thus, the 13 CO clouds at these velocities are in front of the objects.
The absorption spectra of both the SNR candidate and HII region G028.610 +0.020 are nearly identical: the differences are less than the noise level (see Figure 5). This has been verified by examining HI channel maps. The HII region G028.610+0.020 has a RRL velocity of 96.6 km•s −1 and is at a distance of 9 kpc (D in Table 1). It is likely that the SNR candidate is related to the HII region because
We adopt a plasma electron temperature of 1.12 keV from [27] and use the average of the GIS lower emission measure (EM) norm limit and the ROSAT upper EM norm limit together with our distance to estimate a value for EM of 2.6 × 10 57 cm −3 . With the measured radius, electron temperature and emission measure, we apply the SNR models given in [28].
These models yield the age, explosion energy and ISM density that result in a SNR with the observed parameters. Table 2 lists the results of our modelling of G18.9-1.1. The SNR age is intermediate, between 2500 and 5000 yr, so that electron heating by ion collisions not yet complete. Assuming T e = T i (i.e. T e too large) gives a too-large age and too-low explosion energy, in effect making the SNR older and weaker than its actual age in an attempt to match the observed electron temperature. We see that the realistic models, with non-zero ejecta mass, yield ages which depend on assumed ejecta mass. The energy and ISM density also depends on ejecta mass but not as strongly. The SNR models also predict the temperature and emission measure of the reverse-shocked ejecta.
When better X-ray observations are available, observation of the reverseshocked ejecta can distinguish between models with different ejecta mass and   [29] with C/τ = 4. d Standard model of [28], based on unified SNR evolution of [30]. s = 0 is for uniform ISM, n = 7 is for ejecta density profile ∝ r −7 .
ejecta density profiles (the value of n).
For SNR candidate G28.6+0.0, we measure its diameter as 2.9 arcmin using the VGPS radio continuum image. Our derived distance then gives its radius as R = 3.8 pc. Because there is no reported X-ray emission, we cannot apply models which use the X-ray emission to determine explosion energy E 0 , ISM density n, and age [28]. Thus, we apply a basic Sedov model with the assumption of E 0 = 0.5 × 10 51 erg. This is the mean value found for Galactic SNRs and for LMC SNRs ([7] [31]). Then we take trial values of ISM density n = 0.1, 1, 10 and 100 cm −3 to obtain ages of 180, 580, 1800 and 5800 yr, respectively. In short, the small observed radius implies a small age unless the density is high.

Summary
In this work, we analyze HI spectra of SNR G18.9-1.1 and SNR candidate G28.6+0.0. 13 CO emission spectra and HI spectra of HII regions in the area are compared to the spectra of our objects to verify their kinematic distances. The distance of G18.9-1.1 is 2.1 ± 0.4 kpc and of G28.6+0.0 is 9.0 ± 0.3 kpc.
Our derived radius of G18.9-1.1, with published X-ray shocked-ISM temperature and emission measures, allow application of SNR models. The models show that G18.9-1.1 has slightly low explosion energy of ~3 × 10 50 erg and is in a low-density region of the ISM, ~0.1 cm −3 . If the ejecta mass is 1.4 M ʘ , typical of Type Ia SNe, its age is ~3700. If the ejecta mass is 5 M ʘ , typical of core-collapse SN, its age is ~4700 yr. We apply a basic Sedov model to SNR candidate G18.9-1.1. If the ISM density is 1 cm −3 its Sedov age is ~600 yr, but if the ISM density is as high as 100 cm −3 its age is significantly larger, ~6000 yr.