An Investigation of the Interstellar Environment of Supernova Remnant CTB87

Figure 1 shows the averaged CO spectra from a region covering SNR CTB 87. There are two prominent ¹²CO (J = 1–0) emission peaks, at around $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ and $-38\,\mathrm{km}\,{{\rm{s}}}^{-1}$. The ¹³CO (J = 1–0) emission is only prominent at $\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$, and the C¹⁸O (J = 1–0) emission is not significant across the velocity range.

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We made ¹²CO emission channel maps around the two peaks with velocity interval $0.5\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (see Figures 2 and 3) to examine the spatial distribution of the two molecular components. The $\sim -38\,\mathrm{km}\,{{\rm{s}}}^{-1}$ ¹²CO component seems to overlap the SNR at its southeastern "apex" by projection in interval −39 to $-37.5\,\mathrm{km}\,{{\rm{s}}}^{-1}$, but there is no systematic morphological feature correspondence between the molecular gas and the SNR (Figure 2).

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The $\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ component appears to spread over a large area covering the SNR, and a few LSR-velocity dependent structures are noteworthy. (1) In velocity interval −55.5 to $-54.5\,\mathrm{km}\,{{\rm{s}}}^{-1}$, there is a linear structure of ¹²CO (J = 1–0) emission along the eastern edge of the SNR (Figure 3), somewhat similar to that described in Kothes et al. (2003) (in passing, note that a Galactic coordinate system was used there while an equatorial coordinate system is used here, and the shell-like structure shown there is not complete due to the limited field of view). The structure can also be discerned in the ¹³CO (J = 1–0) channel map at $-55\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (Figure 4). (2) In velocity interval −56.5 to $-55.5\,\mathrm{km}\,{{\rm{s}}}^{-1}$, a bar-like molecular structure appears to pass through the SNR along the northeast–southwest orientation and, in particular, through the brightness peak of the radio emission. (3) In interval −58 to $-57\,\mathrm{km}\,{{\rm{s}}}^{-1}$, both ¹²CO and ¹³CO emissions show two patches of molecular material in the eastern and southwestern edges of the SNR, similar to the sub-clumps described in Cho et al. (1994). The integrated ¹²CO (J = 1–0) intensity map in the main velocity range of the $\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ component, −60 to $-54\,\mathrm{km}\,{{\rm{s}}}^{-1}$, is presented in Figure 5 (in green). It is overlaid with Chandra 0.5–7 keV X-ray image of CTB 87 (in blue) and the radio continuum image (in red). The X-ray-emitting part of the PWN, with a southeast–northwest oriented elongation, appears to be located in a gap of molecular gas, where CO emission is weak, between the two patches at −58 to $-57\,\mathrm{km}\,{{\rm{s}}}^{-1}$.

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We have inspected the ¹²CO (J = 1–0) line profiles of the two molecular components toward CTB 87. For the $\sim -38\,\mathrm{km}\,{{\rm{s}}}^{-1}$ component, we do not find any asymmetric broad profiles of the ¹²CO line in the grid of CO spectra. (This component, as mentioned in Section 3.1, has neither morphological feature corresponding to the SNR). For the other component, around $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$, we show a grid of ¹²CO (J = 1–0) and ¹³CO (J = 1–0) spectra in the velocity range of −61 to $-53\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (Figure 6) and find asymmetric broad profiles of the ¹²CO line in the eastern and southwestern edges. Figure 7 shows the average line profiles of some of the pixels at the edges of the SNR (regions "E" and "SW" marked in Figure 6). There is a secondary peak at −56 to $-55\,\mathrm{km}\,{{\rm{s}}}^{-1}$ in region "E" (also seen in most of the pixels therein) besides the main peak at $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$, with little ¹³CO counterpart (≲3σ). Although the broad red (right) wing may include a contribution from real line broadening due to shock disturbation, we cannot exclude contamination by line-of-sight emission in a wide region. We note that there is similar secondary peak on the east of the radio boundary and there is non-negligible intensity at −56 to $-55\,\mathrm{km}\,{{\rm{s}}}^{-1}$ on the northeast of the boundary. However, in the three bottom pixels of region "E," there are unique plateaus in the blue (left) wings (∼−61 to $-59\,\mathrm{km}\,{{\rm{s}}}^{-1}$), which are also clearly ($\gt 4\sigma$) reflected in the average profile for region "E" (top panel, Figure 7). This plateau-like spectral feature is somewhat similar to that detected in the ¹²CO spectra in the western edge of SNR 3C397 (Jiang et al. 2010). In the spectra of the pixels in the southwestern edge, as typified by region "SW," the line profiles are characterized by strong peaks at $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ for both ¹²CO and ¹³CO and a broad red wing of ¹²CO extending to $\sim -54\,\mathrm{km}\,{{\rm{s}}}^{-1}$ without a significant ¹³CO counterpart (bottom panel, Figure 7). The surrounding pixels essentially display different shapes of line profiles, and the emission outside the southern edge becomes weak. Since ¹³CO emission, which is usually optically thin (like that with τ(¹³CO) $\ll \,1$ given in Tables 1 and 2), is yielded in quiescent, intrinsically high-column-density molecular gas, a broad ¹²CO-line wing without a ¹³CO counterpart is very likely to represent perturbed gas deviating from the systemic LSR velocity. Therefore, the asymmetric ¹²CO wings from the edges of CTB 87 (espcially the blue wings in region "E" and the red wings in region "SW") very likely result from Doppler broadening of the $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ line and thus might provide kinematic evidence for interaction between the $\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ MC complex and the SNR.

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Table 1.  Fitted and Derived Parameters for the MCs around $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ in Region "R"^a

Gaussian Components
Line	Center ($\,\mathrm{km}\,{{\rm{s}}}^{-1}$)	FWHM ($\,\mathrm{km}\,{{\rm{s}}}^{-1}$)	${T}_{\mathrm{peak}}$(K)	W(K $\,\mathrm{km}\,{{\rm{s}}}^{-1}$)
12CO(J = 1–0)	−57.5	2.6	3.6	9.9
13CO(J = 1–0)	−57.1	2.0	0.6	1.3
Molecular Gas Parameters
N(H2)(1021 cm−2)b	$n({{\rm{H}}}_{2}){d}_{6.1}(\,{\mathrm{cm}}^{-3})$	${{Md}}_{6.1}^{-2}({10}^{3}{M}_{\odot })$ b	${T}_{\mathrm{ex}}$(K)c	τ(13CO)
1.4/1.8	34/43	4.1/5.1	9.5	0.18

Notes.

^aThe region is defined in Figure 5. ^bSee the text for the two estimating methods. ^cThe excitation temperature is calculated from the maximum ¹²CO(J = 1–0) emission point of the $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ component.

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We fit the CO emissions with Gaussian lines for the $\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ molecular gas in regions "R" and "L" (as defined in Figure 5), and the derived parameters, molecular column density $N({{\rm{H}}}_{2})$, excitation temperature ${T}_{\mathrm{ex}}$, and optical depth of ¹³CO(J = 1–0) τ(¹³CO) are summarized in Tables 1 and 2. The distance to the MC/SNR is taken to be 6.1 kpc, as suggested by Kothes et al. (2003). The column density of H₂ and the mass of the molecular gas are estimated using two methods. In the first method, the conversion relation for the molecular column densities, N(H₂) $\approx \,7\times {10}^{5}N$ (¹³CO) (Frerking et al. 1982), is used under the assumption of local thermodynamic equilibrium for the molecular gas and optically thick condition for the ¹²CO (J = 1–0) line. In the second method, a value of the CO-to-H₂ mass conversion factor, N(H₂)/W(¹²CO) (known as the "X-factor"), $1.8\times {10}^{20}$ cm⁻² K⁻¹ km⁻¹ s (Dame et al. 2001) is adopted.

Table 2.  Fitted and Derived Parameters for the MCs around $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ in Region "L"^a

Gaussian Components
Line	Center ($\,\mathrm{km}\,{{\rm{s}}}^{-1}$)	FWHM ($\,\mathrm{km}\,{{\rm{s}}}^{-1}$)	${T}_{\mathrm{peak}}$(K)	W(K $\,\mathrm{km}\,{{\rm{s}}}^{-1}$)
12CO(J = 1–0)	−57.5	2.1	2.9	6.4
13CO(J = 1–0)	−57.4	0.8	0.6	0.5
Molecular Gas Parameters
N(H2)(1021 cm−2)b	$n({{\rm{H}}}_{2}){d}_{6.1}(\,{\mathrm{cm}}^{-3})$	${{Md}}_{6.1}^{-2}({10}^{3}{M}_{\odot })$b	${T}_{\mathrm{ex}}$(K)c	τ(13CO)
0.4/1.1	21/57	0.3/0.8	7.5	0.21

Note. a, b, c: Same notations as in Table 1.

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By projection, SNR CTB 87 is located within a superbubble, centered at R.A. = 20^h14^m03^s, decl. = 37°13'27'', with a circular boundary (brightened in the south and north) of angular radius ∼37', and nears its eastern edge (shown in Figure 8). We investigate the WISE mid-IR observation toward SNR CTB 87 and find the superbubble is bright at both 12 and 22 μm. The H i emission in this sky region also shows a cavity at the LSR velocity around $-64\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (−61 to $-68\,\mathrm{km}\,{{\rm{s}}}^{-1}$), which happens to be spatially coincident with the mid-IR superbubble and to have the same angular size. Furthermore, such a bubble-like structure has a counterpart in the optical extinction map (also see Figure 8).

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As superbubbles are usually the products of energy and material feedback from massive OB stars into the interstellar medium, we searched for possible OB stars in the direction toward this superbubble.

We built a multi-band photometric stellar sample of over 0.18 million stars within the projected region of the superbubble (i.e., the circle shown in Figure 8) by cross-matching the photometric catalogs of Pan-STARRS, IPHAS, and 2MASS. We used a spectral energy distribution (SED) fitting algorithm, similar to those employed by Chen et al. (2014a) and Mohr-Smith et al. (2015), to obtain the effective temperature ${T}_{\mathrm{eff}}$, optical extinction A_V, and distance modulus μ of the individual stars. We compared the observed SED of each star to the stellar models from the Padova isochrone data base (CMD v3.0; Bressan et al. 2012; Marigo et al. 2017). We considered only the main-sequence (surface gravity $\mathrm{log}g\gt 3.8$ dex), and Galactic thin disk metallicity ([Fe/H] = −0.2 dex) models. The observed magnitudes of a star can be modeled by

$$\begin{eqnarray}&&{m}_{i}^{\mathrm{obs}}\,=\,{m}_{i}^{\mathrm{mod}}+{A}_{i}+\mu ,\end{eqnarray} \tag{ 1 }$$

where ${m}_{i}^{\mathrm{obs}}$ and ${m}_{i}^{\mathrm{mod}}$ are the observed and Padova magnitudes in the ith band (i = 1, 2, ..., 11 corresponding to Pan-STARRS ${g}_{{\rm{P}}},\,{r}_{{\rm{P}}},\,{i}_{{\rm{P}}},\,{z}_{{\rm{P}}},\,{y}_{{\rm{P}}}$, IPHAS r', i', Hα, and 2MASS J, H, and ${K}_{{\rm{S}}}$ bands, respectively), A_i is the extinction in the ith band, and μ is the distance modulus. We adopt the R_V = 3.1 extinction law from Cardelli et al. (1989) to convert the optical extinction A_V to extinction in each individual band A_i. A simple Bayesian scheme based on Markov Chain Monte Carlo sampling is adopted to obtain the parameters $\mathrm{log}{T}_{\mathrm{eff}}$, A_V, and μ for the individual stars. We adopt the likelihood

$$\begin{eqnarray}&&{L}_{r}={\rm{\Pi }}\displaystyle \frac{1}{\sqrt{2\pi {\sigma }_{i}}}\exp \displaystyle \frac{-{({m}_{i}^{\mathrm{obs}}-{m}_{i}^{\mathrm{mod}})}^{2}}{2{\sigma }_{i}^{2}},\end{eqnarray} \tag{ 2 }$$

where ${\sigma }_{i}$ is the uncertainty of the ith band observed magnitude, and the priors

$$\begin{eqnarray}P({T}_{\mathrm{eff}},{A}_{V},\mu )=\left\{\begin{array}{ll}1 & \mathrm{if}\,\left\{\begin{array}{l}4.3\,\leqslant \,\mathrm{log}({T}_{\mathrm{eff}})\,\leqslant \,4.7\\ 0\,\leqslant \,{A}_{V}\,\leqslant \,20\\ 0\,\leqslant \,\mu \,\leqslant \,20\end{array}\right.\\ 0 & \mathrm{else}.\end{array}\right.\end{eqnarray} \tag{ 3 }$$

We adopt the results for the stars with high photometric precisions (${\sigma }_{i}$ ≤ 0.1 mag for each band) and high posterior probabilities ($\mathrm{log}P\geqslant 5$, where 5 is around the median value of the logarithmic posterior probabilities). We note that, due to the lack of information in blue optical wavelengths (i.e., u band), there could be degeneracies between the effective temperatures and the amount of extinction for the individual stars.

As a result, we found 82 OB star candidates. Figure 9 shows the distribution of distance and optical extinction for all these objects. There seems to be a peak of the distribution of distance of these OB candidates at d = 7.3 kpc, with a dispersion of 1.9 kpc, although the selected stars may be an incomplete and contaminated sample of the candidates owing to the lack of the u band data. Notably, this distance range covers that of the Perseus spiral arm in the direction of CTB 87 (Kothes et al. 2003, Figure 8 therein). As some massive stars should be responsible for the superbubble, it is most likely that they are located in the dispersion range of the peak. The distance range of the massive stars or the superbubble also seems to be consistent with the distance to CTB 87, either 6.1 ± 0.9 kpc, derived from the intervening neutral hydrogen column density (Kothes et al. 2003) or ∼6–8 kpc, estimated from the column density of the line-of-sight X-ray absorbing hydrogen atoms and molecules (see Section 4.1).

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Our new CO-line observation shows spatial correspondence of the SNR CTB 87 with an MC complex at LSR velocities around $-58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (−54 to $-60\,\mathrm{km}\,{{\rm{s}}}^{-1}$). We have also demonstrated probable kinematic evidence of the SNR shocking against molecular patches at ${V}_{\mathrm{LSR}}\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$. Hereafter we parameterize the distance to CTB 87 and the associated MC as $d=6.1{d}_{6.1}\,\mathrm{kpc}$. We can make a rough estimate of the density of the associated molecular gas. We adopt angular sizes of the two patches of molecular gas in regions "R" and "L" as ∼75 × 75 and ∼39 × 35 (i.e., $\sim 13.3{d}_{6.1}\,\mathrm{pc}\times 13.3{d}_{6.1}\,\mathrm{pc}$ and $\sim 6.9{d}_{6.1}\,\mathrm{pc}\times 6.2{d}_{6.1}\,\mathrm{pc}$), respectively, and assume line-of-sight sizes 13.3 pc and 6.6 pc for them. The estimated gas density, $n({{\rm{H}}}_{2})$, and gas mass, M, for the two patches are given in Tables 1 and 2, respectively.

The multiwavelength spectrum from radio to γ-rays for CTB 87, which incorporates the TeV emission of VER J2016+371 (Aliu et al. 2014) and the GeV emission from 3FGL J2015.6+3709 (Kara et al. 2012; Acero et al. 2015, 2016), has been explained using a leptonic scenario by Saha (2016), with a combination of a broken power law and a Maxwellian distribution of electrons. Nonetheless, it is also mentioned that the observed GeV–TeV data can be explained by a hadronic scenario as long as there is a dense ambient target medium with a mean hydrogen atom density $\sim 20\,{\mathrm{cm}}^{-3}$ or higher (depending on detailed hadronic interaction mechanisms) (Saha 2016). Actually, it is notable that the centroid of the VERITAS very high energy γ-ray emission (Aliu et al. 2014) is essentially coincident with both the radio emission of the SNR and the $\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$ MC complex. The possible SNR–MC interaction revealed here provides a likely hotbed for the production of hadronic γ-rays. The gas density of the two molecular patches, a few tens cm⁻³, estimated here, appears to satisfy the hadronic scenario.

CTB 87 is very likely to be located within (but near the eastern boundary of) the revealed superbubble, in view of the following issues. (1) The LSR velocity of the ¹²CO emission of the associated MC, −54 to $-60\,\mathrm{km}\,{{\rm{s}}}^{-1}$, is very similar to that of the H i emission for the superbubble, −61 to $-68\,\mathrm{km}\,{{\rm{s}}}^{-1}$. (2) The distance to the SNR, derived both from the foreground intervening neutral hydrogen column density, $6.1\pm 0.9\,\mathrm{kpc}$ (Kothes et al. 2003), and from the X-ray-absorbing hydrogen column density, 6–$8\,\mathrm{kpc}$ (see below), is consistent with the likely peak distance of the OB star candidates that are projected inside the superbubble, 7.3 ± 1.9 kpc, which can be regarded as the distance to the superbubble and the OB star candidates inside. The hydrogen (including atoms and molecules) column density for the entire diffuse X-ray emission region of CTB 87 is N_H = 1.21 –$1.57\times {10}^{22}\,{\mathrm{cm}}^{-2}$ (Matheson et al. 2013). The optical extinction can then be estimated to be A_V = 4.8 ± 0.7 from the empirical relation ${N}_{{\rm{H}}}=(2.87\pm 0.12)\times {10}^{21}{A}_{V}\,{\mathrm{cm}}^{-2}$ (Foight et al. 2016), which indicates a distance of ∼6–8 kpc to CTB 87 using the A_V–distance relation toward this SNR given in Kothes et al. (2003). (3) There is little detection of thermal X-rays related to the SN ejecta and blast wave, which sets an upper limit $0.2\,{\mathrm{cm}}^{-3}$ to the density of the medium (Matheson et al. 2013). This can be readily explained if the SN exploded material is blown into the tenuous and hot gas within the superbubble (in CTB 87, another part of the gas may have shocked into the MC; see Section 4.2). In the superbubble, the SN ejecta can freely move away at a very high speed, and the blast shock will propagate with a small Mach number so that it can be expected to be weak and become thermalized, with most of the SN kinetic energy carried by this part of the material being deposited in the thermal energy of the hot medium (Tang & Wang 2005).

The MC with which the SNR probably interacts may survive the strong radiation of the OB stars in the superbubble. The photodissociation timescale of the MC can be comparable to, or even larger than, the age of the superbubble. For simplicity, the dissociating far-ultraviolet (far-UV) radiation of the OB stars (of number N_OB), to which the MC is exposed, is approximated as being from the bubble center, and the molecular column density along the bubble radial is approximated to the line-of-sight column density $N({{\rm{H}}}_{2})$. The distance from the MC to the center, R_MC, is not less than the projected distance to the center (∼225 or $\sim 40{d}_{6.1}$ pc). Each dissociating photon is absorbed by a hydrogen molecule with a photodissociation probability, p_D ∼0.15 (Draine 2011). Thus, the photodissociation timescale of the MC is given by ${\tau }_{\mathrm{MC}}\gtrsim N({{\rm{H}}}_{2})(4\pi {R}_{\mathrm{MC}}^{2})/({p}_{{\rm{D}}}{N}_{\mathrm{OB}}{S}_{{\rm{D}}})$, where ${S}_{{\rm{D}}}$ is the mean production rate of Lyman band dissociating photons of an OB star ($\sim {10}^{47.5}\,{{\rm{s}}}^{-1}$; Diaz-Miller et al. 1998). It is estimated to be ${\tau }_{\mathrm{MC}}\gtrsim 6.4\times {10}^{6}(N({{\rm{H}}}_{2})/1\times {10}^{21}\,{\mathrm{cm}}^{-2})$ ${({N}_{\mathrm{OB}}/20)}^{-1}{({S}_{{\rm{D}}}/{10}^{47.5}{{\rm{s}}}^{-1})}^{-1}{d}_{6.1}^{2}\,\mathrm{yr}$, where the reference value $1\times {10}^{21}\,{\mathrm{cm}}^{-2}$ is adopted for $N({{\rm{H}}}_{2})$ according to Tables 1 and 2, and ${N}_{\mathrm{OB}}\sim 20$ is adopted as a reference value considering this approximate number of stars are possibly inside and responsible for the superbubble (based on Figure 9). On the other hand, the age of the superbubble is estimated to be (Weaver et al. 1977) ${t}_{\mathrm{SB}}\sim 5.8\times {10}^{6}{d}_{6.1}^{5/3}$ ${({N}_{\mathrm{OB}}/20)}^{-1/3}\,\times {({L}_{w}/{10}^{34}\mathrm{erg}{{\rm{s}}}^{-1})}^{-1/3}$ ${({n}_{0}/0.6{\mathrm{cm}}^{-3})}^{1/3}\mathrm{yr}$, where L_w is the power of the wind of an OB star, and n₀ is the atomic number density of the environment medium into which the superbubble expands. Here we use a density of $0.6\,{\mathrm{cm}}^{-3}$, typical for warm interstellar H i gas (which takes the largest volume fraction next to the coronal gas in the Galactic disk) to be a reference value for n₀ (Draine 2011). Thus their ratio is τ_MC/t_SB ≳ 1.1 × (N(H₂)/1 $\times {10}^{21}\,{\mathrm{cm}}^{-2})$ ${({N}_{\mathrm{OB}}/20)}^{-2/3}{d}_{6.1}^{1/3}$ ${({S}_{{\rm{D}}}/{10}^{47.5}{{\rm{s}}}^{-1})}^{-1}$ ${({L}_{w}/{10}^{34}\mathrm{erg}{{\rm{s}}}^{-1})}^{1/3}{({n}_{0}/0.6{\mathrm{cm}}^{-3})}^{-1/3}$. Here we have ignored the formation of new molecules and the extinction of the dissociating photons by dust grains in the molecular gas. This estimate indicates that, with some typical or proper values of parameters adopted, the photodissociation timescale appears not less than the age of the superbubble, and thus the survival of the MC in the superbubble could be reasonable.

It has been suggested that CTB 87 is inside another superbubble found in H i emission at ${V}_{\mathrm{LSR}}\sim -70\,\mathrm{km}\,{{\rm{s}}}^{-1}$, with a radius in the range ∼38'–70', centered at approximately R.A. = 20^h16^m15^s, decl. = 36°40' (J2000) (Wallace et al. 1997). This superbubble, however, is not as favored as the $\sim -64\,\mathrm{km}\,{{\rm{s}}}^{-1}$ one in view of the bigger offset of the LSR velocity $\sim -70\,\mathrm{km}\,{{\rm{s}}}^{-1}$ from the CO-line velocity of the associated MC ($\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$) than that of $\sim -64\,\mathrm{km}\,{{\rm{s}}}^{-1}$. Incidentally, we do not find the counterpart of this H i superbubble in the WISE near- and mid-IR observations.

The X-ray observation of CTB 87 shows a cometary-like trailing structure, ∼200'' × 300'' or $\sim 5.9{d}_{6.1}\,\mathrm{pc}\times 8.9{d}_{6.1}\,\mathrm{pc}$ in size at 0.3–7 keV (Matheson et al. 2013). The radio emission takes a blow-out, arc-like shape, with the X-ray trail at its symmetric axis, and has a remarkably larger size (∼6' × 8' or $10.6{d}_{6.1}\,\mathrm{pc}\times 14.2{d}_{6.1}\,\mathrm{pc}$, even up to 16') than the X-ray emission and a different brightness peak location from the X-ray one (Kothes et al. 2003; Matheson et al. 2013). The apparent one-sided confinement of the X-ray emission represents a typical structure for a relative oriented motion between the ambient gas and the PWN. But why is the radio emission much more extended than the X-ray nebula, with different locations for the brightness peaks in the two bands?

Matheson et al. (2013) suggest that the radio emission of CTB 87 is a relic PWN that was crushed by the reverse shock propagating back from somewhere external, and the X-ray nebula is the new trailing PWN after the passage of the reverse shock and the subsequent oscillation of the nebula. The pulsar has moved for about 10 kyr southeastward from the explosion site, assumed to be at the location of the radio brightness peak. However, as the authors point out, no sign of a forward shock (especially in the nearby southeastern region) has been found in this scenario. Also, according to the simulation in Blondin et al. (2001), van der Swaluw et al. (2004), and Kolb et al. (2017), relic radio PWNe are apparently swept/left behind the head of the new, small X-ray nebula. In particular, given the physical contact of the eastern and southwestern edges of the extended radio emission with the MC, this seems to leave no room for the reverse shock that shocks against and crushes the suggested radio relic PWN.

With the probable interaction of CTB 87 with the ambient MC revealed here, we suggest instead that the radio emission is mainly a remnant of the blast wave that propagates into the MC at ${V}_{\mathrm{LSR}}\sim -58\,\mathrm{km}\,{{\rm{s}}}^{-1}$, although it may include a contribution from the PWN. This scenario also allows for a reverse shock to have moved backward from the remnant's edge and crushed the PWN, forming the trailing morphology of X-ray emission, as in the case of other, similar PWNe (e.g., N157B, van der Swaluw et al. 2004; Chen et al. 2006). This scenario naturally addresses the question, as noted in Matheson et al. (2013), of the lack of the X-ray emission related to the ejecta and blast wave.

The progenitor exploded somewhere along the symmetric axis of the radio emission or the X-ray trail, with a part of the blast wave shocking against the MC and the other part expanding, and quickly becoming sufficiently faint in a low-density region, possibly a portion of the superbubble. The shock in the MC decelerated and entered the intense radiative stage shortly thereafter, with the shocked gas at that point at too low a temperature to emit X-rays. Actually, the cooling timescale of the shock propagating into the molecular gas is ${t}_{\mathrm{cool}}=2.8\times {10}^{3}{(E/{10}^{51}\mathrm{erg})}^{0.24}{({n}^{a}/80{\mathrm{cm}}^{-3})}^{-0.52}$ yr (Falle 1981), where E is the SNR's explosion energy and n_a is the preshock H atom density. For an average molecular density $n({{\rm{H}}}_{2})\sim 40\,{\mathrm{cm}}^{-3}$ (see Tables 1 and 2), the cooling time is much shorter than the spin-down time $\sim 1\times {10}^{4}\,\mathrm{yr}$ (Matheson et al. 2013). Although the SNR may have a part blown out, for simplicity we crudely estimate the evolution of the radiative part in the MC as a complete sphere with the radius as ${r}_{s}\sim 5{(\epsilon /0.24)}^{5/21}{(E/{10}^{51}\mathrm{erg})}^{5/21}$ ${({n}_{a}/80{\mathrm{cm}}^{-3})}^{-5/21}{(t/{10}^{4}\mathrm{yr})}^{2/7}\,\mathrm{pc},$ where $\epsilon \sim 0.24$ is the energy fraction (McKee & Ostriker 1977; Blinnikov et al. 1982). If we approximate the pulsar's spin-down time of $\sim {10}^{4}\,\mathrm{yr}$ as the age of the remnant and adopt an explosion energy $\sim {10}^{51}\,\mathrm{erg}$, then the radius is $\sim 5\,\mathrm{pc}$, very similar to the size of the radio emission. The shock velocity is ${v}_{s}\,\sim (2/7){r}_{s}/t\sim 140{(\epsilon /0.24)}^{5/21}$ ${(E/{10}^{51}\mathrm{erg})}^{5/21}{({n}_{\mathrm{a}}/80{\mathrm{cm}}^{-3})}^{-5/21}{(t/{10}^{4}\mathrm{yr})}^{-5/7}\,\mathrm{km}\,{{\rm{s}}}^{-1}.$ Since MCs are usually highly clumpy, the observed shocked molecular gas showing broad CO-line wings should be in dense clumps, in which the shock velocity can be well below $50\,\mathrm{km}\,{{\rm{s}}}^{-1}$ (Draine & McKee 1993) so that the molecules are not dissociated. The radio emission as a relic of the blast wave in this scenario indicates a composite nature for SNR CTB 87.