High-resolution X-ray study of supernova remnant J0453.6−6829 with unusually high forbidden-to-resonance ratio

Abstract

Recent high-resolution X-ray spectroscopy has revealed that several supernova remnants (SNRs) in the Large Magellanic Cloud (LMC) show unusually high forbidden-to-resonance (f/r) line ratios. While their origin is still uncertain and debated, most of these SNRs have asymmetric morphology and/or show evidence of interaction with dense material, which may hint at the true nature of the anomalous f/r ratios. Here we report on a detailed spectral analysis of the LMC SNR J0453.6−6829 with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton. We find that the f/r ratio of O vii (⁠$={1.06}_{-0.10}^{+0.09}$⁠) is significantly higher than expected from the previously reported thermal model. The spectrum is suitably explained by taking into account a charge exchange (CX) emission in addition to the thermal component. Analyzing archival ATCA and Parkes radio data, we also reveal that an H i cloud is possibly interacting with J0453.6−6829. These results support the presence of CX in J0453.6−6829, as the origin of the obtained high f/r ratio. Although a contribution of the resonance scattering (RS) cannot be ruled out at this time, we conclude that CX seems more likely than RS considering the relatively symmetric morphology of this remnant.

1 Introduction

Plasma diagnostics of astrophysical objects using line ratios of He-like ions (i.e., G and R ratios; Gabriel & Jordan 1969) will become a part of the mainstream in the upcoming era of high-resolution X-ray spectroscopy. Recent grating observations revealed that several supernova remnants (SNRs) have anomalous line ratios of O vii, in which the forbidden line intensity relative to the resonance line (hereafter, f/r) is significantly higher than expected for an ordinary thermal plasma (e.g., Katsuda et al. 2011; Uchida et al. 2019). While their physical origin is still under debate, two interpretations have mainly been argued: charge exchange (CX) and resonance scattering (RS), both of which were predicted to occur in SNRs by previous calculations (e.g., Lallement 2004; Kaastra & Mewe 1995). The presence of the CX emission and/or RS effect would hinder accurate plasma diagnostics, and, more importantly, these physical processes themselves work as useful probes to obtain key information such as collision and turbulent velocities. It is therefore required to know physical conditions and surrounding environments in which CX and/or RS can occur, that is, to reveal the origin of the anomalously high f/r ratios of O vii found in SNRs.

On the basis of observations with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton, previous analyses of SNRs in the Large Magellanic Cloud (LMC) indicate that several of them tend to have relatively high f/r ratios of O vii; N 23 (Broersen et al. 2011), N 49 (Amano et al. 2020), and N 132 D (Suzuki et al. 2020). They are located in a dense ambient medium as suggested by radio-line (e.g., Banas et al. 1997; Sano et al. 2017) or infrared observations (e.g., Williams et al. 2006), which may hint at the cause of the anomalous spectral features. In this context, we found that no detailed spectroscopy has been performed so far for the middle-aged LMC SNR J0453.6−6829, although this remnant is in a dense environment similar to N 23 and N 132 D according to Williams et al. (2006).

J0453.6−6829 is a relatively compact (∼2″ in diameter) remnant of a core-collapse explosion (Lopez et al. 2009; McEntaffer et al. 2012), containing a pulsar wind nebula (PWN) at the center of the shell (Gaensler et al. 2003). In addition to synchrotron radiation from the PWN, McEntaffer, Brantseg, and Presley (2012) indicated that the X-ray spectrum of J0453.6−6829 obtained with Chandra is well explained by a shock-heated interstellar medium (ISM). A similar conclusion was reached by Haberl et al. (2012), who performed multi-frequency observations of J0453.6−6829, including X-ray band with XMM-Newton. In their spectral fit, the forbidden line of O vii is seemingly higher than a normally expected thermal model. In this paper, we thus revisit the RGS data with particular attention to the He-like lines, in conjunction with an H i observation around the remnant, in order to investigate the relation between the f/r ratio and surrounding environment of J0453.6−6829. Errors are given at the $68\mathrm{\%}$ confidence level throughout the paper. We assume the distance to J0453.6−6829 to be 50 kpc (Pietrzyński et al. 2013).

2 Observation and data reduction

J0453.6−6829 was observed with XMM-Newton on 2001 March 29 (Obs. IDs 0062340101 and 0062340501). Since one of the data sets (Obs. ID 0062340501) was affected by large soft-proton flares, we present results only from Obs. ID 0062340101. The raw data were processed with the XMM Science Analysis Software (SAS) version 18.0.0 and the calibration data files released in 2020 June. In the following spectral analysis, we combine RGS1 and RGS2 data with MOS spectra. After discarding periods of background flares, we obtained MOS and RGS data with effective exposure times of ∼6 ks and ∼20 ks, respectively. We do not analyze second-order spectra because of their poor statistics.

3 Analysis and results

Figure 1 shows a background-subtracted true-color image of J0453.6−6829 taken by EPIC (MOS and pn). We extracted RGS spectra by limiting the cross-dispersion width so as to cover the whole of J0453.6−6829. MOS spectra were obtained from the entire region of the remnant. Off-source regions in the field of view (FOV) were used to extract background spectra for each instrument. We simultaneously fitted the unbinned RGS and MOS spectra using SPEX version 3.06.01 (Kaastra et al. 1996), applying a maximum likelihood method, W-stat (Wachter et al. 1979). Throughout this analysis, the hydrogen column density (N_H) of the Galactic absorption was fixed to 6 × 10²⁰ cm⁻² (Dickey & Lockman 1990) and that of the LMC was left free. We referred to Russell and Dopita (1992) for the elemental abundances of the LMC.

Figure 2 presents the MOS and RGS spectra of J0453.6−6829. McEntaffer, Brantseg, and Presley (2012) reported that the X-ray spectrum of J0453.6−6829 can be explained by a two-temperature non-equilibrium ionization (NEI) model (neij) with a power-law component for the PWN. We first applied this “2-NEI” model, in which photon index Γ and normalization of the power-law component were fixed to 2.0 and 3.5 × 10⁴³ photons s⁻¹ keV⁻¹ (McEntaffer et al. 2012), respectively. Free parameters for the thermal components include the electron temperature (kT_e), ionization timescale (n_et, where n_e and t are the electron number density and the time after shock heating, respectively), and emission measure (n_en_HV, where V is the emitting volume of the plasma). Abundances of C, N, O, Ne, Mg, Si, and Fe were set free and tied between the two components.

The best-fitting model of the 2-NEI model is plotted in figure 2, and its parameters are summarized in table 1. We found that whereas the model can reproduce the MOS spectrum, significant residuals remain in the RGS spectrum especially at the O vii Heα and O viii Lyβ lines. This result implies that some additional considerations are required to explain the fine structures of the spectrum of J0453.6−6829 better. To clarify this point, we quantified the f/r intensity ratio of O vii by adding four Gaussians instead of multiplet line components of O vii Heα, (i.e., resonance, forbidden, and intercombination lines) implemented in the neij code: in this method, the other lines and continua were not changed and were the same as those of the best-fitting model. We compared the obtained value with that expected from the NEI model as indicated in figure 3a. The resultant f/r ratio, ${1.06}_{-0.10}^{+0.09}$⁠, requires kT_e < 0.025 keV. On the other hand, few O^{6 +} (He-like) ions, which emit O vii, are present in such a low-temperature plasma (figure 3b), which is inconsistent with our result. We thus conclude that any single or multiple NEI component(s) cannot reproduce the observed RGS spectrum. Another possible scenario to account for both the f/r ratio and the O viii Lyβ line is an over-ionized plasma. This scenario, however, would make a significant excess of radiative recombination continua, and thus contradicts the observed spectrum.

We next added a CX component to the 2-NEI model (hereafter, 2-NEI+CX model) to enhance the forbidden line intensity of O vii, as Uchida et al. (2019) did for a similar case of the Cygnus Loop. Free parameters of the CX model are normalization (n_Hn_nhV, where n_nh is the density of the neutral materials), and shock velocity (zv). The ionization temperature was tied to kT_e of the low-temperature NEI component. The best-fitting result and parameters are shown in figure 4 and table 1, respectively. As a result, we successfully fitted the RGS spectrum with the 2-NEI+CX model, except that the discrepancy between the data and model is still seen at the O viii Lyβ line.

A similar excess of the O viii Lyβ line has been reported by Amano et al. (2020), who analyzed the RGS spectrum of N 49 and concluded that the line ratios including f/r of O vii can be reasonably explained by taking into account the effect of RS. We thus applied negative Gaussians in addition to the 2-NEI model (hereafter, 2-NEI−Gaus model), in which we assumed that the SNR shell is a slab and all scattered photons will escape from the line of sight (Kaastra & Mewe 1995). The Gaussians were fixed at the centroid wavelengths of the lines whose oscillator strengths are relatively large: resonance lines of Ne ix and O vii, Fe xvii L(3d-2p), Fe xvii L(3s-2p), and O viii Lyα. Normalizations (photons s⁻¹) of these five Gaussians were set free and the other parameters are the same as the 2-NEI model. As shown in figure 4, the 2-NEI−Gaus model globally reduces the residuals. The best-fitting parameters (table 1) are consistent with those expected for a typical middle-aged SNR. We therefore claim that a presence of RS cannot be ruled out in terms of the spectral fitting.

4 Discussion

As indicated in the previous section, the high-resolution X-ray spectrum of J0453.6−6829 suggests the presence of CX or RS in the remnant. Similar cases have often been discussed in the literature (e.g., Uchida et al. 2019; Amano et al. 2020; Suzuki et al. 2020). Although it is in general difficult to distinguish between these two possibilities with the available spectroscopies, the SNR morphology and surrounding environment may provide a clue to the true origin of the high f/r ratio. In table 2, we summarize LMC/SMC SNRs for which the f/r (or G) ratios were measured so far using the RGS to compare our results with those from other SNRs in the discussion below.

4.1 X-ray morphology of J0453.6−6829

For quantitative evaluation of the effect of RS, we calculated a transmission factor τ by applying the same method as Amano et al. (2020). We compared theoretical values of τ for several optical depths according to Kaastra and Mewe (1995) with those estimated from the best-fitting normalizations of the negative Gaussians. Figure 5 shows the result. Although the model fails to explain the observed resonance line of Ne ix, the estimated τ for J0453.6−6829 are roughly consistent with those at N_H = 1.0–5.0 × 10¹⁹ cm⁻², which corresponds to a plasma depth of 3–18 pc under an assumption of the plasma density n_H = 1.1 cm⁻³ (Williams et al. 2006). Since the diameter of J0453.6−6829 is estimated to be ∼36 pc from the apparent angular size of the shell (⁠$\sim 2{}_{.}^{\prime }5$⁠), the line-of-sight plasma depth that contributes to RS should be $10\mathrm{\%}$–$50\mathrm{\%}$ of the diameter. If this is really the case, the shell-type SNR would be required to have a highly asymmetric morphology; for instance, a bright shell is prominent only on one side of the remnant (see figure 6).

If J0453.6−6829 has an ideal spherical symmetric structure, the RS effect will be cancelled out and an enhancement of f/r will not occur. Note that the RGS spectrum of J0453.6−6829 was obtained from the entire region. We can thus postulate that asymmetry of an SNR is a key parameter to evaluate the effect of RS. From a soft-band imaging analysis by Lopez et al. (2011), we found that J0453.6−6829 has a less asymmetric morphology among the six SNRs listed in table 2. Other core-collapse remnants are more “elliptical” (N 23) or “non-uniform” (N 132 D), which are parametrized as P₂/P₀ and P₃/P₀ in their calculation. Although N 49 was not analyzed by Lopez et al. (2011), it would also be categorized as a highly elliptical remnant due to its morphology similar to that of N 23. It is reasonable that N 23, N 49, and N 132 D show relatively high f/r ratios due to RS, as claimed by previous studies (Broersen et al. 2011; Amano et al. 2020; Suzuki et al. 2020). On the other hand, in the case of J0453.6−6829, the effect of RS might be unlikely or insufficient to satisfactorily explain the observed high f/r ratio.

4.2 Surrounding environment of J0453.6−6829

CX is another possibility that causes the enhancement of f/r. An interaction with a dense ambient medium is expected in this case, as in previous studies of Galactic SNRs with the RGS: Puppis A (Katsuda et al. 2012) and the Cygnus Loop (Uchida et al. 2019). While McEntaffer, Brantseg, and Presley (2012) implied a presence of dense gas in the vicinity of J0453.6−6829 because of a spatial correlation between the X-ray and infrared morphologies, the surrounding environment of this remnant has still been unclear (Williams et al. 2006; Lakićević et al. 2015). As shown in figure 7, we compared the ATCA and Parkes (Kim et al. 2003) H i velocity channel map around J0453.6−6829 with the X-ray morphology and found H i clouds located along with the south-western half of J0453.6−6829. Figure 8 shows the integrated intensity maps of H i. We also found the south-western part of the remnant is increasingly covered with an H i cloud. The position–velocity diagram suggests that the SNR shell is expanding into the dense gas (figure 8b).

If the SNR shell interacts with the H i cloud in the south-western region, relatively strong forbidden line emission would be detected there. We thus divided the data into two in the cross-dispersion direction (namely, north-east: NE and south-west: SW) as indicated in figure 1. As displayed in figure 9, the forbidden line intensity of O vii in SW is stronger than that in NE. Applying the same method as the entire region, we obtained f/r ratios of ${0.97}_{-0.14}^{+0.18}$ and ${1.41}_{-0.29}^{+0.40}$ for NE and SW, respectively. Although statistically they are equal within the errors, the trend is consistent with the above expectation and thus strongly supports the presence of CX. The best-fitting models of 2-NEI+CX for these regions are displayed in figure 10. The model parameters are given in table 3. While the CX component is required both in NE and SW, its contribution is relatively dominant in SW. We thus confirmed that the anomalous f/r ratios is due to the CX emission, mainly caused by an interaction with the south-western H i cloud.

According to the discussion above, we presume that the emitting region of CX is the south-western edge of J0453.6−6829, which is in contact with the H i cloud. To examine the possibility of CX quantitatively, we estimate the emitting volume V_CX using the volume emission measure of the CX component EM_CX = n_Hn_nhV_CX, where n_nh is the neutral material density of the surrounding gas. Given that the H i gas has a typical density of n_nh = 10 cm⁻³, we obtain the emitting volume V_CX to be 2 × 10⁵⁸ cm³. Since the total volume of J0453.6−6829 is estimated to be V_SNR ∼ 10⁶⁰ cm³ assuming a diameter of ∼36 pc, we conclude that the CX occurs in $\sim 0.4\mathrm{\%}$ of the SNR radius. The result fits well with the calculation by Lallement (2004) and thus supports the possibility that the observed anomalous f/r ratio is due to CX. Note that the significant residuals seen at ∼16 Å (section 3) is still an open question; such discrepancies around the O viii Lyβ line are often pointed out by many RGS observations (e.g., Amano et al. 2020), and might be due to uncertainties in the atomic data (see also, de Plaa et al. 2012).

5 Conclusions

We performed a high-resolution spectroscopy of J0453.6−6829 with the RGS onboard XMM-Newton and found that the intensity of the forbidden line of O vii is significantly stronger than expected from a simple thermal (2-NEI) model. To account for the obtained high f/r ratio (⁠${1.06}_{-0.10}^{+0.09}$⁠), we examined two possibilities: CX and RS, which have been proposed for explaining similar spectral features found in SNRs. Both models are statistically acceptable, although small residuals remain at ∼16 Å (around the O viii Lyβ line) between the data and the 2-NEI+CX model. Such discrepancies are often pointed out by many RGS observations (e.g., de Plaa et al. 2012; Amano et al. 2020) and are likely due to uncertainties in the atomic data. From the best-fitting result with the RS model, we estimated a transmission factor τ; the result requires a significantly asymmetric shape along the line of sight. This may be inconsistent with the apparent morphology of J0453.6−6829, since a previous systematic X-ray study indicates that this remnant is one of the “least asymmetric” core-collapse SNRs (Lopez et al. 2011). On the other hand, our estimate of the emitting volume for the CX component (⁠$\sim 0.4\mathrm{\%}$ of the SNR radius) agrees well with a theoretical expectation (Lallement 2004). We also found evidence of an interaction between J0453.6−6829 and the dense ambient gas in the ATCA and Parkes H i map, which supports the picture that the observed f/r ratio is due to the CX emission at SNR shock fronts. In conclusion, the presence of CX in J0453.6−6829 is favored in our study, while a slight or significant contribution of the RS effect also cannot be ruled out. Future spatially resolved spectroscopies with high angular resolution missions like Athena will clarify this point.

Acknowledgements

We thank Brian J. Williams for a helpful discussion about the previous multiwavelength studies of the SNR J0453.6−6829. The ATCA and the Parkes radio telescope are all part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi and Wiradjuri people as the traditional owners of the Observatory sites. This work is supported by JSPS/MEXT KAKENHI Scientific Research Grant Numbers JP19K03915 (H.U.), JP19H01936 (T.T.), JP19K14758 (H.S.), JP20KK0309 (H.S.), and JP21H04493 (T.G.T. and T.T.).

References