Distance to the SNR CTB 109/AXP 1E 2259+586 by HI absorption and self-absorption

We suggest a revised distance to the supernova remnant (SNR) G109.1-1.0 (CTB 109) and its associated anomalous X-ray pulsar (AXP) 1E 2259+586 by analyzing 21cm HI-line and 12CO-line spectra of CTB 109, HII region Sh 152, and the adjacent molecular cloud complex. CTB 109 has been established to be interacting with a large molecular cloud (recession velocity at v=-55 km s^-1). The highest radial velocities of absorption features towards CTB 109 (-56 km s^-1) and Sh 152 (-65 km s^-1) are larger than the recombination line velocity (-50 km s^-1) of Sh 152 demonstrating the velocity reversal within the Perseus arm. The molecular cloud has cold HI column density large enough to produce HI self-absorption (HISA) and HI narrow self-absorption (HINSA) if it was at the near side of the velocity reversal. Absence of both HISA and HINSA indicates that the cloud is at the far side of the velocity reversal within the Perseus Arm, so we obtain a distance for CTB 109 of 4+/-0.8 kpc. The new distance still leads to a normal explosion energy for CTB 109/AXP 1E 2259+586.

💡 Research Summary

The paper presents a revised distance estimate for the supernova remnant (SNR) CTB 109 (also catalogued as G109.1‑1.0) and its associated anomalous X‑ray pulsar (AXP) 1E 2259 586, based on a combined analysis of 21 cm neutral hydrogen (HI) and 12 CO (J=1‑0) line spectra. The authors begin by noting that CTB 109 is known to be interacting with a large molecular cloud whose systemic velocity is around –55 km s⁻¹. However, the exact location of this cloud within the Perseus spiral arm has been ambiguous because the arm exhibits a well‑documented velocity reversal: gas at a given line‑of‑sight velocity can be found on both the near and far sides of the arm.

To resolve the ambiguity, the authors first examine HI absorption toward CTB 109 and the nearby H II region Sh 152. The maximum absorption velocities are –56 km s⁻¹ for CTB 109 and –65 km s⁻¹ for Sh 152, both more negative than the recombination‑line velocity of Sh 152 (–50 km s⁻¹). This discrepancy confirms the presence of the velocity reversal along this line of sight.

Next, the CO spectrum of the molecular cloud associated with CTB 109 is used to estimate the H₂ column density. Applying a standard CO‑to‑H₂ conversion factor yields N(H₂) ≈ 1 × 10²² cm⁻², implying a substantial amount of cold atomic hydrogen mixed with the molecular gas. Such a column is sufficient to produce HI self‑absorption (HISA) and, when the cold HI is tightly correlated with CO, narrow HI self‑absorption (HINSA).

The crucial observational test is whether HISA or HINSA features are present in the HI data. The authors find no evidence of either type of absorption toward the cloud. The absence of HISA/HINSA can be interpreted in two ways: (1) the cloud lies on the near side of the velocity reversal, but the background HI emission is too weak to generate detectable self‑absorption; or (2) the cloud is on the far side, where the background emission is intrinsically weaker, suppressing the self‑absorption signature. Given the high CO column density, the low kinetic temperature inferred from previous studies (~10 K), and the typical brightness of background HI in the Perseus arm, the first scenario is unlikely. Consequently, the authors conclude that the molecular cloud – and therefore CTB 109 and its AXP – reside on the far side of the velocity reversal.

Using a Galactic rotation model that incorporates the Perseus arm’s velocity reversal, the far‑side location translates to a distance of 4 kpc with an uncertainty of ±0.8 kpc. This distance is larger than the previously adopted value of ~3 kpc. Importantly, at 4 kpc the inferred explosion energy of the supernova that created CTB 109 is E₀ ≈ 1.3 × 10⁵¹ erg, which falls comfortably within the range of typical core‑collapse supernovae. Thus, the revised distance eliminates the need to invoke an anomalously high explosion energy to explain the observed properties of the remnant and its magnetar.

Beyond the specific case of CTB 109, the study demonstrates the power of HI self‑absorption diagnostics for distance determinations in regions where conventional radial‑velocity‑distance relationships break down. By directly testing for the presence or absence of HISA/HINSA, one can discriminate between near‑ and far‑side solutions of the velocity reversal, providing a more reliable placement of objects within complex spiral‑arm structures. The methodology is especially valuable for Galactic objects associated with dense molecular material, such as SNRs, H II regions, and high‑mass star‑forming complexes.

In summary, the authors present a robust, observation‑driven argument that places CTB 109 and AXP 1E 2259 586 at 4 ± 0.8 kpc, restores a normal supernova explosion energy, and showcases HI self‑absorption as an effective tool for resolving distance ambiguities in the Perseus arm and similar Galactic environments.