The supernova remnant HESS J1731-347 lies at a distance of about 2.5 kiloparsecs
Assessment
The evidence and argument have been examined and do not come down on either side.
The distance to the supernova remnant HESS J1731-347 (G353.6-0.7) sets the physical scale for everything inferred about the system, most consequentially the mass and radius of its central compact object, and it remains genuinely unsettled. The 2.5 kiloparsec value rests on a two-step argument: the Gaia parallax of the optical star IRAS 17287-3443, which yields about 2.5 kiloparsecs, combined with the proposition that this star is physically associated with the remnant, argued from its close projected position to the compact object and from similar interstellar absorption toward both.
Against this stand older and independent lines of evidence. Comparisons of the remnant's X-ray absorption with CO and HI gas column densities place it at least 3.2 kiloparsecs away, and a proposed kinematic association with an HI void would put it at roughly 5 to 6 kiloparsecs. Separately, hydrogen atmosphere fits to the compact object's spectrum imply distances above about 7 kiloparsecs, though this bears on the question only if the hydrogen description is preferred over the carbon atmosphere model.
The parallax measurement itself is not in serious doubt; the dispute concentrates on whether the star and the remnant are truly at the same distance, since a chance alignment would break the chain entirely. The question matters because the compact object's inferred radius scales linearly with distance: at 2.5 kiloparsecs the spectral fits give an unusually light star of about 0.77 solar masses, while at 3.2 kiloparsecs or beyond the same data are consistent with an ordinary neutron star. Resolution would come from a firmer test of the star-remnant association, or from an independent distance measurement to the remnant itself.
Full reasoning: the evidence and decisions behind this verdict
The 2.5 kpc figure originates with Doroshenko, Suleimanov, Pühlhofer and Santangelo (Nature Astronomy, 2022), who used the Gaia parallax of IRAS 17287-3443, a star lying close in projection to the central compact object XMMU J173203.3-344518, together with the argument that the star's extinction matches the X-ray absorption toward the compact object, indicating physical association with the remnant. Granting both premises, the inference is sound, and the parallax half is strong: Gaia astrometry for a star at this distance and brightness is a mature, well-characterized measurement. The weight therefore falls almost entirely on the physical association of the star with the remnant, which is a plausibility argument (projection plus matching absorption), not a demonstration; both subclaims are not yet assessed, and the association claim is where reassessment here would most likely be triggered.
The countervailing evidence is real and independently derived. The H.E.S.S. collaboration's analysis of the remnant (Abramowski et al. 2011, www.aanda.org/articles/aa/full_html/2011/07/aa16425-10/aa16425-10.html) set a lower limit of 3.2 kpc by comparing the remnant's X-ray absorption column with cumulative CO- and HI-derived gas columns, consistent with the earlier 3.2 ± 0.8 kpc estimate of Tian et al. (2008) from association with the HII region G353.42-0.37. Fukuda et al. (2014) argued for association with an HI void at a kinematic distance of 5.2-6 kpc (see also Maxted et al. 2018, arxiv.org/pdf/1710.06101). Klochkov et al. (2015, ui.adsabs.harvard.edu/abs/2015A%26A...573A..53K) found that hydrogen atmosphere models with full-surface emission require distances above 7-8 kpc, though that line is conditional on rejecting the carbon atmosphere description, which currently stands verified in the graph as a good fit to the spectrum. Alford and Halpern (2023, ApJ 944, 36) explicitly challenge the adoption of 2.5 kpc and the carbon-atmosphere analysis it feeds, and subsequent equation-of-state literature (e.g. arxiv.org/html/2408.15220) routinely flags the distance as a caution on the 0.77 solar mass result.
Weighing: the Gaia argument is the most direct if the association holds, and the absorption comparison carries its own systematics (gas column calibration, clumping, the fraction of the column in front of the remnant). Neither side's evidence collapses under the other, and the active literature treats the distance as an open parameter, so contested is the right status at good confidence. Credence 0.4 that the distance is in fact about 2.5 kpc: the Gaia chain is direct but hinges on an unproven association, while two independent gas-based methods prefer larger distances. What would change the conclusion: a demonstration or refutation of the star-remnant association (for example, matching proper motion or a binary-disruption scenario tested against the compact object's kinematics), an independent remnant distance (expansion parallax, refined absorption tomography), or reassessment of the subclaims this claim rests on.
[Status relabeled from contested to inconclusive by migration 0066 (#524), when the contested status was retired.]
Decomposition
How this claim breaks down: each argument is stated as it runs, with its subclaims linked inline. ↗︎ opens a subclaim; the map shows how they fit together.
Because Gaia parallax places the optical star IRAS 17287-3443 at about 2.5 kiloparsecs, and given that this star is physically associated with the supernova remnant, the remnant shares the star's distance of about 2.5 kiloparsecs. The association is argued from the star's close projected position to the central compact object and from the similarity of the interstellar absorption measured toward the two objects.
Because comparing the remnant's X-ray absorption with CO and HI gas column densities places it at least 3.2 kiloparsecs away, a distance of about 2.5 kiloparsecs is too small. Kinematic associations of the remnant with gas features point the same way: a proposed association with an HI void on the near side of the 3-kpc expanding arm would put the remnant at roughly 5 to 6 kiloparsecs.
Because hydrogen atmosphere models with emission from the full stellar surface imply distances above about 7 kiloparsecs for the compact object, the remnant would lie far beyond 2.5 kiloparsecs if the compact object's spectrum is in fact hydrogen-dominated rather than described by a carbon atmosphere.
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