The lightest neutron star?

The mass of the central compact object in supernova remnant HESS J1731-347 estimated using modelling of its X-ray spectrum turns out to be almost too low

Neutron stars are one of the endpoints of stellar evolution and truly extreme objects. This renders them ideally suited to answer many fundamental physics questions such as behaviour of cold dense matter at supra-nuclear densities. The properties of matter under such conditions are normally studied by nuclear physicists, but unfortunately theoretical predictions are often not really testable in the terrestrial labs. One can get, however, important constraints on the equation of state of cold dense matter by observing neutron stars in X-rays and other bands, including the gravitational wave signals from their mergers. Using an astrophysical object as a laboratory comes, however, with many challenges. This includes complexities related to modeling of the X-ray emission in the presence of strong magnetic fields and accretion of matter, distance uncertainties, and so on.

In this regard, a handful of thermally emitting neutron stars at the centers of supernova remnants, so-called central compact objects (CCOs), are especially interesting because they are believed to be as simple as neutron stars can get: they are free of ultra-strong fields, are not accreting, and are yet reasonably bright and thus observationally accessible. The big  "but" here is that they only emit in X-rays and thus it is hard to measure precisely how far they are away, and this uncertainty translates to the uncertainty in measured surface area and neutron star size deduced when modelling their X-ray spectra. The neutron star in the supernova remnant HESS J1731-37 is, however, a special case. Not only is it one of the brightest and most observed CCOs, but more importantly, distance to the source is robustly and accurately measured. That's because the CCO, which is located within a TeV- and X-ray emitting supernova remnant, is also embedded in a thick massive dust shell surrounding the neutron star and its former binary counterpart. This shell is so massive it could have only condensed from ejecta of the same supernova explosion which created the CCO. The normal star emits in the optical and UV bands, heats up the dust and makes it shine in the infrared so we know that the all three objects (or four if one counts the supernova remnant) are indeed physically related to each other. 

A sketch of the supernova remnant and steps leading to NS mass estimate

A false-color image of the supernova remnant HESS J1731-347 hosting the X-ray emitting neutron star observed by XMM-Newton and the optical star observed by Gaia missions (in the center of the shell in the left panel). The image is a result of synergies between various facilities observing all kinds of invisible light from infra-red (orange, Spitzer) to X-rays (green, XMM-Newton) and  ultra-high energy TeV band (blue, HESS). The deep observations by XMM-Newton and Suzaku telescopes allowed us to measure the high-fidelity X-ray spectra of the neutron star (top right) which helped to deduce neutron star’s mass using the neutron star atmosphere models developed in Tübingen. 

The distance to the optical companion star has recently been accurately measured using the parallax by ESAs Gaia mission, and turned out to be lower than previously estimated.  Distance measurement allowed to eliminate the degeneracy of the models describing the observed X-ray spectrum of the CCO with respect to its distance. On the other hand, the fact that actual distance to the source turned out to be lower than previously thought resulted in a revised neutron stars mass estimate, which also turned out to be surprisingly low at just under 0.8 solar masses. That's quite a bit lower than for any other known neutron star, and in fact, also lower than what is believed to be necessary to form a neutron star in the first place. Indeed, porto-neutron star must exceed the Chandrasekhar limit to make a neutron star, so the lightest neutron stars expected to exist shall have masses of 1.2 solar mass or so if one takes details of explosion into the account. The question remains, therefore, how such a light object could be formed, and whether it is indeed a normal neutron star or rather a more exotic object like a strange or quark star (if such a thing exists in nature) which lost part of the mass when transitioning to strange state. Regardless on the origin of the object, a discovery of such a light neutron star is bound to help us better understand structure and processes in the formation of neutron stars, and in particular the origin of CCOs. 

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