Fundamentally, my question is, is this a real and confirmed observation? Or is there a too-high margin of error for now?

Inspirational discoveries, Lightest neutron star ever discovered might include compressed quarks

A 0.77 solar mass neutron star kind of breaks what I've read is possible. It's too low a limit.

I have other questions, especially if this is accurate, but in keeping with the one question at a time rule, I just want to know how well-supported this claim is.


2 Answers 2


The mass of $0.77^{+0.20}_{-0.17}$ solar masses reported by Doroshenko et al. (2022) would be the least massive neutron star reported. The previous record holder had a mass of about $1.174\pm 0.004$ solar masses - but that was in a binary system and is precisely measured (Martinez et al. 2015).

A note of caution - the paper is behind a paywall, so I haven't had chance to look at it yet, but estimating the mass based on an X-ray spectrum looks fraught with difficulties. I will report back.

Edit: I still can't see beyond the paywall, but in terms of how well-supported is the result - well it's the only result of its kind and has not (yet) been corroborated (or refuted) by any other work. It is potentially important since, while it is certainly possible for such a low-mass neutron star to exist, it is hard to see an evolutionary pathway to form such an object.


An important point to note is the following: To make their mass & radius measurement, Doroshenko et al have to make the assumption that the neutron star is emitting uniformly in all directions. If the neutron star had only one or two bright spots, that would lead them to underestimate its size (given they assume uniform emission). They justify this assumption with the fact that they do not see any pulsation from the rotating neutron star.

BUT that could also be explained if our line of sight is aligned with the stars rotation axis. That way the angle under which we see any hot spot stays constant and so does its apparent brightness. They estimate they probability to miss pulsations by chance for such a non-uniform atmosphere and end up with 0.8$-$21.3$\,$%, depending on its rotation period and atmospheric model (see table below).

The 21.3$\,$% corresponds to very short rotation periods that might be unlikely. For the most typical rotation periods of a few hundred milliseconds they get 4.36$-$0.81$\,$% , with 0.90$\,$% being the probability for the more realistic model with smooth temperature profile.

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So they say:

We conclude, therefore, that the absence of observable pulsations due to unfavourable orientation is also unlikely in this scenario, even if cannot be completely ruled out for a single object for arbitrary temperature distributions.

What they mean is, that they also have not seen pulsation from most other similar sources and you would not expect to always get a lucky alignment. So looked at together, these sources probably do have a uniform atmosphere and emission profile, even if you cannot be certain just from this single observation. For this they also reference Wu et al 2021, who conclude with:

We estimate it is unlikely ($< 10^{−6}$) to attribute that we do not see pulsations to an unfavorable viewing geometry for five considered sources.

So, is this a "real and confirmed observation"? Hmm... probably. But I would say their uncertainties on mass & radius might be a bit underestimated as they only account for statistical uncertainties, after assuming perfectly uniform emission, which is not guarantied.


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