<p>Here is a picture from the most recent review I could find - <a href="https://arxiv.org/abs/2011.08157" rel="nofollow noreferrer">Horvath et al. (2020)</a>.</p>
<p>The companion of J0453+1559 is still the least massive of the double neutron stars with very precisely measured masses. I cannot find any citations to the Martinez et al. paper (<a href="https://ui.adsabs.harvard.edu/abs/2015ApJ...812..143M/citations" rel="nofollow noreferrer">here is a list</a>) that either update the mass or shed any further light on its true identity. It appears to have been accepted in the literature as a neutron star - though there is still a school of thought that it could yet be a massive white dwarf (<a href="https://arxiv.org/abs/1909.12318" rel="nofollow noreferrer">Tauris &amp; Janka 2019</a>).</p>
<p>As you can see from the picture there are several other objects with only slightly higher masses, and some of these are pulsars and are unambiguously neutron stars (e.g. a pulsar of mass <span class="math-container">$1.251 \pm 0.021 M_\odot$</span>, <a href="https://arxiv.org/abs/2009.12283" rel="nofollow noreferrer">McKee et al. 2020</a>). There are some neutron stars in X-ray binaries and with white dwarf companions that <em>may</em> have a lower mass; but the uncertainties could push them above J0453+1559.</p>
<p><a href="https://i.sstatic.net/Fofkj.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Fofkj.png" alt="from Horvath et al. (2020)" /></a></p>

CC BY-SA 4.0
2021-05-26T19:48:13.880Z
Quite an impressive collection, and quite a nice summary of the state of the situation, *thanks!*
uhoh
CC BY-SA 4.0
2021-05-26T10:25:17.830Z
2021-05-26T15:23:01.003Z
2021-05-26T15:23:01.003Z
ProfRob
<p>@ProfRob's Physics SE <a href="https://physics.stackexchange.com/a/143174/83380">answer to <em>What is the theoretical lower mass limit for a gravitationally stable neutron star?</em></a> was linked just now in their <a href="https://astronomy.stackexchange.com/a/43989/7982">answer to <em>Can a neutron star ever be less than about 1.44 solar masses (the Chandrasekhar limit)? Why not?</em></a> which notes that a smaller neutron star companion to the J0453+1559 pulsar <em>might (or might not) be</em> the smallest currently known pulsar:</p>
<blockquote>
<p>A number of neutron stars in binary systems have measured masses below <span class="math-container">$1.44 M_\odot$</span> (e.g. a pulsar of mass <span class="math-container">$1.251 \pm 0.021 M_\odot$</span>, <a href="https://arxiv.org/abs/2009.12283" rel="nofollow noreferrer">McKee et al. 2020</a>). I think the current lowest mass contender is <span class="math-container">$1.174 \pm 0.004 M_\odot$</span> (<a href="http://arxiv.org/abs/1509.08805" rel="nofollow noreferrer">Martinez et al. (2015)</a> <em>Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry</em>.</p>
</blockquote>
<p>The abstract of Martinez et al. (2015) includes the sentence:</p>
<blockquote>
<p><strong>If this companion is also a neutron star</strong> (NS), as indicated by the orbital eccentricity of the system (e=0.11), then its mass is the smallest precisely measured for any such object.</p>
</blockquote>
<p>and Section 3, Results explains:</p>
<blockquote>
<p>Given the orbital eccentricity (e = 0.11), <strong>the companion is very likely to be a neutron star</strong> – if it had evolved into a massive white dwarf star there would be no sudden mass loss associated with a supernova explosion and the system would have retained the circular orbit that is characteristic of compact accreting systems. <strong>This is consistent with the non-detection of an optical counterpart of the system in any of the optical catalogs. However, the high eccentricity does not entirely settle the matter:</strong> the recent discovery of a recycled pulsar with a massive (∼ 1M⊙) companion PSR J1727−2946 <a href="https://ui.adsabs.harvard.edu/abs/2015MNRAS.450.2185L/abstract" rel="nofollow noreferrer">Lorimer et al. (2015)</a> and an orbital eccentricity of 0.0456 bridges the previously observed eccentricity gap between systems with NS and massive WD companions. <strong>In the remainder of this paper we will assume, with caution, that the companion is a NS.</strong></p>
</blockquote>
<p>That the companion may be a neutron star is important for this analysis; the next sentence is</p>
<blockquote>
<p>The pulsar’s orbital eccentricity allows a detection of the advance of periastron, ˙ω. <strong>If
both components are compact, as implied by the optical non-detection,</strong> this is given by:</p>
<p><span class="math-container">$$\dot{\omega} = \dot{\omega}_{GR} + \dot{\omega}_{K}$$</span></p>
<p>[...]</p>
</blockquote>
<p>And the conclusion section notes that this is a relatively small eccentricity for double neutron stars listed in Table 1 <em>as of 2015.</em></p>
<hr />
<p>It's now been six years, so I'd like to ask:</p>
<p><strong>Question:</strong> Has the smaller companion to Pulsar J0453+155 been confirmed to be a neutron star by any further work beyond that of Martinez et al. (2015) and is it still the smallest known neutron star?</p>
<p>Has the smaller companion to Pulsar J0453+155 been confirmed to be a neutron star after Martinez et al. (2015) and is it still the smallest known neutron star?</p>

<observational-astronomy><neutron-star><binary-star><pulsar>
Is the smaller companion of Pulsar J0453+1559 still the smallest known neutron star? Was it further confirmed to be a NS after Martinez et al. (2015)?
uhoh
# Is the smaller companion of Pulsar J0453+1559 still the smallest known neutron star? Was it further confirmed to be a NS after Martinez et al. (2015)?

(Asked by: username_0 on 2021-05-25)

@ProfRob's Physics SE [answer to *What is the theoretical lower mass limit for a gravitationally stable neutron star?*](https://physics.stackexchange.com/a/143174/83380) was linked just now in their [answer to *Can a neutron star ever be less than about 1.44 solar masses (the Chandrasekhar limit)? Why not?*](https://astronomy.stackexchange.com/a/43989/7982) which notes that a smaller neutron star companion to the J0453+1559 pulsar *might (or might not) be* the smallest currently known pulsar:

> A number of neutron stars in binary systems have measured masses below $1.44 M\_\odot$ (e.g. a pulsar of mass $1.251 \pm 0.021 M\_\odot$, [McKee et al. 2020](https://arxiv.org/abs/2009.12283)). I think the current lowest mass contender is $1.174 \pm 0.004 M\_\odot$ ([Martinez et al. (2015)](http://arxiv.org/abs/1509.08805) *Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry*.

The abstract of Martinez et al. (2015) includes the sentence:

> **If this companion is also a neutron star** (NS), as indicated by the orbital eccentricity of the system (e=0.11), then its mass is the smallest precisely measured for any such object.

and Section 3, Results explains:

> Given the orbital eccentricity (e = 0.11), **the companion is very likely to be a neutron star** – if it had evolved into a massive white dwarf star there would be no sudden mass loss associated with a supernova explosion and the system would have retained the circular orbit that is characteristic of compact accreting systems. **This is consistent with the non-detection of an optical counterpart of the system in any of the optical catalogs. However, the high eccentricity does not entirely settle the matter:** the recent discovery of a recycled pulsar with a massive (∼ 1M⊙) companion PSR J1727−2946 [Lorimer et al. (2015)](https://ui.adsabs.harvard.edu/abs/2015MNRAS.450.2185L/abstract) and an orbital eccentricity of 0.0456 bridges the previously observed eccentricity gap between systems with NS and massive WD companions. **In the remainder of this paper we will assume, with caution, that the companion is a NS.**

That the companion may be a neutron star is important for this analysis; the next sentence is

> The pulsar’s orbital eccentricity allows a detection of the advance of periastron, ˙ω. **If
> both components are compact, as implied by the optical non-detection,** this is given by:
>
> $$\dot{\omega} = \dot{\omega}\_{GR} + \dot{\omega}\_{K}$$
>
> [...]

And the conclusion section notes that this is a relatively small eccentricity for double neutron stars listed in Table 1 *as of 2015.*

---

It's now been six years, so I'd like to ask:

**Question:** Has the smaller companion to Pulsar J0453+155 been confirmed to be a neutron star by any further work beyond that of Martinez et al. (2015) and is it still the smallest known neutron star?

Has the smaller companion to Pulsar J0453+155 been confirmed to be a neutron star after Martinez et al. (2015) and is it still the smallest known neutron star?

**Tags:** observational-astronomy, neutron-star, binary-star, pulsar

---

## 1 answers

### Answer by username_1 on 2021-05-26. Score: 2 | Accepted answer

Here is a picture from the most recent review I could find - [Horvath et al. (2020)](https://arxiv.org/abs/2011.08157).

The companion of J0453+1559 is still the least massive of the double neutron stars with very precisely measured masses. I cannot find any citations to the Martinez et al. paper ([here is a list](https://ui.adsabs.harvard.edu/abs/2015ApJ...812..143M/citations)) that either update the mass or shed any further light on its true identity. It appears to have been accepted in the literature as a neutron star - though there is still a school of thought that it could yet be a massive white dwarf ([Tauris & Janka 2019](https://arxiv.org/abs/1909.12318)).

As you can see from the picture there are several other objects with only slightly higher masses, and some of these are pulsars and are unambiguously neutron stars (e.g. a pulsar of mass $1.251 \pm 0.021 M\_\odot$, [McKee et al. 2020](https://arxiv.org/abs/2009.12283)). There are some neutron stars in X-ray binaries and with white dwarf companions that *may* have a lower mass; but the uncertainties could push them above J0453+1559.

[![from Horvath et al. (2020)](https://i.sstatic.net/Fofkj.png)](https://i.sstatic.net/Fofkj.png)

> **Comments:**
> **username_0**: Quite an impressive collection, and quite a nice summary of the state of the situation, *thanks!*  

---

