A strangely light neutron star within a supernova remnant

Abstract

To constrain the equation of state of cold dense matter, astrophysical measurements are essential. These are mostly based on observations of neutron stars in the X-ray band, and, more recently, also on gravitational wave observations. Of particular interest are observations of unusually heavy or light neutron stars which extend the range of central densities probed by observations and thus permit the testing of nuclear-physics predictions over a wider parameter space. Here we report on the analysis of such a star, a central compact object within the supernova remnant HESS J1731-347. We estimate the mass and radius of the neutron star to be M=0.77−0.17+0.20M⊙\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$M=0.7{7}_{-0.17}^{+0.20}\,{M}_{\odot }$$\end{document} and R=10.4−0.78+0.86\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R=10.{4}_{-0.78}^{+0.86}$$\end{document} km, respectively, based on modelling of the X-ray spectrum and a robust distance estimate from Gaia observations. Our estimate implies that this object is either the lightest neutron star known, or a ‘strange star’ with a more exotic equation of state. Adopting a standard neutron star matter hypothesis allows the corresponding equations of state to be constrained.

Full text

Nature Astronomy | Volume 6 | December 2022 | 1444–1451 1444
nature astronomy
Article
https://doi.org/10.1038/s41550-022-01800-1
A strangely light neutron star within a
supernova remnant
Victor Doroshenko  , Valery Suleimanov , Gerd Pühlhofer  and
Andrea Santangelo 
To constrain the equation of state of cold dense matter, astrophysical
measurements are essential. These are mostly based on observations of
neutron stars in the X-ray band, and, more recently, also on gravitational
wave observations. Of particular interest are observations of unusually
heavy or light neutron stars which extend the range of central densities
probed by observations and thus permit the testing of nuclear-physics
predictions over a wider parameter space. Here we report on the analysis of
such a star, a central compact object within the supernova remnant HESS
J1731-347. We estimate the mass and radius of the neutron star to be
M077󰁜0󰅂20
󰁝0󰅂17 M
and
R104󰁜0󰅂86
󰁝0󰅂78
km, respectively, based on modelling of
the X-ray spectrum and a robust distance estimate from Gaia observations.
Our estimate implies that this object is either the lightest neutron star
known, or a ‘strange star’ with a more exotic equation of state. Adopting a
standard neutron star matter hypothesis allows the corresponding
equations of state to be constrained.
Central compact objects (CCOs) are isolated, radio-quiet,
non-accreting, thermally emitting neutron stars found at the centres of
supernova remnants (SNRs)
1–3
. Their thermal X-ray emission is believed
to be associated with the cooling of young, weakly magnetized4,5 neu-
tron stars and comes from the atmosphere covering a large fraction of
the surface. Most CCOs exhibit no pulsations, which suggests a rather
uniform temperature distribution over the neutron star surface. This
implies that the uncertainties associated with the geometry of the
emission region, the details of the accretion physics and the radiative
transfer in strong magnetic fields, which are all typical of accreting
neutron stars, are irrelevant for CCOs. Such simplicity makes CCOs ideal
laboratories to investigate the equation of state (EOS) of dense matter6,7.
On the other hand, CCOs are only observed as faint X-ray sources
and often lack reliable distance and luminosity estimates, which trans-
lates to large uncertainties in the emission area and neutron star radius
estimates
8–10
. Furthermore, the lack of detected pulsations does not
necessarily imply that the entire surface emits uniformly, and could
be simply due to an unfavourable orientation of the observer’s line of
sight with respect to the spin and magnetic dipole axes of the neutron
star
11,12
. The composition of the atmosphere also strongly influences
the estimated neutron star parameters. In particular, atmospheres
composed of carbon, or even heavier elements, have been suggested
for a large fraction of CCOs13–15.
We emphasize, however, that the caveats outlined above are now
largely sorted out. With regard to atmosphere composition, hydrogen
atmospheres yield unreasonably small neutron star radii9,10, whereas
emission from atmospheres with even heavier compositions strongly
deviates from the observed black-body-like spectra13. Instead of deduc-
ing unreasonably small neutron star radii, one could assume that the
emission originates from a fraction of the neutron star surface and
pulsations are not observed only because of an unfavourable viewing
orientation. This is, however, in conflict with the existing limits on the
amplitude of potential pulsations (~8% for the CCO in HESS J1731-347
(ref.
12
) and less than ~12% for the CCO in Cas A
16–18
, and in the range
3–50% for other objects). Note that although probabilities quoted
above for individual objects, including the CCO in HESS J1731-347, are
not negligible, missing pulsations from all non-pulsating CCOs are
highly unlikely, with a chance probability estimated to be ≤10
−6
(ref.
16
).
Considering that there are currently no other ideas to explain
the observed thermal non-pulsed emission, uniformly emitting car-
bon atmospheres appear to be the only viable option to explain the
observed spectra of CCOs. The largest remaining source of uncertainty
Received: 31 March 2022
Accepted: 1 September 2022
Published online: 24 October 2022
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Institut für Astronomie und Astrophysik, Tübingen, Germany. e-mail: doroshv@astro.uni-tuebingen.de
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