[Paper Review] PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star

Roger W. Romani, D. Kandel+3 more|arXiv (Cornell University)|Jul 11, 2022

Pulsars and Gravitational Waves Research6 citations

TL;DR

This paper presents the first precise mass measurement of PSR J0952-0607, the fastest-spinning and heaviest known neutron star in the Milky Way, using Keck telescope spectrophotometry and radial velocity measurements of its irradiated companion. The analysis yields a neutron star mass of $2.35 \pm 0.17\,M_\odot$, the highest well-measured mass to date, and implies a lower bound on the maximum neutron star mass of $M_{\rm max} > 2.19\,M_\odot$ at 1σ confidence, significantly constraining the dense-matter equation of state.

ABSTRACT

We describe Keck-telescope spectrophotometry and imaging of the companion of the ``black widow" pulsar PSR~J0952$-$0607, the fastest known spinning neutron star (NS) in the disk of the Milky Way. The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated ``day" half of the orbit. The model fits indicate system inclination $i=59.8\pm 1.9^\circ$ and a pulsar mass $M_{NS} = 2.35\pm 0.17 M_\odot$, the largest well-measured mass found to date. Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit. If the NS started at a typical pulsar birth mass, nearly $1 M_\odot$ has been accreted; this may be connected with the especially low intrinsic dipole surface field, estimated at $6 imes 10^7$G. Joined with reanalysis of other black widow and redback pulsars, we find that the minimum value for the maximum NS mass is $M_{ m max} > 2.19 M_\odot$$(2.09 M_\odot)$ at $1σ$$(3σ)$ confidence. This is $\sim 0.15 M_\odot$ heavier than the lower limit on $M_{ m max}$ implied by the white-dwarf--pulsar binaries measured via radio Shapiro-delay techniques.

Motivation & Objective

• To measure the mass of PSR J0952-0607, the fastest-spinning neutron star in the Milky Way, using optical observations of its irradiated companion.
• To determine whether the extreme spin and low magnetic field of J0952 are linked to significant mass accretion during its evolutionary history.
• To use the mass of J0952 and other spider pulsars to constrain the maximum neutron star mass and test the dense-matter equation of state.
• To assess the role of binary accretion in reducing neutron star magnetic fields, particularly in systems with low intrinsic dipole fields.

Proposed method

• Keck telescope LRIS imaging and spectroscopy were used to obtain multicolor light curves and radial velocities of the companion star over multiple orbital phases.
• A direct-heating model was applied to fit the observed flux variations, assuming the companion is within its Roche lobe and heated only by pulsar irradiation.
• System inclination was constrained via light curve modeling, yielding $i = 59.8 \pm 1.9^\circ$, which enabled mass determination from radial velocity curves.
• The neutron star mass was derived using orbital solutions and the inclination, assuming a circular orbit and Keplerian motion.
• A likelihood-based statistical model was used to estimate the minimum value of the maximum neutron star mass ($M_{\rm max}$) from the observed sample of massive neutron stars.
• A bias-corrected, accelerated bootstrap analysis was applied to the mass measurements of spider pulsars to estimate confidence bounds on $M_{\rm max}$.

Experimental results

Research questions

• RQ1 What is the precise mass of PSR J0952-0607, the fastest-spinning neutron star in the Milky Way?
• RQ2 How much mass has been accreted by PSR J0952, and is this consistent with its exceptionally low intrinsic magnetic field of $6 \times 10^7$ G?
• RQ3 What is the lower bound on the maximum neutron star mass based on the observed sample of spider pulsars, including J0952?
• RQ4 How do the masses of neutron stars in spider binaries compare to those in white-dwarf–neutron star binaries, and what does this imply for the dense-matter equation of state?
• RQ5 Is there a causal link between high mass accretion, low magnetic field, and high neutron star mass in millisecond pulsar systems?

Key findings

• The neutron star mass in PSR J0952-0607 is measured to be $2.35 \pm 0.17\,M_\odot$, the highest well-measured mass for a neutron star to date.
• The system inclination is determined to be $59.8 \pm 1.9^\circ$, which is critical for accurate mass determination from radial velocity curves.
• The analysis implies a $1\sigma$ lower bound on the maximum neutron star mass of $M_{\rm max} > 2.19\,M_\odot$, and a $3\sigma$ bound of $>2.09\,M_\odot$, significantly higher than previous estimates from Shapiro-delay measurements.
• The inclusion of J0952 increases the $1\sigma$ lower bound on $M_{\rm max}$ by $\sim 0.13\,M_\odot$ compared to other spider pulsars alone.
• The data favor a simple direct-heating model for the companion, with minimal complications from reflection or reprocessing, reducing modeling uncertainties.
• The results suggest that at least $0.5\,M_\odot$ and likely $\sim 1\,M_\odot$ has been accreted by J0952, consistent with its low intrinsic magnetic field and high mass.