4FGL J1120.0-2204: A Unique Gamma-ray Bright Neutron Star Binary with an Extremely Low Mass Proto-White Dwarf

Abstract

We have discovered a new X-ray emitting compact binary that is the likely counterpart to the unassociated Fermi-LAT GeV $\gamma$-ray source 4FGL J1120.0-2204, the second brightest Fermi source that still remains formally unidentified. Using optical spectroscopy with the SOAR telescope, we have identified a warm ($T_{\text{eff}}\sim 8500$ K) companion in a 15.1-hr orbit around an unseen primary, which is likely a yet-undiscovered millisecond pulsar. A precise Gaia parallax shows the binary is nearby, at a distance of only $\sim 820$ pc. Unlike the typical "spider" or white dwarf secondaries in short-period millisecond pulsar binaries, our observations suggest the $\sim 0.17M_{\odot }$ companion is in an intermediate stage, contracting on the way to becoming an extremely low-mass helium white dwarf (a "pre-ELM" white dwarf). Although the companion is apparently unique among confirmed or candidate millisecond pulsar binaries, we use binary evolution models to show that in $\sim 2$ Gyr, the properties of the binary will match those of several millisecond pulsar-white dwarf binaries with very short ($<1$ d) orbital periods. This makes 4FGL J1120.0-2204 the first system discovered in the penultimate phase of the millisecond pulsar recycling process.

Accepted to ApJ; 15 pages, 8 figures, 2 tables

References in corpus (27)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Achieving better than 1 minute accuracy in the Heliocentric and Barycentric Julian Dates
• The Evolution of Compact Binary Star Systems
• GROND - a 7-channel imager
• SkyMapper Southern Survey: Second Data Release (DR2)
• The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
• Chandra X-Ray Observations of Nineteen Millisecond Pulsars in the Globular Cluster 47 Tucanae
• The Joker: A custom Monte Carlo sampler for binary-star and exoplanet radial velocity data
• Optical spectroscopy and demographics of redback millisecond pulsar binaries
• The ELM Survey. VIII. 98 Double White Dwarf Binaries
• The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period
• Millisecond Pulsars and Black Holes in Globular Clusters
• B-ducted Heating of Black Widow Companions
• Discovery of Two New Thermally Bloated Low-Mass White Dwarfs Among the Kepler Binaries
• PSR J1012+5307: a millisecond pulsar with an extremely low-mass white dwarf companion
• Gaia pulsars and where to find them
• Search for transitions between states in redbacks and black widows using seven years of Fermi-LAT observations
• Classification of New X-ray Counterparts for Fermi Unassociated Gamma Ray Sources Using the Swift X-Ray Telescope
• Timing of Eight Binary Millisecond Pulsars Found with Arecibo in Fermi-LAT Unidentified Sources
• Face changing companion of the redback millisecond pulsar PSR J1048+2339
• Discovery of a New Redback Millisecond Pulsar Candidate: 4FGL J0940.3-7610
• PSR J1306--40: An X-ray Luminous Redback with an Evolved Companion
• X-ray Spectra and Multiwavelength Machine Learning Classification for Likely Counterparts to Fermi 3FGL Unassociated Sources
• Convection and rotation boosted prescription of magnetic braking: application to the formation of extremely low-mass white dwarfs

Cited by in corpus (1)

• The Third Fermi Large Area Telescope Catalog of Gamma-ray Pulsars