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 γ-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 eff ~ 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 ~820 pc. Unlike the typical "spider" or white dwarf secondaries in short-period millisecond pulsar binaries, our observations suggest the ~0.17 M ⊙ companion is in an intermediate stage, contracting on the way to becoming an extremely low-mass helium white dwarf. Although the companion is apparently unique among confirmed or candidate millisecond pulsar binaries, we use binary evolution models to show that in ~2 Gyr, the properties of the binary will match those of several millisecond pulsar-white dwarf binaries with very short (<1 day) orbital periods. This makes 4FGL J1120.0-2204 the first system discovered in the penultimate phase of the millisecond pulsar recycling process.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2022
- DOI:
- 10.3847/1538-4357/ac4ae4
- arXiv:
- arXiv:2201.03589
- Bibcode:
- 2022ApJ...926..201S
- Keywords:
-
- 1062;
- 633;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted to ApJ