2FGL J0846.0+2820: A New Neutron Star Binary with a Giant Secondary and Variable γ-Ray Emission

Samuel J. Swihart, Jay Strader, Tyrel J. Johnson, C. C. Cheung, David Sand, Laura Chomiuk, Asher Wasserman, Søren Larsen, Jean P. Brodie, Gregory V. Simonian, Evangelia Tremou, Laura Shishkovsky, Daniel E. Reichart, Joshua Haislip

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


We present optical photometric and spectroscopic observations of the likely stellar counterpart to the unassociated Fermi Large Area Telescope (LAT) γ-ray source 2FGL J0846.0+2820, selected for study based on positional coincidences of optical variables with unassociated LAT sources. Using optical spectroscopy from the SOAR telescope, we have identified a late-G giant in an eccentric (e = 0.06) 8.133-day orbit with an invisible primary. Modeling the spectroscopy and photometry together led us to infer a heavy neutron star primary of ∼2 Moand a partially stripped giant secondary of ∼0.8 Mo. Hα emission is observed in some of the spectra, perhaps consistent with the presence of a faint accretion disk. We find that the γ-ray flux of 2FGL J0846.0+2820 dropped substantially in mid-2009, accompanied by an increased variation in the optical brightness, and since then, it has not been detected by Fermi. The long period and giant secondary are reminiscent of the γ-ray bright binary 1FGL J1417.7-4407, which hosts a millisecond pulsar (MSP) apparently in the final stages of the pulsar recycling process. The discovery of 2FGL J0846.0+2820 suggests the identification of a new subclass of MSP binaries that are the likely progenitors of typical field MSPs.

Original languageEnglish (US)
Article number31
JournalAstrophysical Journal
Issue number1
StatePublished - Dec 10 2017
Externally publishedYes


  • binaries: spectroscopic
  • gamma rays: general
  • pulsars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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