3 years ago

# 2FGL J0846.0+2820: A new neutron star binary with a giant secondary and variable $\gamma$-ray emission.

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

We present optical photometric and spectroscopic observations of the likely stellar counterpart to the unassociated \emph{Fermi}-Large Area Telescope (LAT) $\gamma$-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 lead us to infer a heavy neutron star primary of $\sim 2 M_{\odot}$ and a partially stripped giant secondary of $\sim 0.8 M_{\odot}$. H$\alpha$ emission is observed in some of the spectra, perhaps consistent with the presence of a faint accretion disk. We find the $\gamma$-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 \emph{Fermi}. The long period and giant secondary are reminiscent of the $\gamma$-ray bright binary 1FGL J1417.7--4407, which hosts a millisecond pulsar 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 millisecond pulsar binaries that are the likely progenitors of typical field millisecond pulsars.

Publisher URL: http://arxiv.org/abs/1711.02077

DOI: arXiv:1711.02077v1

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