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

Abstract

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 lead us to infer a heavy neutron star primary of ∼ 2M and a partially stripped giant secondary of ∼ 0.8M . Hα emission is observed in some of the spectra, perhaps consistent with the presence of a faint accretion disk. We find 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 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.

Optical Spectroscopy and Demographics of Redback Millisecond Pulsar Binaries

“…They appear to be approximately as common as black widow systems, and depending on whether candidate redbacks are included, make up ∼ 12-21% of fully recycled binary millisecond pulsars for which the identity of the companion is known in the ATNF pulsar database (Manchester et al 2005). Figure 10 also shows the large gap between normal redbacks and the huntsman-type systems 1FGL J1417.7-4407 (Strader et al 2015;Camilo et al 2016) and 2FGL J0846.0+2820 (Swihart et al 2017). 1FGL J1417.7-4407 appears to be in the late stages of the recycling process that will lead to a normal He white dwarf-millisecond pulsar binary, while (if it does indeed contain a pulsar) 2FGL J0846.0+2820 is rather earlier on in the process.…”

“…As in Strader et al (2016) and Swihart et al (2017), we can also estimate the distance to the binary through the normalization of the model light curves compared to the observed source fluxes. We find a distance of 3.4(4) kpc.…”

“…Five would be classified as redbacks (3FGL J0212.1+5320, 1FGL J0523.5-2529, Strader et al 20143FGL J0838.8-2829, Halpern et al 2017b1FGL J0955.2-3949, Li et al 2018;3FGL J2039.6-5618, Salvetti et al 2015 andthis paper). The other is a candidate huntsman with an 8.13 d orbital period (2FGL J0846.0+2820; Swihart et al 2017).…”

A Multiwavelength View of the Neutron Star Binary 1FGL J1417.7–4402: A Progenitor to Canonical Millisecond Pulsars

“…Following similar steps as detailed in Strader et al (2015) and Swihart et al (2017), we estimate the distance to J1417 by comparing its observed magnitude to its intrinsic luminosity derived from the best-fit radius and effective temperature of the secondary from our optical/near-IR light curve models. After applying bolometric corrections to each band as a function of temperature and metallicity from fitting 10 Gyr, [Fe/H] = -0.65 isochrones (Marigo et al 2008), we compared the extinction-corrected magnitudes (corrected using the Schlafly & Finkbeiner (2011) reddening maps) to the predicted absolute magnitude in each band.…”

PSR J1306-40: An X-Ray Luminous Redback with an Evolved Companion

“…Following similar procedures as described in Strader et al (2014) and Swihart et al (2017), we obtained spectra of bright late-G to mid-K giant stars to use as templates Table 1. and convolved these with a set of rotational convolution kernels reflecting a range of v sin i values (including limb darkening).…”