A new γ-ray loud, eclipsing low-mass X-ray binary

Creators: Strader, Jay; Li, Kwan-Lok; Chomiuk, Laura; Heinke, Craig O.; Udalski, Andrzej; Peacock, Mark; Shishkovsky, Laura; Tremou, Evangelia

Abstract

We report the discovery of an eclipsing low-mass X-ray binary at the center of the 3FGL error ellipse of the unassociated Fermi/Large Area Telescope γ-ray source 3FGL J0427.9–6704. Photometry from OGLE and the SMARTS 1.3 m telescope and spectroscopy from the SOAR telescope have allowed us to classify the system as an eclipsing low-mass X-ray binary (P = 8.8 hr) with a main-sequence donor and a neutron-star accretor. Broad double-peaked H and He emission lines suggest the ongoing presence of an accretion disk. Remarkably, the system shows separate sets of absorption lines associated with the accretion disk and the secondary, and we use their radial velocities to find evidence for a massive (~1.8–1.9 M⊙) neutron-star primary. In addition to a total X-ray eclipse with a duration of ~2200 s observed with NuSTAR, the X-ray light curve also shows properties similar to those observed among known transitional millisecond pulsars: short-term variability, a hard power-law spectrum (Γ ~ 1.7), and a comparable 0.5–10 keV luminosity (~2.4 x 10^(33) erg s^(−1)). We find tentative evidence for a partial (~60%) γ-ray eclipse at the same phase as the X-ray eclipse, suggesting the γ-ray emission may not be confined to the immediate region of the compact object. The favorable inclination of this binary is promising for future efforts to determine the origin of γ-rays among accreting neutron stars.

Additional Information

© 2016 The American Astronomical Society. Received 2016 July 27; revised 2016 August 5; accepted 2016 August 11; published 2016 October 28. We thank an anonymous referee for helpful comments that improved the paper. We thank C. C. Cheung, R. Corbet, T. Maccarone, D. Nidever, T. Tauris, and L. van Haaften for useful conversations, F. Walter for providing routines and advice for the reduction of ANDICAM photometry, and R. J. Foley and Y-C. Pan for data acquisition. This project made use of NuSTAR mission data, a project led by the California Institute of Technology and managed by the Jet Propulsion Laboratory. We sincerely thank F. Harrison, K. Forster, and the NuSTAR team for scheduling and executing the NuSTAR observations. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, and NEOWISE, which is a project of the Jet Propulsion Laboratory/California Institute of Technology. WISE and NEOWISE are funded by the National Aeronautics and Space Administration. We acknowledge the use of public data from the Swift data archive. The OGLE project has received funding from the National Science Centre, Poland, grant MAESTRO 2014/14/A/ST9/00121 to AU. Support from NASA grant NNX15AU83G is gratefully acknowledged.

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