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arXiv · 1509.07474

Multi-wavelength observations of 3FGL J2039.6-5618: a candidate redback millisecond pulsar

D. Salvetti·R. P. Mignani·A. De Luca·C. Delvaux·C. Pallanca·A. Belfiore·M. Marelli·A. A. Breeveld·J. Greiner·W. Becker·D. Pizzoccaro
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Abstract

We present multi-wavelength observations of the unassociated gamma-ray source 3FGL J2039.6-5618 detected by the Fermi Large Area Telescope. The source gamma-ray properties suggest that it is a pulsar, most likely a millisecond pulsar, for which neither radio nor $γ$-ray pulsations have been detected yet. We observed 3FGL J2039.6-5618 with XMM-Newton and discovered several candidate X-ray counterparts within/close to the gamma-ray error box. The brightest of these X-ray sources is variable with a period of 0.2245$\pm$0.0081 d. Its X-ray spectrum can be described by a power law with photon index $Γ_X =1.36\pm0.09$, and hydrogen column density $N_{\rm H} < 4 \times 10^{20}$ cm$^{-2}$, which gives an unabsorbed 0.3--10 keV X-ray flux of $1.02 \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. Observations with the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) discovered an optical counterpart to this X-ray source, with a time-average magnitude $g'\sim 19.5$. The counterpart features a flux modulation with a period of 0.22748$\pm$0.00043 d that coincides, within the errors, with that of the X-ray source, confirming the association based on the positional coincidence. We interpret the observed X-ray/optical periodicity as the orbital period of a close binary system where one of the two members is a neutron star. The light curve profile of the companion star, with two asymmetric peaks, suggests that the optical emission comes from two regions at different temperatures on its tidally-distorted surface. Based upon its X-ray and optical properties, we consider this source as the most likely X-ray counterpart to 3FGL J2039.6-5618, which we propose to be a new redback system.

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D. Salvetti, R. P. Mignani, A. De Luca, C. Delvaux, C. Pallanca, A. Belfiore, M. Marelli, A. A. Breeveld, J. Greiner, W. Becker, D. Pizzoccaro. 2015-10-15. Multi-wavelength observations of 3FGL J2039.6-5618: a candidate redback millisecond pulsar. https://arxiv.org/abs/1509.07474

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