2015/09/24 by D. Salvetti, Salvetti, D., R. P. Mignani +19
Physics and Astronomy · #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #astro-ph.HE
paper · pdf · doi:10.48550/arxiv.1509.07474
35 pages, 8 figures, accepted for publication on Astrophysical Journal
arxiv created 2015/10/15 · arxiv updated 2015/10/16
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±0.0081 d. Its X-ray spectrum can be described by a power law with photon index ΓX =1.36±0.09, and hydrogen column density N\rm H < 4 × 1020 cm-2, which gives an unabsorbed 0.3--10 keV X-ray flux of 1.02 × 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'∼ 19.5. The counterpart features a flux modulation with a period of 0.22748±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.