Multi-Wavelength Observations of 3fgl J2039.6–5618: A Candidate Redback Millisecond Pulsar

Abstract

We present multi-wavelength observations of the unassociated γ-ray source 3FGL J2039.6−5618 detected by the Fermi Large Area Telescope. The source γ-ray properties suggest that it is a pulsar, most likely a millisecond pulsar, for which neither radio nor γ-ray pulsations have been detected. We observed 3FGL J2039.6−5618 with XMM-Newton and discovered several candidate X-ray counterparts within/close to the γ-ray error box. The brightest of these X-ray sources is variable with a period of 0.2245±0.0081 days. Its X-ray spectrum can be described by a power law with photon index Γ X =1.36±0.09, and hydrogen column density N H <4×10 20 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 discovered an optical counterpart to this X-ray source, with a time-averaged magnitude g′∼19.5. The counterpart features a flux modulation with a period of 0.22748±0.00043 days 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, which has two asymmetric peaks, suggests that the optical emission comes from two regions with 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.

Classification and Ranking of Fermi Lat Gamma-Ray Sources From the 3fgl Catalog Using Machine Learning Techniques

“…Recently, for example, the young pulsar J1906+0722 was discovered in a gamma-ray blind search of 3FGL J1906.6+0720 (Clark et al 2015) while the MSP J1946−5403 was discovered in a targeted radio search of 3FGL J1946.4−5403 (Camilo et al 2015). In some cases, athough pulsations have not yet been discovered, the presence of a MSP is strongly suggested by multiwavelength observations (e.g., 3FGL J1653.6−0158 (Romani 2014), 3FGLJ0523.3-2528(Strader et al 2014Xing et al 2014), 3FGLJ1544.6-1125 (Bogdanov & Halpern 2015), 3FGLJ2039.6−5618 (Salvetti et al 2015;Romani 2015)), in agreement with our predictions (see Table 6). …”

“…By combining the two methods, a success rate of ∼80% was estimated for the correct classification of the gamma-ray source class ). An artificial neural network approach has also been implemented and applied to the 2FGL catalog with some promising preliminary results (Salvetti 2013).…”

The Einstein@Home Gamma-ray Pulsar Survey. II. Source Selection, Spectral Analysis, and Multiwavelength Follow-up

“…They could also be pulsars with high timing noise, or they could be millisecond pulsars in binary systems. A number of sources in our list were indeed recently identified as candidate binary millisecond Pulsars after we started our search, for instance, 3FGL J0212.1 +5320 (Li et al 2016), 3FGL J0744.1−2523 (Salvetti et al 2017), and 3FGL J2039.6−5618 (Salvetti et al 2015). The discovery of pulsars in binary systems in gamma rays requires initial guesses of the orbital parameters, from, e.g., optical or X-ray observations (see, e.g., Pletsch et al 2012b).…”

“…We performed a standard data processing, using the epproc and emproc tools, and screening for high particle background time intervals following Salvetti et al (2015). For the Chandra data analysis we used the Chandra Interactive Analysis of Observations (CIAO) software version 4.8.…”

“…We performed a standard data analysis and source detection in the 0.3−10 keV energy range of the XMM-Newton-EPIC, Chandra-ACIS, and Swift-XRT observations (e.g., Marelli et al 2015;Salvetti et al 2015). We preferred the XMM and Note.…”