INTEGRAL Observations of the γ-Ray Binary 1FGL J1018.6-5856

Abstract

The Fermi Large Area telescope (Fermi-LAT) collaboration has recently reported that one of their detected sources, namely, 1FGL J1018.6-5856, is a new gamma-ray binary similar to LS 5039. This has prompted efforts to study its multi-frequency behavior. In this report, we present the results from 5.78 Ms International Gamma-Ray Astrophysics Laboratory (INTEGRAL) IBIS/ISGRI observations on the source 1FGL J1018.6-5856. By combining all the available INTEGRAL data, a detection is made at a significance level of 5.4σ in the 18-40 keV band, with an average intensity of 0.074 counts s^-1. However, we find that there is non-statistical noise in the image that effectively reduces the significance to about 4σ and a significant part of the signal appears to be located in a 0.2-wide phase region, at phases 0.4-0.6 (where even the corrected significance amounts to 90% of the total signal found). Given the scarcity of counts, a variability is hinted at about 3σ at the hard X-rays, with an anticorrelation with the Fermi-LAT periodicity. If this behavior were true, it would be similar to that found in LS 5039 and prompt observations with TeV telescopes at phases anticorrelated with the GeV maximum.