A binary counterpart for 2FGL J2039.6-5620

We have identified an optical/X-ray binary with orbital period P_b=5.47h as the likely counterpart of the Fermi source 2FGL J2039.6-5620. GROND, SOAR and DES observations provide an accurate orbital period and allow us to compare with the light curve of an archival XMM exposure. Like many short-period optical X-ray binaries associated with LAT sources this may be a interacting (black widow/redback) millisecond pulsar binary. The X-ray light curve is consistent with the emission associated with an intrabinary shock. The optical light curve shows evidence of companion heating, but has a peculiar asymmetric double peak. The nature of this optical structure is not yet clear; additional optical studies and, especially, detection of an orbital modulation in a gamma-ray pulsar are needed to elucidate the nature of this peculiar source.

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