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We observed the field of the Fermi source 3FGL J0838.8-2829 in optical andX-rays, initially motivated by the cataclysmic variable (CV) 1RXSJ083842.1-282723 that lies within its error circle. Several X-ray sources firstclassified as CVs have turned out to be gamma-ray emitting millisecond pulsars(MSPs). We find that 1RXS J083842.1-282723 is in fact an unusual CV, astream-fed asynchronous polar in which accretion switches between magneticpoles (that are $\approx$120$^{\circ}$ apart) when the accretion rate is atminimum. High-amplitude X-ray modulation at periods of 94.8$\pm$0.4 minutes and14.7$\pm$1.2 hr are seen. The former appears to be the spin period, whilelatter is inferred to be one-third of the beat period between the spin and theorbit, implying an orbital period of 98.3$\pm$0.5 minutes. We also measure anoptical emission-line spectroscopic period of 98.413$\pm$0.004 minutes which isconsistent with the orbital period inferred from the X-rays. In any case, thissystem is unlikely to be the gamma-ray source. Instead, we find a faintervariable X-ray and optical source, XMMU J083850.38-282756.8, that is modulatedon a time scale of hours in addition to exhibiting occasional sharp flares. Itresembles the black widow or redback pulsars that have been discovered ascounterparts of Fermi sources, with the optical modulation due to heating ofthe photosphere of a low-mass companion star by, in this case, an as-yetundetected MSP. We propose XMMU J083850.38-282756.8 as the MSP counterpart of3FGL J0838.8-2829.

astrophysical journal/the astrophysical journal

X-ray and Optical Study of the Gamma-ray Source 3FGL J0838.8$-$2829:  Identification of a Candidate Millisecond Pulsar Binary and an Asynchronous  Polar