In search for the counterpart to the Fermi -LAT source 3FGL J0838.8 - 2829 , we performed a multi-wavelength campaign , in the X-ray band with Swift and XMM–Newton , performed infrared , optical ( with OAGH , ESO-NTT and IAC80 ) and radio ( ATCA ) observations , as well as analysed archival hard X-ray data taken by INTEGRAL .
We report on three X-ray sources consistent with the position of the Fermi -LAT source .
We confirm the identification of the brightest object , RX J0838 - 2827 , as a magnetic cataclysmic variable , that we recognize as an asynchronous system ( not associated with the Fermi -LAT source ) .
RX J0838 - 2827 is extremely variable in the X-ray and optical bands , and timing analysis reveals the presence of several periodicities modulating its X-ray and optical emission .
The most evident modulations are interpreted as due to the binary system orbital period of \sim 1.64 h and the white dwarf spin period of \sim 1.47 h. A strong flux modulation at \sim 15 h is observed at all energy bands , consistent with the beat frequency between spin and orbital periods .
Optical spectra show prominent H \beta , HeI and HeII emission lines Doppler-modulated at the orbital period and at the beat period .
Therefore , RX J0838 - 2827 accretes through a diskless configuration and could be either a strongly asynchronous polar or a rare example of a pre-polar system in its way to reach synchronism .
Among the other two X-ray sources , XMM J083850.4 - 282759 showed a variable X-ray emission , with a powerful flare lasting \sim 600 s , similar to what is observed in transitional millisecond pulsars during the sub-luminous disc state : that would possibly associate this source with the Fermi -LAT source .