The 54-day orbital period of AX J1820.5-1434 unveiled by Swift

Context. The hard X-ray survey that Swift Burst Alert Telescope (BAT) has been performing since late 2004 provides a considerable database for a large number of sources, whose hard X-ray emission are poorly known. Aims. We exploit the BAT survey archive to improve the temporal and spectral characterization of the Galactic hard-X-ray sources. We focus here on the study of the high mass X-ray binary AX J1820.5-1434. Methods. All the data relevant to AX J1820.5-1434 were extracted from the BAT survey archive and analyzed using a folding technique to search for periodical modulations. A broad-band spectral analysis was also performed, complementing the BAT spectrum with the available Swift X-Ray Telescope (XRT) and XMM-Newton pointed observations. Results. A timing analysis revealed the detection of a coherent signal at P-0 = 54.0 +/- 0.4 d, which we interpret as the orbital period of the binary system. When folded with a period of P-0, the light curve shows an asymmetric profile with a minimum roughly consistent with zero intensity. The broadband spectral analysis coupling Swift-XRT, XMM-Newton, and Swift-BAT spectra confirms that the source emission is well modeled with a strongly absorbed power law with no evidence of a high energy exponential cutoff.