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

¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
e-mail: segreto@ifc.inaf.it
² Dipartimento di Fisica e Chimica, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁴ INAF – Brera Astronomical Observatory, via Bianchi 46, 23807 Merate ( LC), Italy

Received: 14 May 2013
Accepted: 11 August 2013

Abstract

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 P₀ = 54.0 ± 0.4 d, which we interpret as the orbital period of the binary system. When folded with a period of P₀, 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.