Swift/XRT orbital monitoring of the candidate supergiant fast X-ray transient IGR J17354–3255

¹ Institut für Astronomie und Astrophysik, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
e-mail: ducci@astro.uni-tuebingen.de
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica – Palermo, via U. La Malfa 153, 90146 Palermo, Italy
³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica – Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ ISDC, Data Center for Astrophysics of the University of Geneva, Chemin d’Ecogia, 16 1290 Versoix, Switzerland
⁵ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ Universities Space Research Association, Columbia, MD, USA
⁷ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA

Received: 3 April 2013
Accepted: 7 June 2013

Abstract

We report on the Swift/X-ray Telescope (XRT) monitoring of the field of view around the candidate supergiant fast X-ray transient (SFXT) IGR J17354–3255, which is positionally associated with the AGILE/GRID gamma-ray transient AGL J1734–3310. Our observations, which cover 11 days for a total on-source exposure of ~24 ks, span 1.2 orbital periods (P_orb = 8.4474 d) and are the first sensitive monitoring of this source in the soft X-rays. These new data allow us to exploit the timing variability properties of the sources in the field to unambiguously identify the soft X-ray counterpart of IGR J17354–3255. The soft X-ray light curve shows a moderate orbital modulation and a dip. We investigated the nature of the dip by comparing the X-ray light curve with the prediction of the Bondi-Hoyle-Lyttleton accretion theory, assuming both spherical and nonspherical symmetry of the outflow from the donor star. We found that the dip cannot be explained with the X-ray orbital modulation. We propose that an eclipse or the onset of a gated mechanism is the most likely explanation for the observed light curve.