Accretion geometry in the persistent Be/X-ray binary RXJ0440.9+4431

¹ ISDC, department of astronomy, Université de Genève, chemin d’Écogia, 16, CH-1290 Versoix, Switzerland
² Dipartimento di Fisica Università di Ferrara via Saragat 1, I-44100, Ferrara, Italia
³ CRESST & University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁵ IAAT, Abt. Astronomie, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
⁶ ESAC, ISOC, Villañueva de la Cañada, Madrid, Spain

^a e-mail: carlo.ferrigno@unige.ch

Published online: 8 January 2014

Abstract

The persistent Be/X-ray binary RXJ0440.9+4431 flared in 2010 and 2011 and has been followed by various X-ray facilities (Swift, RXTE, XMM-Newton, and INTEGRAL). We studied the source timing and spectral properties as a function of its X-ray luminosity to investigate the transition from normal to flaring activity. The source spectrum can always be described by a bulk-motion Comptonization model of black body seed photons attenuated by a moderate photoelectric absorption. At the highest luminosity, we measured a curvature of the spectrum, which we attribute to a significant contribution of the radiation pressure in the accretion process. This allows us to estimate that the transition from a bulk-motion-dominated flow to a radiatively dominated one happens at a luminosity of ~ 2 × 10³⁶ erg s⁻¹. The luminosity dependency of the size of the black body emission region is found to be r_BB ∝ L_X^0.39±0.02. This suggests that either matter accreting onto the neutron star hosted in RXJ0440.9+4431 penetrates through closed magnetic field lines at the border of the compact object magnetosphere or that the size of the black-body emitting hotspot is larger than the footprint of the accretion column. This phenomenon can be due to illumination of the surface by a growing column or by a a structure of the neutron star magnetic field more complicated than a simple dipole at least close to the surface.