RX J0440.9+4431: a persistent Be/X-ray binary in outburst

TL;DR

This study investigates the timing and spectral properties of the persistent Be/X-ray binary RX J0440.9+4431 during outbursts, revealing insights into accretion processes and magnetic field structure through multi-instrument observations.

Contribution

The paper provides the first detailed analysis of RX J0440.9+4431's spectral and timing behavior during outbursts, highlighting the transition from bulk-motion to radiatively dominated accretion.

Findings

Orbital period determined from long-term light curve.

Spectral curvature indicates radiation pressure effects.

Black body emission region scales with luminosity as L_X^{0.39}.

Abstract

The persistent Be/X-ray binary RX J0440.9+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 and the dynamical properties of the system. We have determined the orbital period from the long-term Swift/BAT light curve, but our determinations of the spin period are not precise enough to constrain any orbital solution. 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 ~2e36 erg/s. The luminosity dependency of the size of the black body emission region is found to be $r_{BB} \propto L_X^{0.39\pm0.02}$. This suggests that either matter accreting onto the neutron star hosted in RX J0440.9+4431 penetrates through closed magnetic field lines at the border of the compact object magnetosphere or that the structure of the neutron star magnetic field is more complicated than a simple dipole close to the surface