Pulsed thermal emission from the accreting pulsar XMMU J054134.7-682550

Astronomy & Astrophysics manuscript no. paper c⃝ESO 2018 November 19, 2018 Pulsed thermal emission from the accreting pulsar XMMU J054134.7-682550⋆ A. Manousakis1,2, R. Walter1,2, M. Audard1,2, and T. Lanz3 1 ISDC Data Center for Astrophysics, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland e-mail: Antonios.Manousakis@unige.ch 2 Observatoire de Gen`eve, Universit´e de Gen`eve, Chemin des Maillettes 51, CH-1290 Versoix, Switzerland 3 Department of Astronomy, University of Maryland College Park, MD 20742-2421, USA Received October 3, 2008; accepted February 2, 2009 ABSTRACT Aims. Soft X-ray excesses have been detected in several Be/X-ray binaries and interpreted as the signature of hard X-ray reprocessing in the inner accretion disk. The system XMMU J054134.7-682550, located in the LMC, featured a giant Type II outburst in August 2007. The geometry of this system can be understood by studying the response of the soft excess emission to the hard X-ray pulses. Methods. We have analyzed series of simultaneous observations obtained with XMM-Newton/EPIC-MOS and RXTE/PCA in order to derive spectral and temporal characteristics of the system, before, during and after the giant outburst. Spectral fits were performed and a timing analysis has been carried out. Spectral variability, spin period evolution and energy dependent pulse shapes are analysed. Results. The outburst (LX = 3 × 1038erg/s ≈LEDD) spectrum could be modeled successfully using a cutoffpowerlaw, a cold disk emission, a hot blackbody, and a cyclotron absorption line. The magnetic field and magnetospheric radius could be constrained. The thickness of the inner accretion disk is broadened to a width of 75 km. The hot blackbody component features sinusoidal modulations indicating that the bulk of the hard X-ray emission is emitted preferentially along the magnetic equator. The spin period of the pulsar decreased very significantly during the outburst. This is consistent with a variety of neutron star equations of state and indicates a very high accretion rate. Key words. X-Rays: Binaries – Stars: emission line, Be – Accretion, accretion disks – Magelanic Clouds 1. Introduction Be/X-ray binaries consist of a neutron star orbiting a Be star, which is defined as a non-supergiant B-type star whose spec- trum shows (or showed, at some time) one or more Balmer lines in emission. Be/X-Ray binaries display X-ray pulsations, a sig- nature of the strong magnetic field (B ∼1012 G) of the neutron star orbiting a massive star companion. Most known Be/X-Ray binaries undergo outbursts in which their X-ray luminosity sud- denly increases by a factor of ∼10 −104 with respect to the quiescence level. They can feature two types of outbursts: Type I (or normal) X-ray outbursts of moderate intensity (LX ∼1036 erg s−1) occur- ing during the periastron passage of the neutron star and Type II (or giant) X-ray outbursts of higher intensity (LX ∼1037−38 erg s−1) lasting for several weeks or even months. Generally, Type II outbursts start shortly after periastron passage, but do not show any other correlation with orbital parameters (Finger & Prince 1997). A small fraction of Be/X-ray binares are persis- tent sources (the prototype being X-Per), with a low luminosity LX ∼1034 erg s−1 at an almost constant emitting level (Reig & Roche 1999). During giant outbursts, the spin period of the neutron star has been observed to decrease (neutron star spin-up), indicating that angular momentum is transfered from the accreted material Send offprint requests to: A. Manousakis ⋆Based on observations obtained with XMM-Newton, an ESA sci- ence mission with instruments and contributions directly funded by ESA Member States and NASA to the neutron star, through an accretion disk (Finger et al. 1999; Wilson et al. 2003). Corbet (1986) has shown that Be/X-Ray binaries fall into a narrow area in the Pspin −Porb diagram. This correlation has been interpreted as a result of the rotation of the neutron star at the equilibrium velocity between the spin-up, and the spin-down led by centrifugal effects of the strong magnetic field (Waters & van Kerkwijk 1989). The X-ray spectra of Be/X-Ray binaries are very close to those of accreting pulsars, although these depend on the physi- cal conditions close to the neutron star. The spectra can be char- acterized by cutoffpowerlaws. In a few systems with low in- terstellar absorption, there is evidence for soft excesses at low energies, often modeled as blackbody components (White et al. 1983; Hickox et al. 2004; Paul et al. 2002; Endo et al. 2000). Hickox et al. (2004) showed that the soft excesses observed in luminous X-ray sources (LX > 1038 erg s−1) can only be ex- plained by reprocessing of hard X-rays by optically thick mate- rial, near the inner edge of the accretion disk. Many, if not all, bright sources with low absorption have shown this feature. XMMU J054134.7-682550, located in the Large Magellanic Cloud - LMC, has been proposed as a likely HMXB by Shtykovskiy & G

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