Evidence for a Neutron Star in the non-pulsating massive X-ray binary 4U2206+54

¹ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politénica Superior, Universidad de Alicante, Ap. 99, 03080 Alicante, Spain e-mail: jmt@disc.ua.es
² Institut für Astronomie und Astrophysik – Astronomie, Sand 1, 72076 Tübingen, Germany
³ Integral Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland
⁴ ESA/ESTEC Research and Science Support Division, Noordwijk, The Netherlands
⁵ US Naval Research Laboratory, Space Science Division, 4555 Overlook Av., Washington, USA

Received: 25 November 2003
Accepted: 30 April 2004

Abstract

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54. For the first time, high energy data (≥30 keV) are analyzed for this source. The data are well described by comptonization models (CompTT and BMC) in which seed photons with temperatures between 1.1 keV and 1.5 keV are comptonized by a hot plasma at 50 keV thereby producing a hard tail which extends up to, at least, 100 keV. We offer a new method of identification of neutron star systems using a temperature – luminosity relation. If a given X-ray source is characterized by a low bolometric luminosity and a relatively high color blackbody temperature (>1 keV) it has necessarily to be a neutron star rather than a black hole. From these arguments it is shown that the area of the soft photon source must be small ( km) and that the accretion disk, if present, must be truncated very far from the compact object. Here we report on the possible existence of a cyclotron line around 30 keV. The presence of a neutron star in the system is strongly favored by the available data.