X-ray eclipse time delays in 4U 2129+47

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00044 Rome, Italy e-mail: bozzo@oa-roma.inaf.it
² Dipartimento di Fisica – Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
³ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France
⁴ JILA, University of Colorado, Boulder, CO 80309-0440, USA
⁵ INAF Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy
⁶ INAF Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
⁷ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁸ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy

Received: 8 August 2007
Accepted: 10 September 2007

Abstract

Aims.4U 2129+47 was discovered in the early 80's and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 h binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star. The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached.

Methods.Here, we present timing and spectral analyses of two XMM-Newton  observations of this source, carried out in May and June, 2005.

Results.We find evidence for a delay between two mid-eclipse epochs measured ~22 days apart, and we show that this delay can be naturally explained as being due to the orbital motion of the binary 4U 2129+47 around the center of mass of a triple system. This result thus provides further support in favor of the triple nature of 4U 2129+47.