From outburst to quiescence: the decay of the transient AXP XTE J1810-197

¹ Università degli Studi di Roma “Tor Vergata” via Orazio Raimondo 18, 00173 Roma, Italy
² INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy e-mail: bernardini@oa-roma.inaf.it
³ Dipartmento di Fisica “Enrico Fermi”, Universitá di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
⁴ University of Amsterdam, Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
⁵ Mullard Space Science Laboratory University College of London Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
⁶ Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy
⁷ JILA, Univ. of Colorado, Boulder, CO 80309–0440, USA
⁸ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette, France
⁹ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy
¹⁰ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via Edoardo Bassini 15, 20133 Milano, Italy

Received: 10 August 2008
Accepted: 10 January 2009

Abstract

Aims. XTE J1810-197 is the first transient anomalous X-ray pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude.

Methods. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four year baseline (September 2003–September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse-phase spectroscopic studies were also carried out for observations with a high enough signal-to-noise.

Results. We find that (i) a three-blackbody model reproduces the spectral properties of XTE J1810-197 over the entire outburst statistically better than the two blackbodies model previously used in the literature, (ii) the coldest blackbody is consistent with the thermal emission from the whole surface and has temperature and radius similar to those inferred from ROSAT observations before the outburst onset, (iii) there is a spectral feature around 1.1 keV during six consecutive observations (since March 2005). If this stems from proton resonant cyclotron scattering, it would imply a magnetic field of ~ G. This closely agrees with the value from the spin period measurements.