Letter to the Editor

A new symbiotic low mass X-ray binary system: 4U 1954+319

¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, via Bassini 15, 20133 Milano, Italy e-mail: mattana@iasf-milano.inaf.it
² Università di Milano Bicocca, Dipartimento di Fisica G. Occhialini, P.za della Scienza 3, 20126 Milano, Italy
³ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France
⁴ Università di Pavia, Dipartimento di Fisica Nucleare e Teorica and INFN-Pavia, via Bassi 6, 27100 Pavia, Italy
⁵ Université Paul Sabatier, 31062 Toulouse, France

Received: 31 July 2006
Accepted: 24 September 2006

Abstract

Aims.4U 1954+319 was discovered 25 years ago, but only recently has a clear picture of its nature begun to emerge. We present for the first time a broad-band spectrum of the source and a detailed timing study using more than one year of monitoring data.

Methods.The timing and spectral analysis was done using publicly available Swift, INTEGRAL, BeppoSAX, and RXTE/ASM data in the 0.7 to 150 keV energy band.

Results.The source spectrum is described well by a highly absorbed ( cm^-2) power law with a high-energy exponential cutoff around 15 keV. An additional black body component is needed below 3 keV to account for a soft excess. The derived ~5 h periodicity, with a spin-up timescale of ~25 years, could be identified as the neutron star spin period. The spectral and timing characteristics indicate that we are dealing both with the slowest established wind-accreting X-ray pulsar and with the second confirmed member of the emerging class dubbed “symbiotic low mass X-ray binaries” to host a neutron star.