The identification of the optical/IR counterpart of the 15.8-s transient X–ray pulsar XTE J1946+274

¹ Dipartimento di Fisica, Università degli Studi “La Sapienza”, piazza A. Moro 5, 00185 Roma, Italy
² ASI Science Data Center, ESA-Esrin, via Galileo Galilei, 00044 Frascati, Rome, Italy
³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ Observatoire Astronomique de Strasbourg, rue de l'Université 11, 67000 Strasbourg , France
⁵ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁶ Istituto di Astrofisica Spaziale, CNR, Area della Ricerca di Roma Tor Vergata, via Fosso del Cavaliere, 00133 Roma, Italy
⁷ Astronomy Centre, CPES, University of Sussex, Brighton BN1 9QH, UK
⁸ Dept. of Physics & Astronomy, University College London, London WC1E 6BT, UK
⁹ Astrophysics Research Institute, Liverpool John Moores University, Liverpool CH41 1LD, UK
¹⁰ INAF – Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127, Bologna, Italy

Corresponding author: F. Verrecchia, verrecchia@mporzio.astro.it

Received: 15 May 2002
Accepted: 24 July 2002

Abstract

We report on the discovery of the optical/IR counterpart of the 15.8 s transient X–ray pulsar XTE J1946+274. We re–analyzed archival Beppo SAX observations of XTE J1946+274  obtaining a new refined position (a circle with 22´´ radius at 90% confidence level). Based on this new position we carried out optical and infra–red (IR) follow–up observations. Within the new error circle we found a relatively optical faint () IR bright () early type reddened star (). The optical spectra show strong Hα and Hβ emission lines. The IR photometric observations of the field confirm the presence of an IR excess for the Hα–emitting star (, ) which is likely surrounded by a circumstellar envelope. Spectroscopic and photometric data indicate a B0–1V–IVe spectral–type star located at a distance of 8–10 kpc and confirm the Be–star/X–ray binary nature of XTE J1946+274.