EXTraS discovery of two pulsators in the direction of the LMC: a Be/X-ray binary pulsar in the LMC and a candidate double-degenerate polar in the foreground

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: fwh@mpe.mpg.de
² INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090- GE Amsterdam, The Netherlands
⁵ Scuola Universitaria Superiore IUSS Pavia, piazza della Vittoria 15, 27100 Pavia, Italy
⁶ INFN – Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy
⁷ CNR, Istituto di Matematica Applicata e Tecnologie Informatiche, via de Marini 6, 16149 Genova, Italy
⁸ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 19 September 2016
Accepted: 10 October 2016

Abstract

Context. The Exploring the X-ray Transient and variable Sky (EXTraS) project searches for coherent signals in the X-ray archival data of XMM-Newton.

Aims. XMM-Newton performed more than 400 pointed observations in the region of the Large Magellanic Cloud (LMC). We inspected the results of the EXTraS period search to systematically look for new X-ray pulsators in our neighbour galaxy.

Methods. We analysed the XMM-Newton observations of two sources from the 3XMM catalogue which show significant signals for coherent pulsations.

Results. 3XMM J051259.8-682640 was detected as a source with a hard X-ray spectrum in two XMM-Newton observations, revealing a periodic modulation of the X-ray flux with 956 s. As optical counterpart we identify an early-type star with Hα emission. The OGLE I-band light curve exhibits a regular pattern with three brightness dips which mark a period of ~1350 d. The X-ray spectrum of 3XMM J051034.6-670353 is dominated by a super-soft blackbody-like emission component (kT ~ 70 eV) which is modulated by nearly 100% with a period of ~1418 s. From GROND observations we suggest a star with r′ = 20.9 mag as a possible counterpart of the X-ray source.

Conclusions. 3XMM J051259.8-682640 is confirmed as a new Be/X-ray binary pulsar in the LMC. We discuss the long-term optical period as the likely orbital period which would be the longest known from a high-mass X-ray binary. The spectral and temporal properties of the super-soft source 3XMM J051034.6-670353 are very similar to those of RX J0806.3+1527 and RX J1914.4+2456 suggesting that it belongs to the class of double-degenerate polars and is located in our Galaxy rather than in the LMC.