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

Abstract

Context. The EXTraS project to explore the X-ray Transient and variable Sky 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 source with 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 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 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.

Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6–670353

“…The periods in all three optical bands are consistent to within < 3σ with the Xray period (23.63±0.02 min, Haberl et al 2017). We folded the light curves on a period of 23.6 min and show these in Figure 3.…”

“…light curves of XMMJ0510-67. The period derived from the g ′ r ′ data is entirely consistent with the X-ray period (23.63±0.03 min, Haberl et al (2017). The u ′ band data is of lower quality and the uncertainty on the period is less reliable than the g ′ r ′ estimates.…”

“…Haberl et al (2017) also show that XMMJ0510-67 has a colour g − r=-0.03±0.02. Comparing stars in the field with the Skymapper uv band magnitudes (Scalzo et al 2017) we estimate that XMMJ0510-67 gives u − g ∼0.1-0.2, where the uncertainty stems from the inexact relationship between the UL-TRACAM u ′ filter and the Skymapper passbands.…”

“…• ), it is quite crowded and a slightly brighter star (g ′ =21.0) is only 1.8 ′′ from XMMJ0510-67 (Haberl et al 2017). Haberl et al (2017) also show that XMMJ0510-67 has a colour g − r=-0.03±0.02.…”

Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

“…An intriguing sub-class of XRBs are the ultra luminous X-ray sources (ULXs), bright systems with isotropic luminosities that exceed the Eddington limit for a stellar mass (∼10−20 M ) BH (Kaaret et al 2017). It was initially suggested that these systems hosted intermediate-mass BHs accreting at sub-Eddington rates, but has now been established that at least a few of them host pulsating NSs (Bachetti et al 2014;Carpano et al 2018;Fürst et al 2016;Israel et al 2017). In addition, it has been shown that the spectral properties of pulsating and non pulsating ULXs share similarities and are consistent with theoretical predictions of super-Eddington accretion onto a NS (Koliopanos et al 2017;Walton et al 2018).…”