High-mass X-ray binaries in the Small Magellanic Cloud

Abstract

Aims. The last comprehensive catalogue of high-mass X-ray binaries in the Small Magellanic Cloud (SMC) was published about ten years ago. Since then new such systems were discovered, mainly by X-ray observations with Chandra and XMM-Newton. For the majority of the proposed HMXBs in the SMC no X-ray pulsations were discovered as yet, and unless other properties of the X-ray source and/or the optical counterpart confirm their HMXB nature, they remain only candidate HMXBs. Methods. From a literature search we collected a catalogue of 148 confirmed and candidate HMXBs in the SMC and investigated their properties to shed light on their real nature. Based on the sample of well-established HMXBs (the pulsars), we investigated which observed properties are most appropriate for a reliable classification. We defined different levels of confidence for a genuine HMXB based on spectral and temporal characteristics of the X-ray sources and colour-magnitude diagrams from the optical to the infrared of their likely counterparts. We also took the uncertainty in the X-ray position into account. Results. We identify 27 objects that probably are misidentified because they lack an infrared excess of the proposed counterpart. They were mainly X-ray sources with a large positional uncertainty. This is supported by additional information obtained from more recent observations. Our catalogue comprises 121 relatively high-confidence HMXBs (the vast majority with Be companion stars). About half of the objects show X-ray pulsations, while for the rest no pulsations are known as yet. A comparison of the two subsamples suggests that long pulse periods in excess of a few 100 s are expected for the "non-pulsars", which are most likely undetected because of aperiodic variability on similar timescales and insufficiently long X-ray observations. The highest X-ray variability together with the lowest observed minimum fluxes for short-period pulsars indicate that in addition to the eccentricity of the orbit, its inclination against the plane of the Be star circum-stellar disc plays a major role in determining the outburst behaviour. Conclusions. The large population of HMXBs in the SMC, in particular Be X-ray binaries, provides the largest homogeneous sample of such systems for statistical population studies.

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

“…In our classifications listed in Table 7, many of our sources have luminosities and hardness ratios consistent with multiple compact-object types, including accreting black hole primaries. Sources 1728Sources , 1701Sources , 1666Sources , 2012Sources , and 2035 are found in regions of the diagnostic diagrams (Figures 14, 12, 10, 17, 18) consistent with multiple compact-object types -yet all of these XRBs are associated with known X-ray pulsars (Haberl & Sturm 2016). Hereafter, we refer to these sources as inconsistent pulsars, as relates to the NuSTAR hardnessintensity diagram.…”

“…Source 1705 falls along the boundary between the hard state black hole and pulsar loci on our diagnostic diagrams ( Figure 13) during each epoch of observation. Source 1705 is 0.5 away from CXO J005215.4-731915, listed as source 93 in Haberl & Sturm (2016), and 7.5 away from the known pulsar SXP15.3. Based on the results of our timing analysis (see §4.6) we suggest that during the first observation of source 1705 in March 2017 we detected flux predominantly from CXO J005215.4-731915 and during the next two observations in July and August 2017 we detected flux mostly from SXP15.3.…”

The radio pulsar population of the Small Magellanic Cloud

“…The majority of the isolated pulsars have a spin period distribution that peaks around 1 s, and extends to 10 s. The pulsars in binaries account for only a few percent of all the simulated pulsars, and have a spin period distribution peaking at ∼0.3 s. Unlike the isolated pulsars, very few pulsars in binaries have spin periods beyond ∼1 s. Initially, rapidly spinning pulsars are prevented from accreting material by the propeller mechanism, however once they have spun down sufficiently the NS can accrete material and enter the HMXB phase. The SMC in particular harbours a number of accreting NSs with spin periods below 10 s (see Haberl & Sturm 2016, and references therein). Moreover, the simulations predict a population of ∼60 -160 NSs in binaries with massive companions.…”

Discovery of a very young high-mass X-ray binary associated with the supernova remnant MCSNR J0513-6724 in the LMC

“…The nearest ALLWISE counterpart is much brighter (see below) thereby making this possibility highly unlikely. The Magellanic Clouds, especially the Small Magellanic Cloud, host a large population of HMXBs owing to the relatively recent star formation history (Antoniou et al 2010;Haberl & Sturm 2016;van Jaarsveld et al 2018). To investigate further a possible HMXB nature of the unresolved source, we searched the Magellanic Clouds Photometric Survey (MCPS) catalogue (Zaritsky et al 2004) for a counterpart using the same correlation criterion as equation 1.…”