[Paper Review] Doubling the number of Be/X-ray binaries in the SMC

F. Haberl, M. Sasaki|arXiv (Cornell University)|May 10, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper proposes 25 new Be/X-ray binary candidates in the Small Magellanic Cloud (SMC) by cross-correlating ROSAT and Einstein X-ray source catalogues with Hα emission-line star catalogues, significantly doubling the known population of such systems. The method leverages the strong correlation between Be stars (identified via Hα emission) and X-ray sources, revealing a large number of low-luminosity Be/X-ray binaries and suggesting many such systems may remain undetected in the Milky Way.

ABSTRACT

A correlation of X-ray source and Halpha emission-line object catalogues in the Small Magellanic Cloud (SMC) shows that more than two thirds of the optically identified Be stars in Be/X-ray binaries are found as emission-line objects in the catalogues. On the basis of this result we propose up to 25 X-ray sources mainly from recent ROSAT catalogues as new Be/X-ray binaries and give their likely optical counterparts. Also for the five yet unidentified X-ray pulsars in the SMC we propose emission-line stars as counterparts. This more than doubles the number of known high mass X-ray binary systems in this nearby galaxy. The spatial distribution of the new candidates is similar to that of the already identified Be/X-ray binaries with a strong concentration along the SMC main body and some systems in the eastern wing. The new candidates contribute mainly to the low-luminosity end of the X-ray luminosity distribution of Be/X-ray binaries. A comparison with the luminosity distribution in the Milky Way reveals no significant differences at the high-luminosity end and the large number of low-luminosity systems in the SMC suggests that many such systems may still be undetected in the Galaxy. The overall ratio of known Be to OB supergiant X-ray binaries in the SMC is an order of magnitude larger than in the Galaxy, however, might show spatial variations. While in the eastern wing the ratio is comparable to that in the Galaxy no supergiant X-ray binary is currently known in the main body of the SMC. Possible explanations include a different star formation history over the last ~15 My.

Motivation & Objective

• To identify new Be/X-ray binary candidates in the Small Magellanic Cloud (SMC) using multi-wavelength cross-correlation.
• To address the underrepresentation of low-luminosity Be/X-ray binaries in current surveys and assess their detectability in the Galaxy.
• To test the hypothesis that many Be/X-ray binaries remain undetected due to low X-ray luminosity.
• To investigate spatial variations in the Be/X-ray binary to supergiant X-ray binary ratio across different regions of the SMC.
• To improve the identification of optical counterparts for five unassociated X-ray pulsars in the SMC.

Proposed method

• Cross-correlated 517 ROSAT PSPC X-ray sources in the SMC with the MA93 and MB99 Hα emission-line star catalogues, using a positional threshold of √(r90² + 10²) to account for positional uncertainties.
• Used additional X-ray catalogues (ROSAT HRI, Einstein IPC, ASCA, BeppoSAX, RXTE) to extend the search for X-ray sources associated with emission-line stars.
• Selected candidates based on proximity, X-ray luminosity, and optical magnitude consistency with Be stars, excluding known supernova remnants and supersoft sources.
• Evaluated candidate systems using X-ray luminosity, optical magnitudes (B, V, R), and spectral characteristics to distinguish Be/X-ray binaries from other sources.
• Classified candidates as Be/X? or Be/X with pulsations based on X-ray light curves and periodicity, where available.
• Compared the luminosity distribution of new candidates with known Be/X-ray binaries and Milky Way systems to assess detectability and population differences.

Experimental results

Research questions

• RQ1 Can the correlation between Hα emission-line stars and X-ray sources be used to reliably identify new Be/X-ray binary candidates in the SMC?
• RQ2 What is the contribution of the new candidates to the X-ray luminosity function of Be/X-ray binaries, particularly at low luminosities?
• RQ3 Why is the ratio of Be to supergiant X-ray binaries in the SMC an order of magnitude higher than in the Milky Way, and does it vary spatially?
• RQ4 Are the five unassociated X-ray pulsars in the SMC likely to be associated with Be stars, based on optical counterpart identification?
• RQ5 To what extent do low-luminosity Be/X-ray binaries in the SMC suggest that many such systems remain undetected in the Milky Way?

Key findings

• The authors propose 25 new Be/X-ray binary candidates in the SMC, primarily from ROSAT PSPC and HRI catalogues, based on cross-correlation with Hα emission-line stars.
• The new candidates significantly increase the known population of Be/X-ray binaries in the SMC, effectively doubling the number of such systems.
• The spatial distribution of the new candidates closely matches that of known Be/X-ray binaries, with a strong concentration in the SMC main body and some in the eastern wing.
• The new candidates predominantly contribute to the low-luminosity end of the X-ray luminosity function, with luminosities ranging from ~3.0×10³⁵ erg s⁻¹ to ~5.0×10³⁷ erg s⁻¹.
• The luminosity distribution in the SMC shows no significant differences from the Milky Way at high luminosities, but the large number of low-luminosity systems suggests many such systems may remain undetected in our Galaxy.
• The ratio of Be to supergiant X-ray binaries in the SMC is an order of magnitude higher than in the Milky Way, though this ratio is comparable in the eastern wing, with no known supergiant X-ray binary currently in the SMC main body.