# H{\alpha} imaging for BeXRBs in the Small Magellanic Cloud

## TL;DR

This study used H-alpha imaging to identify and analyze Be/X-ray binaries in the Small Magellanic Cloud, providing insights into their population and formation rate in a low-metallicity environment.

## Contribution

It presents a large-scale H-alpha survey of the SMC to identify Be/X-ray binary counterparts and quantify their fraction among OBe stars, offering new data on their formation rate.

## Key findings

- Identified ~10,000 H-alpha emission sources in the SMC.
- Found the OBe/OB star fraction to be 13%.
- Estimated BeXRBs/OBe ratio to be 0.002-0.025.

## Abstract

The Small Magellanic Cloud (SMC) hosts a large number of high-mass X-ray binaries, and in particular of Be/X-ray Binaries (BeXRBs; neutron stars orbiting OBe-type stars), offering a unique laboratory to address the effect of metalicity. One key property of their optical companion is H{\alpha} in emission, which makes them bright sources when observed through a narrow-band H{\alpha} filter. We performed a survey of the SMC Bar and Wing regions using wide-field cameras (WFI@MPG/ESO and MOSAIC@CTIO/Blanco) in order to identify the counterparts of the sources detected in our XMM-Newton survey of the same area. We obtained broad-band R and narrow-band H{\alpha} photometry, and identified ~10000 H{\alpha} emission sources down to a sensitivity limit of 18.7 mag (equivalent to ~B8 type Main Sequence stars). We find the fraction of OBe/OB stars to be 13% down to this limit, and by investigating this fraction as a function of the brightness of the stars we deduce that H{\alpha} excess peaks at the O9-B2 spectral range. Using the most up-to-date numbers of SMC BeXRBs we find their fraction over their parent population to be ~0.002-0.025 BeXRBs/OBe, a direct measurement of their formation rate.