# The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9

## TL;DR

This paper confirms 47 Tuc X9 as an ultracompact X-ray binary with a white dwarf donor, presents spectral evidence of photoionized gas, and suggests it may be the first ultracompact black hole X-ray binary in our galaxy.

## Contribution

The study provides the first orbital period measurement and spectral evidence confirming a white dwarf donor, and proposes X9 as a potential ultracompact black hole X-ray binary.

## Key findings

- Confirmed 28.18-minute orbital period.
- Detected high oxygen abundance indicating a C/O white dwarf donor.
- Observed X-ray reflection features in a faint LMXB.

## Abstract

47 Tuc X9 is a low mass X-ray binary (LMXB) in the globular cluster 47 Tucanae, and was previously thought to be a cataclysmic variable. However, Miller-Jones et al. (2015) recently identified a radio counterpart to X9 (inferring a radio/X-ray luminosity ratio consistent with black hole LMXBs), and suggested that the donor star might be a white dwarf. We report simultaneous observations of X9 performed by Chandra, Nustar and Australia Telescope Compact Array. We find a clear 28.18$\pm$0.02 min periodic modulation in the Chandra data, which we identify as the orbital period, confirming this system as an ultracompact X-ray binary. Our X-ray spectral fitting provides evidence for photoionized gas having a high oxygen abundance in this system, which indicates a C/O white dwarf donor. We also identify reflection features in the hard X-ray spectrum, making X9 the faintest LMXB to show X-ray reflection. We detect a $\sim$ 6.8 day modulation in the X-ray brightness by a factor of 10, in archival Chandra, Swift, and Rosat data. The simultaneous radio/X-ray flux ratio is consistent with either a black hole primary or a neutron star primary, if the neutron star is a transitional millisecond pulsar. Considering the measured orbital period (with other evidence of a white dwarf donor), and the lack of transitional millisecond pulsar features in the X-ray light curve, we suggest that this could be the first ultracompact black hole X-ray binary identified in our Galaxy.