The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9 (1702.02167v1)

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.

Ultracompact X-ray Binary 47 Tuc X9: Nature and Implications

The paper conducted by Bahramian et al. investigates the black hole candidate X-ray binary 47 Tuc X9, a source located within the globular cluster 47 Tucanae. This research addresses the potential ultracompact nature of the system, its unusual X-ray spectral features, and the radio/X-ray properties that could suggest a black hole accretor.

Major Findings

The authors provide compelling evidence for 47 Tuc X9 being an ultracompact X-ray binary (UCXB), primarily characterized by a periodic modulation in the X-ray light curve indicative of a 28.18-minute orbital period. This short orbital period suggests that the donor star is likely a white dwarf with either carbon/oxygen (C/O) or helium-dominated composition, typical for UCXBs.

An in-depth X-ray spectral analysis reveals emission features below 1 keV, notably strong lines from O VII and O VIII, hinting at high oxygen abundance. The paper utilizes a photoionization model to best fit these features, indicating that a photoionized region with a high oxygen content surrounds the accretor and emits these lines. This abundance supports the hypothesis of a C/O white dwarf donor, reinforcing the UCXB classification.

The radio and X-ray luminosity relationship for 47 Tuc X9 suggests it aligns with the correlation known for black hole low-mass X-ray binaries (LMXBs), though the authors acknowledge this cannot definitively exclude a neutron star accretor. Observations show no clear evidence of coherent millisecond X-ray pulsations, providing no support for a transitional millisecond pulsar (tMSP) model, which leaves a black hole primary as a plausible scenario.

Implications and Future Directions

The research highlights the discovery of an ultracompact black hole X-ray binary candidate within a globular cluster, an environment traditionally thought to disfavor black holes remaining bound due to dynamical interactions. This finding challenges previous theoretical frameworks regarding black hole retention in dense stellar systems.

Observations supporting a high oxygen abundance and the application of a photoionization model mark a significant advancement in characterizing such systems, steering future studies toward understanding the accretion dynamics and chemical compositions of the disk and donor in UCXBs.

The detection of a 6.8-day super-orbital modulation in the X-ray brightness, although tentative, opens avenues for exploring mechanisms like accretion disk precession or superhump instabilities within these compact binaries. These phenomena can alter our comprehension of disk interactions and evolutionary models for UCXBs.

As next steps, further multi-wavelength observations, particularly coordinated radio, X-ray, and possibly infrared observations, are pivotal to confirm the nature of the compact object. The incorporation of future high-resolution spectroscopy could refine our understanding of the system's accretion structure and chemical profile. Moreover, advancing X-ray polarimetry techniques may offer additional insights into the geometry and magnetic environment around the compact object.

Ultimately, the work by Bahramian et al. sets a precedent for the continued discovery and analysis of similar X-ray binaries and underscores the importance of detailed spectral modeling in constraining the nature of distant X-ray sources within globular clusters, which can yield broader implications for stellar evolution models and cluster dynamics.