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

摘要

47 Tuc X9 is a low mass X-ray binary (LMXB) in the globular cluster 47udTucanae, and was previously thought to be a cataclysmic variable. However,udMiller-Jones et al. (2015) recently identified a radio counterpart to X9ud(inferring a radio/X-ray luminosity ratio consistent with black hole LMXBs),udand suggested that the donor star might be a white dwarf. We reportudsimultaneous observations of X9 performed by Chandra, Nustar and AustraliaudTelescope Compact Array. We find a clear 28.18$pm$0.02 min periodic modulationudin the Chandra data, which we identify as the orbital period, confirming thisudsystem as an ultracompact X-ray binary. Our X-ray spectral fitting providesudevidence for photoionized gas having a high oxygen abundance in this system,udwhich indicates a C/O white dwarf donor. We also identify reflection featuresudin the hard X-ray spectrum, making X9 the faintest LMXB to show X-rayudreflection. We detect a $sim$ 6.8 day modulation in the X-ray brightness by audfactor of 10, in archival Chandra, Swift, and Rosat data. The simultaneousudradio/X-ray flux ratio is consistent with either a black hole primary or audneutron star primary, if the neutron star is a transitional millisecond pulsar.udConsidering the measured orbital period (with other evidence of a white dwarfuddonor), and the lack of transitional millisecond pulsar features in the X-rayudlight curve, we suggest that this could be the first ultracompact black holeudX-ray binary identified in our Galaxy.