Seminar

47 Tucanae X9: The first black hole candidate ultracompact X-ray binary in a globular cluster

Ultra-compact X-ray binaries are accreting binaries with very short orbital period (< 80 min). The small separation between the accretor and donor means that both stars are compact stellar remnants (e.g., black hole, neutron star, white dwarf). 47Tuc X9 is a low-luminosity (1e33-1e34 erg/s) X-ray binary in the globular cluster 47 Tuc, identified with a blue optical/UV counterpart, and showing strong emission lines from carbon (in the UV) and oxygen (in the X-ray) that suggest accretion from a C/O white dwarf and thus an ultra-compact nature. Recently we have measured strong radio emission from this object, suggesting that the accretor is a black hole. I will talk about our deep simultaneous study of this source in X-rays (with Chandra and NuSTAR) and in Radio (ATCA). We find evidence for clear signs of photo-ionized emission from oxygen VII and VIII in the system, providing additional evidence that the donor is a white dwarf. We also find a 6.8 day periodic modulation in the X-ray light curve, which indicates the presence of a warped accretion disk. Finally, we present evidence for the presence of reflected X-ray emission from the disk.

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