Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate

Abstract

We report the detection of steady radio emission from the known X-ray source X9 in the globular cluster 47 Tuc. With a double-peaked C IV emission line in its ultraviolet spectrum providing a clear signature of accretion, this source had been previously classified as a cataclysmic variable. In deep ATCA imaging from 2010 and 2013, we identified a steady radio source at both 5.5 and 9.0 GHz, with a radio spectral index (defined as S ν ∝ ν α ) of α = −0.4 ± 0.4. Our measured flux density of 42 ± 4 µJy beam −1 at 5.5 GHz implies a radio luminosity (νL ν ) of 5.8 × 10 27 erg s −1 , significantly higher than any previous radio detection of an accreting white dwarf. Transitional millisecond pulsars, which have the highest radio-to-X-ray flux ratios among accreting neutron stars (still a factor of a few below accreting black holes at the same L X ), show distinctly different patterns of X-ray and radio variability than X9. When combined with archival X-ray measurements, our radio detection places 47 Tuc X9 very close to the radio/X-ray correlation for accreting black holes, and we explore the possibility that this source is instead a quiescent stellar-mass black hole X-ray binary. The nature of the donor star is uncertain; although the luminosity of the optical counterpart is consistent with a low-mass main sequence donor star, the mass transfer rate required to produce the high quiescent X-ray luminosity of 10 33 erg s −1 suggests the system may instead be ultracompact, with an orbital period of order 25 minutes. This is the fourth quiescent black hole candidate discovered to date in a Galactic globular cluster, and the only one with a confirmed accretion signature from its optical/ultraviolet spectrum.

The MAVERIC survey: a hidden pulsar and a black hole candidate in ATCA radio imaging of the globular cluster NGC 6397

“…Thanks to the increased sensitivity of new Karl G. Jansky Very Large Array (VLA) and ATCA receivers, faint radio sources have been detected in a number of GCs (e.g. Strader et al 2012;Chomiuk et al 2013;Miller-Jones et al 2015). There are several possibilities for these sources.…”

“…The dashed grey line shows the L R -L X correlation for black holes from Gallo et al (2014). The larger purple circles are radio-selected black hole candidates (Strader et al 2012;Chomiuk et al 2013;Miller-Jones et al 2015;Tetarenko et al 2016;Bahramian et al 2017;Shishkovsky et al 2018). The light-green triangles mark known transitional millisecond pulsars (Hill et al 2011;Papitto et al 2013;Deller et al 2015;Bogdanov et al 2018).…”

The First Low-Mass Black Hole X-Ray Binary Identified in Quiescence Outside of a Globular Cluster

“…In addition, as some early works on advectiondominated accretion flows (Narayan & Yi 1994) predict that the emission from the hot inner flow can start to fall out of the X-ray band at very low Eddington fraction, it is possible that most of the emission in VLA J2130+12 is at far-UV rather than X-ray frequencies, implying that the observed high radio/ X-ray flux ratio could be explained in this case without invoking an enormous beaming factor. The large radio luminosity may also indicate a higher than average stellar- Gallo et al 2012Gallo et al , 2014Ratti et al 2012;Corbel et al 2013;Munar-Adrover et al 2014;Dzib et al 2015), and the yellow circles represent the BHCs in M22, M62, and 47 Tuc (Strader et al 2012b;Chomiuk et al 2013;Miller-Jones et al 2015). The blue circles represent NS systems in the hard state (Rutledge et al 1998;Moore et al 2000;Migliari et al 2003Migliari et al , 2010Migliari et al , 2011Migliari & Fender 2006;Tudose et al 2009;Miller-Jones et al 2010;Tetarenko et al 2016a).…”

“…The remaining four are found in globular clusters (GCs; Strader et al 2012b;Chomiuk et al 2013;Miller-Jones et al 2015). The vast majority of these Galactic BHXBs are low-mass X-ray binaries (LMXBs) where mass transfer occurs via Roche lobe overflow of a secondary companion with a mass of M 2  3 M e and spectral type A or later.…”

Slow decline and rise of the broad [O iii] emission line in globular cluster black hole candidate RZ2109

“…Kremer et al 2019), so it is not surprising studies are starting to find them, even if their signatures are challenging to observe. Currently there are a handful of accreting black hole candidates in Milky Way globular clusters (Strader et al 2012;Chomiuk et al 2013;Miller-Jones et al 2015;Shishkovsky et al 2018), and there is one dynamically-confirmed black hole candidate in which there is no evidence for accretion (Giesers et al 2018). There are a handful of other black hole candidates in extragalactic globular clusters, including globular clusters in NGC 1399, NGC 4472 and NGC 4649 (Shih et al 2010;Irwin et al 2010;Maccarone et al 2011;Roberts et al 2012;Dage et al 2019).…”