Towards a larger sample of radio jets from quiescent black hole X-ray binaries

Abstract

Quiescent black hole X-ray binaries (X-ray luminosities ≲ 1034 erg s−1) are believed to be fed by hot accretion flows that launch compact, relativistic jets. However, due to their low luminosities, quiescent jets have been detected in the radio waveband from only five systems so far. Here, we present radio observations of two quiescent black hole X-ray binaries with the Australia Telescope Compact Array. One system, GS 1124-684, was not detected. The other system, BW Cir, was detected over two different epochs in 2018 and 2020, for which we also obtained quasi-simultaneous X-ray detections with Chandra and Swift. BW Cir is now the sixth quiescent X-ray binary with a confirmed radio jet. However, the distance to BW Cir is uncertain, and we find that BW Cir shows different behaviour in the radio/X-ray luminosity plane depending on the correct distance. Estimates based on its G-type subgiant donor star place BW Cir at >25 kpc, while initial optical astrometric measurements from Gaia Data Release 2 suggested likely distances of just a few kpc. Here, we use the most recent measurements from Gaia Early Data Release 3 and find a distance $d=7.1^{+4.8}_{-3.9}$ kpc and a potential kick velocity PKV=$165^{+81}_{-17}$ km s−1, with distances up to ≈20 kpc possible based on its parallax and proper motion. Even though there is now less tension between the parallax and donor-star based distance measurements, it remains an unresolved matter, and we conclude with suggestions on how to reconcile the two measurements.

The black hole X-ray binary MAXI J1348–630 in quiescence

“…Sources in quiescence are marked with different colors. The plot shows the location on the diagram in quiescence for A0620-00 (Gallo et al 2006), MWC 656 (Ribó et al 2017), XTE J1118+480 (Gallo et al 2014), GX 339-4 (Tremou et al 2020, BW Cir (Plotkin et al 2021) and V404 Cyg (Plotkin et al 2017), as well as upper limits for GS 2000+25 (Rodriguez et al 2020), GS 1124-684 (Plotkin et al 2021) and Swift J1357.2-0933 (Plotkin et al 2016;Charles et al 2019).…”

“…However, BH LMXBs are for most of their time in a quiescent state, with low accretion rates and X-ray luminosities in the range between 10 30 and 10 34 erg s −1 , corresponding to Eddington fractions of 10 −9 ∼ 10 −5 for a 10 M BH (e.g. Plotkin et al 2013;Reynolds et al 2014;Plotkin et al 2021). While quiescence is the most common state for a BH LMXB, the properties of the accretion flow and of the jets at low luminosities still need to be properly characterized.…”

Astrometry of variable compact radio sources: a search for Galactic black hole X-ray binaries

“…We are also biased against observing the most massive black holes in BHXBs as evidenced by the number of known and dynamically confirmed BHXBs found close to the Galactic Plane and the Galactic Centre (Jonker et al 2021). The stark difference between the number of BHXBs expected in our Galaxy (10 4 ; Jonker et al 2011), the number of known BHXB candidates (77; Tetarenko et al 2016a) and the number of BHXBs detected during quiescence in the radio wavelengths (7; e.g., Plotkin et al 2021;Carotenuto et al 2022) indicates that a large population of BHXBs are likely hidden from conventional discovery techniques. Since none of the sources from the B10 sample have been detected as outbursting X-ray or optical sources, this suggests that some of these could be quiescent BHXBs.…”

No X-Rays or Radio from the Nearest Black Holes and Implications for Future Searches

“…By placing BHs on the Fundamental Plane, the physical process behind BH X-ray and radio emission can be understood. We reproduce the most current compilation of hard state Galactic BHs with measured X-ray and radio luminosities (Bahramian et al 2018;Plotkin et al 2021) with upper limits of Gaia BH1 and BH2 overplotted in Figure 3. There is a minor correction to convert to the same X-ray energy ranges and radio frequency ranges, which we omit since it is of order unity.…”