An HI spectroscopic distance constraint on Swift J151857.0-572147
ATel #16538; B. J. Burridge (ICRAR - Curtin), J. C.A. Miller-Jones (ICRAR - Curtin), A. Bahramian (ICRAR - Curtin), S. Prabu (ICRAR - Curtin), F. Carotenuto (Oxford), T. D. Russell (INAF), F. Cowie (Oxford), R. P. Fender (Oxford/Cape Town), on behalf of the X-KAT collaboration
on 18 Mar 2024; 14:25 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)
Subjects: Radio, Binary, Black Hole, Transient
The recently discovered X-ray transient Swift J151857.0-572147 (GCN #35849, #35835, ATel #16500, #16506, #16516) is speculated to be a black hole X-ray binary candidate (ATel #16519). Its radio counterpart was first detected with MeerKAT (ATel #16503) on 2024-03-04 (MJD 60373.1) at a 1.3-GHz flux density of 10.2 mJy.
The source has subsequently undergone a bright radio flare during which 5.5 GHz and 9 GHz radio observations using the Australia Telescope Compact Array (ATCA) measured preliminary flux densities greater than 1.55 Jy (ATel #16518) on 2024-03-09 (MJD 60378.45).
Following the detection of this bright radio flare we triggered a high spectral resolution observation with MeerKAT as part of the X-KAT program on 2024-03-10 between 03:09:30.7 and 03:24:27.5 UTC (MJD 60379.14). We measured a flux density of 149.4 +/- 1.8 mJy at 1.42 GHz, evidencing a rapid fading since the flare observed by ATCA.
We observed HI absorption towards the source, out to a maximum negative velocity of -73.7 +/- 0.7 km/s. According to Wenger et al. (2018), that requires the source to be at or beyond the near kinematic distance of 4.48 (+0.67/-0.47) kpc. Positive velocity absorption lines towards other sources in the field are absent in the HI absorption for Swift J151857.0-572147, which implies an upper limit of 15.64 (+0.77/-0.60) kpc.
From this preliminary analysis, the velocity is consistent (within 2σ) with the expected velocity of -85.4 (+7.2/-8.1) km/s for the tangent point along the line of sight, at a distance of 6.54 (+0.15/-0.11) kpc. We cannot therefore rule out the absorption arising from the tangent point, and hence we estimate the distance of Swift J151857.0-572147 to lie in the range 4.48-15.64 kpc.
X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme, contact Rob Fender.
