ATCA radio detection of the new X-ray transient Swift J1729.5-3223

ATel #14815; Thomas Russell, Melania Del Santo, Antonino DAi, Elena Ambrosi, Alberto Segreto (INAF/IASF Palermo), Alessio Marino (Palermo University), Arash Bahramian (ICRAR/Curtin)
on 29 Jul 2021; 13:57 UT
Credential Certification: Thomas Russell (t.d.russell@uva.nl)

Tweet

Following the recent discovery of the new X-ray transient Swift J1729.5-3223 (Atel #14791), we observed the field with the Australia Telescope Compact Array (ATCA).

Observations were carried out on 2021-07-21 between 10:39:50 UT and 13:48:50 UT (MJD 59416.51 +/- 0.07). ATCA was in its compact H168 configuration. Data were recorded simultaneously at two basebands, with central frequencies of 5.5 GHz and 9 GHz, and 2 GHz of bandwidth at each frequency band. PKS 1934-638 was used for bandpass and flux calibration, while the nearby (2.84 degrees away) source TXS 1714-336 was used for complex gain calibration. Data were edited and calibrated following standard procedures within the Common Astronomy Software Package (CASA, version 5.1.0; McMullin et al. 2007). Imaging was carried out with a Briggs robust parameter of 0. Due to the compact array configuration of ATCA, the isolated antenna 6 was used. These choices provided a synthesized beam of 7”x0.9” with a position angle of 4.7 deg at 5.5 GHz and 4.3”x0.6” at 4.6 deg at 9 GHz.

We detected a radio counterpart entirely consistent with the enhanced Swift-XRT position (of R.A. (J2000) = 262.39377 deg, and Dec (J2000) = -32.37359 deg, with a 90% error radius of 2.5”). Fitting for a point source in the image plane, our best radio position (measured at 9 GHz) of:
R.A. (J2000) = 17:29:34.50 +/- 0.07”
Dec. (J2000) = -32:22:25.59 +/- 0.44”,
where uncertainties are systematic and statistical errors added in quadrature. A similar position was found at 5.5 GHz.

We measure flux densities of 180 +/- 22 μJy at 5.5 GHz and 100 +/- 20 μJy at 9 GHz, providing a radio spectral index of α = 1.3 +/- 0.7 (where S_ν ∝ ν^α), consistent with a optically-thin synchrotron emission from a transient jet. Our radio detection translates to a 5 GHz radio luminosity of ∼7.8E29 (D/8kpc)² erg s^-1 (L_r = 4 π d² ν S_ν).

Swift-XRT observed the source 2 days later (on 2021-07-23). These X-ray data were well fit with a simple absorbed powerlaw (tbabs x powerlaw) with NH = (3.1 +/- 0.7)x10²² cm^-2 and an X-ray photon index of 1.6 +/- 0.3, providing an unabsorbed 1-10 keV flux of (6.3 +/- 0.5)x10^-11 erg cm^-2 s^-1, marginally fainter than the previous XRT observations reported in ATel #14791. The X-ray flux corresponds to an X-ray luminosity of ∼4.8E35 (D/8kpc)² erg s^-1 (1-10 keV).

The X-ray spectrum and luminosity are consistent with a typical hard X-ray state at low-Eddington luminosity. This state is associated with a compact, steady jet. However, our measured radio spectral index is most consistent with optically-thin spectra expected for a transient jet (α ∼ -0.7), typically observed at much higher X-ray luminosity during the hard to soft state transition. Although the general radio/X-ray correlation does not apply for transient jet emission, the radio and X-ray luminosities would place Swift J1729.5-3223 at a position on the radio/X-ray plane consistent with it being either a black hole or neutron star X-ray binary.

Further observations are planned. Multiwavelength support is encouraged.

We thank Jamie Stevens and ATCA staff for making this observation possible. We acknowledge the Gomeroi people as the traditional owners of the ATCA observatory site. We also thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible.