ATCA radio detection of the transient millisecond X-ray pulsar SRGA J144459.2-604207 at the updated X-ray position from Chandra

ATel #16511; T. D. Russell (INAF-IASF Palermo), F. Carotenuto (Oxford), J. van den Eijnden (U. Warwick), E. Kuulkers (ESTEC/ESA, The Netherlands), C. Sanchez-Fernandez (ESAC/ESA, Spain), N. Degenaar, S. Fijma (UvA), T. Maccarone (Texas Tech), J. C. A. Miller-Jones (ICRAR-Curtin), A. J. Tetarenko (U. Lethbridge), A. Marino (ICE-CSIC), M. Del Santo (INAF-IASF Palermo), N. Gusinskaia (U. Toronto)
on 7 Mar 2024; 21:41 UT
Credential Certification: Thomas Russell (thomas.russell@inaf.it)

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The new X-ray transient SRGA J144459.2-604207 (Atels #16464, #16469, #16471, #16474, #16510) was observed using the Australia Telescope Compact Array (ATCA) between 2024-02-29 10:40:40 UT and 2024-02-29 19:37:40 UT (MJD 60369.63 +/- 0.19). Data were recorded simultaneously at central frequencies of 5.5 GHz and 9 GHz, with 2 GHz of bandwidth at each central frequency. We used PKS B1934-638 for bandpass and flux calibration, and the nearby (3.49 degrees away) source B1414-59 for phase calibration. Data were flagged, calibrated, and imaged following standard procedures with the Common Astronomy Software Application (CASA, v5.1.2; CASA Team et al. 2022). We imaged using a Briggs Robust parameter of 0 to balance sensitivity and resolution. This choice resulted in a synthesised beam of 2”x1” at a position angle of 160 degrees east of north at 5.5 GHz and 1.4”x0.7” with a position angle of 160 degrees east of north at 9 GHz.

A point source was detected at a position consistent with the updated X-ray position from Chandra (ATel #16510). The radio source was detected at both frequencies. We measure a best radio position (at 9 GHz) of:
R.A. (J2000): 14:44:59.0 +/- 0.2” and
Dec. (J2000): -60:41:56.1 +/- 0.4”,
where the uncertainties include the statistical and systematic uncertainties added in quadrature. A consistent position was measured at 5.5 GHz.

For this source, fitting for a point source in the image plane, we measure flux densities, S_{ν, of 200 +/- 15 μJy at 5.5 GHz and 170 +/- 15 μJy at 9 GHz. This corresponds to a radio spectral index, α= -0.3 +/- 0.5, where Sν ∝ ν^α. The measured spectral index can be consistent with emission from either a compact jet or discrete ejecta from an X-ray binary. We note that the observations occurred during poor weather, which may result in phase decorrelation that affects higher radio frequencies more strongly, artificially steepening the radio spectrum (by reducing the 9 GHz radio flux density more than at 5.5 GHz). This has not yet been tested or quantified.}

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.