ATCA radio detection of the new outburst from the neutron star X-ray binary 4U 1608-52 (QX Nor)
ATel #17037; T. Russell (INAF/IASF Palermo), J. van den Eijnden (UvA), N. Degenaar (UvA), F. Carotenuto (INAF/OAR), A. J. Tetarenko (U. Lethbridge), T. Maccarone (Texas Tech.)
on 13 Feb 2025; 21:55 UT
Credential Certification: Thomas Russell (thomas.russell@inaf.it)
Subjects: Radio, X-ray, Neutron Star, Transient
Following the recent report of a possible X-ray brightening of the neutron star low-mass X-ray binary 4U 1608-52 (QX Nor; ATels #17013, #17036), we triggered radio observations with the Australia Telescope Compact Array (ATCA), project code C3580. Observations were carried out between 2025-02-11 16:13 UT and 2025-02-11 19:52 UT, with the array in an EW352 configuration. Data were recorded simultaneously at central frequencies of 5.5GHz and 9GHz, with 2GHz of bandwidth at each frequency. PKS B1934-638 was used for bandpass and flux calibration, and B1600-489 was used for phase calibration. The data were calibrated and imaged following standard procedures within the Common Astronomy Software Application (CASA, v5.3; CASA Team et al. 2022). Imaging used a Briggs weighting scheme with a robust parameter of 0, balancing sensitivity and resolution, while helping to reduce the effects of diffuse emission present in the field.
A radio source was detected at a position consistent with the known X-ray position of 4U 1608-52. Fitting for a point source in the image plane, we measured preliminary flux densities of 200 +/- 30 μJy at 5.5 GHz and 205 +/- 25 μJy at 9 GHz. These flux densities correspond to a radio spectral index, α, of +0.1 +/- 0.3, where Sν ∝ να, consistent with radio emission from a self-absorbed compact jet.
Swift-XRT observed 4U 1608-52 starting 2025-02-13 06:51:00 UT with an exposure time of 580s. Preliminary analysis using the XRT pipeline (Evans et al. 2009, MNRAS, 397, 1177) shows an 0.3-10keV spectrum that can be well fit by an absorbed powerlaw with an NH of (1.82 +/- 0.09)E22 cm-2 and a photon index, Γ, of 1.46 +\- 0.05. We measure an unabsorbed 0.3-10keV flux of (3.7 +/-0.9)E-9 erg cm-2 s-1.
The radio spectral index and X-ray photon index are consistent with values expected from an X-ray binary during a hard X-ray state.
Further radio observations are planned. Multiwavelength observations are 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.
