VLA detection of Aql X-1 during its new outburst
ATel #16823; T. D. Russell (INAF-IASF Palermo), S. Fijma, N. Degenaar (UvA), E. Kuulkers, C. Sanchez-Fernandez (ESA), M. Del Santo (INAF-IASF Palermo), A. Tetarenko (U. Lethbridge), M. Diaz-Trigo (ESO), T. Maccarone (Texas Tech), J. C. A. Miller-Jones (Curtin)
on 20 Sep 2024; 22:22 UT
Credential Certification: Thomas Russell (thomas.russell@inaf.it)
Subjects: Radio, Neutron Star, Transient
Following the announcement of a new outburst from the neutron star X-ray binary Aql X-1 (Atels #16821, #16822) we triggered radio observations with the Karl G. Jansky Very Large Array (VLA), project ID VLA/24A-127. The radio observations were carried out on 2024-09-20 with the telescope on source between 02:48 UT and 02:58 UT. The VLA was in an extended BnA configuration. Data were recorded at central frequencies of 5GHz and 7.5GHz, with 1GHz of bandwidth at each frequency. We used 3C 286 for bandpass and flux calibration, and J1856+0610 for phase calibration. Data were processed using the Common Astronomy Software Applications (CASA) pipeline (version 6.5.4.9; The CASA Team et al. 2022). Imaging was carried out at each frequency band using natural weighting, resulting in a beam size of 1.4"x1.1" at a position angle of 4.5 degrees east of north at 5GHz and 0.9"x0.7" with a position angle of 8 degrees at 7.5 GHz.
The CASA images show a point source at the Aql X-1 position. Fitting for a point source in the image plane, we measure flux densities of 52 +/- 13 μJy at 5GHz and 80 +/- 12 μJy at 7.5GHz. These flux densities suggest a flat-to-inverted radio spectrum, with a spectral index, α, of 1.2 +/- 1.0, where Sν ∝ να. This is consistent with radio emission from a self-absorbed compact jet (Diaz-Trigo et al. 2018), typically associated with the hard X-ray state of an X-ray binary.
Further radio observations are planned. Multiwavelength observations are strongly encouraged.
