Quasi-simultaneous radio and X-ray observations of Aql X-1 : probing low luminosities
Abstract
Aql X-1 is one of the best-studied neutron star low-mass X-ray binaries. It was previously targeted using quasi-simultaneous radio and X-ray observations during at least seven different accretion outbursts. Such observations allow us to probe the interplay between accretion inflow (X-ray) and jet outflow (radio). Thus far, these combined observations have only covered one order of magnitude in radio and X-ray luminosity range; this means that any potential radio-X-ray luminosity correlation, LR ∝ LXβ, is not well constrained (β ≈ 0.4-0.9, based on various studies) or understood. Here we present quasi-simultaneous Very Large Array and Swift-XRT observations of Aql X-1's 2016 outburst, with which we probe one order of magnitude fainter in radio and X-ray luminosity compared to previous studies (6 × 1034 erg s-1 < LX <3 × 1035 erg s-1, I.e. the intermediate to low-luminosity regime between outburst peak and quiescence). The resulting radio non-detections indicate that Aql X-1's radio emission decays more rapidly at low X-ray luminosities than previously assumed - at least during the 2016 outburst. Assuming similar behaviour between outbursts, and combining all available data in the hard X-ray state, this can be modelled as a steep β =1.17^{+0.30}_{-0.21} power-law index or as a sharp radio cut-off at LX ≲ 5 × 1035 erg s-1 (given our deep radio upper limits at X-ray luminosities below this value). We discuss these results in the context of other similar studies.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- February 2020
- DOI:
- 10.1093/mnras/stz3420
- arXiv:
- arXiv:1908.04778
- Bibcode:
- 2020MNRAS.492.2858G
- Keywords:
-
- stars: neutron;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 15 pages, 11 figures, 3 tables, accepted for publication in MNRAS