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 7 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, L R ∝ L X β , 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 (5 × 10 34 < L X < 3 × 10 35 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, 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 L X ∼ < 5 × 10 35 erg s −1 . We discuss these results in the context of other similar studies.

A MeerKAT survey of nearby nova-like cataclysmic variables

“…Tremou et al (2020) for a recent discussion), the picture is more complicated in the case of NS systems. The correlation has been studied in depth in only a few individual systems (Migliari et al 2003;Tudose et al 2009;Tetarenko et al 2016a;Gusinskaia et al 2020), with different values of the power-law index proposed by different groups and in different sources. More recently a 'universal' NS correlation index of 0.44 has been suggested (Gallo et al 2018, although see Tudor et al 2017.…”

The MAVERIC Survey: Variable Jet-Accretion Coupling in Luminous Accreting Neutron Stars in Galactic Globular Clusters

“…XB 1832-330 is a persistent X-ray source, and its deep radio upper limit at L X ∼ 2 × 10 35 is among the most constraining observed at this L X , comparable to that seen in Aql X-1 during its fading after an outburst (Gusinskaia et al 2020b). As in Aql X-1, the XB 1832-330 data are consistent either with a steep radio/X-ray correlation for this binary or with a quenching of the jet at this luminosity.…”

“…Dark green circles show quiescent/hard state black holes. Quiescent/hard state neutron stars are shown as blue squares, with additional data points from Aql X-1 (Gusinskaia et al 2020b), and soft sources shown as orange pluses from (Ludlam et al 2019). Pink stars are accreting millisecond X-ray pulsars and light green triangles are transitional millisecond X-ray pulsars.…”

“…Others, such as the accreting millisecond pulsar IGR J17591-2342, are likely as radio-loud as accreting black holes over some range in X-ray luminosity, perhaps reaching down to L X ∼ 4 × 10 35 -10 36 erg s −1 (Russell et al 2018;Gusinskaia et al 2020a). The deviations occur not only in normalization but in slope: there is good evidence that in at least in some systems, the radio emission becomes much fainter, or is quenched entirely, at L X 10 36 erg s −1 (Gusinskaia et al 2017(Gusinskaia et al , 2020b.…”

The evolving radio jet from the neutron star X-ray binary 4U 1820−30

“…Jets from accreting NS LMXBs show a diverse range of behaviour (e.g., Migliari & Fender 2006;Migliari 2011;Tudor et al 2017;Gusinskaia et al 2017;Gallo et al 2018;Russell et al 2018;Gusinskaia et al 2020), and there has been some debate as to whether NS LMXB jets show a similar pattern of behaviour to BH systems (see, e.g., Migliari & Fender 2006;Marino et al 2020) NS and BH LMXBs can launch two types of jets depending on the state of the source: a steady compact jet or a bright transient jet (e.g., Migliari & Fender ★ E-mail:thomas.russell@inaf.it 2006), depending on the mass accretion rate ( ) and structure of the accretion flow. At lower during the harder X-ray spectral states the compact jet is characterised by optically-thick synchrotron emission exhibiting a flat to inverted radio spectrum ( ∼ 0, where the radio flux density, , is proportional to the observing frequency, , such that ∝ ; Blandford & Königl 1979).…”