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

Abstract

Accreting neutron stars in low-mass X-ray binaries show outflows-and sometimes jets-in the general manner of accreting black holes. However, the quantitative link between the accretion flow (traced by X-rays) and outflows and/or jets (traced by radio emission) is much less well-understood for neutron stars than for black holes, other than the general observation that neutron stars are fainter in the radio at a given X-ray luminosity. We use data from the deep MAVERIC radio continuum survey of Galactic globular clusters for a systematic radio and X-ray study of six luminous (L X > 10 34 erg s −1 ) persistent neutron star X-ray binaries in our survey, as well as two other transient systems also captured by our data. We find that these neutron star X-ray binaries show an even larger range in radio luminosity than previously observed. In particular, in quiescence at L X ∼ 3 × 10 34 erg s −1 , the confirmed neutron star binary GRS 1747-312 in Terzan 6 sits near the upper envelope of the black hole radio/X-ray correlation, and the persistently accreting neutron star systems AC 211 (in M15) and X1850-087 (in NGC 6712) show unusual radio variability and luminous radio emission. We interpret AC 211 as an obscured "Z source" that is accreting at close to the Eddington limit, while the properties of X1850-087 are difficult to explain, and motivate future coordinated radio and X-ray observations. Overall, our results show that neutron stars do not follow a single relation between inflow and outflow, and confirm that their accretion dynamics are more complex than for black holes.

Low-Mass X-ray Binaries

“…Finally, there are a handful of other LMXBs that, like V404 Cyg, are not observed to be particularly luminous but are suspected to be intrinsically accreting around the Eddington limit. Evidence pointing in this direction comes e.g., from their orbital period evolution [309], or their strong jets and disk winds [310]. For these sources it appears that the most luminous inner part of the binary is obscured, e.g., due to a high inclination or because of strong disk winds.…”